GLASS

GLASS, a transparent, brittle, factitious body, produced from sand melted in a strong fire with fixed alkaline salts, lead, slags, &c. till the whole becomes perfectly clear and fine. The word is formed of the Latin glastum, a plant called by the Greeks isatis, by the Romans vitrum, by the ancient Britons guadum, and by the English wood. We find frequent mention of this plant in ancient writers, particularly Caesar, Vitruvius, Pliny, &c. who relate that the ancient Britons painted or dyed their bodies with glastum, guadum, vitrum, &c. i. e. with the blue colour procured from this plant. And hence the factitious matter we are speaking of came to be called glass; as having always somewhat of this bluishness in it.

At what time the art of glass-making was first invented, is altogether uncertain. Some imagine it to have been invented before the flood: but of this we have no direct proof, though there is no improbability in the supposition; for we know, that it is almost impossible to excite a very violent fire, such as is necessary in metallurgic operations, without vitrifying part of the bricks or stones wherewith the furnace is built. This indeed might furnish the first hints of glass-making; though it is also very probable, that such imperfect vitrifications would be observed a long time before people thought of making any use of them.

Neri traces the antiquity of glass as far back as the time of Job. That writer, speaking of the value of wisdom (chap. xxviii. verse 17.), says, that gold and crystal cannot equal it. But this word, which Neri will have to signify factitious glass, is capable of a great many different interpretations, and properly signifies only whatever is beautiful or transparent. Dr Merret will have the art to be as ancient as that of pottery or the making of bricks, for the reasons already given, viz. that by all vehement heat some imperfect vitrifications are produced. Of this kind undoubtedly was the fossil glass mentioned by Ferant. Imperator. to have been found under ground where great fires had been. But it is evident, that such imperfect vitrifications might have passed unnoticed for ages; and consequently we have no reason to conclude from thence, that the art of glass-making is of such high antiquity.

The Egyptians boast, that this art was taught them by their great Hermes. Aristophanes, Aristotle, Alexander Aphrodiseus, Lucretius, and St John the divine, put it out of all doubt that glass was used in their days. Pliny relates, that it was first discovered accidentally in Syria, at the mouth of the river Belus, by certain merchants driven thither by a storm at sea; who being obliged to continue there and dress their victuals by making a fire on the ground, where there

was great plenty of the herb kali; that plant, burning to ashes, its salts mixed and incorporated with the sand, or stones fit for vitrification, and thus produced glass; and that, this accident being known, the people of Sidon in that neighbourhood essayed the work, and brought glass into use; since which time the art has been continually improving. Be this as it will, however, the first glass-houses mentioned in history were erected in the city of Tyre, and here was the only staple of the manufacture for many ages. The sand which lay on the shore for about half a mile round the mouth of the river Belus was peculiarly adapted to the making of glass, as being neat and glittering; and the wide range of the Tyrian commerce, gave an ample vent for the productions of the furnace.

Mr Nixon, in his observations on a plate of glass found at Herculaneum, which was destroyed A. D. 80, on which occasion Pliny lost his life, offers several probable conjectures as to the uses to which such plates might be applied. Such plates, he supposes, might serve for specula or looking glasses; for Pliny, in speaking of Sidon, adds, siquidem etiam specula excogitaverat: the reflection of images from these ancient specula being effected by besmearing them behind, or tinging them through with some dark colour. Another use in which they might be employed, was for adorning the walls of their apartments, by way of wainscot, to which Pliny is supposed to refer by his vitrea camera, lib. xxxvi. cap. 25. § 64. Mr Nixon farther conjectures, that these glass plates might be used for windows, as well as the lamina of lapis specularis and phengites, which were improvements in luxury mentioned by Seneca and introduced in his time, Ep. xc. However, there is no positive authority relating to the usage of glass windows earlier than the close of the third century: Manifestus est (says Lactantius*), mentem esse, quæ per oculos ea quæ sunt opposita, transpiciat, quasi per fenestras lucente vitro aut Dei, cap. 5. speculari lapide obductas.

