is a vessel or building, for the purpose of containing combustible materials, whether of coal or wood, and so constructed that great heat may be produced and concentrated. There is a great variety of furnaces, and they are variously constructed, according to the views of the operator, and the purposes to which they are applied. But in all furnaces there are four things which require to be particularly attended to. 1. To be able to concentrate the heat, and direct it as much as possible to the substances which are to be acted upon. 2. To prevent the dissipation of the heat after it is produced. 3. To obtain the greatest quantity of heat from the smallest quantity of fuel; and 4. To be able to regulate at pleasure the necessary degree of heat, or to have it under proper management.
1. To accomplish the first object, namely to concentrate the heat, it is usual to confine the fire in a chamber or cavity properly constructed, furnished with a door or opening, by which the fuel is introduced; a grate for supporting it, and allowing a free passage to the air, as well as for the ashes to fall through into the cavity below, called the ash-pit. In this way the heat produced by the combustion of the fuel is confined by the sides of the furnace, and so concentrated that its force is chiefly spent on the substances inclosed.
2. The dissipation of the heat is prevented by keeping the door of the furnace shut, by constructing the chimney no wider than to allow a passage for the smoke, and placing the substance to be acted upon in such a manner that the fire may have its full effect as it goes up the chimney.
3. The third object, which is not the least important, is to produce the greatest quantity of heat from the smallest quantity of fuel. In an economical point of view, this object is worthy of the greatest attention, though it is often difficult to attain it. In this view much depends upon the proportion between the spaces between the bars of the furnace, and the wideness and height of the chimney. This is obvious from considering the circumstances which regulate the process of combustion; for this depends on the current of air passing through the combustible matter. When the fuel in the furnace is kindled, a certain degree of heat is produced; but without a current of fresh air passing through the burning matter, the fire is instantly extinguished; and without this stream of fresh air the inflammation cannot go on. But when this takes place, the air within the furnace is rarefied, and being no longer a balance for the external air, it is driven up the chimney by a current of denser air, rushing in at the openings. This having passed through the fuel, is also rarefied, and passes off, giving place in its turn to a new current, so that in this way there is a constant flux of air up the chimney. From this it must appear, that the greater the rarefaction of the air in the fire-place is, the greater will be the intensity of the heat produced. By constructing a furnace in a particular way, the heat may be so managed that the under part of the chimney may be nearly as strongly heated as the fire-place itself; so that, although a strong current of air passes through the fuel, yet as the heat is uselessly spent on the chimney, there is a great and unnecessary waste of fuel. To prevent this, there is a contrivance by which the throat of the chimney is occasionally contracted, by means of a sliding plate, which, when it is pushed in, closes up the whole vent; but may be drawn out in such a way as to form a larger or smaller opening as may be thought necessary. Till the fuel is thoroughly kindled, and the furnace fully heated, the plate should be quite drawn out, so that the largest column of air which the furnace will admit, may pass through the fuel. The plate is then put in to a certain length, and so regulated that the smoke may be prevented from issuing at the door of the furnace. The current of air increases in proportion to the rarefaction of the air in the fire-place, and this increases the inflammation of the fuel; and the heat now being reflected from every point of the furnace, excepting the narrow passage by which the smoke passes off, becomes extremely intense. If a large quantity of fuel be introduced at once, it will consume slowly, and require little attention, in comparison with those furnaces where this precaution is not observed. When the intensity of the heat is not very great, the sliding-plate may be of cast iron; but to resist great degrees of heat, it will be found more convenient to have it made of fire clay. But it must be observed, that the advantage derived from the sliding-plate is lost to those furnaces which are of a large construction, and where great quantities of metal are to be melted; and there it is commonly found, that the waste of fuel is very great.
4. To attain the fourth object, namely, to be able to regulate conveniently the degree of heat, a certain proportion of air only is to be allowed to pass through the fuel. With this view it is necessary to have the command of the furnace below, because the parts above are often filled with small quantities of foot. To manage this in the most effectual manner, the door of the ash-pit is to be perfectly closed, and furnished with a series of round holes which have a certain proportion to each other. In the furnaces constructed according to Dr Black's direction, the areas of these holes are as 1, 2, 4, 8, 16, &c. in geometrical progression. Seven or eight of these in the door of the ash-pit give a sufficient command over the fire. When the utmost intensity of heat is required, all the passages are thrown open, and the height of the chimney is increased, so that the height of the column of rarefied air being augmented, the motion of the current of air through the fuel is proportionably more rapid, and consequently the heat of the furnace becomes more intense. In the construction of a furnace recommended by Macquer, another tube is applied to the ash-pit, having the extremity most distant from the furnace widest, and gradually tapering as it approaches it. By this contrivance, it was proposed to increase the velocity of the current of air as it passes from a wider into a narrower tube. But it is found that the air will not ultimately move with greater velocity than if the tube were not applied. It may indeed be useful where the furnace is placed in a small apartment, and the tube itself forms a communication with the external air.
After these preliminary observations on the general principles of furnaces, we propose in the following treatise to give a short account of the construction and application of some of the more important furnaces which are employed in the arts and manufactures.
But before we enter into the detail and description of particular furnaces, we shall lay before our readers the description of one which was invented by Meffrs Robertsons of Glasgow, for the purpose of consuming its own smoke, and saving fuel.
"To construct furnaces (says the editor of the Philosophical Magazine, from which this account is taken), on such a principle as should enable them to consume their own smoke has long been a desideratum; and we believe the public in general, but especially those who have been annoyed by the smoke of steam engines, foundries, and similar erections in their neighbourhood, will be glad to learn that a furnace has been contrived which effectually gains this end.
"The construction is extremely simple, and will be easily understood by the following description, and the plate to which it refers.
Fig. 1. represents a vertical section, and fig. 2. a front view of a steam-engine boiler, furnished with one of Meffrs Robertsons furnaces; and the same letters refer in both to the same parts of the construction.
"The opening A, through which the fuel is introduced into the furnace, is shaped somewhat like a hopper, and is made of cast iron built into the brick-work H, H. From the mouth it inclines downward to the place where the fire rests on the bottom grate B. The coals in this mouth-piece or hopper answer the purpose of a door (A), and those that are lowest are by this means brought into a state of ignition before they are forced into the furnace. Below the lower plate of the hopper K, e the furnace is provided with front bars G (b), which not only serve to admit air among the fuel, but offer a ready way to force the fuel back, from time to time, from c to d (c), to make room for fresh quantities to fall into the furnace from the hopper or mouth-piece. By this arrangement the fuel is brought into a state of ignition before it reaches the farther side of the bottom grate, where it is stopped by the rising breast, b, of the brick-work, so that any smoke liberated from the raw coals in the mouth-piece, must pass over these burning coals before it can reach the flue FFF. But this, though it would cause a large quantity of the smoke to be burnt, would not completely prevent the escape and ascent of smoke up the chimney; for it is not merely necessary that the smoke should be exposed to a heat sufficient to ignite it before it escapes: unless, at the same time, a quantity of fresh air, able to furnish a sufficiency of oxygen for the combustion of the smoke, can be brought into contact with it, it will still escape in an undecomposed state. The judicious admission of fresh air, in such a manner that it can reach the smoke, without previously passing through the fire, and parting with its oxygen in its passage, and in such quantity as not to cool the bottom of the boiler, but merely to cause the smoke to burn, constitutes the chief merit of this invention; and to us it appears that it will fully answer the proposed end. Below the upper side of the mouth-piece or hopper, and at about the distance of three-fourths of an inch from it, (this space being a little more or less, according to the size of the furnace), is introduced a cast iron plate a n. This plate is above the fuel, and the space between it and the top of the hopper is open for the admission of a thin stream of air, which, rushing down the opening, comes first in contact with that part of the fire which is giving off the greatest part of the smoke, viz. the fuel that has been last introduced, mixes with it before it passes over the fuel in the interior, which is in a high state of combustion, and enables it to enflame so completely, that not a particle of smoke ever escapes undecomposed.
"The quantity of air thus admitted to pass over the upper surface of the fire, is regulated by a very simple contrivance. The plate a n rests at each end on a fluid, or pin, projecting from the cheeks of the mouth-piece A, or is furnished at each end with a pivot which works in the cheeks; the said pins or pivots being placed about midway between the outside and inside of the mouth-piece or hopper, so that, by elevating or depressing the edge a of the plate, the opening at n is enlarged or diminished. When that degree of opening which produces the best effects are obtained, which is easily known, the plate a n is kept in its place by means of a piece of iron introduced above it, and answering the purpose of a wedge.
"Under the grates is the ash-hole I, the upper part of which is furnished with doors SS, which, when shut, prevent the heat from the front bars G from coming out into the apartment, and incommoding the workmen.
"Invited by an advertisement, we went to Meffrs Bunnell
(a) "In the management of this furnace, what is chiefly to be attended to is, that the hopper be kept full of coal, and either wholly or in part small coal, to prevent, as much as possible, air getting in by that passage; it is also necessary at some times to use a shutter of thin plate-iron, to be applied to the mouth of the hopper to exclude the entrance of air by that passage.
(b) "These bars are, in fact, a grated door, kept in their position by a catch L, and which may be opened at pleasure for cleaning the fire out. In small furnaces an opening here is all that is necessary; the bars may be dispensed with.
(c) "Between the back end, d, of the bottom bars, and the breast brickwork b, is represented in the plate a section of a shutter, which is sometimes opened for the purpose of getting out the refuse of the fuel." FUR
Bunnell and Silver, Bedford-street, Covent Garden, to see one of these furnaces at work, and we were not a little gratified in observing that the smallest appearance of smoke could not be perceived issuing from the top of the chimney. The advantages of such an improvement can hardly be better illustrated than by mentioning what had actually happened with this steam engine. The smoke, before the improved furnace was employed, inconvenienced the neighbourhood so much, that it was flopped as an intolerable nuisance. Now it is so far from disturbing any one, that, without being admitted to see the engine, it would be actually impossible to know when it is at work.
"These furnaces, we understand, have also been adopted by many intelligent manufacturers at Leeds and at Manchester. At the latter place, if we may credit newspaper reports, several manufacturers have had their works indicted as nuisances for not having adopted the improvement; the magistrates arguing, that, though the welfare of the place required that such inconveniences should be submitted to while no possible cure for them was known, the health and comfort of the inhabitants equally demand, now that the evil can be done away, that smoking furnaces should not be permitted in the place.
"We earnestly recommended to owners of steam engines, and also to those who are annoyed by them, to endeavour to bring this improvement into general use. Indeed, we entertain no doubt of its being universally adopted sooner or later; for it yields advantages not only in point of cleanliness, comfort, and health, but also in point of interest; all the smoke usually discharged at the top of the chimney, being in fact, so much good fuel, that only wanted the contact of fresh air to inflame it under the boiler. It is a fact well known, that the flame which is often seen issuing from the chimneys of founders, &c. has no existence except at the top of the chimney: while ascending the flue it is only dense smoke, consisting of the azote of the atmospheric air decomposed in passing through the fire, of hydrogen, coal tar, and carbonaceous matter, of such a high temperature, that it only wants oxygen to make it flame spontaneously: this it obtains from the atmospheric air into which it ascends, and then presents such appearances as would make a hasty observer adopt the opinion that the flame had ascended, as flame, from the fuel in the furnace; which is by no means the case. A consideration of this simple fact will convince any person that it is not an inconsiderable proportion of the fuel that is thus wasted. Nor is this the only loss sustained; the quantity of heat required not merely to render such a portion of the fuel volatile, but to give it to a temperature able to produce the effect of which we have taken notice, is itself furnished at the expense of an extra and unnecessary quantity of fuel. The whole waste in many cases is, we are persuaded, not less than for an eighth of the whole fuel employed."
One of the most important furnaces, particularly for this country, where, although great and essential improvements have been made by industry and ingenuity, the manufacture is yet in its infancy, is that for the melting of iron.
We shall therefore enter more fully into the detail of the history, construction, and general principles of the operation of blast furnaces; and in tracing their progressive history, it may be observed, that in this country it has experienced a revolution, of which no analogous instance has occurred in other countries.
In the early and barbarous periods of society, before the introduction of agriculture, the surface of a country is usually covered with extensive forests. From this circumstance wood, as being most accessible, abundant, and of easiest application, is usually employed by mankind for the purposes of fuel. In the progress of population and improvement, other advantages were derived from the general use of wood as fuel; and among these the improvement of the climate, and clearing land for the purposes of agriculture, were none of the least. The application of wood as fuel to different manufactories, had no doubt also an early origin; and in the manufacture of iron, if conducted on a scale of any extent, the demand for fuel of this kind must have been very great. If, then, during the gradual improvement and prosperity of this country, this manufacture, in place of remaining stationary, or declining, from diminished consumption, has increased in capital and extent, without some substitute for wood, the art would have been long before this time entirely lost, because it depended on a stock which must have rapidly declined, and even its very existence was often far from being compatible with the views and interest of landholders. Such were the circumstances in which Great Britain was placed, from the reign of Charles II. to the middle of the 18th century. During this period, being in a prosperous state, the manufactures and commerce of the country increased the demand for iron, while the supply of wood, one of the most necessary materials in its manufacture, was greatly diminished. It is true, indeed, that, previous to this period, pit-coal had been employed as a substitute; but the prejudice of some, and the selfish views of others, and especially the want of sufficient mechanical powers, obstructed the progress of this mode of manufacture. When, however, these difficulties were surmounted, and it was found that the change of fuel in the blast furnace was likely to prove beneficial, this manufacture acquired new vigour, and improvements succeeded each other in rapid succession. In a period of about 50 years, a complete revolution was effected, not only in relinquishing the mode of making iron with charcoal and in employing pit-coal in the blast furnace, but also in the immense increase of the manufacture.
At what period the manufacture of iron commenced and grew in Britain, cannot be precisely ascertained. It has, however, been supposed, that the Phoenicians, who wrought the tin mines of Cornwall, may have introduced into the country men who were skilled in metallic ores, and were capable of estimating their value, by converting these mineral riches to such purposes as their own necessities, or the wants of the inhabitants, might require. It is probable also, that the invasion of England by the Danes, and their establishment in this country, added something to their former knowledge in the art of mining and manufacturing the ores of iron. In support of this conjecture, the large heaps of scoria found in many parts of England, and having a considerable thickness of soil upon them, have been denominated from time immemorial, "Danes cinders;" and indeed so early as the year 1620, large casks were found in a state of decay, upon the tops of some of those hills But although these may have been very ancient manufactures, it is the less probable that the production of these cinders is to be ascribed to the blast furnace; for at that remote period the manufacture was chiefly directed to the fabrication of small portions of malleable iron, in what were called foot-blasts and bloomeries. The art of casting or moulding in iron was either altogether unknown, or in so rude a state, that it could not be prosecuted with much prospect of advantage. Pig or cast iron, if it was at all produced, was then of the most refractory nature for being converted into malleable iron. It was not till a future period, when improvements had been made in machinery, and the advantages of a division of labour were known, that different furnaces were constructed; one for manufacturing pig iron, and another for converting it into malleable iron. To this the blast furnace seems to have owed its existence, and it is to be considered as an improvement of the advantages which are derived from a division of labour. The blast furnaces being exclusively appropriated to the making of pig iron, the attentive manufacturer would soon perceive that the products of the furnace were often different from each other. Repeated observation and experience would enable him to ascertain what was the cause of this difference. Observing that an additional quantity of fuel rendered the forged pig iron more fusible, this circumstance would suggest the practicability of casting it into shape. Hence probably arose the art of moulding, which afterwards, as well as the bar-iron forge, became an appendage to the blast furnace. After this new manufacture became familiar, the advantage of dividing the product of the blast furnace into gray melting iron, or into forged pigs, according to the demand, would be obvious.
In the year 1615, according to Dudley, who has stated the fact in his Metallum Martis, there were no less than 300 blast furnaces in England for smelting iron ore with charcoal, and each furnace was supplied with fuel upon an average of 40 weeks in the year. Taking the average produce of pig iron at each furnace of 15 tons per week, or 600 tons per annum, the total annual quantity will amount to 180,000 tons, which is a greater quantity than has ever been produced in Britain since that period. It is supposed that this quantity may be greatly exaggerated, but at the same time it is allowed that the iron manufacture was, at this early period, highly prosperous and productive. But in the progress of agriculture and the increase of population, it was necessary to clear the land for the purpose of cultivation. From this circumstance, as well as from the great consumption of wood for the navy, the supply of fuel was greatly diminished; so that the iron manufacture became consequently less productive.
It is curious to remark that, although pit-coal was known long before this period, and was wrought at Newcastle previous to the year 1272, and great quantities of it were annually exported to Holland and the Low Countries, and was used in the smith's forge, and other manufactures which require a strong continued heat, yet in England the prejudice against its use in the manufacture of cast iron was so inveterate, that when it was first proposed and attempted, every obstacle which could be devised was thrown in its way. During the reign of James I. several patents were granted for the exclusive privilege of manufacturing iron with pit-coal.
None of the adventurers, however, succeeded in their Furnace attempts till the year 1619, when Dudley made pig-iron in a blast furnace, but produced only three tons in the week. At this time the price of iron had risen, in consequence of many of the iron works having stopped for want of wood as fuel. To those manufacturers, therefore, who could still be furnished with a supply of wood, the manufacture was highly profitable, so that they opposed any new attempt by which the price of iron was likely to be diminished.
After this period, the progress of the iron manufacture was greatly interrupted from other causes. Amidst the distraction occasioned by the civil wars which raged in England, little improvement was to be expected. It appears, however, that patents were granted during the Commonwealth, for the exclusive privilege of manufacturing iron in the new way; and in one of these, it was believed at the time, that the Protector himself had a share. All these experienced the fate of the former, and no manufacture of any extent was successfully established. In the year 1663, Dudley in his application for his last patent, stated that he could produce at one time seven tons of pig iron in the week with a furnace of an improved construction, 27 feet square, and with bellows which one man, without much fatigue, could work for an hour.
Thus, as the demand for wood for the purposes of fuel in this manufacture increased, and the growth of timber was greatly diminished, the manufacturer was forced by necessity to have recourse to the use of pit-coal; and when various valuable improvements had been made on machinery, and particularly when the beneficial effects of the steam engine had been ascertained, the iron manufacturer saw himself in possession of a command of power in the management of his materials, of which he had formerly no conception. The small furnace supplied with air from bellows constructed of leather, which was moved by means of oxen, horses, or men, went into disuse, when larger furnaces were introduced, with an increase of the column of air, for the purpose of exciting combustion. But at this period, when the manufacture derived new vigour from the introduction of the steam engine, and the general improvement in machinery, it seemed, from the operation of other causes, and particularly from the deficiency of fuel, to decline rapidly. The demand for iron in the manufactured state, and particularly for bar iron, had increased, while the quantity produced gradually diminished. Recourse was now had to foreign markets for a supply, and the importation of Russian and Swedish iron then commenced. Of the 300 blast furnaces spoken of by Dudley, 59 only existed; and estimating their annual produce at about 295 tons to each furnace, the total amount did not much exceed 17,000 tons.
Such was the state of the manufacture of iron in England and Wales, before the introduction of pit-coal; and thus it appears, that in a period of from 100 to 130 years, it had suffered a diminution of more than 50,000 tons annually. It proved of singular benefit to this manufacture, that the steam engine, which had then become a powerful machine, was introduced, for the purpose of raising and compressing the air, and could be employed in those places where materials were abundant, but where there was a deficiency of water for moving the machinery. Besides, experience now taught FUR
the manufacture, that the produce of his furnace could be increased by enlarging the diameter of the steam cylinder, for rendering the vacuum under the piston more perfect; and it was soon found that, by increasing these effects, such a quantity of pig iron could be produced from the coal of pit-coal, as would be attended with a suitable profit. It is scarcely to be wondered at, that this circumstance should have long remained a secret; for a small quantity of air only being necessary to ignite the charcoal furnace, whether it arose from the peculiar inflammability of the fuel, or the small capacity of the furnace, it was always under the eye of the manufacturer, and he would more frequently experience the inconveniences of overblowing than underblowing the furnace. It seems too extremely probable, that pit-coal, being considered in every respect inferior to charcoal, the manufacturer would proceed with great caution in enlarging the column of air, or increasing its density; and thus the advantages to be derived from its use would be in a great measure lost. When, however, experience had taught them a different lesson, the limits to the quantity of air that might be directed to a coal blast furnace, before any injurious effects arose, were not very observable. It was found, indeed, that the density of air diminished the quantity of the produce, and the same law seemed to hold with regard to pit-coal as well as to wood,—that the softer qualities might be overblown, while the strata of a denser and more compact consistence remained undiminished before a heavier blast.
Between the years 1750 and 1760 the use of pit-coal was pretty generally substituted for charcoal, in the blast furnace. The iron manufacture assumed new vigour, and in a period of 30 years it experienced in England and Wales a very remarkable progress. From the general and increasing use of pit-coal, it is probable that many of the charcoal works were sooner relinquished than they would otherwise have been. The history of the celebrated foundry of Carron in Scotland, affords us a curious instance of the progress of the use of pit-coal in this manufacture. These extensive operations commenced about the year 1760. The blowing, as was the practice at the time, was performed by means of large bellows, moved by a water wheel. But as there was a scanty supply of air, and as this was deficient in density, the weekly produce of the furnace rarely exceeded 10 or 12 tons, and often in summer this quantity was considerably diminished. With a view to improve the operation, immense quantities of wood charcoal were prepared, and it was found that the process of melting succeeded much better with this kind of fuel than with the mineral coal which was dug out in the neighbourhood. But in the improvement of machinery, more effectual means were discovered to procure a blast of sufficient force and density for the ignition of pit-coal, wheels of greater power were constructed; the use of the bellows was relinquished, and in their place large iron cylinders, so contrived as to blow both up and down, were introduced. Thus, a larger column of air, of three or four times the former density, was obtained, and the beneficial effects arising from the improvements were soon perceived; for the same furnace which formerly produced 10 or 12 tons in the week, sometimes yielded 40 tons in the same time; and on an annual average, not less than 15,000 tons of Furnace metal.
About the end of the reign of Queen Elizabeth, we are informed by Dudley, that blast furnaces had been constructed on so large a scale, and with such a power of machinery, as to yield a daily produce of more than two tons of charcoal iron; but it is probable that so large a produce could only be obtained in situations where there was a copious supply of water, and where the water wheels and bellows employed were of large size. In the more ordinary modes of conducting this process, furnaces of a much smaller size were employed, and these received the supply of air from hand bellows which were moved by men, and sometimes by cattle. From the superiority of the manufacture of iron guns, mortars, &c. England possessed at this time a considerable export trade; but as pit-coal had not yet been applied to any departments in the manufacture of iron, it seems probable that these articles were cast from the large blast furnaces, because the flame of wood, comparing it with that of pit coal, possessing but feeble effects, would render the application of the reverberating furnace (if it was then known), of no use in the casting of guns and mortars. The want of pit-coal in every department of the foundry, greatly retarded the perfection to which the art of moulding might have arrived, and even obstructed its improvement. The backward state in which the art of casting and moulding long remained in this country, shewed that the want of this material of the smelting fuel in the blast furnace was long severely felt; and owing to this, other nations, who in many other respects enjoyed fewer advantages, made more rapid progress in the improvement of this manufacture. Before this period, it is not improbable that the use of pit-coal might have been suggested to the manufacturer, and that this material, employed as a fuel, might have been considered as an auxiliary, or as a substitute in various departments of the process. The inflammability of this substance, and its tendency to be converted into a cinder, as well as the general decay of wood, would afford sufficient ground for what might be considered by many as a useless speculation. The benefits of this manufacture as it then stood, had been carefully investigated, and fully appreciated by those who were interested in it. The supply of wood only seemed to limit its extent, but for want of a sufficient supply of materials, the establishment of new works became impracticable, those already engaged in the business were anxious to preserve the supply they enjoyed, however limited, rather than encourage any innovation or change in the process, which, by the substitution of pit-coal for charcoal from wood, would probably give to new adventurers and speculators a superiority of the market. Besides, many of the furnaces which were then going, were at a great distance from pit-coal, so that the general use of this fuelance, and the advantages to be derived from it, would be highly injurious to their interests.
