taken in its general acceptation, is a chemical process by which any ore, or other metallic compound, is analysed, and its constituent parts determined. But the term more particularly denotes the peculiar art by which gold and silver are examined, and their qualities ascertained, in relation to their state of purity. By the former the whole contents of the substance under examination are separated and collected; by the latter it is only necessary in practice to find, by the destruction or separation of the alloys, the amount of pure metal contained in the specimen operated on, so that a value may be given, by computation, to the whole mass. To this specific branch we purpose to confine ourselves in the following observations.
The art of assaying the precious metals must be esteemed of considerable importance in many points of view; but more especially to commercial nations trading extensively in these commodities. For although the ultimate destination of these metals be their conversion into coin, plate, or other articles of use and ornament, still there are vast treasures of bullion consigned to the stores of different countries merely as commodities, as convenient representatives of value, or in security for nominal wealth, the marketable value of which is determined solely by the skill and accuracy of the assayer. Every one having experience of such matters knows that, unlike other things of a mercantile nature, bullion cannot be valued by its bulk, weight, or any simple external characteristic, but by ascertaining in some way the amount of fine metal contained in a given quantity. And implicit confidence being placed in the honesty, accuracy, and fidelity of the assayer, who, by examining a small portion only, gives a certificate of its quality upon which the whole is estimated, merchants are enabled to buy and sell bullion without risk of loss, and with the most perfect assurance of the value being maintained. By means of the art of assaying, as applicable to small as to large things, we also possess a certain safeguard against fraud in the manufacture of plate and other articles of personal or domestic use, which must be regarded as of some importance in a country rich and affluent like Great Britain, where gold and silver have become so common amongst the middle classes as scarcely to be esteemed luxuries. We could only wish that the laws which prevail in reference to plate were more generally extended in their application to the manufacture of jewellery, and other personal ornaments; for in these things the public have no guarantee against fraud and knavery beyond the character or assurance of the dealer.
If the art of assaying be deemed valuable and important as regards bullion and plate, securing a certain and reliable test of value, as well as a sufficient check upon fraudulent designs, we cannot surely question its even greater importance in reference to the metallic currency of a country, the standard accuracy of which affects materially the interests of all, and so far tends to promote the welfare of mankind. Whatever this standard be, it is obvious the value of all property in exchange must be regulated and determined by it, whether in land, houses, commodities, or the wages of labour; and if we had no reliance on the integrity of our coin, as a measure of value, we could enjoy no security in our property. In all countries, therefore, claiming a character for honesty, the integrity of the standard should be a fundamental principle. Any slight deviation from it will instantly be discovered, followed by a loss of credit, and violent fluctuations in the exchange, which of all things are most injurious to trading communities. For the exchange between one country and another is not determined by coin merely as a circulating medium, but, in reality, by the exact amount of fine metal contained in the coin; and, therefore, it is necessary to know that the proportion is maintained called the standard, which, in our gold currency, consists of eleven-twelfths of pure metal and one-twelfth of alloy. This fact can only be ascertained by the process of assay. But when nations acquire a character for honesty and integrity, the currency will freely circulate all over the world without suspicion, at its reputed value, and the currency of one be easily converted into the currency of another. But we should fail to secure this great advantage, this implicit confidence, without the assayer's skill and check, by which the due proportions of metal are guaranteed, and any depreciation detected.
We need now no longer apprehend any of those capricious and dishonourable changes in the currency, not unknown to our history before the reign of Elizabeth, which enriched the monarch at the expense of his subjects, created sudden and violent changes in the value of property, and often spread dismay and poverty amongst all classes, without any clear knowledge of the cause. That patriotic and sagacious queen, of whom England is justly proud, among other great and durable merits, is honourably distinguished by the restoration of our currency to purity, and by fixing our present standard of value, from which, happily, no deviation can be notified in succeeding ages. In this respect science and knowledge, if not always the handmaids of integrity, are the best guarantees against fraud and evil designs; and we can scarcely believe it possible, in these times, to suffer from a dishonourable depreciation of the currency, while any vague apprehension of error is instantly dissipated by the numerous checks, public and private, on the purity of our coin. Amongst these we may briefly instance the ancient ceremony called the Trial of the Pyx, Trial of which, in the most public manner, secures an impartial examination of the coinage, and a verdict, as public, of its legal or standard purity. In no other country have we been able to discover an institution analogous to this, which, after all intermediate tests have been tried in the process of manufacture, affords an ultimate and judicial appeal for the public satisfaction. And it may be added, to the credit of the officers of the Mint, that, whatever may happen in future times, amid the sudden and hasty revolutions in public affairs, during the past we are unable to adduce any instance of this ordeal being passed without honour and integrity; and instead of the allowance or remedy by law for errors, unavoidable in manufacture, being taken advantage of, as a protec- Assaying tion to carelessness, we may pronounce that the coinage has always been proved to be as high to the legal standard as is possible, the deviation being on the average scarcely worth notification.
The Trial of the Pyx takes place once in about every three years, but no specified period is fixed by law. It is so denominated from the pyx, or chest, in which the specimen coins are deposited in the Mint for future examination; these specimen coins being supposed a fair representation of the whole money coined within a certain period. Out of each bag of coin, whether gold or silver, two pieces are taken, one for the trial by assay within the Mint, the other for the general pyx; and these are carefully sealed up in paper by three officers, and deposited in the chest. It should be remarked, that previously to the issue of coin to the public, a minor pyx takes place within the Mint, intended for the examination of the coined money by appointed officers, as regards both weight and fineness, and no coin is permitted to be delivered before it has passed this necessary ordeal.
It having been notified to the government that a trial of the pyx is called for, the Lord Chancellor issues his warrant to summon a jury of goldsmiths, who, on the appointed day, proceed to the Exchequer Office, Whitehall, and there, in presence of several privy-councillors, and the officers of the Mint, receive the solemn charge of the Lord Chancellor, who directs them in their important functions, and requests them to deliver to him a verdict of their finding. A piece of gold and silver, cut from the trial-plates deposited in the Exchequer, supposed to be of the exact legal standard, are delivered to the foreman of the jury, who is required to declare to what degree the coin under examination deviates from them. This being done, the jury proceed to Goldsmith's Hall, London, where assaying apparatus, and all other necessary appliances, are in daily use for the trial of gold and silver plate; and sealed packets of the specimen coins being delivered to them by the officers of the Mint, they are first tried by weight, and then a certain number are taken from the whole and melted into a bar, from which the assay trials are subsequently taken.
The verdict of the jury, founded on the results of these proceedings, proving favourable, the Master of the Mint and subordinate officers are released from all further responsibility, while the country receives, by the publicity of the verdict, an attestation of the standard purity of the coinage.