The first time we hear of glass made among the Romans was in the reign of Tiberius, when Pliny relates that an artist had his house demolished for making glass malleable, or rather flexible; though Petronius Arbiter, and some others, assure us, that the emperor ordered the artist to be beheaded for his invention.

It appears, however, that before the conquest of Britain by the Romans, glass-houses had been erected in this island, as well as in Gaul, Spain, and Italy.—Hence, in many parts of the country are to be found amulets of glass, having a narrow perforation and thick rim, denominated by the remaining Britons gleineu naid-reedh or glass adders, and which were probably in former times used as amulets by the druids†. It can scarcely be questioned that the Britons were sufficiently well versed in the manufacture of glass, to form out of it many more useful instruments than the glass beads. History indeed assures us, that they did manufacture a considerable quantity of glass vessels. These, like their amulets, were most probably green, blue, yellow, or black, and many of them curiously streaked with other colours. The process in the manufacture would be nearly the same with that of the Gauls or Spaniards. The sand of their shores being reduced to a sufficient degree of fineness by art, was mixed with three-fourths

of its weight of their nitre (much the same with our kelp), and both were melted together. The metal was then poured into other vessels, where it was left to harden into a mass, and afterwards replaced in the furnace, where it became transparent in the boiling, and was afterwards figured by blowing, or modelling in the lath, into such vessels as they wanted.

It is not probable that the arrival of the Romans would improve the glass manufacture among the Britons. The taste of the Romans at that time was just the reverse of that of the inhabitants of this island. The former preferred silver and gold to glass for the composition of their drinking vessels. They made indeed great improvements in their own at Rome, during the government of Nero. The vessels then formed of this metal rivalled the bowls of porcelain in their dearness, and equalled the cups of crystal in their transparency. But these were by far too costly for common use; and therefore, in all probability, were never attempted in Britain. The glass commonly made use of by the Romans was of a quality greatly inferior; and, from the fragments which have been discovered at the stations or towns of either, appears to have consisted of a thick, sometimes white, but mostly blue green, metal.

According to venerable Bede, artificers skilled in making glass for windows were brought over into England in the year 674, by Abbot Benedict, who were employed in glazing the church and monastery of Wermouth. According to others, they were first brought over by Wilfrid, bishop of Worcester, about the same time. Till this time the art of making such glass was unknown in Britain; though glass windows did not begin to be common before the year 1180; till this period they were very scarce in private houses, and considered as a kind of luxury, and as marks of great magnificence. Italy had them first, next France, from whence they came into England.

Venice, for many years, excelled all Europe in the fineness of its glasses; and in the thirteenth century, the Venetians were the only people that had the secret of making crystal looking glasses. The great glass works were at Murano, or Murano, a village near the city, which furnished all Europe with the finest and largest glasses.

The glass manufacture was first begun in England in 1557: the finer sort was made in the place called Crutched Friars, in London; the fine flint glass, little inferior to that of Venice, was first made in the Savoy house, in the Strand, London. This manufacture appears to have been much improved in 1635, when it was carried on with sea coal or pit coal instead of wood, and a monopoly was granted to Sir Robert Mansell, who was allowed to import the fine Venetian flint glasses for drinking, the art of making which was not brought to perfection before the reign of William III. But the first glass plates, for looking glasses and coach windows, were made, 1673, at Lambeth, by the encouragement of the duke of Buckingham; who, in 1679, introduced the manufacture of fine glass into England, by means of Venetian artists, with amazing success. So that within a century past, the French and English have not only come up to, but even surpassed the Venetians, and we are now no longer supplied from abroad.

The French made a considerable improvement in the art of glass, by the invention of a method to cast very

large plates, till then unknown, and scarce practised yet by any but themselves and the English. That court applied itself with a laudable industry to cultivate and improve the glass manufacture. A company of glassmen was established by letters patent; and it was provided by an arret, not only that the working in glass should not derogate any thing from nobility, but even that none but nobles should be allowed to work therein.