Such was the state of this manufacture when the use of pit-coal in this process was discovered, or when it was proposed to employ it for this purpose. With this view, James I, in the year 1612, granted a patent to Simon Sturtevant, for the exclusive manufacture of iron with pit-coal, for the period of 31 years. In obtaining this privilege, privilege, the patentee obliged himself to publish a full account of his discoveries, and this appeared in a work in quarto, under the title of "Metallica." It appears, however, that Sturtevant had not succeeded in his schemes; for in the following year he gave up his privilege, but it is not known to what causes the failure is to be ascribed.
After Sturtevant, a John Ravenson, embarked in the same hazardous undertaking; and although he procured a patent without much trouble, he had soon to encounter difficulties in the way of ultimate success, analogous to those which had prevailed over the perseverance of Sturtevant, and induced him to relinquish the farther prosecution of his schemes. He obtained his patent on conditions similar to those on which his predecessor procured it, in consequence of which he published his "Metallica" in 1613. All his successors were like him, obliged to resign their patents from the want of adequate success.
Dudley procured his patent in the year 1619, and notwithstanding he affirmed that he manufactured not more than three tons per week, he found it a lucrative undertaking. This discovery he brought to perfection at the works of his father in Worcestershire; but by the influence of those who wished to share in the emoluments arising from the manufacture of iron with pit-coal, his patent was limited to 14 instead of 31 years. He informs us himself, that, during the greater part of this period, he was enabled to fell pig and bar iron much cheaper than any of his competitors; but as his remarkable success drew their envy upon him, his devoted works were at length destroyed by a lawless mob, urged on, it is supposed, to perpetrate so atrocious a deed by his rivals in business. In this unmerited treatment of the fanguiue but unfortunate Dudley, the coke pig process unquestionably experienced an irreparable loss. He had so many rivals to contend with, by virtue of the original ground he occupied as a manufacturer, and his attachment to the cause of royalty was so sincere, that his improvements were effectually prevented from arriving at lasting or general utility. Could he have procured a new patent after the restoration, there is little doubt but he would have again entered with avidity on the laborious paths of discovery. In petitioning for the recovery of his ancient privileges, we find him declaring that instead of three, he was enabled to manufacture seven tons per week of coke pig iron, in consequence of a large furnace, and an improved bellows.
To stand clear as much as possible of the method of operation which Dudley had discovered, one Captain Buck, Major Wildman, and some others, constructed large air-furnaces in the forest of Dean, into which they put clay pots, for containing the requisite preparations of ore and charcoal. Pit-coal was employed for the purpose of heating the furnaces; and it is highly probable that these new adventurers were fanguiue enough to believe that, by tapping the pots below, the separated metal would flow out. This strange method of assaying was soon found impracticable; for the heat was not of sufficient intensity to produce an entire separation; the pots gave way, and the prosecution of this ridiculous scheme was speedily relinquished.
The manufacture of iron received no farther improvements for about a century after this period. It was found to be practicable; but how to procure such a quantity as to produce a lucrative return, was not to be derived from the mere knowledge of the particular proportions of the raw materials. Had machinery reached that degree of perfection in the time of the ill-fated Dudley which it has since done, we have good reason to believe that the rapid progress of the pig iron manufacture would have dated its origin from the era of that enterprising genius.
We shall conclude this historical account of the iron Produce of manufacture, with a view of the progressive quantity furnaces in produced at the different furnaces in Great Britain at different periods.
<table> <tr> <th></th> <th>Tons.</th> </tr> <tr> <td>In 1620, the 300 blast furnaces mentioned by Dudley, which existed in England and Wales, produced each at an average</td> <td>250</td> </tr> <tr> <td>At a later period, but previous to the use of pit-coal, 59 furnaces produced each on an average</td> <td>294</td> </tr> <tr> <td>In 1788, 24 charcoal furnaces, which were then going in England, produced each on an average</td> <td>545</td> </tr> <tr> <td>In 1788, 53 blast furnaces, in which coal from pit-coal was used, yielded each on an average, nearly</td> <td>907</td> </tr> <tr> <td>In 1788, eight furnaces in Scotland produced on an average, each</td> <td>875</td> </tr> <tr> <td>In 1796, there were in England and Wales, 104 furnaces, from each of which was obtained on an average</td> <td>1048</td> </tr> <tr> <td>In 1796, 17 furnaces in Scotland produced each on an average</td> <td>946</td> </tr> </table>
But from the above statement we are not enabled to draw an accurate conclusion of the degree of improvement which has been introduced in blowing machinery; for among the furnaces mentioned in 1796, were included a number of charcoal blasts, which yielded only a small produce. But the average produce of iron manufactured at pit-coal blast furnaces, at no less an amount than
<table> <tr> <th></th> <th>Tons.</th> </tr> <tr> <td>At melting furnaces</td> <td>1200</td> </tr> <tr> <td>At forge pig works</td> <td>2000</td> </tr> </table>
To what we have now said, we shall only give a view of the prices of the produce of this manufacture, and different the channels of consumption for this immense quantity of materials.
<table> <tr> <th></th> <th>Per Ton.</th> </tr> <tr> <td>Charcoal pig iron sold in 1620 for</td> <td>L.6 0 0</td> </tr> <tr> <td>Ditto for melting in 1788</td> <td>8 0 0</td> </tr> <tr> <td>Ditto in 1798</td> <td>9 10 0</td> </tr> <tr> <td>Coak pig iron in the time of Dudley</td> <td>4 0 0</td> </tr> <tr> <td>Ditto in 1788,</td> <td>5 10 0</td> </tr> <tr> <td>Ditto in 1798,</td> <td>7 10 0</td> </tr> <tr> <td>Melting iron in 1802,</td> <td>8 10 0</td> </tr> </table>
The produce of pig iron in England and Wales, and in Scotland, from 168 furnaces, has been calculated at the immense quantity of 172,000 tons. It will be impossible to say with absolute precision what are the channels into which this immense quantity of raw materials passes for consumption; but the following views will enable the reader to account for part of it.
<table> <tr> <th></th> <th>Tons.</th> </tr> <tr> <td>Annual consumption in the erection of new furnaces, forges, &c.</td> <td>5000</td> </tr> <tr> <td>M m Annual</td> <td></td> </tr> </table> FUR
Annual consumption at forges in Britain, for the manufacture of bar iron Purchased by government in the state of cannons, mortars, &c on an average of three years, including the waste in melting, &c. with what is employed in the navy as ballast - 70,000 Ditto by the India Company - 14,899 Ditto for merchantmen - 5,700 Ballast for India and merchantmen - 11,000 Ballast for India and merchantmen - 5,000
Let us now consider the construction and general principles of the blast furnace. The term blast is employed at iron foundries, to signify the column of air which is forced into the furnace for the purpose of promoting combustion. The velocity of this blast is produced by the blowing machine impelling the contents of the air-pump through one or two small apertures, and in this way a column of air of various density is produced.
Here we propose to avail ourselves of what has been done by Mr Muhet, formerly of the Calder iron works near Glasgow, a manufacturer himself, who with much philosophical discrimination joins a great deal of excellent practical observation. The many valuable hints which he has suggested, will, we trust, not only be acceptable, but prove highly beneficial in directing and assisting the views and operations of those concerned in this important manufacture.
To have a clear view of his reasonings and observations on the nature and principles of the blast furnace, we shall first give his description of the building and apparatus, and then detail what he has said concerning its management and mode of operation.
Fig. 3. represents a blast furnace with part of the blowing machine. A, the regulating cylinder, eight feet diameter and eight feet high. B, the floating piston, loaded with weights proportionate to the power of the machine. C, the valve, by which the air is passed from the pumping cylinder into the regulator: its length 26 inches, and breadth 11 inches. D, the aperture by which the blast is forced into the furnace. Diameter of this range of pipes 18 inches. The wider these pipes can with convenience be used, the less is the friction, and the more powerful are the effects of the blast. E, the blowing or pumping cylinder, five feet diameter, nine feet high: travel of the piston in this cylinder from five to seven feet per stroke. F, the blowing piston, and a view of one of the valves, of which there are sometimes two, and sometimes four, distributed over the surface of the piston. The area of each is proportioned to the number of valves: commonly they are 12+46 inches. G, a pile of solid stone building, on which the regulating cylinder rests, and to which the flanch and tilts of the blowing cylinder are attached. H, the safety-valve, or cock; by the simple turning of which the blast may be admitted to, or shut off from the furnace, and passed off to a collateral tube on the opposite side. I, the tuyere, by which the blast enters the furnace. The end of the tapered pipe, which approaches the tuyere, receives small pipes of various diameters, from two to three inches, called nofe pipes. These are applied at pleasure, and as the strength and velocity of the blast may require. K, the bottom of the hearth, two feet square. L, the top of the hearth two feet six inches square. KL, the height Furnace, of the hearth fix feet fix inches. L is also the bottom of the bohies, which here terminate of the same size as the top of the hearth; only the former are round, and the latter square. M, the top of the bohies, 12 feet diameter and eight feet of perpendicular height. N, the top of the furnace, at which the materials are charged; commonly three feet diameter. MN, the internal cavity of the furnace from the top of the bohies upwards, 30 feet high. NK, total height of the internal parts of the furnace, 44½ feet. OO, the lining. This is done in the nicest manner with fire bricks made on purpose, 13 inches long and three inches thick. PP, a vacancy which is left all round the outside of the first lining, three inches broad, and which is beat full of coke-dust. This space is allowed for any expansion which might take place in consequence of the swelling of the materials by heat when descending to the bottom of the furnace. QQ, the second lining, similar to the first. R, a cast-iron lintel, on which the bottom of the arch is supported. RS, the rife of the arch. ST, height of the arch; on the outside 14 feet, and 18 feet wide. VV, the extremes of the hearth, ten feet square. This and the both-stones are always made from a coarse gritted freestone, whose fracture presents large rounded grains of quartz, connected by means of a cement lets pure.
Fig. 4. represents the foundation of the furnace, and Fig. 4 a full view of the manner in which the false bottom is constructed.
AA, the bottom stones of the hearth. B, stratum of bedding sand. CC, passages by which the vapours, which may be generated from the damps, are passed off. DD, pillars of brick. The letters in the horizontal view, of the same figure, correspond to similar letters in the dotted elevation.
Fig. 5. AA, horizontal section of the diameter of the bohies, the lining and vacancy for stuffing at M. C, view of the top of the hearth at L.
Fig. 6. vertical side-section of the hearth and bohies; Fig. 6. shewing the tuyre and dam-stones, and the tuyre and dam-plates. a, the tuyre-plate. b, the tuyre-plate, which is wedged firmly to the stone, to keep it firm in case of splitting by the great heat. c, dam-stone, which occupies the whole breadth of the bottom of the hearth, excepting about fix inches, which, when the furnace is at work, is filled every cast with strong sand. This stone is surmounted by an iron plate of considerable thickness, and of a peculiar shape d, and from this called the dam-plate. The top of the dam-stone and plate is two, three, or four inches under the level of the tuyre hole. The space betwixt the bottom of the tuyre and the dotted line is also rammed full of strong sand, and sometimes fire-clay. This is called the tuyre-stopping, and prevents any part of the blast from being unnecessarily expended.
The square of the base of this blast-furnace is 38 feet; the extreme height from the false bottom to the top of the crater is 55 feet.
Having given the above description of the construction of the furnace, Mr Muhet next proceeds to take a view of its mode of operation and management. "The operations (he observes) I am about to describe have never as yet received any explanation consonant to true philosophy or chemical facts; yet there are few which present a more beautiful chain of affinities, decomposition, and recombination, than the manufacture of iron in all its various stages. An extensive foundry is a laboratory fraught with phenomena of the most interesting nature in chemistry and natural philosophy: are we not then justly surprised to find that prejudice still reigns there; and that the curious manipulations of these regions are still throned with error and misconception; as if their dingy structure forbade the entrance of genius, or consigned her laborious unlettered sons to an endless stretch of mental obscurity?"
Having described the furnace, he continues, "I shall proceed to detail the train of preparation necessary before the furnace is brought to produce good melting iron.
"The furnace being finished, the bottom and sides of it, for two feet up the square funnel, receive a lining of common bricks upon edge, to prevent the stones from shivering or mouldering when the fire comes in contact with it. On the front of the furnace is erected a temporary fire-place, about four feet long, into the bottom of which are laid corresponding bars. The side-walls are made so high as to reach the under surface of the tymp-stone; excepting a small space, which afterwards receives an iron plate of an inch and a half thick, by way of a cover: This also preserves the tymp stone from any injury it might sustain by being in contact with the flame. A fire is now kindled upon the bars, and is fed occasionally with small coals. As the whole cavity of the furnace serves as a chimney for this fire, the draught in consequence is violent, and the body of heat carried up is very considerable. In the course of three weeks the furnace will thus become entirely free from damp, and fit for the reception of the materials: when this is judged proper the fire place is removed, but the interior bricks are allowed to remain till the operation of blowing commences. Some loose fuel is then thrown upon the bottom of the furnace, and a few baskets of cokes are introduced; these are allowed to become thoroughly ignited before more are added. In this manner the furnace is gradually filled; sometimes entirely full, and at other times 5-8ths or 3-4ths full. The number of baskets full depend entirely upon the size of the furnace: that in the plate will contain 900 baskets. If the coal is splint, the weight of each basket-full will be nearly 11cbl. × 900 = 99,000lb. cokes. As this quality of cokes is made with a lot of nearly 50 per cent, the original weight in raw coals will be equal to 198,000lb. When we reflect that this vast body of ignited matter is replaced every third day, when the furnace is properly at work, a notion may be formed of the immense quantity of materials requisite, as also the consequent industry exerted to supply one or more furnaces for the space of one year.
"When the furnace is sufficiently heated throughout, specific quantities of cokes, iron-stone, and blast-furnace cinders are added: these are called charges. The cokes are commonly filled in baskets, which, at all the various iron-works are nearly of a size. The weight of a basket, however, depends entirely upon the nature and quality of the coal, being from 7c to 112lb. each (d). The iron-stone is filled into boxes, which, when moderately heaped, contain 56lb. of torrefied iron-stone; they often exceed this when the stone has been severely roasted. The first charges which a furnace receives, contain but a small proportion of iron-stone to the weight of cokes: this is afterwards increased to a full burden, which is commonly four baskets cokes, 320lb.; two boxes iron-stone, 112lb.; one box of blast-furnace cinders, 60 or 70lb. (e). At new works, where these cinders cannot be obtained, a similar quantity of limestone is used.
"The descent of the charge, or burden, is facilitated by opening the furnace below two or three times a day, throwing out the cold cinders, and admitting, for an hour at a time, a body of fresh air. This operation is repeated till the approach of the iron-stone and cinder, which is always announced by a partial fusion, and the dropping of lava through the iron bars, introduced to support the incumbent materials while those on the bottom are carried away. The filling above is regularly continued, and when the furnace at the top has acquired a considerable degree of heat, it is then judged time to introduce the blast; the preparations necessary for which are the following:—
"The dam-stone is laid in its place firmly imbedded in fire-clay; the dam-plate is again imbedded on this with the same cement, and is subject to the same inclination. On the top of this plate is a slight depression, of a curved form, towards that side farthest distant from the blast, for the purpose of concentrating the scoria, and allowing it to flow off in a connected stream, as it tends to surmount the level of the dam. From this notch to the level of the floor a declivity of brick-work is erected, down which the scoria of the furnace flows in large quantities. The opening betwixt the dam and side-walls of the furnace, called the fauld, is then built up with sand, the loose bricks are removed, and the furnace bottom is covered with powdered-lime or charcoal-dust. The ignited cokes are now allowed to fall down, and are brought forward with iron bars nearly to a level with the dam. The space between the surface of the cokes and the bottom of the tymp-plate is next rammed hard with strong binding sand; and these cokes, which are exposed on the outside, are covered with coke-dust. These precautions being taken, the tuyere-hole is then opened and lined with a soft mixture of fire-clay and loam: the blast is commonly introduced into the furnace at first with a small discharging-pipe, which is afterwards increased as occasion may require. In two hours
(d) "This same variety in the coal renders it almost impossible, under one description, to give a just idea of the proportions used at various blast furnaces: to avoid being too diffuse, I shall confine my description connected with a coal of a medium quality, or a mixture of splint and free-coal, a basket of which will weigh from 78lb. to 84lb.
(e) "A preference at first is always given to blast-furnace cinders in place of lime; being already vitrified, they are of much easier fusion, and tend to preserve the surface of the hearth by glazing it over with a black vitrid crust." hours after blowing, a considerable quantity of lava will be accumulated; iron bars are then introduced, and perforations made in the compressed matter at the bottom of the furnace; the lava is admitted to all parts of the hearth, and soon thoroughly heats and glazes the surfaces of the fire-flone. Shortly after this it rises to a level with the notch in the dam-plate, and by its own accumulation, together with the forcible action of the blast, it flows over. Its colour is at first black; its fracture dense and very ponderous; the form it assumes in running off is flat and branched, sometimes in long streams, and at other times less extensive. If the preparation has been well conducted, the colour of the cinder will soon change to white; and the metal, which in the state of an oxide formerly coloured it, will be left in a disengaged state in the furnace. When the metal has risen nearly to a level with the dam, it is then let out by cutting away the hardened loam of the fauld, and conveyed by a channel, made in sand, to its proper destination; the principal channel, or runner, is called the sow; the lateral moulds are called the pigs.
"In six days after the commencement of blowing, the furnace ought to have wrought herself clear, and have acquired capacity sufficient to contain from 5000 to 7000 weight of iron. The quality ought also to be richly carbonated, so as to be of value and estimation in the pig-market. At this period, with a quality of coal as formerly mentioned, the charge will have increased to the following proportions:—Five baskets cokes, 4colb.; five boxes iron-flone, 336lb.; one box limestone, 100lb.
"An analysis of the smelting operation, and the tendency which the individual agents have to produce change in the quality and quantity of the iron, come next under consideration. Let us, however, first notice the characteristic features exhibited by the different kinds of iron while in fusion, whereby the quality of the metal may be judg'd defined.
"When fine (No 1.) or super-carbonated crude iron is run from the furnace, the stream of metal, as it issues from the fauld, throws off an infinite number of brilliant sparkles of carbone. The surface is covered with a fluid pellicle of carburet of iron, which, as it flows, rears itself up in the most delicate folds: at first the fluid metal appears like a dense, ponderous stream, but, as the collateral moulds become filled, it exhibits a general rapid motion from the surface of the pigs to the centre of many points; millions of the finest undulations move upon each mould, displaying the greatest nicety and rapidity of movement, conjoined with an uncommonly beautiful variegation of colour, which language is inadequate justly to describe. Such metal, in quantity, will remain fluid for 20 minutes after it is run from the furnace, and when cold will have its surface covered with the beautiful carburet of iron, already mentioned, of an uncommonly rich and brilliant appearance. When the surface of the metal is not carbureted, it is smooth like forged iron, and always convex. In this state iron is too rich for melting without the addition of coarse metal, and is unfit to be used in a cupola furnace for making fine castings, where thinness and a good skin are requisite.
"No 4., or oxygenated crude iron, when issuing from the blast-furnace, throws off from all parts of the fluid surface a vast number of metallic sparks: they arise from a different cause than that exerted in the former instance. The extreme privation of carbone renders the metal subject to the combination of oxygen to soon as it comes into contact with atmospheric air. This truth is evidently manifested by the ejection of small spherules of iron from all parts of the surface; the deflagration does not, however, take place till the globule has been thrown two or three feet up in the air; it then inflames and separates with a flight hissing explosion, into a great many minute particles of a brilliant fire. When these are collected they prove to be a true oxide of iron, but so much saturated with oxygen, as to possess no magnetic obedience. The surface of oxygenated iron, when running, is covered with waving flakes of an obscure smoky flame, accompanied with a hissing noise; forming a wonderful contrast with the fine rich covering of plumagio in the other state of the metal, occasionally parting and exhibiting the iron in a state of the greatest apparent purity, agitated in numberless minute fibres, from the abundance of the carbone united with the metal.
"When iron thus highly oxygenated comes to rest, small specks of oxide begin to appear floating upon the surface; these increase in size; and when the metal has become solid, the upper surface is found entirely covered with a scale of blue oxide of various thicknesses, dependent upon the stage of oxygenation or extreme privation of carbone. This oxide, in common, contains about 19 per cent. of oxygen, and is very obedient to the magnet. In place of a dark blue smooth surface, convex and richly carbonated, the metal will exhibit a deep, rough, concave face, which, when the oxide is removed, presents a great number of deep pits. This iron in fusion stands less convex than carbonated iron, merely because it is less susceptible of a state of extreme division; and indeed it seems a principle in all metallic fluids, that they are convex in proportion to the quantity of carbone with which they are saturated. This iron flows dead and ponderous, and rarely parts in shades but at the distance of some inches from each other.
"This is a slight sketch of the appearance of the two extreme qualities of crude or pig iron, when in a state of fusion. According to the division formerly made, there still remain two intermediate stages of quality to be described: these are, carbonated and carbo-oxygenated iron; that is No 2. and 3. of the manufacturers. Carbonated iron exhibits, like No 1., a beautiful appearance in the runner and pig. The breakings of the fluid, in general, are less fine; the agitation less delicate; though the division of the fluid is equal, if not beyond that of the other. When the internal ebullition of the metal is greatest, the undulating shades are smaller and most numerous: sometimes they assume the shape of small segments; sometimes fibrated groups; and at other times minute circles, of a mellower colour, that the ground of the fluid. The surface of the metal, exposed to the external air, when cooling is generally slightly convex, and full of punctures: these, in iron of a weak and fusible nature, are commonly small in the diameter, and of no great depth. In strong metal the punctures are much wider and deeper. This criterion, however, is not infallible, when pig-iron of different works, is taken collectively. At each individual work, however, that iron will be strongest whose honeycombs are largest and deepest.
"Carbo-" "Carbo-oxygenated, or No. 3. pig-iron, runs smoothly, without any great degree of ebullition or disengagement of metallic sparks. The partings upon its surface are longer, and at greater distances from each other than in the former varieties; the shape they assume is either elliptical, circular, or curved. In cooling, this metal acquires a considerable portion of oxide; the surface is neither markedly convex nor concave; the punctures are less, and frequently vanish altogether. Their absence, however, is no token of a smooth face succeeding: in qualities of crude iron oxygenated beyond this, I have already mentioned that a concave surface is the consequence of the extreme absence of carbone; and that, in proportion as this principle is absent, the surface of the iron acquires roughness and asperity.
"It may perhaps be proper here to mention, once for all, that although, for convenience, the manufacturer has, from a just estimation of the value of the metal in a subsequent manufacture, affixed certain numbers for determinate qualities of iron, yet it is difficult to say at what degree of saturation of carbone each respective term commences: suffice it then to say, that the two alternative principles, oxygen and carbone, form two distinct classes, that in which oxygen predominates, and that in which carbone predominates; the latter comprehends No. 1. and 2. of the manufacturers, the former includes oxygenated, white and mottled; and the equalization of these mixtures form, as has already been noticed, the variety of carbo-oxygenated crude iron.