In times of comparative ignorance, the art of assaying was esteemed a mystery, and, like some other crafts, the practice of it was retained in few hands. There were supposed secrets in the conduct of the processes which none but the initiated were permitted to know; but now it is admitted that those secrets are nothing but the knowledge acquired by long experience, amounting in reality to certain allowances or adjustments in the results of the operations. The uncertain tests and appliances employed in ancient times, which afforded only a wide approximation to truth, and exposed the public to extortion and fraud, have in more scientific times been superseded by chemical processes as accurate as they are delicate and beautiful. The exquisite and varied laws of nature, in connection with metals and their transformations, are made instrumental to the use and knowledge of mankind; and science, so called, enables man to nicely balance and estimate the vast treasures found in the bowels of the earth, and constitute them measures of value more unchanging than any other product of nature. As science has progressed, so has the art of assaying improved, while in modern times new fields have been opened up for its use and application. Along with increased accuracy, it has become more varied and extensive in its practice. The amount of the precious metals have not only increased immeasurably, thereby magnifying the importance of the art, but in recent times changes in the mode of refining or separating these metals have created a new branch of business little practised in former ages. The application of sulphuric acid to separate gold from silver, and silver from gold, by which the operation is effected with great economy, and nearly all the contents recovered at comparatively little cost, has led to an extensive business in parting assays, which did not formerly exist. In this manner the holder of bullion, of a mixed character, has a higher value put upon his metal by reason of the gold or silver contained in it; and in the market he is able to realize the whole value by assay, less the deduction made to cover the charge of refining. The parting assay is different from the simple assay in this, that it declares upon the certificate of a gold assay the amount of silver combined with it, and of a silver assay the number of grains of fine gold contained in each pound.
The bullion to be valued having been melted into ingots or bars, small pieces are cut from each and folded separately in slips of paper with a corresponding mark or number of the bar, so as to preserve the identification of the assay reports with the bars. On these slips of paper the assayer writes his report, which declares the quality of the gold and silver, and this is the certificate upon which the bullion is bought and sold in the market. The Bank of England, however, and the Mint, in order to guard against any surreptitious change, or fraud, require the assays upon which they receive bullion to be cut off in presence of appointed officers. The assayer reports gold by carats, and silver by pennyweights. In the one case the Troy pound is divided into 24 parts or carats, and British standard being $\frac{1}{2}$ths fine gold and $\frac{3}{8}$ths alloy, the carat will thus represent 10 dwts. Troy, the standard being therefore 22 carats fine and 2 carats alloy. In the other the Troy pound is divided into 240 dwts., and the standard of silver being 222 dwts. fine and 18 dwts. of alloy, the pennyweight will represent the $\frac{1}{2}$ths of the pound.
Carats are subdivided into four carat grains, = 60 Troy Assay grains each, and these are again further subdivided into weights eighth of a carat grain, = $\frac{7}{8}$ grains Troy. The lowest and trade report of gold is one-eighth of a carat grain, and of silver, half a pennyweight. In reporting gold, the practice in general use is to take 2 carats as the representative of fine gold for bullion better than standard, and 24 carats for bullion worse than standard. Thus a bar reported better 1.3$\frac{3}{4}$, or one carat, three carat grains, and three-quarters of a grain, is within one-quarter of a carat grain of purity, or 15 grains Troy. But if a bar were found to contain only one-half of fine gold, the report would not be one carat worse, but worse 12 carats, or $\frac{1}{2}$ths. We may observe, however, that this complex mode of enumeration, so great a mystery to the uninitiated, will probably in a few years be entirely superseded by the decimal system of notation in general use on the continent. Already it is partially adopted by assayers in England, who are now required to append the decimal report, to the ordinary one, on the certificate. Instead of carats and pennyweights, the numeral 1000 will represent fine gold and silver, and any deviation in purity from this will be expressed by a decimal instead of a vulgar fraction.
It has been already remarked that the lowest denomination of the trade report is $\frac{1}{4}$th of a carat grain, or $\frac{7}{8}$ grains Troy, as respects gold, and half a pennyweight, or 12 grains, as respects silver; but practically an assayer can arrive at a much nearer approximation to the truth. As in the Royal Mint, in making the combination for standard coin, he can report to a single grain, or $\frac{1}{2}$ths, in each case; but in buying and selling bullion some protection to the purchaser is deemed necessary as an indemnity against errors and irre- Assaying, gular mixture of the alloy, and hence arises the above latitude in the assay report. It is probable, however, that the general use of decimal notation will eventually cause a more accurate report, and deprive the bullion dealer of a share of that advantage which obviously is greater than is necessary.
An ordinary assay report of gold and silver expresses the variation from the standard, and not the fine metal contained in it; and it is, therefore, marked as either better or worse than standard. The standard of gold being 22 carats fine and 2 alloy, or $\frac{1}{3}$ths fine, an ingot of gold found to contain only 21 carats pure gold would be reported worse 1 carat; if it contained 23½ carats, it would then be reported better 1 carat, 3 grains, and half a grain. The standard of silver being 11 oz. 2 dwts. fine, and 18 dwts. alloy, an ingot of silver found to contain only 11 oz. of fine would be reported worse 2 dwts., but if it contained 11 oz. 4½, the report would then be better 2½ dwts.
In buying or selling, the betterness or worseness of the bar is added or deducted from the gross weight; and the value is computed on this, the standard weight, at the market price of the day.
When the assay required is a parting assay, or an assay of gold containing silver, a report is given of the weight of fine silver in the pound; and when the silver exceeds 15 dwts. per pound, all above that is usually added to the value of the gold, that being an allowance made by general agreement for the cost of separation or refining. So likewise with an assay of silver holding gold. The report declares the number of grains of fine gold in the pound, and all above 3 or 4 grains is added to the computed value of the silver.
In these delicate operations we need scarcely impress upon the reader how important an instrument an accurate balance must be in securing a certain and uniform result. The specimen taken by an assayer is no more than 12 or 15 grains of the mass, and if 12 grains, each grain would represent an ounce, or $\frac{1}{12}$th. In the Royal Mint, the fine balances in use are sensible to the $\frac{1}{1000}$th of a grain.
The principle of assaying gold and silver is very simple theoretically, but in practice great experience is necessary to insure accuracy; and there is no branch of business which demands more personal and undivided attention. The result is liable to the influence of so many contingencies, that no assayer who regards his reputation will delegate the principal processes to one not equally skilled with himself. Besides the result ascertainable by weight, there are allowances and compensations to be made which are known only to an experienced assayer, and if these were disregarded, as might be the case with the mere novice, the report would be wide from the truth.
With regard to silver assaying, the chief principles in the process are—1. The power of lead to oxidate, or destroy the metallic property of the alloys, which it does by combining with them and sublimating them in fumes; 2. The absorbing power of the cupels in which the assays are made, and by which a great portion of the lead is taken up.
With regard to gold assaying, the same principles prevail in the preliminary process of cupellation, the result being a compound of gold and silver in their pure state. In the second process of parting the two metals, the first principle is the property of nitric acid to bring silver into a state of solution while it leaves gold untouched. The silver thus separated from the gold, which remains in a pure state, enters not into the calculation unless a parting assay is required, and therefore it is not recovered for any purpose connected with the art.
But before proceeding to the detailed description of the various delicate and interesting processes comprised in the art of assaying, we shall first describe the furnaces and implements formerly in use, as well as those changes and improvements which have been recently introduced and adopted, the result of experience and the promptings of scientific research, by which it may be said assaying has approached as near to certainty as can be expected in any chemical operation.
Plate LXXIV, A A A A, fig. 1, is a front elevation of an assay Descriptive furnace; a, a view of one of the two iron rollers on which the function of furnace rests, and by means of which it is moved forward or backward; b, the ash-pit; c, c, the ash-pit dampers, which are moved in a horizontal direction towards each other for regulating the draught in use; d, the door or opening by which the cupels and assays are introduced into the muffle; e, a movable funnel or chimney, by which the draught of the furnace is increased.