An extensive manufactory of this elegant and valuable branch of commerce was first established in Lancashire, about the year 1773, through the spirited exertions of a very respectable body of proprietors, who were incorporated by an act of parliament. From those various difficulties constantly attendant upon new undertakings, when they have to contend with powerful foreign establishments, it was for some time considerably embarrassed; but government, of late, having taken off some restrictions that bore hard upon it, and made some judicious regulations relative to the mode of levying the excise duty, it now bids fair to rival, if not surpass, the most celebrated continental manufactures, both with respect to the quality, brilliancy, and size of its productions.

With regard to the theory of vitrification, we are almost totally in the dark. In general, it seems to be that state in which solid bodies are, by the vehement action of fire, fitted for being dissipated or carried off in vapour. In all vitrifications there is a plentiful evaporation: and if any solid substance is carried off in vapour by the intense heat of a burning speculum, a vitrification is always observed previously to take place. The difference, then, between the state of fusion and vitrification of a solid body we may conceive to be, that in the former the element of fire acts upon the parts of the solid in such a manner as only to disjoin them, and render the substance fluid; but in vitrification the fire not only disjoins the particles, but combines with them in a latent state into a third substance; which, having now as much fire as it can contain, can receive no further change from that element except being carried off in vapour.

But though we are unable to effect this change upon solid bodies without a very violent heat, it is otherwise in the natural processes. By what we call crystallization, nature produces more perfect glasses than we can make with our furnaces. These are called precious stones; but in all trials they discover the essential properties of glass, and not of stoner. The most distinguishing property of glass is its resisting the force of fire, so that this element cannot calcine or change it as it does other bodies, but can only melt it, and then carry it off in vapours. To this last all the precious stones are subject. The diamond (the hardest of them all) may be dissipated in a less degree of heat than what would dissipate common glass. Nor can it be any objection to this idea, that some kinds of glass are capable of being converted into a kind of porcelain by a long-continued cementation with certain materials. This change happens only to those kinds of glass which are made of alkaline salt and sand; and Dr Lewis hath shown that this change is produced by the dissipation of the saline principle, which is the least fixed of the two. Glass, therefore, we may still consider as a substance upon which the fire

Glass. has no other effect than either to melt or dissipate it in vapour.

The other properties of glass are very remarkable, some of which follow:

1. It is one of the most elastic bodies in nature. If the force with which glass balls strike each other be reckoned 16, that wherewith they recede by virtue of their elasticity will be nearly 15.

2. When glass is suddenly cooled, it becomes exceedingly brittle; and this brittleness is sometimes attended with very surprising phenomena. Hollow balls made of unannealed glass, with a small hole in them, will fly to pieces by the heat of the hand only, if the hole by which the internal and external air communicate be stopped with a finger. Some vessels, however, made of such unannealed glass have been discovered, which have the remarkable property of resisting very hard strokes given from without, though they shiver to pieces by the shocks received from the fall of very light and minute bodies dropped into their cavities. These glasses may be made of any shape: all that needs be observed in making them is, that their bottom be thicker than their sides. The thicker the bottom is, the easier do the glasses break. One whose bottom is three fingers breadth in thickness flies with as much ease at least as the thinnest glass. Some of these vessels have been tried with strokes of a mallet sufficient to drive a nail into wood tolerably hard, and have held good without breaking. They have also resisted the shock of several heavy bodies, let fall into their cavities, from the height of two or three feet; as musket balls, pieces of iron or other metal, pyrites, jasper, wood, bone, &c. But this is not surprising, as other glasses of the same shape and size will do the same: but the wonder is, that taking a shiver of flint of the size of a small pea, and letting it fall into the glass only from the height of three inches, in about two seconds, the glass flies, and sometimes at the very moment of the shock; nay, a bit of flint no larger than a grain, dropped into several glasses successively, though it did not immediately break them, yet when set by, they all flew in less than three quarters of an hour. Some other bodies produce the same effect with flint; as sapphire, diamond, porcelain, hard tempered steel; also marbles such as boys play with, and likewise pearls.