"I shall now observe some things relative to the various faces which crude iron assumes. Nos. 1. and 2. with their intermediate qualities, possess surfaces more or less convex, and frequently with thin blisters: this we attribute to the presence of carbone, which being plentifully interperled betwixt and throughout the particles of the metal, the tendency which the iron has to shrink in cooling is entirely done away; it tends to dilute the aggregate of the mass, and to give a round face, by gradually elevating the central parts of the surface, which are always last to lose their fluidity.
"Again, that quality of iron known by the name of No. 3. or carbo-oxygenated, is most commonly found with a flat surface. If we still farther trace the appearance of the surface of pig-iron, when run from the furnace, we shall find No. 4. either with a white or mottled fracture, possessed of concave faces rough and deeply pitted. Beyond this it may be imagined that every degree of further oxygenation would be productive of a surface deeper in the curve, and rougher, with additional aperities. The contrary is the case: when crude iron is so far debased as to be run from the furnace in clotted lumps highly oxygenated, the surface of the pigs is found to be more convex than that of No. 1. iron; but then the fracture of such metal presents an impure mass covered on both faces with a mixture of oxidated iron, of a blueish colour, nearly metallic. In short, this quality of iron is incapable of receiving such a degree of fluidity as to enable us to judge whether the convexity of its surface is peculiar to its state, or is owing to its want of division as a fluid, whereby the gradual consolidation of the metal is prevented.
"These features sufficiently distinguish betwixt the various qualities of crude iron after they are obtained from the blast furnace: there are, however, criterions not less infallible, whereby we can prejudge the quality of the metal many hours before it is run from the furnace. These are the colour and form of the scoria; the colour of the vitrid crust upon the working bars, and the quantity of carburet which is attached to it. The variety of colour and form in the cinder almost form the universally indicate the quality of the metal on the hearth. Hence, from a long course of experience, have arisen the following denominations: "Cinder of sulphury iron;" "Cinder of No. 1. No. 2. and No. 3.;" and "Cinder of ballast iron." Although at different works, from local circumstances, the same kind of scoria may not indicate precisely the same quality of iron, yet the difference is so small that the following description of the various cinders may convey a very just idea of their general appearance.
"When the scoria is of a whitish colour and short form, branching from the notch of the dam, and emitting from its stream beautiful sparks of ignited carbone, resembling those ejected from a crucible of cast steel in fusion, exposed to external air, or to the combustion of fine steel filings in a white flame; if, when issuing from the orifice of the furnace, it is of the purest white colour, possessing no tenacity, but in a state of the greatest fluid division, and, when cold, resembles a mass of heavy torrefied spar, void of the smallest vitrid appearance, hard and durable, it is then certain that the furnace contains sulphury iron, i.e. super-carbonated iron. At blast furnaces, where a great quantity of air is thrown in per minute, super-carbonated crude iron will be obtained with a cinder of a longer form, with a rough, flinty fracture towards the outside of the column.
"That cinder which indicates the presence of carbonated iron in the hearth of the furnace, forms itself into circular compact streams, which become consolidated and inserted into each other; these are in length from three to nine feet. Their colour when the iron approaches the first quality, is a beautiful variegation of white and blue enamel, forming a wild profusion of the elements of every known figure; the blues are lighter or darker according to the quantity of the metal and the action of the external air while cooling. When the quality of the pig-iron is sparingly carbonated, the blue colour is less vivid, less delicate; and the external surface rougher, and more filled with a mixture of colour. The same scoria, when fused in vessels which are allowed to cool gradually, parts with all its variety and shade, and becomes of a yellowish colour, sometimes nearly white when the quantity of incorporated metals has been small.
"The cinder which is emitted from the blast furnace when carbo-oxygenated (or No. 3.) iron is produced, assumes a long zig-zag form. The stream is slightly convex in the middle; broad, flat, and obliquely furrowed towards the edges. The end of the stream frequently rears itself into narrow tapered cones, to the height of six or eight inches: these are generally hollow in the centre, and are easily demolished, owing to their excessive brittleness. The colour of this lava is very various; for the most it is pale yellow, mixed with green. Its tenacity is so great, that if, while fluid, a small iron hook is inserted into it at a certain degree of heat, and then drawn from it with a quick but steady motion, 20 to 30 yards of fine glass thread may be formed with ease. If the colours are vivid and variegated, the thread will possess, upon a minute scale, all Furnace. the various tints of colouring which is found in the lunar mafs. When by accident a quantity of this lava runs back upon the discharging-pipe, it is upon the return of the blast impelled with such velocity as to be blown into minute delicate fibres, smaller than the most ductile wire; at first they float upon the air like wool, and when at rest very much resemble that substance.
"The presence of oxygenated crude iron (No 4.) on the furnace hearth, is indicated by the lava resolving itself into long streams, sometimes branched, sometimes columnar, extending from the notch to the lowest part of the declivity; here it commonly forms large, flat, hollow cakes, or inclines to form conical figures: these are, however, seldom perfect; for the quantity of fluid lava, conveyed through the centre of the column, accumulates faster than the internal sides of the cone are consolidated; and thus, when the structure is only half finished, the small crater vomits forth its superabundant lava, and is demolished. The current of such lava falls heavily from the dam as if surcharged with metal, and emits dark red sparks resembling the agitation of straw embers. Its colour is still more varied than the former descriptions of scoria, and is found changing its hues through a great variety of greens shaded with browns. Another variety of scoria, which indicates the same quality of iron, assumes a similar form; but has a black ground colour mixed with browns, or is entirely black. When the latter colour prevails, the texture of the cinder becomes porous; the quantity of iron left is now very considerable, and such as will be easily extracted in the assay-furnace with proper fluxes. In cases of total derangement in the furnace, the scoria will still retain this black colour, although the quantity of metal may amount to 25 per cent.; the fracture, however, becomes dense, and its specific gravity increases in proportion to the quantity of metal it holds incorporated.
"The next source of information, as to the quality of the iron in the furnace, is to be got from the colour of the scoria upon the working bars, which are from time to time inserted to keep the furnace free from lumps, and to bring forward the scoria. When supercarbonated crude iron is in the hearth, the vitrid crust upon the bars will be of a black colour and smooth surface, fully covered with large and brilliant plates of plumbago.
"As the quality of the metal approaches to No 2. (carbonated), the carburet upon the scoria decreases both in point of quantity and size.
"When carbo-oxygenated iron (No 3.) is in the furnace, the working bars are always coated with a lighter coloured scoria than when the former varieties exist; a speck of plumbago is now only found here and there, and that of the smallest size. When the quality of the metal is oxygenated (No 4.), not only have the plates of carburet disappeared, but also the coally colour on the external surface of the scoria; what now attaches to the bars, is nearly of the same nature and colour as the lava emitted at the notch of the dam.
"These criterions are infallible; for, as the fusibility or carbonation of the metal is promoted in a direct ratio to the comparative quantity of the coally principle in the furnace, so in the same proportion will the vitrid crust encircling the working bars exhibit the presence of that principle in the furnace.
"In the smelting operation a just proportion and association of materials and mechanical construction ought to be blended in order to produce the best possible effects. Under the former are comprehended the cokes, iron-stone, limestone, and blast; by the latter is understood the furnace, the power of the blowing-machine, or the compression and velocity under which the air is discharged into the furnace, and the genius or mechanical skill of the workmen. According to this division I shall endeavour to point out the very various effects which disproportion in any case produces, and vice versa.
"In the preceding observations the coal and iron stone have been traced through their various stages of preparation, and that stage pointed out in which they were most suitable for the profitable manufacture of the metal. It will be necessary to carry along with us this fact, that in the exact proportion which the quantity of carbone bears to the quantity of metal in the ore, and its mixtures, so will be the fusibility, and of course the value of the pig-iron obtained. The importance of this truth will still farther appear when we consider the very various qualities of pit-coal, the different proportions of carbone which they contain, and the various properties attached to every species of this useful combustible.
"Among the many strata of coal which I have distilled, some I have found to contain 70 parts in the 100; This large proportion is peculiar to the clod-coal, used at some of the iron-works in England, and justly preferred for the purpose of manufacture, to the purist and hardest variety of splint-coal. The latter I have found to average from 50 to 59 parts of carbone in the 100; and the soft, or mixed qualities of coal, from 45 to 53 parts. Such various proportions of carbone plainly point out that the operations to be followed at each individual iron-work ought not to rest upon precedent, unless borrowed from those works where exactly the same quality of coal is used. This analysis also lays open part of the source from whence originates the widely different quantities of metal produced per week at various blast-furnaces, and the great disproportions of ore used to different coals.
"Experience has shewn that the three qualities of coal just mentioned, will smelt and give carbonation to the following proportions of the same species of torrefied iron stone:
<table> <tr> <th>112 lb. of clod-coal cokes</th> <th>will smelt</th> <th>130 lb.</th> </tr> <tr> <th>112 lb. of splint-coal cokes</th> <th>will smelt</th> <th>105 lb.</th> </tr> <tr> <th>112 lb. mixed soft and hard coal cokes</th> <th>will smelt</th> <th>84 lb.</th> </tr> </table>
"Let the iron stone be supposed in the blast furnace to yield 40 per cent. then we find that the one-twentieth of a ton of the respective qualities of cokes will smelt and carbonate the following proportions of iron, viz. 112 lb. clod-coal cokes, 130 lb. iron stone, at 40 per cent. = 52 lb. iron; 112 lb. of splint-coal cokes, 105 lb. of the stone = 42 lb. of iron; and 112 lb. soft and hard coal cokes, 84 lb. of the iron stone = 33 \( \frac{6}{7} \) lb. of iron. We then have for the quantity of metal produced by one ton of each quality of cokes:
<table> <tr> <th>Clod-coal</th> <th>52</th> <th>× 20 = 1040 lb.</th> </tr> <tr> <th>Splint ditto</th> <th>42</th> <th>× 20 = 840 lb.</th> </tr> <tr> <th>Mixed ditto</th> <th>33 \( \frac{6}{7} \)</th> <th>× 20 = 702 lb.</th> </tr> </table>
"This furnishes a datum whereby we easily obtain the FUR
the quantity of the various cokes necessary to produce one ton of carbonated crude iron by common proportion: for if 1040 lb. of metal are produced by one ton, or 2240 lb. of clod-coal cokes, the quantity of the same cokes requisite for the production of one ton, or 2240 lb. of metal will be—
<table> <tr> <th></th> <th>T. C.Q.lb.</th> </tr> <tr> <td>Splint-coal cokes</td> <td>840:2240::2240:5973.3lb.=2 3 0 8</td> </tr> <tr> <td>Mixed ditto</td> <td>702:2240::2240:7147.5lb.=3 3 3 7</td> </tr> </table>
"If to the quantity of cokes necessary to manufacture one ton of crude iron, we add the quantity of volatile matter driven off in the process of charring, which may be thus estimated upon the average of each quality:
Clod-coal \( \frac{1}{3} \) or 37\( \frac{1}{2} \) per c. produce in cokes \( \frac{6}{5} \) or 62\( \frac{1}{2} \) per c. Splint coal \( \frac{4}{5} \) — 50 Mixed coal \( \frac{3}{5} \) — 62.5
"Then, for the quantity of the respective coals used in the raw state, we have the following results in proportion:
<table> <tr> <th></th> <th>T. C. Q. lb.</th> </tr> <tr> <td>Clod-coal</td> <td>5:4824.6::8:7719\( \frac{3}{4} \)=3 8 2 19</td> </tr> <tr> <td>Splint coal</td> <td>4:5973.3::8:11946=5 6 2 18</td> </tr> <tr> <td>Mixed coal</td> <td>3:7147.1::8:16158\( \frac{1}{2} \)=8 11 0 16</td> </tr> </table>
"These great disproportions of quantity, used to fabricate one ton, or 2240 averdupose pounds of the same quality of crude iron, will convey a striking and impressive idea of the multifarious qualities of coal which may be applied and made to produce the same effects. It should also convince the manufacturer that the study and analysis of his own materials is the first and radical approach to true knowledge, and certainty of operation. Divest him of this knowledge, and view him guided by the customs and rules prevalent at another manufactory, where the coals and ores may be as different as has been already mentioned, and we will no longer wonder at the uncertainty of his results, and the numberless errors of his direction.
"Before I enter into the practical discussion of the application of coal, I beg leave to indulge myself in the following calculations.—We have already seen that the production of 2240 lb. of carbonated crude iron requires 4824 lb. of clod-coal cokes; these may be averaged to contain 4.5 per cent. of ashes, which, deducted from 4824, gives 4607 lb. of carbone used for one ton of metal: this sum, divided by 2240, farther gives, for one lb. of cast iron thus manufactured, 2.056 lb. of carbone.
"We next find that 2240 lb. of the same metal requires of splint coal cokes 5973.3 lb.; we farther find, from a table of the analysis of coal, furnished in a former paper, that 100 parts of the raw coal contained 4.2 parts of ashes. As it is there stated to lose 50 per cent. in charring, 100 parts of cokes will contain 8.4 of ashes; and 8.4 per cent. deducted from 5973.3, gives 5472 lb. of carbone. This again, reduced by 2240 lb. gives for each pound of metal manufactured, 2.442 lb.
"Again, 7147.1 lb. of cokes obtained from soft mixed coals are consumed for every ton of 2240 averdupose pounds of crude iron produced; every 100 parts of the same coals contain 3.3 parts of ashes; and 100 parts of cokes contain nearly 6.5 per cent. of ashes, which, deducted from 7147.3, gives 6672.6 of carbone, which divided by 2240, gives, for the quantity used for one pound of cast iron, 2.978 lb.
"From these calculations it appears, that 2240 lb. of carbonated iron, requires of carbone from clod-coal 4607 lb.; of carbone from splint-coal, 5472 lb.; and of carbone from mixed coal, 6672 lb.: that one pound of carbonated iron requires of carbone from clod-coal cokes 2.056 lb.; from splint, 2.442 lb.; from mixed, 2.983 lb.; and that carbonated crude iron may be obtained when widely different quantities of carbone have been consumed.
"In seeking for a solution of the latter fact, we must have recourse to the different degrees of inflammability of the carbone, according to the various laws of continuity imposed upon it in its fossil construction. It can easily be conceived, that, owing to this structure, and the nature of the interposed ashes, the particles of carbone of some cokes will be more easily oxygenated that those of others; in the same way that we find splint-coal, when exposed to ignition in contact with open air, affords one-third of more cokes than are obtained from soft mixed coals, though the latter, when distilled, yield more pure carbone than the former.
"By experiment it is proven that 100 grains of carbonic acid gas is composed of 72 parts of oxygen, united with 28 parts of carbone: if the quantity of the carbone of clod-coal, viz. 2.056 lb. used for the manufacturing of every pound of cast iron, is reduced to grains, we will find it to consist of 1439.2 grains; this, divided by 28, gives the acidifiable principle of 51.4 × 100 = 51400 grains of carbonic acid gas (r): hence as one cubic foot of this gas, at 29.84 of barometrical pressure, and 54.5 of temperature, weighs nearly 761 grains, we find that in the formation of every pound of cast iron \( \frac{51400}{761} = 67.54 \) cubical feet of carbonic acid gas will be formed; and in the production of one ton of metal, the astonishing quantity of 151289.60 cubic feet. This quantity, however incredible it may seem, is only what would be formed under the above pressure, and at the above temperature: when we take into the account the high temperature at which the decomposition and recombination are effected, with the consequent
(f) "This is supposing, for the moment, that the whole of the carbone is oxygenated, either by the oxygen contained in the ore, or obtained from the discharging-pipe by the decomposition of the atmospheric air: this, however, is not strictly true, as the metal takes up a small portion, by weight, of the carbone; and when, by accident, moisture has been introduced into the furnace, either through the medium of the blast, or of the materials, its decomposition furnishes a portion of both oxygen and hydrogen, which may dissolve, and also carry off, a part of the carbone. Atmospheric air being found to hold water in solution, a small quantity of hydrogen will, even in the driest weather, be present in the blast-furnace. FUR
Furnace. quent increase of elastic force and of volume, our ideas are almost unable to commensurate the sum of the gas hourly formed, and thrown off, ignited to the highest degree of heat.
"If the same mode of calculation is adopted with the other qualities of coal, we will have the following results:
"For the splint-coal 2,442 lb. or \( \frac{17994}{28} = 615.5 \times 100 = 61050 \) grains of carbonic acid, which gives \( \frac{61050}{761} = 82.85 \) cubic feet for 1 lb. and \( 82.85 \times 2240 = 185,584 \) cubic feet for one ton. For the mixed coal \( \frac{22881}{28} = 710 \times 100 = 71000 \) grains carbonic acid; that is, \( \frac{71000}{761} = 93.3 \) cubical feet for 1 lb.; and \( 93.3 \times 2240 = 208,992 \) cubical feet for one ton. By the same calculation we may attain a pretty accurate notion of the quantity of atmospheric air necessary to produce 1 lb. or one ton of cast iron; an average of the three varieties of coal will be sufficiently accurate for this purpose; thus \( \frac{14392 \times 17994 \times 22881}{3} = 17455^{\frac{1}{2}} \) or 2,4935 lb. of carbone are consumed upon the average of each pound of pig-iron: this is found to produce of carbonic acid gas \( \frac{17455^{\frac{1}{2}}}{28} = 62.341 \times 100 = 62,30041 \) grains; which again divided by 761, the grains in one cubic foot, gives 81.86 cubic feet for the gas discharged in manufacturing one pound of cast iron. As carbonic acid contains, as has already been noticed, 72 parts of oxygen in 100, then we have for the quantity of oxygen gas 100 : 72 :: 62,40041 : 44856.29 grains oxygen gas; and as, at the ordinary temperature and pressure of the atmosphere, a cubic foot of oxygen gas weighs 591 grains, we find 44856.29 divided by 591 = 75.89 cubic feet of oxygen gas necessary to form the acidifying principle of 81.86 cubic feet of carbonic acid gas; and that the same quantity of oxygen gas is necessary to the production of one pound of carbonated crude iron. This leads us to the following statement for the quantity of atmospheric air used during the same operation; first premising that the constituent parts of atmospheric air are nearly 73 of azote and 27 of oxygen gas; of atmospheric air then necessary, we have 27 : 100 :: 75.89 : 281 cubic feet.
"I shall now proceed from mere calculation to matter of fact, and attempt to prove the correctness of the former by the approximation of the latter to its results. Let a blast-furnace be supposed to produce 254 tons of pig-iron per week, = 45360 avoirdupoise pounds; this divided by days, hours, minutes, and seconds, gives per day 6,480 pounds, per hour 270, per minute 3\(\frac{1}{3}\) lb. and per second 52.5 grains.
"From this it is evident that one pound of cast iron is produced in 13\(\frac{1}{3}\) seconds; experience has shewn that a blast-furnace, producing, in any of the above periods, the respective quantity of metal, requires a discharge of air per minute nearly equal to 1350 cubic feet; this, divided by 4.5 lb. the quantity produced per minute gives, for one pound of iron, 300 cubic feet. The quantity, by calculation, we have seen to be 281 cubic feet, difference 19; a sum no way considerable when we reflect upon the inequality of the movements of a blowing machine, and when it is recollected that some allowance ought also to be made for what air may pass through the furnace undecomposed, or may be lost at the place of entrance.
"From this coincidence of theory with practice, we cannot help admiring the rigorous principles on which the Lavoisierian system is founded; nor are we less pleased to find, that, small as the operations of the chemist may be, yet they are a just epitome of what takes place in the philosophy of extensive manufactories. The following table exhibits the quantity of carbone which may be used upon an average, with the relative quantity of carbonic acid formed, and air used:
"In the manufacture of 1 lb.—1 ton of iron, The pure carbone requisite is 2.49—5585.44 lb. Carbonic acid formed 81.86—183360.40 cub.ft. Oxygen gas used 75.89—169993.60 cub.ft. Atmospheric air employed 281.00—629440.00 cub.ft.
"From the foregoing particulars upon coal may be learned how much is dependent upon the native construction of coal and its constituent parts; I shall next advert to the effects produced by its improper preparation.
"When coals intended for the blast-furnace are sufficiently charred, they ought, in point of colour, to be well-charged of a silver-gray; their fracture will appear lamellated red coal, and porous if [plint-coals have been used; softer coals form themselves into branches slightly curved, and, when properly prepared, are always very porous. I have frequently found that the better the cokes were charred, the more water they will absorb. Coals half burnt do not take up half so much water, because their fracture continues in part to be smooth and less porous than when thoroughly burnt.
"When half-prepared cokes are introduced into the furnace, the metal formerly carbonated will lose its gray fracture, and approach to the quality of oxygenated iron. Their presence is easily detected by the unusual quantity of thick vapour arising along with the flame. Besides, the water and sulphur, which raw coals introduce into the furnace, and which always impair the quantity of carbone by the various solutions effected by the presence of oxygen, hydrogen, &c. the fines of the coal for combustion, and the support of the ore, is much diminished by this second course of ignition and disengagement of bitumen. The pressure of the incumbent ores also fracture and reduce the cokes into small pieces, which produce a considerable portion of coke dust; this is partly carried to the top of the furnace before the blast; sometimes below it appears in immense quantities, ignited to whiteness, and liquid as sand. Coal thus detached from the mass, exposed to the action of a compressed current of air, is unfit for conveying the carbonic principle to the metal; and as it frequently belongs to the just proportion of charcoal necessary to melt the ores, and to carbonate their iron, its loss must be felt, and the quality of iron impaired.
"When cokes of any quality are exposed to a moist atmosphere, so as to absorb water, their effects in the blast-furnace become much reduced, and the presence of the water is productive of the most hurtful consequences in the production of carbonated crude iron. I have found, by repeated experiment, that one pound of well prepared cokes will, when laid in water, take up 1 1/2 ounce in the space of half an hour; at this rate, a basket of cokes weighing 80 lb. saturated with water, will contain 140 ounces of water, or 8 1/2 lb. If the charge contains fix baskets, then we see that upwards of 50 lb. of water is introduced regularly along with the charge, furnishing an additional quantity of oxygen equal to 42 1/2 lb. and of hydrogen equal to 7 1/2 lb.; but it frequently happens that the cokes contain a larger portion of water than is here stated. When cokes thus furcharged are introduced in quantity into the blast furnace, the quality of the metal is not always instantaneously changed, and frequently the colour and form of the cinder remain long without any great alteration. The contact of wetted cokes with the ore is first seen by the great discharge of pale blue gas, with the whiter flame at the top of the furnace; next, the accumulating oxide upon the surface of the pig when consolidating indicates their presence. Iron thus oxygenated frequently exhibits, while fluid, that agitation and delicate partings peculiar to carbonated metal: the remelting of this iron is never attended with advantage, and is always unprofitable to the founder.
"From the properties which have been assigned to pit-coal, the following facts may be deduced:—That charcoal is the basis of the manufacture of crude iron; that its proper application produces the most valuable qualities of pig-iron; that, by diminishing its relative proportion, or contaminating its quality by heterogeneous mixtures, the value and fusibility of the metal is lost; but that, by a proper increase, and always in proportion to this increase, will the fusibility and value of the iron be mended. From the whole, an important lesson may be learned of the pernicious effects of water in the furnace, and how absolutely necessary it is to prepare the cokes without using water, either to damp the fires, as in the usual mode, or to cool the cinders obtained from the tar kilns, to prevent their consuming in the open air: in all this hurtful operation considerable quantities of water become fixed in the cokes, which require a very great degree of heat to expel.