B B B B, fig. 2, a perpendicular section of fig. 1; f, a, end view of the rollers; b, the ash-pit; c, one of the ash-pit dampers; d, the grate; e, the plate upon which the muffle rests, and which is covered with loam nearly one inch thick; f, the muffle in section representing the situation of the cupels; g, the mouth plate, and upon it are laid pieces of charcoal, which, during the process, are ignited, and heat the air that is allowed to pass over the cupels, and which will be more fully explained in the sequel; h, the interior of the furnace, exhibiting the fuel.
The total height of this furnace is 2 feet 6 inches; from the bottom to the grate 6 inches; the grate, muffle, plate, and bed of loam with which it is covered, 3 inches; from the upper surface of the grate to the commencement of the funnel, e, fig. 1, 2½ inches; the funnel 6 inches. The square of the furnace which receives the muffle and fuel is 11½ inches by 15 inches. The external sides of the furnace are made of plates of wrought iron, and are lined with a 2-inch fire-brick.
C C C C, fig. 3, is a horizontal section of the furnace over the grate, showing the width of the mouth-piece or plate of wrought iron, which is 6 inches, and the opening which receives the muffle-plate.
Fig. 4 represents the muffle or pot, which is 12 inches long, 6 inches broad inside; in the clear 6½; in height 4½ inside measure, and nearly 5½ in the clear.
Fig. 5, the muffle-plate, and which is of the same size as the bottom of the muffle.
Fig. 6 is a representation of the sliding door of the mouth-plate, as shown at d in fig. 1.
Fig. 7, a front view of the mouth-plate or plate, fig. 1.
Fig. 8, a representation of the mode of making, or shutting up with pieces of charcoal, the mouth of the furnace.
Fig. 9, a view of the cupel, which is generally 1 inch by ¼ths of an inch.
Fig. 10, the teaser for cleaning the grate.
Fig. 11, a larger teaser, which is introduced at the top of the furnace, for keeping a complete supply of charcoal around the muffle.
Fig. 12, the tongs used for charging the assays into the cupels.
Fig. 13 represents a board of wood used as a register, and is divided into 45 equal compartments, upon which the assays are placed previous to their being introduced into the furnace. When the operation is performed, the cupels are placed in the furnace in situations corresponding to these assays on the board. By these means all confusion is avoided, and without this regularity it would be impossible to preserve the accuracy which the delicate operations of the process require.
Figs. 14, 15, 16, 17, represent sections of a smaller assay furnace invented by MM. Le Roye and D'Arrest of Paris, called by them Le petit fourneau à compelle, and which may be advantageously used by the experimental assayer. It is composed of a chimney or pipe of wrought iron a, and of the furnace b. It is 17½ inches high, and 7½ inches wide. The furnace is formed of three parts: of a dome A; the body of the furnace B; and the ash-pit c, which is used as the base of the furnace, figs. 14 and 15. The principal piece or body of the furnace has the form of a hollow cylinder, flattened equally at the two opposite sides, parallel to the axis, and in such a manner that the horizontal section is elliptical.
The section of the furnace, fig. 15, presents several openings; the principal, which is that of the muffle, is placed in i; it is shut with the semicircular door m, fig. 14, and as is seen in the section se, fig. 15. In front of this opening is the table or shelf upon which the door of the muffle is made to advance or recede; the latter g, fig. 15, shows the face, side, and cross section of the shelf, which makes part of the furnace.
Fig. 19 is a plan of the grate of the furnace; and fig. 20 a horizontal view of it.
Figs. 21, 22, 23, are views of the muffle.
Fig. 24 is a view of a crucible for annealing gold.
Figs. 25, 26, 27, are cupels of various sizes, to be used in the furnace. Assaying.
Figs. 23 and 29 are views of the hand-shovels used for filling the furnace with charcoal.
Fig. 30, the smaller pincers or tongs by which the assays are charged into the cupels.
Fig. 18, the teaser for cleaning the grate of the furnace.
Fig. 16 is a representation of the furnace first constructed by MM. Aubry and D'Arcet, and which was worked by means of a pair of bellows, which forced a current of air through the brass tube b, entering the ash-pit under the grate at the circular hole c, fig. 15. The strength of the blast or current of air can be regulated at pleasure by the stop-cock d, fig. 16.
As the accuracy of the art of assaying depends in a great measure upon the construction of the furnace, as well as its management, and other implements and appliances, we proceed now to describe the improvements introduced recently in the Royal Mint by Mr H. W. Field, the Resident Assay-master of that establishment, which tend to secure greater accuracy, and allow of the processes to be conducted with greater economy. Instead of the expensive product, charcoal, being used in the cupellation-furnace, anthracite coal has been substituted; and the old sand-bath, employed to boil the acid, has been superseded by the use of gas.
In the furnace already described, figs. 1 and 2, the usual mode of lighting and feeding it is to throw the charcoal in at the top, igniting the fuel by means of a faggot of wood placed on the charcoal. In this manner two to three hours are necessary to raise the furnace to the required temperature to carry on the process.
In the new furnace, Plate LXXV., figs. 32 and 33, the fuel is ignited by a small quantity of live charcoal scattered round the sides and end of the muffle; and a few minutes having elapsed, the furnace is then filled up to the door h, with anthracite coal, broken into fragments of about the size of a two-inch cube, and perfectly free from dust. The doors of the furnace should then be closed, excepting that of the ash-pit, and damper in the chimney, intended to give circulation to the air.
When the coal is found to be in a state of combustion, either the door at the top, x, or the furnace door, h, or both, should be opened, as experience may dictate; because it must be borne in mind that, after a certain point, if the heat were urged too suddenly, either the muffle would crack and be spoilt, or the bottom be too hot and the roof too cold while at the same time clinkers formed in the furnace might impede the draught. If these simple precautions are attended to, and the heat gradually raised, the annealing of the muffle, cupels, and furnace will be uniform, and both trouble and expense avoided. By the use of anthracite coal, the process of cupellation may be carried on as successfully as by charcoal, when a saving is made of 550 per cent.
Plate LXXV., fig. 32, is the front elevation of Mr Field's furnace; fig. 33, is a view of the front iron-roller, on which it rests; b, the ash-pit; c, e are the dampers moving horizontally from side to side towards each other, meeting exactly in the centre; d, the muffle-door by which the assays are introduced; e, e, the door slides. So far the body of the furnace is similar to the old one, except that the bars on which the muffle stands run from front to back, and are movable, rendering the removal of the brick-work unnecessary. By this means the muffle stand is easily introduced, and having steady pins on the under side, it is raised about an inch above the bars.
The furnace measures 2 feet 10 inches in height; 1 foot 7 inches in width; and 1 foot 11 inches in depth. Instead of the furnace, as formerly, being fed at the top, the fuel is charged by the door h, which also affords the means of regulating the draught, and of throwing a current of air through the muffle by the door d. This door has a bar, k, traversing about two-thirds of it, running easily from the top towards the bottom within i i, with a ketch, s, on each side, to keep it close. These are made on an incline, and about 3½ inches long, so as to allow the traversing-bar to slide freely when the door is not required to be closed. In this manner the door may be opened from a quarter of an inch to the extent of three inches.