"The preparation of iron stone has already been fully attended to, and the phenomena which it exhibits under every stage minutely described. In consequence of various experiments we are authorized to draw the following conclusions: That when pure calcareous iron-stone is used, it admits of having the local quantities of cokes diminished; that argillaceous requires a larger portion than the calcareous genus; and that siliceous iron-stone requires a greater proportion of fuel than any variety of the former genera. We have also seen that fusibility, either connected with strength or otherwise, is derived from the mixture of the ores; and that excessive brittleness, intimately connected with infusibility, is also derived from the same source. From a review of these facts, we are forcibly impressed with the importance of combining the prepared iron-stones with proportions of fuel suited to their various natures, in order to produce all the varieties of iron with the greatest possible economy. Contemplating farther the same subject, it is easy to be conceived that a want of knowledge of the component parts of iron-stones, and the effects which individually they produce, must lead to great uncertainty of operation in the smelting process, Furnace. wherein the beautiful economy of nature, and even real property, will be often unprofitably sacrificed to precedent.
"Besides the above causes of alteration, dependent upon mixtures of the earths, the existence of oxygen gen of the in various quantities in the ores ought never to be overlooked in proportioning the cokes to the iron-stone. This powerful agent, whose form and substance constantly eludes our vision; whose existence is only ascertained by the wonderful changes produced by its various combinations with the iron; and whose presence in the fame iron-stone, in various quantities, may produce such variety of result as to characterize the ores, as containing good or bad iron, surely forms the most interesting mixture which ores or iron-stones possess. It will be a momentous epoch in the manufacture of iron when the existence of such a principle shall be fully admitted by the manufacturer, and its agency, from certain visible effects produced, adopted to explain its accompanying phenomena. Till that period he will not perceive the utility of ascertaining the quantity of oxygen, and devising economical methods of taking it from the ore. An attention to this powerful principle can alone root out those prejudices so inimical to the real interests of the manufacturer, and which seem to glance at nature, as having improvidently combined her most useful metal with mixtures which could resist the ingenuity of man, or fet his comprehensive intellect at defiance. In the progress of this great inquiry, is it not possible that the present expensive exertions may in part be superseded. Is it not possible, that, by laying open the sources of information to individuals at large, a greater mass of intellect may engage in the practice of this art? While the present extensive and lofty buildings are necessary, the business is entirely confined in the hands of men of great capital: the extent of their manufactures require that a large tract of country be devoted to their supply; a natural consequence is, that innumerable tracts of land are overlooked, or held unworthy of notice, merely because they cannot, in a period necessary to clear a great capital and insure a fortune, afford the necessary supply of materials. Such situations, according to the present state of the iron business, must remain unexplored. Should, however, a desire for truth once gain footing in the manufactories of iron, and should this natural impulse of the unprejudiced mind keep pace with other branches of intellectual information, we may not despair of seeing many imperfections removed, which were the unavoidable consequence of the period of their creation.
"In the application of iron-stone in the blast furnace, the following particulars ought rigorously to be attended to:—
"1. Their mixture, whether clay, lime, or flex; their relative proportions to each other, judging according to the rules formerly laid down; which of them may admit of a diminution of fuel; which of them will afford the quality of iron at the time requisite; and which of them will be most likely, by a judicious arrangement, to give the greatest produce of metal, united with value and economy. Iron stones, united with large portions of flex, have already been stated to require a greater proportion of fuel to carbonate their metal than the other genera. When ballast or forge-pigs are wanted, it FUR
Furnace. is obvious that siliceous iron-stones ought to be used; not that they contain a greater quantity of iron, but because they form a substitute for the other kinds, which may be more advantageously melted for the production of more valuable qualities.
"2. The quantity of metal which each individual iron-stone may contain, is another object of consideration. Besides the proportion of mixtures, which chiefly contribute to the fusibility of iron-stones, a second degree of fusibility is dependent upon the richness of the ore in iron; this is so obvious in the use of the Cumberland and Lancashire ores, that the consequences of their introduction will be perceived, by the change of the scoria and metal, in half the time that change would be effected by ordinary iron-stones. It has been frequently noticed, that crude iron contained pure carbone in proportion to its fusibility; then the more fusible or supercarbonated qualities must take up, comparatively, a considerable portion of the carbonaceous principle from the fuel. From this results a striking consequence, that the quantity of fuel should, over and above its relation to the mixtures, bear a just proportion to the quantity of iron in the stone: for example, let the weight per charge of fuel at a blast furnace be 400lb. and let this be supposed sufficiently to fuse and carbonate the iron contained in 360lb. of iron-stone; let the quantity of metal be supposed 35 per cent. then the produce will be 126 lb. Should a change take place, and iron stone richer in iron be applied, though the same by weight, and should this iron stone yield of torrefied stone 45 per cent. its produce will be 162lb. or 40lb. more than the former. As there exists no greater proportion of carbone in the furnace, it is evident that the existing quantity, being distributed over nearly one-third of more metal, must therefore be in more sparing quantity in the whole, and the value of the metal consequently reduced.
"3. The weight of oxygen contained in iron stones is the next object of serious consideration. I have already shewn, from experiment, that our iron stones naturally contain from 9 to 14 per cent. of oxygen, which remains after torrefaction; it has also been shewn, that this quantity of hurtful mixture may be easily doubled by over-roasting or under-roasting the stone; and that the bad effects entailed are in the ratio of its combination with the iron. From a review of the facts which have been adduced on this subject, its agency and effects will easily be credited by men of science; its property of constituting the acidifying base of all the ids readily explains the unalienable consequence of its presence with acidifiable bases. The effects are still more pernicious when the oxygen is furnished by the decomposition of water in raw iron stone; the hydrogen in this case, set free, also seizes a portion of the carbone; and these abstractions, united to that produced by the native portion of oxygen in the stone, form an aggregate which frequently reduces the value of iron 40 per cent. So long as the principles of science are overlooked in the manipulations of the foundry and forge, the existence of such agents will be treated as chimeras of the philosopher and chemist, and the effects hourly produced by them indutriously attributed to causes which, in point of unity or consistency, will not bear the slightest touch of investigation*."
* Phil. Mag. vol.v.
The compression, velocity, and effects of the air are of the utmost importance in blast furnaces. The production, management, and direction of these effects are therefore serious objects of consideration to the manufacturer of iron, since on their proper application the success of his operation chiefly depends. And here we shall renew our obligations to Mr Muffet for his interesting observations on this subject. "When it is considered," he says, "that in the smelting operation the reduction of immense quantities of materials is effected by a compressed current of air impelled by the whole power of a blowing machine, the consequences of the change of air, either in quantity or quality, must be very obvious: when, farther, we contemplate the metal called into existence by means of combustion thus excited; when we consider iron as having the most powerful affinity for the base of that part of the air which maintains combustion; and when we view the debased state to which the metal is reduced by coming into improper contact with it, we must conclude, that the application of blast in the manufacturing of iron calls for the most minute and thorough investigation. In order to take a comprehensive view of this subject, the following division will be requisite:—
"1st, The intimate connection which the quantity of blast bears to the area of the internal cavity of the furnace, and to the nature of the pit-coal.
"2d, The various modes by which air is procured, and how these respectively affect the quality of the air.
"3d, The various changes to which air is subjected by a change of temperature in the atmosphere, with the consequent effects.
"4th, How far increased or diminished velocity and compression alter the results of the furnace.
"5th, The form and diameter of the discharging-pipe.
"1st, Then, in the construction of a blast-furnace and blowing-machine, the quantity of air to be used ought to depend upon the internal dimensions of the former; which, again, ought to be formed according to the quality of the pit-coal. Upon the softness or hardness of the coal, ought more immediately to depend the rate, and height of the blast-furnace. This necessary precaution has given rise to a vast variety of furnaces, of different capacities, from 30 to 50 feet in height, and from nine to 16 feet diameter at the boles. Furnaces from 30 to 36 feet are used for the softer qualities of coal, such as a mixture of free-coal and splint. Furnaces from 36 to 45 are appropriated to the burning of splint-coal cokes; and in Wales, such is the superior strength and quality of the pit-coal, that the furnaces admit of being reared to the height of 50 feet.
"These various qualities of coal, it has been formerly shewn, have appropriate weights of iron-stone, and to use the language of the manufactory, are capable of "supporting a greater or less burden of mine." The former qualities admit not of having the air discharged in great quantity, unless it is impelled under an uncommon degree of compression and consequent velocity, incompatible with the operations of a steam-engine. The reason is obvious: when air, loosely compressed, or comparatively so, is thrown into a body of ignited fuel, the mechanical structure and continuity of whose particles are soft, the air is much more easily decomposed; the ignition, of course, is more rapid: the descent of the materials is promoted beyond their proper ratio, and FUR
long before the carbonaceous matter has penetrated the ore, or united to the metal, to constitute fusibility. I shall adduce an example, as being the most illustrative of this doctrine.
"Suppose a blast furnace, 35 feet high, 11 wide at the bofhes, properly burdened, and producing No. 1 pig-iron. Let the discharge of the air be supposed equal to a preflure of two pounds and a half upon the square inch, or equivalent to one-sixth of the atmosphere, or five inches of mercury: under these circumstances let it farther be supposed, that 1500 cubical feet of air are discharged in one minute; and that the diameter of the discharging pipe is 2.625, the area of which is equal to 6.892625 circular inches. Let the discharging pipe be increased to three inches diameter, and let the same quantity of air be passed into the furnace; it is evident that as the area of the discharging pipe is increased to nine circular inches, or nearly one-third more than formerly, the compression of air must be proportionally diminished. The alteration is soon perceived by its effects; the quantity of scoria increases from the furnace, whilst the consumption of the materials above is also considerably augmented. In a few hours the scoria will have undergone a complete change, from pure white, enamelled with various blue shades, to a green, brown, or black colour, considerably charged with the oxide of iron (c). The same effects will continue, in greater or lesser degree, till all the materials are reduced which were existing in the furnace at the period of diminished compreflion. The philosophy of this fact may be investigated in the following manner:—
"While the just affiocation of proportions remained, the air was discharged under such a degree of compreflion as to excite proper combustion: the decomposition of the air by means of the ignited fuel, was not effected in immediate contact with the separating metal, but had, by its uncommon degree of density, resisted decomposition in the ignited passage, and had been decomposed upon the cokes at a greater elevation in the furnace. As a proof of this, we frequently see a tube formed throughout the whole breadth of the furnace, quite black and apparently cold, formed of the fused materials; when this is removed, a considerable descent momentarily takes place of cokes heated visibly beyond the common pitch: these inflame rapidly, but are soon again cooled to blackness by the incessant discharge of air upon them. The defending mixture of iron and lava is in like manner cooled along the line of blast; the tube is again formed, and, if not removed, will remain for days together, while the furnace will be otherwise working in the best manner.
"When by accident or design the compreflion and velocity of blast are diminished, the tube begins to burn, and throws off a great many fiery-coloured sparks, the fides and roof fail, and are carried before the blast in all directions. Sometimes considerable clots of imperfect iron are recoiled with such violence as to escape the vortex of blast, and issue from the tuyere-hole with such velocity as to inflame the air, and fall down in the state of oxide. In the end the tuyere will appear to flame, and all the passage inwards shews an astonishing degree of whiteness. The decomposition of the air is instantaneously effected upon its entering the ignited passage; the iron by this means is exposed to the oxygen that is disengaged; and the vast quantity of caloric set free, in consequence of its union with the iron and carbone, produces the astonishing heat now visible, but which formerly took place at a more proper height in the furnace.
"From this it will appear, that although a greater apparent degree of heat is visibly produced by the sudden decomposition of the air, and a more rapid decelfent of materials for some time is the consequence, yet, as the quality of the iron is impaired, and as in the end the furnace will return to its old consumption of materials as to quantity, the effects of a loose soft blast are obvioufly pernicious.
"It sometimes happens, that when a loose blast is fur-charged with a considerable portion of moisture, or effects of comes in contact with cokes which had been wet when introduced into the furnace, the inflammation which takes place at the tuyere is prodigious: fine fire clay will be melted down and blown to flag in a few minutes; the fides of the furnace, composed of very infu- fible fire stone, is next attacked, and in a few hours will be so completely destroyed as to stop the working, and require immediate repair. Effects similar to those now described will be felt when the blast is improperly proportioned to coal of a stronger continuity of fracture and superior quality. Besides the effects produced by the sudden decomposition of iron, others of like nature are produced where a soft coal is used, a small furnace, and a great discharge of blast.
"It has been found that crude iron, to be properly matured, ought to remain in the blast furnace, according to circumstances, 48 to 60 hours; that is, from the period that the iron stone is introduced till such time as the metal begins to occupy its place in the hearth in a state of perfect separation. When the contrary is the case, the mixtures arrive at the hottest parts of the furnace before the metal has taken up a sufficient quantity of carbone from the fuel; the action of the blast, and the immediate heat by which the ore is surrounded, forces the iron from its connections to the bottom of the furnace. The quality is de-carbonated, and reduced in its value: to restore this again, the local portion of fuel is increased; this adds to the expence of manufacturing, and diminishes, in some measure, the melting of the furnace.
"When splint-coal cokes are used in the blast furnace, the blast admits of being thrown in under the highest possible pitch of compreflion; the uncommon density of the charcoal sustains a very powerful discharge of blast before it is diffipated to facilitate the general descent. Most frequently, large masses of these cinders pass through the whole ignited cavity, and are thrown out below, poifenting all the acuteness of their original form and fracture.
"This quality of coal is used in all the Curfon blast furnaces, where, to ensure a respectable produce, the air is discharged under a pressure equal to 3 1/2 pounds upon the square inch, or 6 1/2 inches of mercury.
(c) "The metal will have lost nearly all its carbone, and have become inferior in value 25 to 30 per cent. "The same quality of coal was used at the Devon iron works, where at one time, having all the blast of a 48 inch cylinder engine thrown into one furnace, the column of mercury supported was upwards of seven inches; the quantity of air discharged under such an impelling power, I found to exceed 2600 cubical feet per minute.
"The coals used at the Cleugh, Cleland, and Clyde iron works, are nearly of the same quality at each—a mixture of fplint and soft coal. The Muirkirk and Glenbuck iron works have a coal different from any of the former, and in some particular spots it considerably resembles the English clod coal.
Methods of directing air into the furnace, "2d, The various methods of procuring air for the blast furnace may be reduced to the following:—1st, That procured by cylinders, and discharged into the furnace by means of a floating piston heavily loaded, and working in a large receiver or regulating cylinder: 2d, That wherein pumping cylinders only are used, and the air thrown into chelts inverted in water, called the water vault: 3d, That mode wherein the air is discharged from the pumping or forcing cylinder into an air-tight house, called the air vault.
"The first method is the original mode of blowing, and is still much used at those iron works whose erection has been prior to the last fifteen years. By this mode the quality of the air is less subject to alteration by a change of atmosphere. The principal objection to this manner of blowing, is the want of capacity in the receiving cylinder; which cannot be increased so much as to take away the considerable intervals which occur at different parts of the engine stroke. This effect is sensibly seen by the speedy and irregular ascent and descent of the column of mercury. In water blowing machines, where the air is raised by three or four cylinders worked by means of a crank, and where the air is received into an air chest, and forced into the furnace by the continual action of the blast of each successive cylinder, the current of the air is steady, and supports the column of mercury with great uniformity.
"The use of the water vault has of late years become very general among new erected works. Its properties are, a steady and very cold blast: the largeness of the receiving cisterns gives them a sufficient capacity to retain every pound of air raised by the furnace, and distribute it to the greatest advantage. This is not the case with the floating pistons, where a certain quantity of spare wind is thrown out at every return of the engine, left the great piston and weight should be blown out of the cylinder altogether; which, indeed, sometimes happens. The only objection which remains in force against the use of the water-vault, is the tendency which the air has to take up a considerable portion of water in solution, and introduce it into the furnace. A judicious arrangement of the conducting pipes would in some measure obviate this, as well as the more dangerous tendency which water has to rise in a pipe speedily emptied of its air by the stopping of the engine: a stream of water thus conveyed to the furnace, would be productive of the most awful consequences.
"The air afforded by the air vault is much inferior to that obtained in the former methods. This immense magazine of compressed air generates a considerable portion of heat, which greedily seizes the damps, which are unavoidable in underground excavations, and conveys them to the furnace. The blast is, however, steady and uniform; and when the inside of the building is completely secured against the passage of air, it is productive of considerable effects in the furnace. In the summer months, however, the air becomes so far debauched as to affect the quality of the iron, and change it from gray to white. Every change in the temperature of the atmosphere during this period, is indicated by various changes in the furnace.
"The largest air-vault hitherto in use was excavated out of solid rock at the Devon iron works: the fissures of the rock admitted considerable quantities of water; and the same degree of damp would always prevent the possibility of making the side walls and roof air-tight by means of pitch and paper, &c.
"Besides the various natures of blast, as to the strength and equality of the current afforded by different modes of constructing the blowing machines, and state of variety in the quality of the air obtained is also an variable consequence: this is sufficiently known by the effects which it produces in the blast furnace, and ought to be subject to scrupulous examination.
"In this, as in other countries, larger produces of cast iron are obtained in the winter months than during the summer and autumn seasons: the quality of the metal is also much more carbonated, and with a less proportion of fuel. In many parts of Sweden, where the summer heats are intense, the manufacturer is obliged to blow out or stop his furnace for two or three months: not only is he unable to make carbonated metal, but is frequently incapable of keeping the furnace in such trim as to make a produce of any quality whatever. In Britain, during the months of June, July, and August, more especially in dry seasons, the quality of the iron, with the local proportion of fuel, will be depreciated 30 per cent, and the quantity reduced to two-thirds or three-fourths.
"In seeking for a solution of this universally acknowledged fact, our attention is naturally directed to an examination of the various states of air. That the quality of the air in winter is more fit for combustion than in summer, is a truth which requires no farther demonstration. Greater coolness, whereby an almost complete refrigeration of moisture takes place, and the presence of perhaps a greater relative proportion of oxygen, may account for this phenomenon. On the contrary, the quality of air during the summer months becomes much contaminated for combustion, by holding in solution a much greater quantity of moisture: the abundance of nitrous particles may also diminish the usual proportion of oxygen.
"This will account for the inferior effects of combustion both in common fires and in the blast furnace; it will also in a great measure tend to solve the curious phenomenon of the pig-iron taking up less carbone in summer, although reduced with a superior quantity of fuel. The air discharged most probably contains less oxygen; yet the metal is much less carbonated than at other times, when contrary proportions of these exist. Most probably the deficient carbone is carried off by dissolving in hydrogen, forming a constant stream of hydro-carbonic gas, while the oxygen that is set free unites to the iron; and while it reduces its quality, at the same time the quantity is reduced by a portion of the metal being lost in the scoria (H).
"To correct these occasional imperfections in the quality of the air, and to devise methods to procure air always fit for proper combustion, ought to be an object of much consideration to the manufacturer of cast iron. Whether such a consideration has given rise to the different modes of receiving and discharging the air now in use, I cannot say; I rather think not: a great quantity of air has hitherto been a greater object than a certain and uniform quality; and in a country where there is more temperate and cold weather than hot, it is by far the most important object: to unite both, however, would be an attainment of the greatest utility, and would rank the discoverer amongst the well-deserving of his country. How far the mechanism of our present machinery has been adapted to the exigencies of our atmosphere, will appear upon examining the nature and properties of the air, judged by its effects upon the blast furnace.
"The air produced by the blowing and receiving cylinder is less changed, and less subject to change, than that produced and lodged in contact with a vast body of air or water. If the blowing cylinder is fixed in a dry cool spot, the only difference which the air undergoes is an increase of temperature; this is so very considerable, that upon entering the blowing cylinder immediately after stopping the engine, I have found the thermometer rise \(15\) to \(17\frac{1}{2}\) degrees higher than the surrounding air. That this heat is generated in the cylinder is unquestionable; but whether it is occasioned by the friction of the piston leather upon the sides of the cylinder, or expressed from the air by its severe compression, I have not yet been able to decide. It very probably arises from both causes, although the latter is sufficient to produce a much greater degree of heat. What effect this increase of temperature has upon combustion we are unable to say, as the degree of heat accumulated will at all times bear a reference to the temperature of the surrounding air; and as there is no method likely to be devised where heat would not be generated by the action of the particles of air upon each other. When the bulb of a thermometer is held in the middle of the current of blast, as it issues from the discharging pipe, a temperature is indicated as much lower than the temperature of the surrounding air, as the temperature of the cylinder was higher; and it is most probable that a much lower degree would be obtained, were it not for the previous expression of some heat in the blowing cylinder. Upon the whole, I think, the quality of the air obtained in this way of blowing uniformly most fit for combustion, provided the numerous pauses and irregularities of the current of air were done away.
"Air forced into the furnace under water pressure always contains a considerable portion of moisture; the blast of course is colder, as it issues from the discharging pipe. The temperature differs so much from that of the external air as to sink the thermometer from \(54^\circ\) down to \(28^\circ\) and \(30^\circ\). Such effects are produced by air coming into contact with water, that, although the temperature of the atmosphere is \(60\), \(65\), to \(70\), yet the blast at the orifice seldom rises above \(38\): the cold produced in this manner is much increased if the air is further charged with so much water as to be visible in the state of a fine spray. The leading feature, therefore, of the water vault, as to its effects upon the quality of the air, seems to indicate an almost uniform degree of temperature in the blast: this can only be occasioned by the warm air in summer taking up a greater portion of the water in solution, the escape of which at a small orifice, and under a great degree of compression, produces the very great depression of the thermometer. I have already hinted at the bad effects produced by moist blasts, and shall, in a proper place, more minutely attend to them.
"The most inferior quality of air used in the blast from the furnace is that thrown into the air vault, and afterwards air vault, expressed from thence by its own elasticity and the successive strokes of the engine. The capacity of such a building is from 60 to 75,000 cubical feet; this, when filled, generates a much superior degree of heat to that sensible in the blowing cylinder. As this heat is produced many feet distant from any mechanical motion, it is most evident that it is extricated from the air, and will readily unite with the moisture which penetrates the building: the quality of the air introduced into the furnace will therefore be in proportion to the quantity of moisture taken up; this will be much more in summer than in winter, as the temperature of the former exceeds that of the latter. The sensation, on entering the air vault in the coldest months, immediately after stopping the engine, is exactly similar to that experienced upon entering a crowded room in the hottest summer day; the walls are covered with damp, and the superior regions of the vaults readily obscure the flame of a candle. The feeling, upon remaining in the air vault when the engine is at work, is less marked than would be expected where so great a compression of air existed; the sense of hearing, owing to the moisture in the conducting medium, is considerably impaired, and respiration is performed with some difficulty; the light of a candle is faint, and not visible at the distance of a few feet.
"I have explained the necessity of just proportions existing betwixt the area of the interior of the blast furnace, the quantity of air thrown per minute, and the quality of coal. The various modes of blowing, and their respective effects, deduced from strict observation, were also attended to. We have now, thirdly, to adduce examples where the various changes of the atmosphere, as to heat and pressure, occasion the most sensible difference in the quantity of materials consumed, and in the quality and quantity of metal produced.
"It has been already demonstrated, that the air in winter, by containing less moisture, is more proper for combustion, and more calculated to produce carbonated crude iron, than the air exiting at any other season. From this superior quality the manufacturer obtains advantages, which induce him to wish for a continuance of
(H) "May not the superabundant azote of the summer atmosphere produce part of these effects, by dissolving a portion of the carbone, and forming carbonated azotic gas, as has been proved by M. Lavoisier." Furnace of cool air throughout the whole year. These effects are not, however, uniform; they depend greatly upon a light or heavy atmosphere. The keener and more still the air, the more rapid the combustion. During a Effects of a severe frost, the deficient of the materials is facilitated from change of one-tenth to one-fifteenth more than in rainy or hazy weather, and at the same time the quality of the iron is rather improved than impaired. When a change from frost to snow or rain takes place, the effects frequently become almost immediately obvious; the colour of the flame at the furnace head is changed; the tuyere of the furnace inflames, and burns with great violence; the lava, as it flows from the notch of the dam stone, becomes lengthened and tenacious; the form of it is changed, and the colour undergoes the most visible alterations; the iron no longer retains its complete saturation of carbone, but flows out sensibly impaired of its fluidity; and, when cold, the privation of carbone is most evident by the examination of its fracture.
"When such consequences arise from the transition so frequent in winter from frost to thaw, it will be easily conceived that the change effected during the milder and warmer months must produce proportionally additional effects. The increase of temperature by taking up, and holding in solution, a much greater portion of aqueous vapour, will account for the ordinary effects which are annually observable in every work. Where these pernicious consequences approach to extremity, a solution of the phenomenon will likely be obtained by the examination of the blowing apparatus. If air is fitted for combustion in proportion as it is free from watery solutions, we are not to expect similar results from these blast furnaces in summer, which are blown by air from the regulating cylinder, and those blown by air from a water or air vault. I have for years seen this fact verified, and superior quantity and quality of iron during the hot weather, obtained from a furnace excited by means of blast, from the simple regulating cylinder, with a less proportion of fuel than from furnaces whose air was expressed by means of the water or air vault. Observations thus made, where every day the effects of the different means could be justly estimated and compared, have led me to the following conclusion: That the quality of the air, as furnished us by nature in our atmosphere, is uniformly more fit for the manufacture of crude iron to profitable account, when discharged simply by means of cylinders and pistons, than when brought into contact with moisture either in the water vault or air vault.
"So imperfect has the quality of the summer air been found in this country for combustion, where the water vault was used, that experiments have been made to repair the deficiency of effect by introducing steam into the furnace by means of an aperture above the tuyere. The inducing motive to this act, was a belief, that combustion was diminished in consequence of a diminution of oxygen gas during the summer; that, by introducing water upon a surface of materials ignited to whiteness, decomposition would ensue, a larger quantity of oxygen would then be presented to the fuel, and superior effects, as to combustion, obtained in this manner than hitherto witnessed. The idea was ingenious, and, in its application to the manufacture of cast iron, original; but the whole train of facts, which have been detailed, as to the effects of a superabundant quantity of oxygen, was overlooked. The event proved in the most complete manner, and on a great scale, the pernicious effects of moisture. The furnace gradually became cooled where the steam entered; the heat, set free by the decomposition of the water and the disengagement of the oxygen, increased to an alarming pitch a considerable way up the furnace; the quality of the iron became brittle, and as white in the fracture as silver; the introduction of the steam was still continued, the defending materials were instantly robbed of their heat to facilitate the decomposition of the water, and by and by the furnace closed entirely over, and the experiment ceased.
"This experiment, performed in a furnace 18 feet high, is a complete proof that heat is disengaged from bodies while they pass from the fluid to the aeriform state. The first instant of the discharge of steam, a very considerable portion of heat would be withdrawn from the fusing materials and united to the water. This, in its turn, would be ignited to whiteness, and decomposed upon the metals and cokes, in a superior region of the furnace. The process continuing for several hours, the materials at the tuyere were at last so completely deprived of the caloric by the continual torrent of steam, that they lost fluidity, cooled rapidly, and at last became black. Had another aperture for steam and for air been opened above these, now entirely shut up by the consolidated materials, the same effects would have been produced; the immense quantity of caloric, disengaged by the decomposition of the ignited water, would now approach nearer to the top of the furnace, another stratum of fusing materials would again become consolidated, till in the end the whole furnace would be set fast from top to bottom. From the introduction of steam into the blast furnace, either as such, or under a superior degree of expansive force, the following important truths may be learned: That the quantity of oxygen which enters into our atmospheric compound is generally more fit for the manufacture of the superior qualities of crude iron than any mixture which may be furnished by the addition of water; that, although the decomposition of water, by furnishing a superior quantity of oxygen, and by throwing off a relative proportion of caloric, increases the effects of combustion immediately in the vicinity of this chemical analysis; yet, as the water had previously abstracted the heat necessary to its decomposition from the inferior strata, a greater quantity by no means exists in the furnace. The water, in fact, only serves as a medium to convey the heat from one particular spot; but, by attempting to fly off with it, meets decomposition, and renders up not only the abstracted heat, but that which was contained in the oxygen of its decomposition.
"4th, The compression and velocity of the air dif. Compre- charged into the furnace, considerably affect the results and velocity of the melting operations. In the consideration of this subject, the various qualities of coals will be found to have an intimate connection with the area of the discharging pipe and the compression of the blast. It has already been more than once observed, that a soft or mixed quality of coal is more susceptible of combustion than either the splint or clod coal: the consequence of this is, that, unless the necessary compression of air is used, decomposition is too early accomplished, and the cokes become oxygenated by combustion in a greater ratio ratio than is proper for the carbonation of the metal.
To avoid this, the column of air ought to be discharged, in the case of soft coals being unavoidably used, under such a degree of compression, as to resist entire decomposition in the ignited passage. In that case, the iron does not so immediately come into contact with oxygen, as the decomposition is chiefly effected in the superior strata of the separating materials. Under the former circumstance, of a loose unconnected stream of air being thrown upon cokes easily combustible, the quality of the metal, with the same quantity of fuel, becomes oxygenated, the tuyere becomes fiery, and frequently emits sparks of metallic oxide. The separating iron may be viewed as it oozes from the ore in small globular masses, frequently on fire, changing its state to that of an oxide. The combination of oxygen, by altering its density, makes it subject to the re-action of the blast, which at times gives it a direction from the tuyere with considerable violence. Those parts of the iron (by far the greatest) thus oxidated, which escape not at the tuyere, mix along with the fused earths of the ores and limestone, alter their colour, and flow from the furnace more unrevived than at their first introduction. It is, however, very different, even with this inferior quality of coal, where the density of the blast is proportioned to the inflammability of the fuel. Qualities and quantities of crude iron may be produced from this, equal to those from coals reckoned of a superior nature. The metal becomes as highly saturated with carbonic principle as that made from clod or splint coal. The tuyere evinces that decomposition is effected in its proper place. The fluid masses of iron, as they become expressed from the ore, are shivered into spray, before the dense column of air, without exhibiting the least symptom of decomposition. They again unite under the level of the blast, increase in size, and sink through the fluid stratum of earths to the bottom of the furnace. This fact holds out one of the strongest proofs of the great affinity which carbons and iron mutually possess towards each other. In the case of the iron separating in an oxygenated state deftite of carbone, it immediately falls a prey to its affinity for oxygen. In the latter case, the iron, being completely carbonated, resists decomposition by the sacrifice of a very small portion of its carbone. It further proves, that the affinity of oxygen is greater to carbone than to iron; and that, before iron becomes oxidated, all the carbone is taken up.
"The continuity of the particles of splint coals renders the cokes of difficult combustion, capable of withstanding a most powerful discharge of air, in quantity and in the degree of compression, without entailing effects similar to those produced with the use of softer coals: this renders the operations with splint coal less subject to casualty and to change. Carbonated iron with a proper blast is more uniformly obtained, and frequently a very superior quantity. Similar effects are produced with the clod coal, but in a more eminent degree. Discharging pipes are used four inches in the diameter, and the compression only equal to two pounds on the square inch; yet the same fatal effects are not known as in the use of soft coal, which, with such a column of air, would require the pressure to be equal to three pounds and a half upon the square inch at least.
"5th, Upon the form and construction of the discharging pipe effects of more considerable importance depend than is either generally allowed or even conceived. At some iron works, no peculiar shape is adopted: if the tube is sufficient to convey the air, and the mouth of it nearly of the size wanted, the interior construction is entirely overlooked. This indifference, however, is by no means general: variously constructed pipes are used at different works, and at some places it is preferred to throw in the air from two pipes whose areas are only equal to one of the usual size.
"To understand properly the objectionable parts of the construction of nose pipes, it must be recollected, that much has been said to depend upon the blast reaching the opposite extremity of the furnace, as little impaired of the compactness and velocity of its original discharge as possible. When it is otherwise, the results in the internal operations of the furnace must be consequently altered. If the compression is diminished one-half or two-thirds when it reaches the opposite wall, decomposition in that portion must be effected before the air has attained its elevated situation in the furnace. It is even possible to disperse the whole column of air in such a manner that the ignited materials of the opposite side may receive little of its effects to promote combustion.
"A discharging pipe is frequently used, in length, 12 inches or more, the discharging aperture 3 inches, the other end 5 inches; but this is arbitrary, depending upon the size of the adjoining pipe. From a pipe thus constructed, the air disperses or diverges too suddenly; and at a small distance from the orifice, a considerable portion of it anfvers but imperfectly the purposes of combustion. Part of it is speedily decomposed, and the oxygen brought into immediate contact with the iron. The quantity of metal is reduced by the former, and the quality injured by the latter. Though long custom, by a continued use of such shaped pipes, has prevented their pernicious effects from being observed, yet they must prove in many cases detrimental to the economical distribution of air, and the manufacture of iron.
"A nose pipe, of another construction, even more exceptional, is also used; and the air disperses still more suddenly, in a degree somewhat proportionate to the more sudden contracting of the pipe, a considerable quantity never enters the furnace, but, striking on the exterior wall, is thence repelled.
"A discharging pipe, of the following best form would obviate, in a great measure, the imperfections of the two former: the length of the tapered piece is 12 inches, of the straight pipe, six inches; extreme diameter, as in the others, five inches; diameter of straight pipe, three inches. From such a pipe it is conceived that the blast will proceed to the greatest possible distance unimpaired in compression and velocity. So far, therefore, as the absolute force of the blast and breadth of the furnace will permit, decomposition will be prevented on the level of the pipe, and the manufacturer freed from the evils which I have above detailed, as attendant upon decomposition in that quarter."
The following is a description, also taken from Mr. Description Muhlet, of an air and a water vault which is employed of an air vault to equalize the discharge of air into a blast furnace.
"Fig. 7 represents a vertical section of the elevation Fig. 7, of an air vault 60 feet long and 30 feet wide, consisting FUR Furnace. of four arches of regularly progressive sizes. This building is generally constructed under the bridge-house, where the materials are daily collected for filling the furnace. AB, represents the acclivity to the furnace top. The space betwixt the arch tops and the level of the floor is filled with materials as dense as can be procured. The walls of the under part are three feet thick, besides a lining of brick and plaster from 18 inches to two feet. Still further precautions are necessary, and alternate layers of pitch and flout paper are requisite to prevent the escape of the compressed air. C, a view of the arched funnel which conveys the air from the cylinder to the vault. Large iron pipes with a well fitted door, are preferable, and less apt to emit air. D, an end view of the pipe by which the blast is carried to the furnace.
"Fig. 8. is a horizontal section of fig. 7. at the dotted line ab, representing the width of the cros arches, which are thrown in each partition to preserve an easy communication betwixt the vaults. D, is a section of the first range of pipes, meant to conduct the air to the furnace. In like manner pipes may be taken off from any part of the vault for the different purposes of blowing furnaces, fineries, hollow fires, &c.
"Fig. 9. represents a vertical longitudinal section of what is generally called the water-vault. The walls of this building may be erected to the height of eight or nine feet, their thickness similar to those of the air vault. A brick lining, and even puddling with clay betwixt it and the stone building, is necessary to prevent the water from oozing by the accumulated pressure. A, is an end view of the horizontal range of pipes which conveys the blast from the blowing cylinder to the inverted chest. BBB, the range which conducts the air to the interior of the inverted chest, and conveys it to the furnaces, proceeding along the extremities of the columns broken off at BB. C, an inverted chest made of wood, iron, or even of well-hewn flags set on end and tightly cemented, is 54 feet within length, 18 feet wide, and 12 feet high. The dimensions, however, vary at different works. When the chest is made of wood or iron, it is generally bolted by means of a flange to the logs on which it is supported, lest the great pressure of the air should overcome the gravitation of the chest, and displace it. DD, view of the centre log, and ends of the cros logs, on which the chest is laid. These should measure 18 inches in height, so that the mouth of the chest may be that distance from the surface of the floor, and the water allowed to retreat from the interior of the chest with the least possible obstruction. EE, the outside walls of the building. FF, the brick-works, made perfectly water tight. The dotted line G, represents the surface of the water when at rest. Let the depth of the water, outside and inside of the chest, be estimated at four feet. When the engine is at work, should the pressure of the air have forced the water down to the dotted line H, three feet and a half distant from the line G, and only fix inches from the mouth of the chest, it follows, that the water must have risen in the outer building, or chest, three feet and a half above G, and have its highest surface nearly at rest at I. In this case the strength of the blast is reckoned equal to seven feet of water, or nearly fix inches of mercury. The space betwixt the chest and outside building is three feet. When the engine is at rest, and the water has assumed its level, Furnace. the quantity of water within the chest should be equal to that without.
"Fig. 10. is a ground plan of fig. 9. The cros logs Fig. 12. on which the cistern is supported are dotted within, but drawn full in the space betwixt the flange of the chest and outer building. The breadth of the flange-tops of the binding bolts, and thickness of the metal of the chest, are also drawn. The letters bear a reference to those in fig. 9."
An account of some curious phenomena observed by Singular Mr Roebuck in the air vault of a blast furnace has been published in the 5th volume of the Transactions of the Royal Society of Edinburgh. This, as well as some remarks of practical utility on the management of blast furnaces, we doubt not, will be interesting to our readers. We shall therefore give it in his own words. It is addressed in the form of a letter to Sir James Hall.
"I have (says he) examined my memorandum, concerning the observations I made on the condensed air in the air vault of the Devon iron works, near Alloa, on the north side of the frith of Forth; and, according to your request, I now transmit you an account of them; and also of an experiment I made, when a partner and manager of these works, in order to increase the produce of blast furnaces.
"The two blast furnaces at Devon are of large dimensions, each being 44 feet high, and about 13 feet wide in the boles, or widest part, and are formed on a steep bank, by two pits sunk in a very solid stratum of coarse-grained freestone.
"These pits were afterwards shaped and lined in the usual manner of blast furnaces, with common bricks and fire bricks, and the hearth was laid with large blocks of the stone that had been dug out, and which serve the purpose of fire stones. At the back of the two furnaces, next the bank, the air vault is excavated, and formed by a mine driven in the solid rock, distant from the furnaces about 16 feet. The bottom of the air vault is only about four feet higher than the level of the bottom of the furnaces. This vault has an aperture at one end to receive the air from the blowing machine, and has two at the opposite end, one of which receives the eduction pipe, and the other is a door to give admittance occasionally into the vault. As the rock is extremely close and solid, the vault is dry, except that a little water oozes very gently from the side next the bank in small drops, and does not appear to exceed an English pint in 24 hours.
"These furnaces are provided with air, or blast, as it is termed, by the means of a fire-engine of the old, or Newcomen's construction. The diameter of the steam cylinder is 4 8/4 inches; and the square area of its piston being about 1866 1/2 square inches, the power of this sort of engine cannot be rated at more than 7 lb. to the square inch, amounting in all to about 13,621 lb. This power was employed to work an air pump, or blowing cylinder, of 78 inches diameter, and about seven feet long. The number of square inches on the piston of the air pump is 4778, and therefore this area, being multiplied by 2 1/3, will produce 13,139, being a refinance that nearly balances the above-rated power, and shows that the air, which was expelled from the air pump, could not be condensed more in the ordinary way way of working, than with a compressing power of about 2 1/2 lb. on each square inch. As the engine was not regulated, at first, to make a longer stroke than about four feet eight inches, only one furnace being used, the quantity of air expelled at each stroke of the machine was about 155 cubic feet, which it discharged through a valve into the air vault, about 16 times in a minute. When two furnaces afterwards were blown, the engine was regulated to work much quicker, and with a longer stroke. The air vault is 72 feet long, 14 feet wide, and 13 feet high; and contains upwards of 13,000 cubic feet, or above 80 times the contents of the air pump. The top, sides, and bottom of this vault, where the least fissure could be discovered in the beds of the rock, were carefully caulked with oakum, and afterwards plastered, and then covered with pitch and paper. The intention of blowing into the vault is to equalize the blast, or render it uniform, which it effects more completely than any machinery ever yet contrived for the same purpose. The air is conducted from the vault by the eduction pipe, of 16 inches diameter, into an iron box or wind chest, and from this it goes off to each furnace, in two smaller pipes that terminate in nozzles, or blow-pipes, of only 2 1/2 to 3 1/4 inch diameter, at the tuyere of the furnace.
"When the furnace was put in blast, after having been filled with coals, and gently heated for more than six weeks, the keepers allowed it to have but little blast at first, giving it a small blow-pipe of about 2 1/2 inches diameter, and likewise letting off a very considerable quantity of air, at the escape or safety valve on the top of the iron wind chest, as it is a received though erroneous opinion among them, that the blast must be let on very gradually for several months. From the construction of this valve, it was impossible to ascertain the exact proportion of the blast which was thus lost, but I believe it was very considerable. The consequence was, that the furnace, after it had been in blast for several days, never seemed to arrive at its proper degree of heat, but was always black and cold about the tuyere in the hearth, and appeared in danger of choking, or gobbing as it is termed.
"After various experiments tried in vain, by the keepers and the company's engineer, and others, (indeed they tried every thing, except giving the furnace a greater quantity of air, which, as I afterwards ascertained, was all that it wanted), they concluded, that the air vault was the cause of the whole mischief; and, to confirm their opinion, they said they had now discovered that water was, in considerable quantities, driven out of the air vault through the blow-pipe, which cooled the furnace; and they insisted, that the power of the engine was such as to force water out of the solid rock; so that this method of equalizing the blast never would succeed. The other managing partner was so much alarmed by these representations, that he began to consult with the engineer, and others, about finding a substitute for the air vault at any expense.
"As the plan of the blowing apparatus had been adopted at my recommendation, and was now so loudly condemned on account of the water, I had other motives, than mere interest, for trying to become better acquainted with the phenomena attending it. I accordingly determined to go into the air vault, and to remain inclosed in the condensed air while the engine was blowing the furnace. It is an experiment that perhaps never was made before, as there never existed such an opportunity. I could not persuade the engineer, or any other of the operative people about the work, to be my companions, as they imagined that there was much danger in the experiment. Mr Neil Ryrie, however, one of the clerks of the Devon company, had sufficient confidence in my representations to venture himself along with me.
"The machine had been stopped about two hours previous to our entering the vault, and we found a dampness and mildness in it, which disappeared soon after the door was that fast upon us, and the engine began to work in its usual manner. After four or five of the strokes of the engine, we both experienced a singular sensation in our ears, as if they were stopped by the fingers, which continued as long as we remained in the condensed air. Our breathing was not in the least affected. I had no thermometer with me, but the temperature of the air felt to us the same as that without the vault. Sound was much magnified, as we perceived, when we talked to each other, or struck any thing; particularly, the noise of the air escaping at the blow-pipe, or waste-valve, was very loud, and seemed to return back to us. There was no appearance of wind to disturb the flame of our candles; on the contrary, I was surprised to find, that when we put one of them into the eduction pipe, which conveys the wind from the vault to the furnaces, it was not blown out. There was not the smallest appearance of any drops of water issuing out of this pipe. The oozing and dropping of water from the side of the rock, next the bank, seemed the same as before the condensation was made in the vault. In short, everything appeared, in other respects, the same as when we were in the common atmosphere. Having remained about an hour in the condensed air, and satisfied ourselves that no water, during that time, that we could in the least discover, was agitated and forced out of the rock and vault by the power of the blast, as was imagined and insisted on, we gave the signal to stop the engine. As soon as it ceased to work, and the condensation abated, and before the door of the vault was unfastened, the whole vault in a few seconds, became filled with a thick vapour, so that we could hardly see the candles at four or five yards distance. The door being now opened, the work people, anxious to know our situation, and what had occurred, came into the vault, and prevented any further observations.
"I now endeavoured to account for this curious appearance of the water, which only flowed itself occasionally, in very small quantities, at the tuyere, at a hole I ordered to be made in the bottom of the wind chest to collect it more accurately, for it never was observed, but either when the engine, after working slowly, was made to work quicker, or, after having been stopped for a few minutes, was set to work again.
"I considered the vapour which we had discovered in the vault to arise from the moisture of the side of the rock next the furnace, which being expelled by the great heat of the furnace, and converted into water, was able to force its way through the pores of the rock into the vault, but that being in a manner confined within the rock, by the prelude of the condensed air, it found itself at liberty to come into the vault, vault, only when the condensation abated considerably, or was totally removed by the going flow, or slopping of the engine. It also occurred to me, that the air, in a state of condensation, might possibly be capable of holding a greater quantity of water in solution, which might precipitate suddenly into vapour or mist when the condensation abated. I imagined, therefore, that the very small quantities of water we at times discovered, proceeded from nothing else but this vapour, in its passage to the furnace along with the blast, being conducted into water, by the coolness of the eduction pipe and iron wind chest. The quantity of water did not appear to amount to a gallon in twenty-four hours.
"A few days after I had made this experiment, the water ceased entirely to make its appearance, either at the tuyere, or at the hole in the wind chest, but the furnace did not come into heat for a long while after, and indeed not till the keepers let much more air into it by a larger blow-pipe, and allowed less air to escape at the safety valve. It is probable that the rock was now become perfectly dry by the continued heat of the furnace.
"My experiment had the good effect to remove all the prejudices against the plan I had adopted of blowing the furnaces, and likewise prevented the other partner from laying out a large sum of money, by stopping the works, and altering the blowing machinery. Indeed, it has since been admitted, by all who have seen it at work, to be the most simple and effective method of equalizing the blast which has yet been put in practice.
"This experiment led me, some time afterwards, to apply a wind gauge that I contrived, to ascertain precisely the state of the condensation of the air thrown into the furnaces. I found that a column of quicksilver was raised five inches, and sometimes, though seldom, fix inches, and, in the interval of the engine to receive air into the air pump, it fell only half an inch. At this time only one furnace was worked. But when two furnaces were in blast, the engine only raised the mercurial gage about four inches, because the Devon company, for several reasons, did not, while I continued a partner, think proper to allow the blowing machinery to be completed, by putting to work their second boiler of 20 feet diameter for the fire engine, according to my original design, which, by adjusting the machinery, would have enabled us to blow two furnaces with two boilers, with as much effect, in proportion, as one furnace with one boiler. This instrument had the advantage of enabling the work people to discover the real power of their blast, and know the exact condition of the air valves, and the gearing of the blowing piston; for if these were not tight, and in order, (although the engine might, to appearance, be doing well, by making the same number of discharges of the air pump as usual per minute), yet the wind gage would not rise so high, and would shew that there was an imperfection somewhere, by reason of a quantity of air escaping at the valves, or piston, that could not so easily otherwise be known. This contrivance was considered as of much use, and was afterwards always quoted in the company's journal books, to show the actual state of the blowing machine, in comparing the daily produce of the furnaces.
"I hope you will not think me tedious, when I explain to you another experiment, which appears to me to be of considerable importance to all manufacturers of cast iron.