This feeding and regulating door is fixed by hinges, l l, to the front part of the iron frame covering the brick lining on the top of the furnace. On this frame rests the square dome, the front of which, w, is removable by two handles, n n; and by taking out the Assaying two thumb-screws, o o, the door and part of the frame come away, leaving a large opening, so as to enable the furnace to be cleared, the muffle repaired, &c.
The furnace should be placed in a recess under a chimney, with a movable iron ceiling, r, about one foot above the dome, fitting close in every part, so that the draught of air may pass through the furnace. A door, or flap, x, is attached to the iron ceiling by a hinge, opening on the side of the recess, with means to fix it at any point required, so that the current of air may be regulated by the operator; s, a swivel door affords another mode for damping the furnace.
Fig. 33 is a section of the furnace fig. 32; a a, the two rollers on which the furnace is placed; b b, the slides on which the ash-pit doors run; c, the door and ash-pit; d, the iron casing to the furnace; e, the brick lining; f, the ash-pit; g g, the two bars inserted in the brick lining, one in front, one at the back, support the furnace bars, which can easily be removed at pleasure; h shows one of the bars on which the muffle-plate rests; i, a movable tray on which the mouth coal is placed; k is a section of a muffle charged with its full complement of 50 cupels; showing also the rows of holes over each row of cupels, through which a current of air passes. Similar holes are placed at the back in three rows. They are not pierced horizontally, but slope towards the ceiling of the muffle, at such an angle as to exclude the ashes; l represents the extra covering of fire-clay; m, the anthracite coal, showing the level; n, the feeding and regulating door; o, the ketch, or inclined plane on which the sliding bar travels; p, the door, with running staples in which the bar slides; q, the mode in which the movable front is brought round, and fixed by thumb-screws, r r s s, the hood; t, handle for removing the front; u, the damper and handle; v, the iron ceiling.
Fig. 34 represents the upper internal view of the furnace bars, with the muffle stand or plate, showing also the space intended for the fuel.
Fig. 35 is the mouth of the muffle door, representing the mode of regulating the current of air by cylinders of charcoal. Fig 40 is the movable muffle door.
Fig. 38 is a representation of a muffle, 1½ inches long, 7½ inches wide, until it begins to taper at about 1½ inches from the front (see fig. 39), when it does not exceed 5½ inches. The height is 6½ inches, in the clear 6½ inches. Its sides are perforated with holes about a quarter of an inch diameter.
Fig. 36, an annealing-iron for softening the assays after they are flattened and rolled. It resembles a square box of iron about ¾th of an inch thick, having strengthening pieces riveted at each end, and two in the middle, b b b b, between which are receiving places for the assays, a a a a. The apertures are made diagonally, as shown by c c e, that the assays may not fall completely to the bottom of the box, so that they may be conveniently removed. The under part of the box has a kind of double keel, d, riveted on it, so that in taking it from the furnace there be no danger of upsetting it on the annealing trident.
Fig. 37 represents the trident for removing the annealing-iron from the furnace.
Fig. 41 is a view of the apparatus for boiling the gold assays in Gas acid. Though, it may be remarked, there is no peculiar novelty in the apparatus, boiling by the agency of gas, this is a very useful and ingenious application of the principle; and, we may add, that the apparatus here represented, constructed by the Assay-master of the Royal Mint, is superior to any in use. By it 45 gold assays can be operated upon at the same time, but its greatest advantage is the perfect control which it gives to the assayer over the process, by which great uniformity in the result is obtained.
The whole apparatus, with the exception of the wooden feet and slides upon which the flasks rest, is constructed of brass, and consists of three tubes, b b b, firmly fixed at each end to two sides of mahogany, a a, by screws: each tube has a cap soldered thereon, with small stopcocks; the other end is open, in order to secure its connection, by an insulated India-rubber tube, with three fixed pipes having stop-cocks; and these pipes again unite in one main pipe, also furnished with a cock.
From the end of the front and back tubes or bars, b b, rise the metal pillars g g g g, to which are attached the inclined supports k k on each side, having notches for the movable bars f f f f, which are so arranged as to allow the operator to see over each bar the row of burners behind it, as well as inspect the ebullition of the liquid. From the three tubes, bars b b b rise the gas-burners, c d e, each row being fixed at a distance sufficient to allow the flasks when resting over the burners and against the movable bar to be secure from falling. The burners have each a separate stopcock, and as the tubes, or bars, are arranged at different heights, the highest being in front, the operator can easily reach the stop-cock of any particular burner, or, if necessary, a key can be applied. The movable bars \( f \) have cross handles at each end to remove them, and on the front of the bars a piece of wood is screwed, having semi-circular notches exactly opposite each burner in which the neck of the flask rests, while the body of the flask is supported over the burner by brass rings passing through the bar above, attached and fixed by thumb or milled screws, thus affording the means of adjustment as to distance from the burner, etc. These particulars may be seen by a reference to fig. 42, \( a \) b e d.
Fig. 43 is a section of the apparatus; \( a \), the wooden stand; \( b \) b, the three tubes or bars by which the gas passes to the several burners; \( g \), the pillars rising from the front and back bars, which at \( A \) hold the sloping notched supports \( k \), for the movable bars \( f \); \( e \), faintly indicate the burners \( b \), the middle line of burners, showing also the stop-cocks with the rings supporting the flasks over the flame; \( m \), the notches for front and back movable bars \( f \), the handle to the bar to which the ring supports are fixed by screws.
Fig. 44, a tray for the flasks when charging the acid, or washing the assay.
Fig. 45, tray to stand in front of the operator in case one or more flasks require to be removed before the whole are sufficiently boiled.
Fig. 46 represents a stand for holding the movable bars when taken from the apparatus to charge the acids or otherwise.
Fig. 47 shows a burner unscrewed from the bar.
Fig. 48, the wire support.
Fig. 49, a portion of a movable bar; \( a \), a burner; \( b \), part of supporting bar; \( c \), a flask; \( d \), rings for carrying flasks.
Fig. 49, a glass-flipped vessel in which the acid is placed before filling the flasks, \( b \), a pipette by which an equal quantity of acid is supplied to each flask.
By this description, the mode of working may be easily seen. The movable bars are first charged with the flasks containing acid. The three small cocks \( h \) b, at the end of the tubes, are then opened, for speedy escape of the air; next the main or chief cock; and then the other three cocks communicating with the tubes. Each individual burner can be regulated by its own stop-cock, the whole tube by its cock, and the whole apparatus by the main.
Having thus fully described the various implements in use amongst assayers, without which the process could not be conducted with any practical utility, it is necessary to make some preliminary observations here with regard to the lead, cupels, &c., before the operation of the assay commences.
The cupels are made of the ashes of calcined bones, or phosphate of lime, and besides being uninjured by the ordinary heat of an assay furnace, they possess the peculiar property of absorbing the lead used to refine the silver. Hence the term cupellation applied to the process of purifying gold and silver by fire on cupels.