"I had reason to conjecture, from my own observations on the effects of blowing machinery on blast furnaces, as well as from the knowledge I had acquired from my father Dr Roebuck, and from my communications with other experienced iron masters, that a great part of the power of such machinery was misapplied in general practice, by throwing air into furnaces with much greater velocity than necessary, and that, if this velocity was, to a certain degree, diminished, the same power, by properly adjusting the blowing machinery, of whatever nature, would be capable of throwing into the furnace a proportionally greater quantity of air. For, Since the quantities of any fluid, issuing through the same aperture, are as the square roots of the pressure; it follows, that it would require four times the pressure, or power, to expel double the quantity of air, through the same aperture, in the same time; but if the area of the aperture was doubled, then the quantity of air expelled by the same power, and in the same time, would be increased in the ratio of the square root of 2 to 1, though its velocity would be diminished exactly in the same proportion. Again: I considered that the quantity and intensity of heat, produced in blast furnaces, and consequently its effects in increasing the produce, might be only in proportion to the quantity of air decomposed in the process of combustion, without regard to its greater velocity; that is to say, whether or not the same quantity of air was forced, in the same time, into the furnace through a small pipe, or through one of larger dimensions; for, in attending to the process of a common air furnace for remelting of iron, where there is a very large quantity of air admitted through the large areas between the bars, it is well known, that a much greater intensity of heat is produced than takes place in a blast furnace; and yet the air does not enter into the fire through the bars with increased density or great velocity. I therefore thought it probable, that increasing the quantity of air thrown into the blast furnace in a considerable degree, although the velocity or density might be much less, would have the effect of increasing its heat, and operations, and produce. And as, from the principles above stated, with regard to the machinery, I saw I could greatly increase the quantity of air thrown into the furnace, by enlarging the diameter of the blow-pipe, and regulating the engine accordingly, without being obliged to employ more power, I was anxious to make this experiment.
"A system of management, of which I did by no means approve, was adopted by the other partners of the Devon company, soon after the works were begun to be erected; and, in the prosecution of it, they ordered their second furnace to be put in blast, without permitting those measures to be taken that were necessary to provide and maintain a sufficient stock of materials; and also without allowing their blowing machine to be completed, according to the original design, by the addition of its second boiler. As might have been expected, a trial of several months to carry on two furnaces, with only half the power of steam that was necessary, and an inadequate stock of materials, proving unsuccessful, Furnace. unsuccessful, the company, as a remedy, instead of making up the above deficiencies, ordered one of the furnaces to be blown out, and stopped altogether. This improper measure, however, afforded me the opportunity of immediately putting in practice the plan I have mentioned.
"When one of the furnaces was stopped, the other continued to be blown by a blow pipe of 2 1/2 inches diameter, and the produce of the furnace, for several weeks thereafter, was not 20 tons of iron per week at an average. The engine at this time was making about 16 strokes a minute, with a stroke of the air pump, about 4 feet 8 inches long; but when I altered the diameter of the blow-pipe, first to 3, and immediately after to 3 1/4 inches diameter, and regulated the working gears of the engine, so as to make a stroke of 5 feet 2 inches long, and about 19 strokes in a minute, on an average, the produce was immediately increased. It continued to be, on an average of nine months immediately after this improvement, at the rate of 33 tons of iron per week, of as good quality as formerly; for, during this period, from the 21st November 1795 to July 32, 1796, this one furnace yielded 1188 tons of iron. No more coals were consumed in working the blast engine, or other expenses about the blowing machine incurred, and therefore no more power was employed to produce this great effect. It is also of much importance to remark, that the consumption of materials, from which this large produce was obtained, was by no means so great as formerly." The furnace required very considerably less fuel, less iron stone, and less lime stone, than were employed to produce the same quantity of iron by the former method of blowing; and according to the statements made out by the company's orders, as great a change was effected in the economical part of the business.
"From the success of this experiment, so well authenticated, and continued for several months, I am led to be of opinion, that all blast furnaces, by a proper adjustment of such machinery as they are provided with, might greatly and advantageously increase their produce, by assuming this as a principle, viz.: 'That with the given power it is rather by a great quantity of air thrown into the furnace, with a moderate velocity, than by a less quantity thrown in with a greater velocity, that the greatest benefit is derived, in the melting of iron stones, in order to produce pig-iron.' However, it is by experiment alone, perhaps, that we can be enabled Furnace. to find out the exact relations of power, velocity, and quantity of air requisite to produce a maximum of effect (1)."
In order to illustrate what is said above, a ground plan of the air vault and furnaces of the Devon Iron Works is given in Plate CCXXVI.; of which the explanation follows.
Explanation of Fig. 11.
A, The air vault, formed by a mine driven in the solid rock of coarse-grained freestone. B, The blowing cylinder. C, The pipe that conveys the air from the blowing cylinder to the air vault. D, The eduction pipe that carries the air from the air vault to the iron wind-chest. E, The iron wind chest (about 2 1/2 feet cube), in which is inserted a wind-gauge, represented in fig. 12. FF, The two blow-pipes for each furnace, which terminate in apertures of 3 1/2 inches diameter at the tuyeres of the furnaces. GG, The two blast furnaces, placed in two pits sunk in the solid rock. HH, The tuyeps of the furnaces from whence the cast iron is run off into the casting room, LL. O, The door to give occasional admittance into the air vault. M, The excavation, in which is placed the blowing machine.
Explanation of Fig. 12.
A, The end of the wind-gauge (about 12 inches long), which is open to the atmosphere, being half filled with quicksilver. B, The end that is inserted in the iron wind chest, and exposed to the pressure of the condensed air of the air vault.
To Mr Muflet we are also indebted for the following Description account of air furnaces, which are employed in iron and air founderies for the purpose of casting large pieces of furnace ordnance, and other heavy articles.
These furnaces, he observes, "are employed for melting pig iron with the flame of pit coal. Furnaces of this kind are constructed of various sizes according to circumstances. The small sizes will run down from
(1) "If Q be the quantity of a fluid, issuing in a given time through an aperture of the diameter D, V its velocity, and P the power by which it is forced through the aperture: then the area of that aperture being as D^2, the quantity of the fluid issuing in the given time will be as VD^2, or VD^2=Q.
"Again, this quantity multiplied into its velocity, will be as the momentum of the fluid expelled, or as the power by which it is expelled, that is, V^2 D^2=P, or VD=\sqrt{P}.
"Here, therefore, if D is given, V is as \sqrt{P}, as Mr Roebuck affirms. Also, because V=\frac{Q}{D^2}, and also V=\frac{\sqrt{P}}{D}, Q=D\sqrt{P}, so that, while P remains the same, Q will increase as D increases, and V will diminish in the same ratio.
"The problem, therefore, of throwing the greatest quantity of air into the furnace, with a given power, strictly speaking, has no maximum, but the largest aperture of which the engine can admit must be the best. It is probable, however, that there is a certain velocity with which the air ought to enter into the furnace; this will produce a limitation of the problem, which, as Mr Roebuck suggests, is not likely to be discovered but by experiment." Note by Mr Playfair. Furnace, seven to ten hundred weight, and are used in small founderies for what the trade call flogging.
"Fig. 13. (Plate CCXXVI.) A ground plan of two large air furnaces, and chimney for melting pig or cast iron with the flame of pit coal.
"The letters ABCD point out the exterior dimensions of the flak or chimney, which is first erected, leaving two openings or arches into which the fore-part of the furnaces are afterwards built. The breadth of the chimney at the particular place which the plan exhibits is 16 feet from A to B, and from A to D or from B to C fix feet fix inches. The plan is drawn at that elevation where the flame enters the chimney by the flue or throat, narrowed on purpose to throw back part of the flame, and keep the furnace equally hot throughout, as may be more particularly viewed in the vertical section, fig. 14.
"EE, the furnace bars on which the coals rest, and where the combustion is maintained.
"FF, openings called teasing holes, by which the coals are introduced to repair the fire.
"GG, fire brick buildings called bridges. These are meant to concentrate the flame, that it may act as violently on the metal as possible. Upon the height of the bridge much depends in fusuing the metal speedily, and with little loss. The height of this may be seen in the vertical section, fig. 14. G.
"HH, the charging doors, by which the metal is introduced in the shape and state of pig iron, lumps, scraps, &c. &c. The iron generally occupies the furnace across to I, called the back wall, and is never meant to approach the bridge nearer than the dotted line, lest the metal in melting should run back into the grates, in place of descending into the general reservoir or cavity below. The corners or notches, h,h,h,h, receive a stout cast iron frame lined with fire bricks. This is hung by means of a chain and pulley, and can be raised and depressed at pleasure. This frame is, properly speaking, the charging door, and is always carefully made air tight by means of moistened sand.
"KK, the flues or openings by which the flame enters the chimney. These are 15 inches by 10. On maintaining these openings of a proportionate size to the other parts depend in a great measure the powers and economy of the furnace.
"LL, lading doors, by which ladles are introduced, in the case of small furnaces, to lift out the metal and distribute it to the various moulds.
"MMMM, binding bolts to limit within proper bounds the expansion which takes place in the building when the furnace is highly heated.
"Fig. 14. vertical section of one of the furnaces, and its appropriate flak or chimney.
"E, the grates. "F, the teasing hole. "G, the bridge. "H, the charging door. "K, the flue or opening into the chimney. "L, the lading door. "MM, the binder or binding bolt. "N, the interior of the flak or chimney, 30 inches square. "OO, the fire brick work, nine inches thick. "PP, space of two inches for stuffing with sand. "QQ, common brick building.
"RR, cast iron lintels, over which are thrown double nine inch arches, so that at any time the inferior building can be taken down to make repairs, without shaking or in the least injuring the chimney.
"S, The dotted lines here are meant to represent what is called the tapping hole. When a large piece of goods is to be cast, lifting the metal with ladles would be impracticable. A sharp pointed bar is driven up this opening. The iron then flows freely out into a large basin of sand made for its reception. It is then conducted, by collateral channels, into the mould.
"The space under the curved dotted lines from G to L, by S, is filled with a mixture of sand and ashes. When the surface is prepared to melt, the whole of the bottom receives a stratum of sharp clean sand about two inches thick. This is broken up at night, and fresh sand is substituted for it before the fire is kindled in the morning.
"Fig. 15. is a horizontal section of the chimney or Fig. 15 stalk, taken where the flues assume a perpendicular direction. The letters in this figure correspond to those in the vertical section, fig. 14. The height of the chimney ought not to be less than 45 feet : if 50 feet, the effect will be sooner and of course better produced.
"The effect wished to be produced in air furnaces is the fusion of a certain portion of pig or cast iron, for the purpose of being poured or run into moulds to form articles of almost every description.
"The preparation previous to melting is as follows: Prepare After the bottom of the furnace is laid, and smoothed out with fresh land, and all the openings made air tight, the furnace man introduces a kindling at the teasing hole, accompanied with new pit coal. In a few minutes a considerable volume of dark flame mixed with smoke is produced. The fire quickly gathers strength; more coal is introduced; and the furnace now becomes filled with a yellow-coloured flame. By continuing this operation for an hour, or an hour and a quarter, the furnace and flame will have become completely white; the latter steady, and at times apparently without motion. The furnace man now judges the bottom to have been sufficiently hardened for receiving the pig iron without any risk of finking. The charging door is now opened, and the pig metal thrown carefully and regularly upon that part of the bottom formerly described as being appropriated for its reception. The door is again closed and made air tight, and the operation of firing continued with unremitting care and attention.
"The time of melting depends entirely upon the quantity of metal introduced. The furnaces described above are capable of melting from 50 to 60 hundred weight of metal each, and when there is a moderate circulation of air they will perform this work in 2 1/2 or 3 hours. In half an hour after the metal is introduced it assumes a blackish red colour. It then begins to brighten with every additional fire, and in about one hour appears white, and begins to lose shape, and resemble a wreath of snow.
"An eye accustomed to such heats will now discern the metal beginning to drop, and run down the inclined plane in very beautiful streamlets resembling quick-filter. Eight or ten of these are visible at a time, and after proceeding half way down begin to form junctions with each other, and flow connected into the general cavity or reservoir. By-and-by this becomes filled, and literally forms a beautiful molten mirror, in which sometimes part of the interior furnace is reflected.
"The furnace man, by searching at the bridge with his fire-iron or tealer, judges when the metal is nearly all gone. Of this he is certain by looking up from the peep-hole of the lading door. If the streamlets of the running metal have ceased, then the whole is melted, and ready for running out.
"In the operation of melting, the three following circumstances ought to be particularly attended to: the thinness or hotness of the metal; the waste or loss sustained in melting; and the quantity of coals employed.
"The first is of the utmost importance, as many articles in the foundry business require the metal in a state of the greatest division; otherwise they will be found imperfect when taken from the sand, and unfit for sale. The furnace man, therefore, is always on the watch to replace the fire as it decays, and keep a large and sharp volume of flame constantly paling over the metal.
"The waste or loss of real metal is also an object of great importance. This always bears a relation to the quality of the iron, the strength and cleanliness of the coals, and the judgment and attention of the melter. Strong iron is found always more difficult to fuse; this necessarily exposes it for a long period in contact with the flame. The reverse happens with metal that is more fragile, and easier broken in the pig. The length of the exposure in fusing depends on this; and other circumstances being alike, the loss or waste of metal will also be in the same ratio.
"There are, however, other facts not unworthy of notice. No. 1. pig iron, or richly carbonated metal, when run from an air furnace, will be found in point of quality little better than No. 2. or carbonated iron. This is owing to a quantity of its carbons being destroyed during the fusion. The loss in melting No. 1. iron, therefore, chiefly consists of carbons; and the deficiency of metal ought never, with a clean bottom, to exceed 1 cwt. in 25.
"Carbonated or No. 2. iron also becomes deprived of a considerable portion of its carbonaceous mixture in fusion; and when run from the air furnace is seldom better than No. 3. metal. The loss sustained in melting may be averaged at 7 1/2 per cent.
"No. 3. pig iron is, after melting in an air furnace, found whitish or mottled. It is seldom susceptible of the same nice degree of division as the superior qualities, and loses in fusion a much larger proportion of metal, seldom under 10 per cent. and frequently 12 1/2 or 15.
"The quantity of coals requisite to melt a given quantity of iron is various, as much depends upon the quality and fusibility of the metal. If the furnace goes one heat a day with No. 1. or 2. iron, the quantity of coals will be from 20 to 25 cwt. for a ton of iron. If two or three heats a day, or as many tons of iron are melted at one kindling, the proportion of coals will be nearly weight for weight of the iron melted when the coals are mixed with a fair proportion of small; with strong large splint coals, one ton of good pig iron may be completely reduced with from 12 to 15 cwt. included. Furnace, ing the previous heating of the furnace."
In the reduction and fusion of ores, the improvement of the blowing apparatus, or the machinery contrived for the purpose of forcing a current of air into furnaces, where a high degree of temperature was necessary, has always been an important object of consideration to the manufacturer; and indeed, it appears that the history and of blowing improvement of this kind of machinery have progressively advanced, in some cases have exceeded the improvement of other departments of the manufactures of this country.
In melting some metallic ores, as for instance, those of lead and tin, the magnitude and powers of blowing machines have been less attended to, because the requisite temperature for that purpose is far inferior to what is necessary for the reduction of the ores of iron. Lead and tin being naturally fusible, and easily volatilized in a temperature beyond a bright red heat, have hitherto fixed the limits with regard to the size of the furnace, and the quantity of blast. The air furnace is generally employed in the manufacture of copper, excepting in small blast furnaces, in which the precipitated oxide of this metal is received, and they are similar to the furnaces called cupolas, and used at iron foundries.
The lead mill, as it is called, or machine for the reduction of the ores of lead, is of a very simple construction. In the middle of a square building a water wheel is erected, and to the shaft of this wheel, four small wheels of cast iron, about 18 inches in diameter, are attached. Two pairs of bellows placed at equal distances, and on each side of the shaft, are supported on a strong frame of wood. During the revolution of the shaft of the water wheel, the small wheels are also carried round, and alternately depress the end of the lever which is attached by means of an iron chain, to an equally balanced beam. When this lever descends, the opposite end of the beam is elevated, and to this end there is attached by another iron chain, the moveable surface of the bellows. The blast produced in this way is soft, and far inferior, either with regard to quantity or density, to the blast necessary for an iron furnace. The length of the bellows is usually about 10 feet, the breadth across the breech about five or six, and they move at the rate of about 30 strokes a minute.
But in the manufacture of iron, and particularly since which must the use of pit-coal was introduced, it is absolutely necessary for the manufacturer to have a more powerful blowing machinery, power in this, therefore, has always been an essential requisite, the manufacture has been a constant object in this manufacture; for iron, in proportion to the quantity of air thrown into the furnace, the produce and quantity of metal is increased. In the earlier periods of this manufacture, when the fuel employed was charcoal from wood, the process was more easily managed. Furnaces which were built of small size, and which were then called bloomeries, were considered of sufficient capacity to yield profit, if they produced a bloom or two of iron in the day, each bloom amounting to about 90 or 120 lbs. For smaller operations, hand-bellows, and what were called fuel blasts, were deemed of sufficient power; but when the refining furnace began to be employed, and the iron manufacture branched out into the making of pig iron, and the refining refining of it into bar or malleable iron, the advantage and necessity of a powerful blast were immediately seen. The first moving power introduced was that of the water wheel; and this working two or more pairs of leathern bellows, was found to produce effects sufficiently powerful for the purpose.
Machinery constructed in this way, and set in motion by the power of water, continued to be employed for this purpose, till the principles of the steam engine were fully understood, and this powerful machine came into general use. The steam engine, besides many other advantages, could be employed in situations where the want of water prevented furnaces being erected, but otherwise commodious, in being near the necessary materials of ore and fuel. The first substitute for the leathern bellows were cylinders composed of wood, closely jointed, and strongly hooped. These in their turn gave place to cylinders of cast iron, smoothly and accurately bored; and this kind of apparatus being discovered and applied in the manufacture of iron, the blowing machine now assumed a more perfect and more manageable form.
But without attempting to describe any of the blowing machines in our own country, the powers and effects of which are familiar to those to whom this knowledge is most interesting, we shall give a short description of an apparatus of this kind, which is set in motion by the pressure of a column of water, and is erected near Namur in the Netherlands. The account of this machine is given by Baillet, inspector of the mines, who observes, that its construction is simple, and not very expensive, and that it may be kept up without requiring much repair. This machine, besides, can be employed to blow several furnaces at once. It does not require any great moving power, and the consumption of water is much less than in the blowing apparatus of leather or wood. In consequence of these advantages, the number of furnaces has been greatly increased since this apparatus was first erected, and the extent of the manufacture has been doubled. This apparatus possesses another superiority over the ordinary blowing machines. The latter, to be put in motion, require a water wheel; but the apparatus which is here alluded to, is set in motion merely by the pressure of a column of water.
The following is the description of this blowing machine, as it was first erected at Marche upon the Meuse. It was invented and constructed by Jannins, proprietor of the forges, and it consists of two cylinders of three feet eight inches diameter, and of thirty inches high, placed vertically near each other. One of these cylinders is represented at fig. 16. A piston of wood covered with leather, (fig. 17,) moves in each cylinder, and forces the air through the tubes o, o, o, which are fitted to the upper part of the cylinders, and are conducted to the different furnaces where combustion is to be excited. The base of these tubes is furnished with valves, to prevent the return of the air. The piston is, besides, furnished with two lids or covers, w, w, (fig. 18. and 19.) which open when it descends, and shut when it rises. The piston is surrounded with a band of leather in the usual way, to make it tight.
The moving power in this apparatus, is a water wheel erected on the horizontal shaft, s. On this shaft are fixed the arms t, t, projecting from its circumference, which alternately elevate the stalk of the piston.
The descent of the piston is regulated by the weight f, which acts as a counterpoise; and the spring of wood, g, which is balanced when the stalks of the piston are at their lowest descent, serves to retard the velocity, and to prevent any sudden or violent stroke.
Two of these cylinders, erected at one of the forges at Marche, furnish air to two furnaces, which employ charcoal from wood, and one with coke from pit-coal. The stroke of the piston is about 18 inches, and 25 strokes in a minute, and with this length of stroke and velocity, the two pistons produce nearly about 400 cubic feet of air. The consumption of water, having a fall of about 10 feet, is about 80 cubical feet.
Two similar cylinders, erected at another furnace at the same place, move with the velocity of 19 strokes per minute. The length of each stroke is about 22 inches, so that it produces about 360 cubical feet of air. For this, with a fall of 10 feet, 75 cubical feet of water are necessary.
In the construction of this blowing machine, no peculiar difficulty occurs. It is not necessary that the cylinders should be accurately turned in the inside. All that is required is, to grind or polish their inner surface with sand stone. It was in this way that the cylinders and apparatus, just described, were prepared.
The piston, which is made of wood, has in the middle of it a mortoise, u, fig. 17. and 19. to admit the stalk, p, which is kept in its place by four bands or straps of iron, x, fig. 17.
The band of leather, es, is about three lines in thickness, and about five inches broad. It is nailed to the piston, and ought to be raised above the groove or gutter, v.
The grooves y, y, are sunk in the piston, in proportion to the thickness of the leather, and their external diameter should be somewhat smaller than that of the cylinder. The large lids or covers of the piston are of wood, lined with sheepskin ; and their hinges, which are made of leather, are fixed with screws to the wood : a bridle of leather limits the extent of the opening.
The final valves, which are fixed at the upper opening of the cylinders, at the end of the tubes for conducting the air, are also of wood, and covered with sheepskin.
The tubes or pipes which conduct the air are made of iron plates, or of tinned iron, and they terminate in pipes of a convenient diameter, and proportioned to the different furnaces. They should also be furnished with keys or cocks, for regulating at pleasure the quantity of the air.
The frame which supports these cylinders is of a very simple construction, as will appear by inspecting fig. 16. It is attached and secured to part of the wall of the building.
All that is necessary to keep this apparatus in order, is with a brush to cover the internal surface of the cylinders with oil once every 10 days.
The following are the dimensions of the principal parts in the old French measure.
The large valves of the piston, 8 inches by 6. The interval between these valves, 14 inches. Stalk of the piston, 6 inches square. The rollers on the axis { Length, 12 inches. of the wheel. Diameter, 36 inches. Diameter Diameter of the cylinder, 38 inches. Height of ditto, 26 do.
Baillet, who has given the above description, proposes a new application of the moving force to this kind of blowing machine; and he observes, that a very important advantage may be derived from these cylinders, since the simple pressure of a column of water may be substituted for the moving power. In fig. 29. the apparatus is so arranged as to shew in what way this effect may be produced.
The stalk, \( f_3 \) of the cylindrical apparatus \( e_3 \), is common to the piston of the small cylinder \( d_1 \), in which it can convey the column of water \( b_1 c_1 \). When the cock \( h_1 \) is open, and that at \( l_1 \) is shut, the pressure of the column must elevate the stalk \( f_3 \) and the piston of the blowing cylinder. Then the cock \( h \) being shut, and that at \( l \) being open, the water of the cylinder \( d \) will flow out, and the stalk \( f \) and the piston of the cylinder will descend. These alternate motions can be easily managed by means of levers, or regulators at \( i \), fitted to the stem of the piston, and in the same way as in the steam engine. The openings at \( h \) and \( l \) may be regulated according to the velocity which is required in the motion of the piston, and the diameter of the cylinder \( d \) will be proportioned to the fall of water \( b_1 c_1 \), and the volume of air which is wanted.
EXPLANATION OF THE FIGURES.
Fig. 16. exhibits a section and elevation of the blowing machine. a, the wall of the building. b, the opening in the wall for the balance beam. c, one of the two beams which receive the gudgeons on which the balance beam moves. d, e, the balance beam; f, the weight which acts as a counterpoise; g, the spring of wood. h, a brace or strap of leather, which is attached to the curved head of the beam. i, k, l, m, the frame which supports the cylinders. n, the blowing cylinder of cast iron. o, o, o, tubes for conveying air to the furnace. p, stalk of the piston. q, a knee or catch attached to the stalk. r, the horizontal axis of the water wheel. s, s, arms attached to the axis, with rollers which raise the knee or catch q, and the piston. t, t, similar arms and rollers for moving the piston of the second cylinder. Fig. 17. Section of the piston. Fig. 18. The piston seen from above. Fig. 19. View of the under surface of the piston. Fig. 17, 18, and 19. p, stalk of the piston. w, w, lids or valves. v, v, groove in the circumference of the piston. u, mortise to receive the stalk p. x, x, straps of iron to support the stalk p. y, y, the band of leather surrounding the piston. Fig. 20. a, a reservoir of water; b, c, a column of water. d, a cylinder for water. e, the blowing cylinder. f, the stalk common to the pistons of the two cylinders, d and e. g, the pipe for conducting the air. h, l, cocks for receiving and letting out the water. i, t, the regulators, for the purpose of opening and shutting the cocks. k, a second blowing cylinder*.
The following is a description by Torelli-Narci, of a Three-blast furnace, which was constructed in the chemical laboratory of the French school of mines.
"This furnace (says the author) is defined for fusing different mineral substances, in order to ascertain the nature of them; and the experience of six years has shown that it answers the intended purpose. By its means a very intense heat is obtained, and it was employed by C. Clouet for repeating his experiments on the conversion of forged iron into cast steel, which were attended with full success.
"Chemists who have seen this furnace seemed desirous of being better acquainted with the construction of it; the council even transmitted drawings of it to several persons; and what has hitherto prevented a description of it from being given was a desire to ascertain its power by longer use.
"I long ago conceived the idea of a fusing furnace, in which the wind was distributed in three tuyeres placed in its circumference, and at equal distances from each other; but I had no opportunity of realizing this idea till I became attached to the council of mines.
"Nearly seven years ago a plan was in agitation for constructing in the laboratory of the school a fusing furnace capable of producing a very great degree of heat, in order to operate with facility and speed on larger quantities of mineral, and consequently to obtain more precision in the trials which might be made than had been obtained by the small furnaces before employed for docimastic experiments.
"I proposed my ideas: they were approved by the council of mines; and I was ordered to cause the furnace I am about to describe to be constructed. The principal difference between it and those before used for the same purpose is, that in the present one the wind is introduced through three tuyeres, placed at equal distances from each other in its circumference, whereas in common furnaces it enters only by one.
"This furnace is round, both outside and inside, and constructed of very refractory bricks, secured by iron hoops in such a manner that they cannot be displaced. It rests on a square base of strong mason'work, raised to a sufficient height above the ground to render it easy to manage.
"The bellows are four feet in length, and the mean breadth of them is about 20 or 21 inches: they are of wood, and the joints are covered with white leather. The upper part consists of five folds and two half folds; the inferior, of two folds and two half folds. They are placed eight or nine feet (k) above a wooden box, the joints of which are covered with leather, and into which the wind
(k) "This height is arbitrary; it depends in part on the manner in which the bellows are disposed, and on the height of the chamber in which the furnace is placed." wind as it comes from the bellows is conveyed by a copper pipe, three inches in diameter, adjusted to the upper part of the box. The box itself is supported by two iron bars built into the wall. From the lower part of this box descend, in a vertical direction, three pipes of copper, two inches in diameter, bent at right angles about 45 inches below it, to bring them into a horizontal position, and to convey the wind to the furnace, which is about six feet distant. The extremities of these pipes are fitted into three tuyeres of forged iron, fixed at equal distances around the circumference of the furnace: these three pipes are more or less curved or bent, to convey the wind into the furnace by the three apertures made for that purpose.
"About fix inches below the box is adjusted, on each of the three tubes, which defend in a vertical direction, a bras cock about three inches of interior diameter: these cocks serve to intercept entirely the communication between the bellows and the furnace; and by opening them all more or less, or each of them separately, any required quantity of wind may be obtained (1).
"These cocks are well fixed to the tubes, and kept in their place by two clips of iron suited to the diameters of the tubes, and forming a kind of three collars, which by means of four screws embrace and confine them: these pieces of iron are themselves made fast to two crutches of iron, which support the box and are fixed to it by screws. The box is kept on the crutches by two straps, which embrace it at each extremity, and are fixed by female screws, which are fitted to screws on the ends of these straps after they have passed through the horizontal part of the two crutches.
"To give the proper strength to this furnace, a solid square was constructed of mason-work, about a foot larger on each side than the exterior diameter of the sides of the furnace, which were from 21 to 22 inches from outside to outside. Bricks were placed on the ground in the middle of this erection for the extent of 18 inches, in order to form a bottom, and on this base were placed the sides of the furnace constructed in the manner about to be described.
"I caused to be forged two iron hoops fix lines in thickness, from 2 to 2 1/2 inches in breadth, and about 22 inches of exterior diameter: these two circles were fastened together by three bars of iron, the distance of their exterior edge being kept at about nine inches, the height of the bricks: these bars are pierced with holes towards the end riveted on the circles, and placed at equal distances on their circumference. One of the extremes of each of these three bars is left of a sufficient length to pass beyond the lower circle about an inch, in order to make them enter into three holes formed in the brick-work which forms the bottom of the furnace, and by these means to prevent the furnace from becoming deranged.
"This kind of iron frame was filled with bricks similar to those employed for the bottom of the furnace: they were rubbed one on the other to smooth them, and the corners were a little rounded; so that, being placed upright with their broad sides applied to the iron hoops, the narrow side flood inward. By these means all these bricks were adjusted in such a manner as to touch each other by their broadest faces, and to form the sides of the furnace, the thickness of which was equal to the breadth of the bricks, and its depth to their length. Three apertures were reserved for the tuyeres which terminate the three tubes that convey the wind, by cutting from as many bricks a portion equal to the thickness of a brick.
"These bricks thus adjusted were taken from the iron frame, and then replaced, putting between them a cement to connect them firmly and to fill up the joints. The dust produced by cutting the bricks was reserved for this purpose; and I desired the workmen to mix with it a small quantity of clay diluted in a great deal of water, in order to make a puddle for daubing over the bricks, and in particular to put between them no more than was necessary for filling the joints and the small space left between their faces in consequence of any inequality left in dressing them.
"The furnace thus constructed was then placed on its base, a stratum of the same mortar employed for filling up the joinings of the bricks being first interposed. The extremities of the three iron bars projecting beyond the lower circle were placed in the holes left in the base to receive them. The body of the furnace encircled with iron, both by its weight and the gentle blows given to the iron hoops above the bars which connected them, expelled the excess of the mortar, and caused a part of it to enter and unite with that which filled up the joints of the brick work of the circumference, which rendered it immovable.
"The bellows is secured as usual by crutches of iron and supporters fixed in the wall and to the floor: the handle is disposed in such a manner, that the rope which makes it act may be pulled by the same person who manages the fire of the furnace, which in certain cases is necessary.
"The tuyeres of forged iron which receive the ends of the copper tubes are secured in their proper apertures in the circumference of the furnace by pieces of brick and mortar similar to that employed for filling up the joints; and the ends of the copper pipes introduced into these tuyeres are luted with the same mortar, a little thickened with brick dust.
"The apertures of these tuyeres towards the interior of the furnace is only nine lines in diameter; on which account,
(1) "Care must be taken, when the action of the bellows ceases, to shut the cocks, especially when coals are used in the furnace; for the hydrogen disengaged from that mineral substance ascends into the box, and when the bellows are again made to act, may inflame, and cause a violent explosion, or even burst the bellows. This accident once took place in the furnace here described: the box burst with a loud noise on the first stroke of the bellows, the gas which filled them having suddenly inflamed; but by good fortune no person was hurt. The same thing happened at the house of C. Gorlier, locksmith of Paris; one of his bellows burst with a horrid explosion at the moment when they were put in motion." account, as the volume of air furnished by the bellows cannot pass so quick as it is produced, it becomes condensed in the box placed above the cocks. By these means a very uniform blast is obtained, which can also be regulated by opening more or fewer of the cocks.
"During more than fix years, since this furnace was constructed, it has suffered no derangement: it is not even cracked. It is however worn in the inside by the violence of the heat it has experienced, which has increased its diameter about two inches. The parts round the three tuyeres have also got hollowed, so that it has need of being repaired. It is intended to make it deeper, and to have a kind of moveable muffs or linings made of fire clay, in order that its diameter may be reduced at pleasure: it is meant also to construct it in such a manner, as to deposit the rest or support for the crucible, not on the bottom of the furnace, but on bars of forged iron placed at the distance of some inches from that bottom, so as to leave below them a vacuity in which the blast of the bellows may be diffused, and from which it may rise, passing between the bars to traverse the mass of charcoal which surrounds the crucible. The blast will then produce a more uniform fire, and the flame can no longer be directed against the sides of the crucibles; so that the risk of their breaking by sudden inequalities in the heat will be much less.
"This alteration is going to be immediately carried into execution, and the method proposed for doing it is as follows:
"A round frame will be made of forged iron, in which bricks will be placed in the same manner as above described. In the lower part of the furnace an aperture will be referred for raking out the ashes, which will be closed by means of a door of baked earth carefully luted with clay. Some inches above the bottom of the furnace will be placed a grate of forged iron, and between this grate and the bottom of the furnace the tuyeres will terminate, and the blast be introduced. Muffs or linings of very refractory earth will then be introduced, so as to defend to this grate. There will be two of them, one within the other, and both within the body of the furnace. At the lower part these muffs will be furnished with a rim, projecting outward so as to leave between the body of the furnace and the muffs a vacuity, which will be luted at the bottom with clay, and which will be filled with pounded glass, or any other substance a bad conductor for heat.
"The interior muff, or both of them, may be removed at pleasure to obtain a furnace of greater or less capacity according to the operations to be performed. It is proposed to make the muffs wider at the top than at the bottom.
Explanation of the Figures.
"Fig. 21. Plan of the bellows and of the furnace. AB, the bellows made of wood, the folds of which are also of wood covered with leather on the joints. CD, the handle which serves for moving the bellows. E, a copper tube which conveys the wind of the bellows into the box FG, in which it is condensed. FG, a box of wood serving as a reservoir for the wind condensed by the bellows. HI, KL, MN, three pipes adapted to the box FG, and which convey the wind into Furnace. The inside of the furnace by three tuyeres, I, L, N. OP, mason work to support the horizontal pipes. Q, the furnace properly so called, the former of which is circular, and which is placed on the square mason work R, S, T, U.
"Fig. 22. Elevation of the furnace, the pipes which Fig. 22. convey the blast, the cocks, the condensing box, and the bellows. AB, the bellows mounted in their place, and supported by the iron-work necessary for securing it which is fixed in the wall and to the floor. CD, the handle which serves for moving the bellows. E, the copper pipe which conveys the blast of the bellows to the box FG in which it is condensed. At G is a hole shut by a large cork stopper, which can be opened at pleasure. This box is supported by two crutches of iron f, g, and h, i, built into the wall, and on which it is fixed by two iron stirrups l, m.
"Fig. 23. One of the crutches and its stirrup are seen Fig. 23. represented sidewise at f, g, l; the extremities, n, o, are built into the wall, and the two ends, p, q, of the iron piece which keeps the box on the horizontal traverse of the crutch, are tapped, and receive screws which make them fast to the crutch f, g. HI, KL, MN, are three pipes which convey the wind into the interior of the furnace. Q, R, S, T, U, mason work on which is placed the furnace Q, and which serves it as a bottom. OP, masonry which serves to support the three pipes that convey the wind to the furnace. XYZ, fig. 22, are the three cocks fixed to the three pipes which proceed from the box to convey the wind to the furnace.
"In fig. 24, the dimensions of which are double those Fig. 24. of fig. 22, may be seen the details of one of these cocks.
"At r, s, t, the body of the cock is seen in front; the stopper being taken out shows at r and at t the two holes which receive the tubes that communicate either with the box or with the tuyeres. u Exhibits the body of the cock seen on one side; v the key with its aperture x, and its head y. This key, turned round more or less in its socket, serves to give more or less wind. 1, 2, 3, Iron clips which secure the cocks at the distance they ought to be from each other, and connect them at the same time to the iron crutches which support the air-box.
"Fig. 25. a plan of these two clips. They are bent Fig. 25. at the places marked 1, 2, 3, to embrace the body of the three cocks, and secure them in such a manner that they cannot be deranged when they are opened or shut.
"Fig. 26. and 27. represent the plan and section of Fig. 26. and the changes and additions proposed to be made when 27. the furnace is re-constructed. At I, L, and N, are seen the extremities of the three pipes that enter the forged iron tuyeres, and convey the wind to the interior of the furnace. a, b, and c, indicate the thickness at the upper part of each of the muffs and of the body of the furnace, between which there are two vacuities filled with pounded glass or some other bad conductor of heat. d, the grate on which are deposited the rests of baked earth destined to receive the crucibles. e, the crucible, luted and attached with clay to a rest of baked earth (M)."
(m) "The advantage arising in large foundries from the application of two or three tuyeres instead of one, is well known; but I do not believe that such an arrangement was ever adopted in small furnaces." Mr Collier, in a paper communicated to the Manchester Philosophical Society, has delivered some important observations on iron and steel, with a more correct account of the process for the manufacture of the latter than has hitherto been given. To this account he has added the description of a furnace for the conversion of iron into steel. As his observations and reasonings are extremely valuable, we shall lay the whole before our readers in his own words.
"After examining (says Mr Collier) the works of different authors who have written on the subject of making iron and steel, I am persuaded that the accounts given by them of the necessary processes and operations are extremely imperfect. Chemists have examined and described the various compound minerals containing iron with great accuracy, but have been less attentive to their reduction. This observation more particularly applies to steel, of the making of which I have not seen any correct account.
"It is singular to observe, how very imperfectly the cementation of iron has been described by men of great eminence in the science of chemistry. Citizen Fourcroy states the length of time necessary for the cementation of iron to be about twelve hours; but it is difficult to discover whether he alludes to cast or to bar steel: for he says, that short bars of iron are to be put into an earthen box with a cement, and closed up. Now steel is made from bars of iron of the usual length and thickness: but cast steel is made according to the process described by Citizen Fourcroy, with this essential difference; the operation is begun upon bar steel and not bar iron.
"Mr Nicholson is equally unfortunate in the account given in his Chemical Dictionary. He says, that the usual time required for the cementation of iron is from six to ten hours, and cautions us against continuing the cementation too long; whereas the operation, from the beginning to the end, requires sixteen days at least. In other parts of the operation he is equally defective, confounding the making of bar with that of cast steel, and not fully describing either. In speaking of the uses of steel, or rather of what constitutes its superiority, Mr Nicholson is also deficient. He observes, that 'its most useful and advantageous property is that of becoming extremely hard when plunged into water.' He has here forgotten everything respecting the temper and tempering of steel instruments, of which, however, he takes some notice in the same page. 'Plunging into water' requires a little explanation: for if very hot steel be immersed in cold water without great caution, it will crack, nay, sometimes break to pieces. It is, however, necessary to be done, in order to prevent the steel from growing soft, and returning to the state of malleable iron; for, were it permitted to cool in the open air, the carbure which it holds in combination would be dissipated (N).
"I shall, at present, confine my remarks to the operation performed on iron in Sheffield and its neighbourhood: from whence various communications have been transmitted to me by resident friends, and where I have myself seen the operations repeatedly performed.
"The iron made in that part of Yorkshire is procured from ores found in the neighbourhood, which are of the argillaceous kind, but intermixed with a large proportion of foreign matter. These, however, are frequently combined with richer ores from Cumberland and other places. The ore is first roasted with cinders for three days in the open air, in order to expel the sulphureous or arsenical parts, and afterwards taken to the furnaces: some of which are constructed so that their internal cavity has the form of two four-sided pyramids joined base to base; but those most commonly used are of a conical form, from 40 to 50 feet high. The furnace is charged at the top with equal parts of coke, cinder and lime-stone. The lime-stone acts as a flux, at the same time that it supplies a sufficient quantity of earthy matter to be converted into scoriae, which are necessary to defend the reduced metal from calcination, when it comes near the lower part of the furnace. The fire is lighted at the bottom; and the heat is excited by means of two pair of large bellows blowing alternately. The quantity of air generally thrown into the furnace is from a thousand to twelve hundred square feet in a minute. The air passes through a pipe, the diameter of which is from two inches and a quarter, to two and three quarters, wide. The compression of air which is necessary is equal to a column of water four feet and a half high. The ore melts as it passes through the fire and is collected at the bottom, where it is maintained in a liquid state. The flag, which falls down with the fused metal, is let off, by means of an opening in the side of the furnace, at the direction of the workmen.
"When a sufficient quantity of regulus, or imperfectly reduced metal, is accumulated at the bottom of the furnace (which usually happens every eight hours), it is let off into moulds; to form it for the purposes intended, such as cannon or pig iron.
"Crude iron is distinguished into white, black, and gray. The white is the least reduced, and more brittle than the other two. The black is that with which a large quantity of fuel has been used; and the gray is that which has been reduced with a sufficient quantity of fuel, of which it contains a part in solution.
"The operation of refining crude iron consists in burning the combustible matter which it holds in solution; at the same time that the remaining iron is more perfectly reduced, and acquires a fibrous texture. For this purpose, the pigs of cast iron are taken to the forge; where they are first put into what is called the refinery: which is an open charcoal fire,
"At Treibach, in Carinthia, C. Le Febre, and Hassenfratz member of the council and inspector of mines, saw, about twenty years ago, a large furnace with two tuyeres; drawings of which they brought to France, and which they represented in the third plate of l'Art de fabriquer des Canons, by Monge: two pairs of bellows supply wind through two opposite tuyeres, and since that arrangement the daily product of metal has been double."
(N) "It is the opinion of some metallurgists, that a partial abstraction of oxygen takes place, by plunging hot metal into cold water." fire, urged by a pair of bellows, worked by water or a steam engine; but the compression of air, in the refinery, ought to be less than that in the blast furnace. After the metal is melted, it is let out of the fire by the workmen, to discharge the scorie; and then returned and subjected to the blast as before. This operation is sometimes repeated two or three times before any appearance of malleability (or what the workmen call coming into nature) takes place; this they know by the metal's first assuming a granular appearance, the particles appearing to repel each other, or at least to have no signs of attraction. Soon afterwards they begin to adhere, the attraction increases very rapidly, and it is with great difficulty that the whole is prevented from running into one mass, which it is desirable to avoid, it being more convenient to stamp small pieces into thin cakes: this is done by putting the iron immediately under the forge hammer and beating it into pieces about an inch thick, which easily break from the rest during the operation. These small pieces are then collected and piled upon circular stones, which are an inch thick, nine inches in diameter, and about ten inches high. They are afterwards put into a furnace, in which the fire is reverberated upon them until they are in a semi-fluid state. The workmen then take one out of the furnace and draw it into a bar under the hammer; which being finished, they apply the bar to another of the piles of semi-fluid metal, to which it quickly cements, is taken again to the hammer, the bar first drawn serving as a handle, and drawn down as before. The imperfections in the bars are remedied by putting them into another fire called the chafery, and again subjecting them to the action of the forge hammer.
"The above method is now most in use, and is called flourishing; but the iron made by this process is in no respect superior to that which I am going to describe. It is, however, not so expensive, and requires less labour.
"The process for refining crude iron, which was most common previously to the introduction of flourishing, is as follows.
"The pigs of cast iron are put into the refinery, as above, where they remain until they have acquired a consistency resembling paste, which happens in about two hours and a half. The iron is then taken out of the refinery and laid upon a cast iron plate on the floor, and beaten by the workmen with hand hammers, to knock off the cinders and other extraneous matters which adhere to the metal. It is afterwards taken to the forge hammer and beaten, first gently, till it has obtained a little tenacity; then the middle part of the piece is drawn into a bar, about half an inch thick, three inches broad, and four feet long; leaving at each end a thick square lump of imperfect iron. In this form it is called ancony. It is now taken to the fire called the chafery, made of common coal; after which the two ends are drawn out into the form of the middle, and the operation is finished.
"There is also a third method of rendering crude Furnace iron malleable, which, I think, promises to be abundantly more advantageous than either of the two for. An immer, as it will dispense both with the refinery and process, chafery; and nothing more will be necessary than a reverberating furnace, and a furnace to give the metal a malleable heat, about the middle of the operation. The large forge hammer will also fall into disrepute, but in its place must be substituted metal rollers of different capacities, which, like the forge hammer, must be worked either by a water wheel, or a steam engine.
"It is by the operation of the forge hammer or metal rollers, that the iron is deprived of the remaining portion of impurity, and acquires a fibrous texture.
"The iron made by the three foregoing processes is equally valuable, for by any of them the metal is rendered pure; but after those different operations are finished, it is the opinion of many of the most judicious workers in iron, that laying it in a damp place, for some time, improves its quality; and to this alone, some attribute the superiority of foreign iron, more time elapsing between making and using the metal. To the latter part of this opinion I can by no means accede, as it is well known that the Swedish (o) ores contain much less heterogeneous matters than ours, and are generally much richer, as they usually yield about 70 per quintal of pure iron, whereas the average of ours is not more than 30 or 40 (p): add to this, that the Swedish ores are smelted in wood fires, which gives the iron an additional superiority.
"Iron instruments are case-hardened by heating them in a cinder or charcoal fire; but if the first be used, a quantity of old leather, or bones, must be burnt in the fire to supply the metal with carbone. The fire must be urged by a pair of bellows to a sufficient degree of heat; and the whole operation is usually completed in an hour.
"The process for case-hardening iron, is in fact the same as for converting iron into steel, but not continued so long, as the surface only of the article is to be impregnated with carbone.
"Some attempts have been made to give cast iron, by case-hardening, the texture and ductility of steel, but they have not been very successful. Table and penknife blades have been made of it, and, when ground, have had a pretty good appearance; but the edges are not firm, and they soon lose their polish. Common table knives are frequently made of this metal.
"The cementation of iron converts it into steel:—a substance intermediate between crude and malleable iron.
"The furnaces for making steel are conical build. Furnaceings; about the middle of which are two troughs of making brick or fire stone, which will hold about four tons of iron in the bar. At the bottom is a long grate for fire.
"A layer of charcoal dust is put upon the bottom of
(o) "Steel is commonly made of Swedish iron." (p) "The iron made from the ore found in the neighbourhood of Sheffield, contains a great deal of phosphate of iron, or siderite, which renders the metal brittle when cold." Furnace. the trough; and, upon that, a layer of bar iron, and so on alternately until the trough is full. It is then covered over with clay to keep out the air; which, if admitted, would effectually prevent the cementation. When the fire is put into the grate, the heat passes round by means of flues, made at intervals, by the sides of the trough. The fire is continued until the conversion is complete, which generally happens in about eight or ten days. There is a hole in the side by which the workmen draw out a bar occasionally, to see how far the transmutation has proceeded. This they determine by the blisters upon the surface of the bars. If they be not sufficiently changed, the hole is again closed carefully to exclude the air; but if, on the contrary, the change be complete, the fire is extinguished, and the steel is left to cool for about eight days more, when the process for making blistered steel is finished.
" For small wires, the bars are drawn under the tilt hammer, to about half an inch broad and three-fifteenth of an inch thick.
Tilted steel. "The change wrought on blistered steel by the tilt hammer, is nearly similar to that effected on iron from the refinery by the forge hammer. It is made of a more firm texture, and drawn into convenient forms for use.
German steel. "German steel is made by breaking the bars of blistered steel into small pieces, and then putting a number of them into a furnace; after which they are welded together and drawn to about 18 inches long; then doubled and welded again, and finally drawn to the size and shape required for use. This is also called shear steel, and is superior in quality to the common tilted steel.