The cupels are formed in a circular mould made of forged steel, nicely turned, by which means they are easily freed from the mould when struck. The bone-ash being moistened with a quantity of water, just sufficient to make the particles adhere, is put into the mould and pressed down level with the surface. It is then struck with a pestle or rammer, smoothly turned and polished, the end being of a convex shape, and of the exact size of the cupel required. (See Plate LXXV, figs. 50 and 51, the section.) This cavity forms the cup in which the assay is made. These cupels are allowed to dry in the air for some time before they are used, and are annealed in the furnace before the assay is charged in.
The lead used in cupellation should be of the greatest purity, because, as most lead contains a small portion of silver, this silver would necessarily combine with the assay, and vitiate the accuracy of the result. Lead revived from litharge is supposed to be the purest, as it contains scarcely any silver. Another important consideration in the process of assaying is the quantity of lead to be used with each assay, but such information can only be acquired by experience; for the assayer must first ascertain the approximate quality of the metal to be examined before he can determine the amount of lead necessary.
This in ancient times was effected by the use of what were called touch-needles, or slips of metal composed of pure silver alloyed with definite proportions of copper in a regularly-increasing series. The silver to be assayed was examined in comparison with these touch-needles in colour, tenacity, and other external characters, and its alloy was estimated by that of the needle to which it showed the closest resemblance. These needles are now seldom used, the assayer relying on his general experience for the quality of the metal. Having ascertained this point so far as is necessary, he apportions the lead deemed requisite (ranging from 10 to 20 times the weight of silver), which is first flattened into a thin plate from small bullets cast for this purpose.
We will now proceed to describe and explain the process of assaying silver by cupellation.
The small specimen of silver sent to an assayer cut from the bar to be valued, usually weighs from 3 to 4 dwts.: it is folded in a slip of paper on the head of which is written the date, the mark or number of the bar from which it was severed, and the kind of assay required. This piece is flattened upon a polished anvil by means of a hammer, which should be examined and freed not only from dirt but from any particles of metal flattened from previous assays. From the silver, thus flattened to about the \( \frac{1}{2} \) th of an inch thick, the centre portion is cut out, as being the most compact, and is then adjusted in a "preparing balance," by cutting and filing to the weight of what is technically denominated the "assay pound." After this preliminary weighing, it is then weighed most accurately in one of the superior balances.
At this stage of the operation it is necessary to ascertain the approximation to purity of the metal, so that the proper amount of lead be added; but, as we have before remarked, practice alone can guide the assayer in this particular. The chief indications relied on are the specific gravity, colour, and even sound; for the purer the silver is the less it rings, while at the same time whiteness, softness, ductility, and superior gravity are tokens of purity; but, on the contrary, when sonorous, yellow, and specifically light, it may be inferred that the metal is base in the degree that these peculiarities are indicated.
The amount of lead having been determined, the assay is wrapped up in a known quantity (say one-half of that required for its purification), formed into a case somewhat resembling a thimble, great care being taken to make the joints firm and close, so that no particle of silver shall escape. When a number of assays are made at the same time, they are arranged, enveloped in their cases of lead, on a board divided into compartments, corresponding in number and position with the cupels into which they are intended to be charged. As the assayer makes generally two or more trials of the same piece, so that great accuracy may be secured, it is his practice to give one assay a side place in the muffle, and the second a middle one, in order to check any irregularity in the result.
When a sufficient number of assays are weighed, and arranged upon the board, in the manner referred to, and the furnace raised to the necessary point of heat as well as the cupels, the charging tongs are then taken, and the rest of the lead apportioned to each assay placed individually upon the cupels, beginning at the back of the muffle. The lead added in this case is not flattened, but a piece or bullet of known weight, various sizes of which, as well as cases, are kept in stock by the assayer. The lead so placed in the furnace rapidly melts and becomes covered with a grey oxide, but soon after appears fluid and brilliant; and at this point the assays are charged by means of a pair of tongs, great attention being given that no part overhang or touch the edge of the cupel. The assays are thus drawn into the mass of molten lead, and any particles of silver are in this manner prevented from adhering to the sides of the cupels in charging; sufficient despatch being used to obviate the fusion of the assay in its transition. The assays being charged in order on their respective cupels, and the furnace Assaying previously filled with fuel, cylinders of charcoal, about six inches long, are inserted in the mouth of the muffle, so as to fill up the orifice in the furnace. The object of this is, that the stream of air admitted to pass over the surface of the cupels, and which is necessary for the rapid oxidation of the lead, may not chill the muffle and retard the progress of the assay. By displacing one or more of these pieces of charcoal, the assayer can increase the current of air, while at the same time he is enabled to inspect the operation as it proceeds.
In the first instance, dense fumes will be observed to rise from the melted metal, indicating the oxidation and subsequent volatilization of the lead. These continuing some time are then followed by the appearance of small luminous points on the surface, which increase in size and brilliancy as the operation progresses. And then a minute stream of red fused matter is seen to flow from the top of the silver globule, and circulate around it, which is carried down and absorbed by the cupel. This arises from the vitrification of the lead by the air, which at the same time oxidizes the copper.
As the cupellation advances, the fumes gradually lessen in density till they disappear altogether. The melted button at this stage is observed to become more convex and round; and as the last vestiges of the lead and alloy are being carried off, it assumes a cloudy appearance on the surface, changing to large bright points of the fused oxide, till at length it is nearly freed from all impurity. At this point the noble metal displays some singular and beautiful characteristics. Deprived of all the base alloys save the last minute portion that tarnished its lustre, it has become bright and pure, and gives forth from its surface iridescent circulating rays of light, which indicates the successful completion of the process.
During the operation, the assayer's attention should be directed to the heat of the furnace, which must be neither too hot nor too cold: if too hot, minute portions of silver will be carried off with the lead, and so vitiate the assay; moreover, the pores of the cupel being more open, greater absorption will ensue, and there is liability to loss from that cause. One indication of an excess of heat in the furnace, is the rapid and perpendicular rising of the fumes to the ceiling of the muffle, the mode of checking and controlling which has been pointed out in the description of the improved furnace.
When the fumes are observed to fall to the bottom of the muffle, the furnace is then too cold; and, if left unaltered, it will be found that the cupellation has been imperfectly performed, and the silver will not have entirely freed itself from the base metals.
The white, or dark appearance of the cupels, may also afford some guide to the operator; but in such cases practice and experience are the best instructors; and it is not possible, in a general description, to measure out the precise degree of heat necessary to the perfection of the process.
The cupellation completed, great care is required with regard to the cooling of the globules, for if that process be not allowed to take place gradually, they will "spring" or burst, and throw out particles of pure metal from the interior. This arises, as is generally supposed, from the exterior portion cooling too rapidly, which forms a sort of shell that, shrinking as it cools, presses on the still liquid fluid and makes it spurt out; but others have suggested that the cause of "springing," as it is called, is to be attributed to the disengagement of a small portion of oxygen combined with the melted silver.
The globules being cool are removed from the cupels with a pair of pincers, and struck on the edge with a hammer on the anvil, in order to remove any oxide or extraneous matter adhering to the under surface, and are further cleaned with a "scratch-brush," made of fine wire bound firmly together. After this, having been carefully placed in their compartments, they are weighed with the greatest nicety, the loss Assaying, in weight indicating the amount of alloy abstracted; and affording the apparent quantity of pure metal in the pound.