Cast steel. "Cast steel is also made from the common blistered steel. The bars are broken and put into large crucibles with a flux. The crucible is then closed up with a lid of the same ware, and placed in a wind furnace. By the introduction of a greater or smaller quantity of flux, the metal is made harder or softer. When the fusion is complete, the metal is cast into ingots, and then called ingot steel; and that which afterwards undergoes the operation of tilting, is called tilted cast steel.
"The cast steel is the most valuable, as its texture is the most compact and it admits of the finest polish.
"Sir T. Frankland has communicated a process, in the Transactions of the Royal Society, for welding cast steel and malleable iron together; which, he says, is done, by giving the iron a malleable, and the steel a white heat; but, from the experiments which have been made at my request, it appears, that it is only soft cast steel, little better than common steel, that will weld to iron: pure steel will not; for, at the heat described by Sir T., the best cast steel either melts or will not bear the hammer.
"It may here be observed, as was mentioned before, that steel is an intermediate state between crude and malleable iron, except in the circumstance of its reduction being complete; for, according to the experiments of Reaumur and Bergman, steel contains more hydrogen gas than cast iron, but less than malleable iron;—less plumbago than the first, but more than the latter;—an equal portion of manganese with each;—less siliceous earth than either—more iron than the first, but less than the second. Its fusibility is likewise intermediate, between the bar iron and the crude. When steel has been gradually cooled from a state of ignition, it is malleable and soft, like bar iron; but when ignited and plunged into cold water, it has the hardness and brittleness of crude iron.
"From the foregoing facts, we are justified in drawing the same conclusions with Reaumur and Bergman, but which have been more perfectly explained by Vandermonde, Berthollet, and Monge, that crude iron is a regulus, the reduction of which is not complete; and which consequently will differ according as it approaches more or less to the metallic state. Forged iron, when previously well refined, is the purest metal; for it is then the most malleable and the most ductile, its power of welding is the greatest, and it acquires the magnetic quality soonest. Steel consists of iron perfectly reduced and combined with charcoal; and the various differences in blistered steel, made of the same metal, consists of the greater or less proportion of charcoal imbibed.
"Iron gains by being converted into steel, about the hundred and eightieth part of its weight.
"In order to harden steel, it must be put into a clean Hardening charcoal, coal, or cinder fire, blown to a sufficient degree of heat by bellows. The workmen say, that neither iron nor steel will harden properly without a blast. When the fire is sufficiently hot, the instrument intended to be hardened must be put in, and a gradual blast from the bellows continued until the metal has acquired a regular red heat; it is then to be carefully quenched in cold water. If the steel be too hot when immersed in water, the grain will be of a rough and coarse texture; but if of a proper degree of heat, it will be perfectly fine. Saws and some other articles are quenched in oil.
"Steel is tempered by again subjecting it to the action of the fire. The instrument to be tempered will suppose to be a razor made of cast steel. First rub it upon a grit stone until it is bright; then put the back upon the fire, and in a short time the edge will become of a light straw colour, whilst the back is blue. The straw colour denotes a proper temper either for a razor, graver, or penknife. Spring knives require a dark brown; scissors, a light brown, or straw colour; forks or table knives, a blue. The blue colour marks the proper temper for swords, watch-springs, or any thing requiring elasticity. The springs for penknives are covered over with oil before they are exposed to the fire to temper.
Explanation of the Figures.
"Fig. 28. is a plan of the furnace, and fig. 29. is a section of it taken at the line A.B. The plan is taken at the line CD. The same parts of the furnace are marked with the same letters in the plan and in the section. EE are the pots or troughs into which the bars of iron are laid to be converted. F is the fireplace; P, the fire bars; and R, the ashpit. GG, &c. are the flues. HH is an arch, the infide of the bottom of which corresponds with the line IIII, fig. 28. and the top of it is made in the form of a dome, having a hole in the centre at K, fig. 29. LD, &c. are fix chimneys. MM is a dome, similar to that of a glass-houle, covering the whole. At N there is an arched opening, at which the materials are taken in and out of the furnace, nace, and which is closely built up when the furnace is charged. At OO there are holes in each pot, through which the ends of three or four of the bars are made to project quite out of the furnace. These are for the purpose of being drawn out occasionally to see if the iron be sufficiently converted.
"The pots are made of fire tiles, or fire stone. The bottoms of them are made of two courses, each course being about the thickness of the single course which forms the outside of the pots. The insides of the pots are of one course, about double the thickness of the outside. The partitions of the fires are made of fire brick, which are of different thicknesses, as represented in the plan, and by dotted lines in the bottom of the pots. These are for supporting the sides and bottoms of the pots, and for directing the flame equally round them. The great object is to communicate to the whole an equal degree of heat in every part. The fuel is put in at each end of the fire-place, and the fire is made the whole length of the pots, and kept up as equally as possible."
In a memoir published by Du Hamel, the inconvenience and expense which attend the process commonly in use, for refining lead or separating the silver from this metal, are pointed out, and a more economical process is proposed. This process, which is known by the name of cupellation, is performed in a vessel called the cupel, which is made of the ashes of the bones of animals, or of vegetables, after separating, by means of water, the saline parts which adhere to them. But the difficulty and expense of obtaining a sufficient quantity of these materials, led him to contrive something else as a substitute, which might be less costly and more easily obtained.
For the purpose of performing the process in the way here recommended, it is not necessary to make any alteration in the general construction of the furnace. All that is required is, to have a sufficient number of canals or openings towards its base, to allow the escape of the moisture. These canals are covered with a bed of scoria, on which is raised a pavement formed of the most porous bricks, and about a brick in thickness. On this floor or area, which should be a little concave, in the same way as the ordinary cupels are formed when they are made of ashes, is placed a quantity of casting or moulding sand, slightly moistened; and if the sand has not a sufficient quantity of earth, some clay is added, to give it consistence, and the whole is carefully mixed together. This sand is beaten together, and a concave vessel is made of it, of an equal thickness in all its parts. When the bason has been uniformly beaten, it will be proper to sift over its whole surface a small quantity of wood ashes, well washed with water, and these are also beaten down with a pebble.
The cupel being thus prepared, the head of the furnace is put on, and a moderate fire is kindled and kept up for some hours, to carry off part of the moisture of the sand. The remainder is dissipated without inconvenience, by means of the canals, during the process. After it has been sufficiently dried, the head of the furnace is again taken off, and allowed to cool a little. A quantity of straw or hay is put upon the bason or cupel, to prevent any injury from the weight of the bars of lead on the sand. To avoid this still more, it is recommended to have the lead to be purified cast into the form of hemispheres, in place of bars.
A sufficient quantity of lead being introduced into the furnace, the head is luted on with baked clay, and the fire is applied in the usual way. As soon as the lead is completely fused, the bason appears covered with the burnt straw: this is removed by means of an iron instrument, and this operation is repeated several times. When the lead begins to grow red, the action of the bellows commences, at first softly, and the blast is so directed that it may strike the centre of the cupel. To effect this more completely, a small round plate of iron is attached to the extremity and upper part of the pipe by means of a hinge, so that at each blast it is half raised, and the current of air is directed to the surface of the fused metal.
After the whole of the scum that rises has been removed, and the lead is covered with a stratum of litharge, a small gutter is made by means of a hook for the purpose, in the fond of the cupel. This is gradually and cautiously hollowed, till it is on a level with the surface of the bath, and then the litharge driven by the blast towards the anterior part of the furnace, will flow this way, and spread itself on the floor in the usual way. When the operator perceives that the litharge has been removed, he flops up the gutter with moistened ashes, till another quantity of litharge appears on the surface. He then re-opens the gutter, which is now made deeper in proportion to the diminution of the fused metal, but at the same time taking care that no part of the lead escapes, especially towards the end of the process, because then a considerable portion of silver would be carried off.
In this way the process is conducted till the separation of the silver begins to take place, observing at the same time to increase the heat as the quantity of fused metal diminishes, because then the silver is collected together; and since it is much more difficult to keep it in fusion than the small portion of lead which remains combined with it, the separation would be very imperfect, without the application of a sufficient temperature. Instead of having only one-twentieth of lead, which is the usual proportion in the common process, the quantity would be much greater, and this would render the second operation, the refining of the silver much more difficult.
Du Hamel observes, that a cupel of sand, well made, will answer for the repetition of the process several times, without renewing it at the end of each operation, as is the case with those of ashes. The only precaution to be observed is, to remove the kind of varnish of oxide of lead which remains on the sides of the gutter by which the litharge flowed out, that the new sand with which it is to be filled up may combine easily with the old.
The length of time which the reverberatory furnace may be employed in melting the ores of lead, and even in reducing litharge, is a proof that the oxide of lead acts only on the surface of the cupel, and penetrates a very small thickness. After the process has been several times repeated, this crust is removed, and it is fused for the purpose of obtaining the lead. This process will be as easy as the reduction of the metal contained in the ashes of the ordinary cupels, and in much smaller quantity, Furnace. quantity. By the new method, therefore, a greater proportion of litharge is obtained; and it may be added, that the sand absorbing a smaller quantity of oxide of lead, it will contain also a smaller proportion of silver; for it is well known that the lead which is reduced from the ashes, contains always much more than that which is produced from the reduction of litharge.
In place of sand, argillaceous earth may be employed in the construction of cupels; but it is necessary that this earth be well beaten together, and that this process should be several times repeated, for several days, without which the clay would be apt to crack, and the melted lead would flow into the crevices; an inconvenience which does not arise from the use of sand, even although it should be mixed with a little earth. And besides, it is to be observed, that the cupel constructed of this substance, becomes too hard to allow a gutter to be easily made for the passage of the litharge. In this case it would be necessary that the place by which the oxide is to flow out, be made up of sand, or of ashes.
In the formation of the bason or cupel, which is here proposed, it seems to be advantageous to employ two kinds of sand, the one fine, like that which is used by the founders, the other coarser, and free from earth. It is of the latter, the coarse kind, that the first stratum is formed; and this, after being made of sufficient thickness, is well beaten with pebbles for the purpose; on this the fine sand is to be placed, containing a proper proportion of earth, and it is to be beaten together in the same way. Both the coarse and the fine sand are to be moistened a little, that they may adhere together, and afterwards acquire a sufficient degree of solidity under the pebbles. The sand of the inferior layer being coarser than the other, will absorb the moisture from it as it evaporates, and will allow it to pass off easily, by means of the canals or openings which are left for that purpose. This stratum, too, is to remain in its place, when the fine sand of the cupel is removed, and that the surface of the stratum of coarse sand may not be disturbed when the other is removed, a thin layer of ashes may be thrown upon it, and beaten down, before the other stratum is laid on*.
The French school of mines appointed a commission, composed of Hassenfratz, Brochant, and Miché, to consider the best form for the construction of a furnace for burning lime-stone, or platter of Paris. After considering different forms of furnaces, and reasoning on their effects, they propose in their report to adopt the following, which is represented in fig. 30. and 31.
Fig. 30. is a plan of the furnace proposed. D, the fire-place. E, E, openings for taking out the substances which are converted into lime or plaster. P, half of the plan taken at the height of the line AK of fig. 31. Q, half of the plan taken at the height of the line XY of fig. 31.
Fig. 31. exhibits a section of the same furnace. B, C, are places which remain empty after the introduction of the substances to be exposed to heat. B, D, the fires. E, the opening for the extraction of the substances after they are converted into lime or plaster.
O, the throat or vent. a, b, openings for regulating the heat.
We shall now conclude this article with a short account of the construction and management of furnaces for chemical purposes.
The following is a description of an essay or cupel-chemical ling furnace. 1. A hollow, quadrangular prism, 11 inches broad and nine inches high, is constructed with iron plates, and it ends at top in a hollow quadrangular pyramid, seven inches high; the latter terminating in an opening seven inches square. The prism is closed at bottom with another iron plate, which serves as a bottom.
2. Near the bottom a door three inches high and five inches broad, is opened. This leads to the ash hole.
3. Above this door, and six inches from the basis, another door is opened, of the figure of a segment of a circle, four inches broad at the bottom, and three inches and a half high in the middle.
4. Three iron plates are then to be fastened to the fore-part of the furnace, the first of them should be 11 inches long, half an inch high, and to fastened with three or four rivets, that its lower edge may rest against the bottom of the furnace. Between this plate and the side of the furnace a space must be left, so wide that the sliders of the lower door, which are made of a thicker iron plate, may move easily in the groove. The second iron plate, which is 11 inches long, and three inches high, is fastened parallel to the first, in the space between the two doors. Both the upper and lower edges of this plate form grooves with the side of the furnace, for receiving the sliders which shut the doors. The third plate of the same dimensions with the first, is riveted close above the upper door, and forms a groove for receiving the edge of the sliders which move that door.
5. For the purpose of closing the doors, two sliders of iron-plates must be adapted to each of them. These sliders are moved in the grooves. The two sliders belonging to the upper door have each a hole near the top; in the one there is a small hole \( \frac{1}{2} \) of an inch broad, \( \frac{1}{2} \) inch long; and the other a semicircular opening one inch high and two broad. To each slider there is a handle attached, to lay hold of it when it is moved.
6. Five round holes, an inch diameter, are bored in the furnace, two in the back part, and two in the fore part, five inches from the bottom; but \( \frac{3}{2} \) inches from each side of the furnace. The fifth hole is at the height of an inch above the upper edge of the upper door.
7. The inside of the furnace must be armed with iron hooks, about three inches from each other, and projecting \( \frac{1}{2} \) inch. The use of these hooks is to secure the lute with which the furnace is to be lined.
8. A moveable, hollow, quadrangular pyramid, also of iron, and 3 inches high, is to be fitted to the upper opening of the furnace, 7 inches broad, and ending above in a hollow tube, 3 inches in diameter, about 2 inches high, nearly cylindrical, but converging a little at the top. This tube serves to support a funnel for conveying the smoke into the chimney. This cover has two handles to lay hold of it. To secure the cover on the furnace, an iron plate is riveted to the right and left of its upper edge, and turned down towards the inside, so that a furrow may be made, open bef Furnace. fore and behind, for receiving the lateral edges of the cover.
9. A square ledge, made of thick iron plate, is fixed at the top of the upper edge of the lower door, for supporting the grate and the lute, and that it may be easily introduced into the cavity of the furnace, it should be of two pieces.
10. Iron bars are then to be fixed in the inside of the furnace, for supporting the fuel. These must be equal in length with the diameter of the furnace, about \( \frac{1}{2} \) inch thick, and \( \frac{3}{4} \) inch distant from each other. They are supported at their extremities by a square iron ledge.
11. To prevent the dissipation of the heat, and the destruction of the iron, by being repeatedly made red hot, the inside of the furnace must be lined with lute, about a finger's breadth, or rather more, in thickness.
For luting furnaces, Doctor Black recommended a simple mixture of sand and clay. The proportions for retarding the violence of fire are four parts of sand to one of clay; but when designed for the lining of furnaces, he uses fix or seven of sand to one of clay, the more effectually to prevent the contraction of the latter; for it is known from experiments, that clay, when exposed to a strong heat, contracts the more in proportion to its purity. The sand settles into less bulk when wet, and does not contract by heat, which it also resists as well as the clay itself.
Besides this outside lining next the fire, Dr Black uses another to be laid on next the iron of the furnace; and this consists of clay mixed with a large proportion of charcoal dust. It is more fit for containing the heat, and is put next to the iron, to the thickness of an inch and a half. That it may be pretty dry when first put in, he takes three parts by weight of the charcoal dust, and one of the common clay, which must be mixed together when in dry powder, otherwise it is very difficult to mix them perfectly. As much water is added as will form the matter into balls; and these are beaten very firm and compact by means of a hammer upon the inside of the furnace. The other lute is then spread over it to the thickness of about half an inch, and this is also beaten solid by hammering; after which it is allowed to dry slowly, that all cracks and fissures may be avoided; and after the body of the furnace is thus lined, the vent is screwed on and lined in the same manner. It must then be allowed to dry for a long time; after which a fire may be kindled, and the furnace gradually heated for a day or two. The fire is then to be raised to the greatest intensity; and thus the luting acquires a hardness equal to that of free-stone, and is afterwards as lasting as any part of the furnace.
To perform an operation in this furnace, two iron bars an inch thick, and of sufficient length to project a little beyond the holes of the furnace, are passed through four lower holes, which are placed before and behind, directly opposite to each other. These bars support the muffle, which is introduced through the upper opening of the furnace, and placed upon the bars, in such a way that the open side of it may be near the inner border of the upper door. The fuel is introduced through the top of the furnace, and the best fuel is charcoal made of hard wood. It should be reduced to small pieces, that they may readily fall between the muffle and the fides of the furnace. The muffle is to be covered with fuel, to the height of several inches. The pieces of charcoal should not be too small, because they may fall immediately through the interstices of the grate, or be too rapidly consumed, and thus increasing the quantity of ashes, obstruct the current of air.
As the management of the fire is of great importance, Manager for the success of operations in the furnace, the following directions may be attended to. To increase the heat to the utmost, the door of the ash-hole may be left open; the sliders of the upper door drawn towards each other, so as to touch in the middle, and the cover and funnel adapted to its tube, placed on the top of the furnace. The heat is still farther increased by putting red burning coals into the open upper door. By shutting the upper door with the slider, which has a narrow oblong hole in it, the heat is diminished, and it is still farther diminished by shutting the door with the other slider, having the semicircular hole. The heat is also diminished by removing the funnel at the top of the cover; and the heat is left by partially or totally shutting the door of the ash-hole, because then the current of air necessary to excite combustion is obstructed.
The heat of the furnace is also increased in proportion to the diminished size of the muffle. The heat is stronger too, according as the muffle has more and larger segments cut out of it, as the fides of it are thinner, and as the number of vessels placed in the hinder part of it is increased; and the contrary. It may be here observed, that when many of the conditions necessary to produce strong heat are wanting, the operator, with all his sagacity, will scarcely be able to excite combustion in such a degree in common assay furnaces as to succeed well in his operations; and even when he employs bellows, and introduces coals by the upper door. The grate, therefore, ought to be placed nearly three inches below the muffle, that the air ruffling through the ash-hole, may not cool its bottom, and that the smaller coals, almost already consumed, and the ashes, may more easily fall through the interstices of the grate; larger coals, fit for keeping up the requisite degree of heat, must be used. The funnel is added, that the blowing of the fire being increased by means of it as much as possible, may be brought to the degree that is wanted; for the fire may be at any time diminished, but without the assistance of proper apparatus, it cannot always be increased at pleasure.
Explanation of Fig. 32, 33, and 34.
Fig. 32, a, a, b, b, body of the assay furnace. Fig. 32. b b, c c, top of the fame. d, opening at the top of the furnace. e, door leading to the ash-hole. f, upper door. g g, h h, i i, the iron plates rivetted on the furnace, which form the grooves in which the doors slide. k k, l l, the sliding doors. m, the hole in one of the doors; n, the semicircular hole. o o, the holes for receiving the bars which support the muffle. Furnace.
p, a hole above the upper edge of the upper door, for introducing a rod to stir the fire. g, the pyramidal cover. r, tube or funnel at the top. s s, its handles.
Fig. 33. represents a longitudinal section of a reverberatory furnace, 18 feet long, 12 broad, and 9½ high.
a, the building. b, the ash-hole. c, channel for the evaporation of the moisture. d, the grate. e, the fire-place. f, the inner part of the furnace. g, a bafon formed of sand. h, the cavity containing the melted metal. i, a hole through which the scoria is removed. k, the passage for the flame and smoke, or the lower part of the chimney, to be carried to the height of 30 feet. l, a hole in the roof, through which the ore is introduced into the furnace.
Fig. 34. is a longitudinal section of a refining furnace.
a a, the building. b, the channels to carry off the moisture. c, other small channels, which meet in the middle of the bafon. d, the bafon made of bricks. e, a layer of ashes. f, the hollow or bafon containing the melted metal. g, the hole for the smoke and flame. h h, two openings for admitting the pipes of the bellows. i, the vault or dome of the furnace. k, the fire-place. l, the grate. m, a hole below for the admission of air. n, a hole in the vault, which serves to cool the furnace.
A convenient portable blast furnace, contrived by Mr Aikin, and described by him in the 17th vol. of the Philosophical Magazine, will probably be useful to some of our chemical readers. "It is (he says) particularly adapted to those who, like myself, can only devote a small room and a moderate share of time to these pursuits.
"Dr Lewis, in his Commerce of the Arts (page 27), describes a very powerful bait furnace formed out of a black-lead pot, which has a number of holes bored at small distances in spiral lines all over it, from the bottom up to such a height as the fuel is designed to reach to. This is let half way into another pot, which last receives the nozzle of the bellows, so that all the air sent in is distributed through the spiral holes of the upper pot, and concentrates the heat of the fuel upon the crucible, which is placed in the midst.
"The furnace which I am going to describe resembles very closely this of Dr Lewis; with this difference, however, that the air-holes are only bored through the bottom of the pot, and this merely stands upon another piece, instead of being let into it. It is on this account somewhat more commodious, and I imagine not less powerful.
"Fig. 35. is a view, and fig. 36. a section, of the Fig. 35. furnace. It is composed of three parts, all made out & 36. of the common thin black-lead melting pots sold in London for the use of the goldsmiths. The lower piece, A, is the bottom of one of these pots cut off so low as only to leave a cavity of about one inch, and ground smooth, above and below. The outside diameter over the top is 5½ inches. The middle piece or fireplace, B, is a larger portion of a similar pot with a cavity about fix inches deep, and measuring 7½ inches over the top, outside diameter, and perforated with fix blast holes at the bottom. These two pots are all that are essentially necessary to the furnace for most operations: but when it is wished to heap up fuel over the top of a crucible contained within, and especially to protect the eyes from the intolerable dazzle of the fire when in full heat, an upper pot, C, is added of the same dimensions as the middle one, and with a large side opening cut out to allow an exit to the smoke and flame. It has also an iron stem with a wooden handle (an old chief will do very well), to lift it off and on.
"The bellows (which are double) are firmly fixed, by a little contrivance which will take off and on, to a heavy flood, as is represented in the plate; and their handle should be lengthened, to make them work easier to the hand. To increase their force on particular occasions, a plate of lead may be tied on the wood of the upper flap. The nozzle is received into a hole in the pot A, which conducts the blast into its cavity. From hence the air passes into the fire-place, B, through fix holes, of the size of a large gimlet, drilled at equal distances through the bottom of the pot, and all converging in an inward direction, so that, if prolonged, they would meet about the centre of the upper part of the fire. The larger hole through the middle of the bottom of the same pot is for another purpose. Fig. 37.-Fig. 39. is a plan of the same, showing the distribution of these holes.
"As a stand or support for the crucible, I have found no method so good as to fit an earthen flopper into the bottom of the pot B, through the large centre hole which is made for this purpose. This keeps the crucible in its proper place, in stirring down the coals and managing the fuel. These floppers are made with great ease and expedition out of the softened fire-brick soil in London. A piece of this brick, made to revolve a few times within a portion of iron or earthenware tube, presently takes the form of its cavity, and comes out a very neat portion of a cylinder or cone, according to the shape of the tube, from which the floppers may readily be fashioned. Fig. 38. represents Fig. 38. one of these floppers, which is also seen in its proper place in fig. 36. supporting a crucible.
"As the construction of this furnace (exclusive of the bellows and its flood) is easy to any one at all used to these little manual operations, I trust that the working chemist will allow me to add a few words on the method which I have found the most convenient and economical. Almost any broken pot of the proper width will furnish the lower piece A; and often the middle and upper pieces may be contrived out of the fame refuse matter. Dr Lewis advises a saw to cut these pots;