But into the assayer's calculation certain considerations enter, derived from long experience and observation during the process, which make the actual result of the assay button only an approximation to the actual quality of the metal under examination. For, supposing the operation to have been perfectly performed, he finds it necessary, for example, to make some compensation for a certain amount of silver, not lost, but absorbed in the cupel itself, which he knows to have been abstracted from the assay.
This fact may be ascertained by reviving the oxide of lead from the cupel, and cupelling the lead by itself, when it will be found that the small globule of silver left on the cupel considerably exceeds the proportion of this metal in the lead, and will correspond nearly with the loss of silver in the assay. And it is manifest, therefore, if this were not taken into calculation in the assayer's report, the metal would appear less pure than it really is, all deficiency of weight being reckoned as alloy. It has been calculated, that when no more lead is used than is necessary to separate the alloy, there is carried down into the cupel as much silver as, when the whole is again reduced, would make the noble metal $\frac{1}{3}$ th of the mass, when the natural admixture of the silver is only $\frac{1}{12}$ th. But if an excess of lead be employed, this loss of silver is somewhat greater, though it does not increase in the ratio of the excess of the lead.
French chemists, who impugn the accuracy and scientific nature of the process of cupellation as applied to the assay of silver, have generally adopted another very ingenious and delicate mode, called "La voie humide," or humid method, in contradistinction to the dry. This system is maintained to be more certain and accurate in its results by the French, but as English assayers are divided upon the point, it is not generally practised, and therefore we need not enter elaborately into a description of it.
This ingenious process was originally discovered by M. Gay-Lussac, the distinguished French chemist, by whose influence it was introduced into the Paris Mint and other establishments. In his "Instruction sur l'Essai des matières d'Argent," it is thus described:— "Le nouveau procédé d'essai que nous allons décrire consiste à déterminer le litre des matières d'argent par la quantité d'une dissolution de sel marin titrée, nécessaire pour précipiter exactement l'argent contenu dans un poids donné d'alliage."
The metal being dissolved in nitric acid, a quantity of salt is poured into it from a graduated vessel till the whole silver is precipitated in the state of chloride, a compound insoluble in water or even in acid. The quantity of chloride of silver precipitated is determined, not by weight, which would render the process tedious, but by the weight or the volume of the dissolved salt necessary to precipitate precisely the silver in the acid. The complete precipitation of the silver is easily recognized by the cessation of the cloudiness in the liquid, when the solution of salt is gradually converted into nitrate of silver. In this process the presence of copper, lead, or any other metal in the solution of silver, does not in a sensible degree affect the quantity of salt necessary to the precipitation; that is to say, the same quantity of silver, whether pure or alloyed, requires a fixed and constant quantity of salt to effect its separation from the acid.
Though the theory of the process appear thus so extremely simple, in the practice of it, nice manipulation, and long experience as to the presence of other metals equally affected by the salt, are absolutely necessary; and these have carried it to such perfection that the results can be obtained with as much rapidity, and, according to its advocates, with more certainty and confidence than by the ancient mode of cupellation. It remains for us now to notice briefly the silver parting assay, or that by which the amount of gold contained in silver is discovered and determined: an assay which has become very common in modern times, in consequence of the discovery of the art of refining by sulphuric acid, whereby gold can be separated from silver at a comparatively small cost.
The button or globule resulting from the cupellation of the silver is simply digested with nitric acid in a glass bottle, or matrass, on a sand-heat, and the acid taking up the whole silver, leaves the gold at the bottom in the shape of a fine brown powder. The acid being poured off, the residuum is washed in warm water, and then allowed to fall into a small clay-vessel, or crucible, in which it is brought to a red heat, in order to expel any moisture. The weight is then ascertained, and the gold being deducted from the silver, shows the proportion of each metal in a given weight. The report of this assay is stated as follows:—B. 15 dwts. for silver; gold 12 grains per pound Troy. When the amount of gold exceeds one-quarter of the assay-specimen, or 2½ dwts., it is found necessary in practice to treat it as a gold assay.
We proceed now to describe the process of assaying gold, which, as greater values are affected, demands even more care and attention than that of silver.
The preliminary process exactly resembles the one already described, the object of cupellation in this case, as in the other, being to destroy the base metal or alloy contained in the gold. If gold contained only copper as alloy, the assaying of gold would be as simple as that of silver, the globule of fine metal on the test indicating by weight the quality of the specimen operated on, as in the case of silver. But as gold generally is found to contain a portion of silver in combination, and as silver is not destroyed by cupellation, it will be manifest that another process is required to separate the silver from the gold. To effect this object recourse is had to the "parting process." This is done by means of nitric acid, which entirely dissolves the silver, and leaves the gold perfectly pure. But as gold does not commonly contain sufficient silver to allow of the action of the acid and complete dissolution of the silver, it is found necessary in all gold assays to add from 2 to 3 parts of fine silver on the cupel. In this manner the particles of gold are disseminated, and no longer protecting the silver from the acid, the dissolution is easily accomplished.
In the first stages of the operation the same remarks are applicable as were previously made on the assay of silver. The specimen to be examined is rolled or hammered out, and a piece taken from it free from extraneous matter; it is then nicely weighed against the integer, or assay pound, the assayer exercising his judgment on the metal as to its various characteristics of colour, &c., so as to determine the addition of pure silver necessary, with other alloys, to effect the solution in boiling acid, the proportion required being full 2½ ds of the whole.
The silver added is then placed with the gold assay in a case of lead weighing half that necessary to complete the process; and if there be a number, they are arranged on the board as formerly described, ready to be charged seriatim into the cupels prepared in the furnace. The remainder of the lead is then added, and the process of cupellation proceeds as before.
The assays having passed the cupel successfully, they are flattened on a polished anvil, three well-directed blows with a heavy hammer being sufficient to reduce them to the proper form and thickness to pass through the flattening-mill. This is worked by hand, and reduces the pieces to thin plates or laminae about 3/64th of an inch thick, which are annealed or softened, and, to avoid confusion, they are placed on the "annealing iron" (fig. 36), from which they are taken one by one and carefully rolled up with round-nosed pliers, in such a manner that the acid may penetrate between each fold. They now resemble coils of silver, and are ready for Assaying.
From time immemorial sand-baths have been used for boiling the acid, deemed the safest and best for matrasses or bottles of glass, liable at all times to break by undue expansion caused by the sudden and irregular application of heat. But as gas in modern times has been applied to many useful purposes, so amongst others it has been successfully employed in boiling the parting assays.
The sand-bath is a somewhat clumsy contrivance, consisting of a square copper pan covered with fine sand about an inch in depth. This is placed over an open charcoal fire, and three-quarters of an hour are required to properly heat the sand. Flat-bottomed, conical glasses, containing the assays and acid are placed on the bath, and the liquid allowed to boil; and by changing the glasses from time to time, so as to equalize the ebullition of each, very satisfactory results have generally been attained. But this plan has been recently improved upon by Mr H. W. Field of the Royal Mint, by the introduction of his apparatus by which gas is used to boil the acid, and which is effected more expediently, with as much safety, and with more economy. This apparatus will be seen by reference to fig. 41 to 49. Plate LXXV.
The flasks or bottles which are round-bottomed, lighter, and more tapering than the old matrasses, are placed on the gas-jets, and in five or ten minutes the acid is hot enough to receive the assays. By means of this 45 assays can be operated upon at the same time. The assays being charged, the acid is seen instantly to attack the alloy with great vehemence, which becomes oxidized by abstracting a portion of oxygen from the nitric acid, giving forth at the same time dense nitrous fumes. So long as these dark red fumes are visible the decomposition continues, but gradually they become fainter and fainter, till they disappear altogether. Lest some silver remain in the gold, protected from the action of the acid, the liquid is decanted, and its place supplied by some stronger acid, of the specific gravity of 1·35. This removes effectually any particles of alloy that may be incorporated with the gold.
In this process, the assayer's attention should be chiefly directed to the strength of the acid; for if it be too strong the cornet of gold is liable to be broken and reduced to powder, rendering the subsequent collection of the grains a work of difficulty; if too weak, a portion of silver or alloy will remain incorporated with the gold, and vitiate the accuracy of the assay. The specific gravity of the acid applied in the first instance should be about 1·2, three or four times the weight of the assay being added; in the second boiling, it should be about 1·35; but experience will be found a safer guide in such particulars than any definite rules. The principal object is to employ acid of the exact strength that will, after the silver has been extracted, maintain the cornet unbroken and of a spiral form. Regard must also be had to the proper proportion of silver added to the assay; for if that be in excess, the nitric acid is liable to reduce the gold to powder.
When the acid is in the act of violent ebullition, the assay will be found to rise in the glass and strike against it, and in order to prevent this the French assayers generally put into the liquid a piece of charcoal about the size of a pea, which forms a kind of nucleus, around which the acid boils; but this application, however simple, discolours the acid and renders it useless. At the Royal Mint, in lieu of charcoal, the assayer uses small porous clay balls about ¼th of an inch in diameter, which have not that objection, and being washed in boiling water after use they can be employed again and again.
Before the cornets are removed from the glasses, the hot acid is carefully decanted, and the flasks filled with warm distilled water, so as to wash away the remaining acid, and any residue of silver. This should be repeated until the Assaying. Liquid is seen to pour off perfectly clear. After this, the flasks being filled with water, they are one by one inverted over a small clay annealing pot, and the cornet of gold is allowed to fall gently through the water into the pot. These vessels are arranged in such a manner as to obviate any confusion, and secure the identification of the assay. The superfluous water being poured out of the pots, they are then placed in the furnace and annealed under a bright heat.
The cornets, by the action of the acid and by the separation of the silver, are thoroughly corroded, though their original form remains unaltered. They are in this state extremely brittle and porous, and when examined microscopically appear to resemble a very fine sponge. They bear a nearer likeness to pieces of brown earthenware than to gold. But after annealing, they regain all the properties of that truly noble metal; and being allowed to cool, they are carefully weighed against the assay pound, and the diminution of weight recorded. If they are weighed hot, it has been ascertained that a variation will occur of nearly 4th of a carat grain, and, therefore, such an error should be guarded against.
It is a matter of the greatest importance that the silver used in this process should contain no gold, otherwise a source of very material error would arise in the delicate operations of the assayer. The silver cannot be too pure, and a careful examination should be made of it before employing it for that purpose. The most certain mode of attaining that object is to precipitate silver from the nitrate, which cannot possibly contain any gold.
As in the case of the silver assay, certain compensations are found by experience necessary before an accurate report can be made of the gold assay. In the cornet it must not be supposed that we have before us an assay perfectly pure, as a previous paragraph may have suggested, or that no loss of precious metal has taken place during the process. Notwithstanding all the care and attention used by the assayer, it has been discovered that the cornet is not altogether free from silver; and it is manifest if this were not allowed for, the report would declare the gold to be finer than it really is. The amount of silver so remaining is estimated by the Mint Assayer to range between 4 and 9 grains Troy. But in this particular it is probable no two assayers will agree, as the allowance to be made depends on the practice of each. And what is gained to the assay by silver, is to some extent lost again, in the cupel and by the action of the acid. For, by the experience of Mr Field, an assayer of long practice, nitric acid does take up a small portion of gold, contrary to the general notion of chemists; which has been proved by laying aside the second or strong acid (employed more than once) in a conical-shaped bottle where it was permitted to remain. After some time a coating of gold was apparent at the bottom of the glass, and under the influence of light the whole interior of the bottle was coated up to the stopper when the acid reached so high.
Two bottles of acid that had been repeatedly used to digest gold assays for some years, yielded in this manner full 30 grains of gold. This fact has recently been confirmed by experiments made by Mr C. Sterry of the Mint, and also by that careful chemist and assayer Mr G. H. Makins of London.
Independently of these allowances and compensations, another must be made for variations caused by the particular process itself, as indicated by the trial pieces put into the furnace along with the assays, and which in all respects passes the same ordeal. This standard piece of metal consists of a certain weight of gold of a known quality, and whatever variations it undergoes, the assays themselves are supposed to be subject to the same.
Gold is found to contain other metals beside silver and copper, such as platinum and the allied metals; and when that happens the assayer has to exercise his judgment in ascertaining the character of the metal and its amount. In ordinary cases, when the assay after cupellation contains gold and silver only, it assumes the appearance of silver; but if platinum be combined with it in any notable proportion, it will easily be detected by the crystalline surface of a dead white colour, covered with bright rough points; if palladium, the button assumes the appearance of an ordinary assay, but in the final clearing of the assay in the furnace it resembles an assay cooled too quickly; and while it tinged the acid a deep rich red colour, the former is marked by a pale yellow. Again, if iridium be in combination, it will mix mechanically with the gold in metallic grains; but, after passing through the fire, it will leave streaks of black powder on various parts of the cornet.
Small portions of platinum and palladium are removable by the acid; but, as iridium is insoluble in any acid, no means have yet been devised of separating it from the gold in the process of assaying.
The "gold parting assay" is simply the process by which gold parting the silver is separated from the gold, so as to enable the assayer to report the contents, that the bullion-holder may realize the value. This is effected by dissolving the silver in the nitric acid, as before described in the common assay, and then ascertaining the amount of silver resulting in excess of the quantity used in the process, which indicates the amount of silver contained in the assay pound, and by computation in the Troy pound.
The gold coming from Australia, and particularly from California, is found to contain a considerable amount of silver, which can be separated easily and economically by the sulphuric acid process of refining, and recently this branch of assaying has also augmented far beyond what was required in former times.
The silver in solution resulting from the parting assay may be precipitated in the metallic state, by diluting the liquid with water, and then putting into it plates of copper. The silver being thrown down and washed carefully until the water be nearly tasteless, may then be dried and melted. This method, however, is considered offensive, because, in the action, there is a copious discharge of dark red noxious fumes of nitrous gas.
The plan now pursued by the Mint Assayer is to throw down the silver in solution as a chloride, by the addition of either common salt or hydrochloric acid, until all cloudiness disappear. The residuum is then collected in proper earthen pans, well washed, and slightly acidulated with hydrochloric acid, immediately immersing in the liquid plates of iron, which reduces the chloride to a metallic state. The plates are then taken out, and the silver first washed with hot water, containing a little hydrochloric acid, and then repeatedly with hot-distilled water till it pours off tasteless. The powder is then dried and melted.
In conclusion, we append a table of the gold assay weights, and their corresponding decimals; premising that in a few years the decimal system will probably supersede the ancient complex mode of notation.
| Carats | Carat Grains | Eighths | Troy Weight | Decimals | |--------|--------------|---------|-------------|----------| | 24 | | | | | | 12 | | | | | | 6 | | | | | | 3 | | | | | | 2 | | | | | | 1 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
Troy Weight:
- 1 lb. - 1 oz. - 1 dwt. - 1 grn.
Decimals:
- 1.000000 - 0.500000 - 0.250000 - 0.125000 - 0.063333 - 0.041666 - 0.020833 - 0.010416 - 0.005208 - 0.002604 - 0.001302 A description of the process of assaying might be deemed incomplete without some reference to the assay-balance, which may be said truly to constitute the right hand of the assayer, and on the delicacy and accuracy of which depend, in a great measure, the truth of the results. With that view, we may be allowed to give a detailed description of a beautiful and ingenious balance recently adopted in the Royal Mint, constructed on improved principles by Mr G. H. Mains, practical Chemist and Assayer, Coleman Street, London, and manufactured by an excellent workman, Mr Oertling of Store Street. It seems to combine all the advantages of modern improvements, both in design and workmanship, and in practice must amply fulfill the most sanguine expectations of the inventor.
In a good assay-balance three essentials are indispensable—1. It should be quick in its action; 2. It should be constant and uniform—that is, oscillate to the same point of the index with the same weight to any number of weighings; 3. It should be extremely sensible and delicate, indicating the minutest shades of difference. As, however, the sensibility of a balance is found to diminish as the quickness of action is increased, the centre of gravity being lowered, one of the excellencies of the balance alluded to consists in the nice adjustment of these two opposing qualities.
The beam is what is generally called a skeleton-beam, 10 inches long, ½ an inch deep at the fulcrum, tapering off at each end to the ¼d of an inch. At the centre it is about ⅛th of an inch in thickness, decreasing at the ends to the ¼th of an inch. The weight of the beam is only 85 grains; and therefore it contains little more metal than is necessary to secure the several parts together at the points of suspension.
The end adjustments, indicated by a, in fig. 52, Plate LXXV, for the length of arm, are similar to many other delicate balances, namely, a saw-cut with screws, to open and shut by pressure. The pendants, b, at each end, are supported by two hard steel-points, c, having a fine screw cut on each, with nuts to fix them, so as to obtain a perfectly straight line, touching all the points of suspension. These pendants or pans, d, with the skiffs, e, are hung on these points by means of steel-plates, having a cup-shaped cavity for the one, and a groove for the other. It would be impossible for the most skilful workman to form two cups so accurately as to plumb or adjust themselves with mathematical precision to the points of suspension.
As the weight used with this balance rarely, if ever, exceeds 20 grains, the parts of contact where friction occur have been reduced to mere points, and in consequence the wear is very slight.
One peculiarity of this balance is the use of a weight called a rider, represented by f, supposed to be a German invention, which is placed or rests on the beam itself, and depresses it; the beam being divided in its length into ten parts, and the rider placed on any part, the weight is ascertained, and thus allows of an easy and simple mode of weighing decimally. The rider, f, is carried from one point or division of the beam to another, or may be altogether removed by a movable arm, g, worked by the hand from the outside, h, of the case or lantern. It travels or traverses the beam on a stage, i, fixed from side to side of the lantern at k; and in this manner the small or fractional weights may be ascertained without subjecting the beam to disturbance by the admission of air upon opening the glass-door, l, of the case B.
A, represents the outline drawing of the balance and apparatus; the supports being of massive construction in order to obviate any tremulousness. They consist of two pillars, m, fixed on a base, n, and on the top of these columns a table is fixed, o, with two upright pieces, p, rising from it, to which are attached the agate bearings, formed of an elliptical shape, and presenting therefore the smallest points of contact to the knife edges. A corresponding table, q, is fixed to the rods, p, which move up and down the pillars, m, and by means of the lever handle, r, give action to the beam. The table has two mortices in it exactly fitting the upright pieces, s, which carry the agate-bearers, and upon these uprights the table slides up and down. On the outside of this second table, q, is a crutch or Y, t, which raises the beam from its bearings when it is thrown out of action.
The beam, on being depressed, acts first upon the arms of two rollers, fixed under the lantern, whose opposite arms depress the ivory tables supporting the pans; and at the point when the tables are free, the lever has reached a connecting stirrup between the movement rods, and then begins to lower the beam upon the agate bearers. The tops of the ivory tables that support the pans are of a spherical shape, v, fixed upon the tables w, so as to admit of the least possible contact; and the pans have the curve of a radius exactly equal to the distance between the ivory table and the point of suspension; so that, if they are found to swing out of the perpendicular during the act of weighing, they will be caught by the tables when they rise upwards; y y are spirit-levels.
In this balance there are adjustments for the amount of fall of the fulcrum to the agates, as well as for the rise of the ivory-tables to the pans; but the former is small in amount. The pillars being placed at the distance of 1½ inch apart, and the index-needle, x, long—extending 6 inches downwards—a good scale of division is obtained; while the motion of the index-needle being nearly level with the pans, a great advantage is secured in use.
(R. M.—T.)
ASELYN, Hans, a distinguished Dutch painter, was born at Antwerp in 1610, and became the disciple of Esaias Vandervelde, the battle-painter. He distinguished himself in historical painting, battles, landscapes, and animals, particularly horses. He travelled into France and Italy, and was so pleased with the manner of Bamboccio that he always followed it. He was one of the first Dutch painters who introduced a fresh and clear manner of painting landscapes in the style of Claude Lorraine; upon which all the painters imitated his style, and reformed the dark brown they had hitherto followed. Asselyn's pictures were in high esteem at Amsterdam. He died in that city in 1660. Twenty-four pieces of landscapes and ruins, which he painted in Italy, have been engraved by Perelle.
ASEMANI, the surname of three learned Maronite Syrians, who flourished in Italy in the last century. The eldest, and the most learned of the three, Giuseppe Simone, was born at Tripoli in Syria, in 1687. Having been sent to Rome to receive a classical education, he soon distinguished himself by his learning and industry, and was chosen by Pope Clement XI. to go and visit the convents of Egypt and Syria, in search of ancient MSS. with which to enrich the library of the Vatican. He executed his commission with great success, and in reward for his services was made archbishop of Tyre, and librarian of the Vatican. His principal work is his Bibliotheca Orientalis Clementino-Vaticana, in 4 folio volumes, 1719–28, which contains valuable biographical notices of Syrian Christian authors. He also published, in 6 vols. fol. (Lat., Greek, and Syriac), 1732–34, the works of St Ephraem, a Syrian father of the Church. He died at Rome in 1768 at the age of 80. Stefano Evodia, his nephew, was created bishop of Apamea, and succeeded his uncle as librarian. He also published two folio volumes of Oriental Catalogues of Vatican MSS. and other archeological works of merit. Simone, the grand-nephew of Giuseppe, born at Tripoli in 1752, was long professor of Oriental languages in the University of Padua, where he died in 1821. He is best known by his masterly de- Assembly, section of the literary imposture of Vella, a pretended history of the Saracens in Sicily, which was confirmed by the German Hager.