These experiments were made before the Royal Society; and succeeded equally when the glasses were held in the hand, when they were rested on a pillow, put in water, or filled with water. It is also remarkable, that the glasses broke upon having their bottoms slightly rubbed with the finger, though some of them did not fly till half an hour after the rubbing. If the glasses are everywhere extremely thin, they do not break in these circumstances.

Some have pretended to account for these phenomena, by saying, that the bodies dropped into the vessels cause a concussion which is stronger than the cohesive force of the glass, and consequently that a rupture must ensue. But why does not a ball of iron, gold, silver, or copper, which are perhaps a thousand times heavier than the flint, produce the same effect? It is because they are not elastic. But surely iron is more elastic than the end of one's finger. Mr Euler has endeavoured to account for these appearances from

his principles of percussion. He thinks that this experiment entirely overthrows the opinion of those who measure the force of percussion by the vis viva, or absolute apparent strength of the stroke. According to his principles, the great hardness and angular figure of the flint, which makes the space of contact with the glass extremely small, ought to cause an impression on the glass vastly greater than lead, or any other metal; and this may account for the flint's breaking the vessel, though the bullet, even falling from a considerable height, does no damage. Hollow cups made of green bottle glass, some of them three inches thick at the bottom, were instantly broken by a shiver of flint weighing about two grains, though they had resisted the shock of a musket ball from the height of three feet.

That Mr Euler's theory cannot be conclusive more than the other, must appear evident from a very slight consideration. It is not by angular bodies alone that the glasses are broken. The marbles with which children play are round, and yet they have the same effect with the angular flint. Besides, if it was the mere force of percussion which broke the glasses, undoubtedly the fracture would always take place at the very instant of the stroke; but we have seen that this did not happen sometimes till a very considerable space of time had elapsed. It is evident, therefore, that this effect is occasioned by the putting in motion some subtle fluid with which the substance of the glass is filled; and that the motions of this fluid, when once excited in a particular part of the glass, soon propagate themselves through the whole or greatest part of it, by which means the cohesive power becomes at last too weak to resist them. There can be little doubt that the fluid just now mentioned is that of electricity. It is known to exist in glass in very great quantity; and it also is known to be capable of breaking glasses even when annealed with the greatest care, if put into too violent a motion. Probably the cooling of glass hastily may make it more electric than is consistent with its cohesive power, so that it is broken by the least increase of motion in the electric fluid by friction or otherwise. This is evidently the case when it is broken by rubbing with the finger; but why it should also break by the mere contact of flint and the other bodies above mentioned, has not yet been satisfactorily accounted for.

A most remarkable phenomenon also is produced in Rotation of glass tubes placed in certain circumstances. When these glass tubes are laid before a fire in a horizontal position, having their extremities properly supported, they acquire a rotatory motion round their axis, and also a progressive motion towards the fire, even when their supports are declining from the fire, so that the tubes will move a little way up hill towards the fire. When the progressive motion of the tubes towards the fire is stopped by any obstacle, their rotation still continues. When the tubes are placed in a nearly upright posture, leaning to the right hand, the motion will be from east to west; but if they lean to the left hand, their motion will be from west to east; and the nearer they are placed to the perfectly upright posture, the less will the motion be either way.

If the tube is placed horizontally on a glass pane, the fragment, for instance, of coach window-glass, in-

Glass. stead of moving towards the fire, it will move from it, and about its axis in a contrary direction to what it had done before; nay, it will recede from the fire, and move a little up hill when the plane inclines towards the fire. These experiments are recorded in the Philosophical Transactions*. They succeeded best with tubes about 20 or 22 inches long, which had in each end a pretty strong pin fixed in cork for an axis.

* No. 476.
§ 1.
7
Attempts to account for it. The reason given for these phenomena, is the swelling of the tubes towards the fire by the heat, which is known to expand all bodies. For, say the adopters of this hypothesis, granting the existence of such a swelling, gravity must pull the tube down when supported near its extremities; and a fresh part being exposed to the fire, it must also swell out and fall down, and so on.—But without going farther in the explanation of this hypothesis, it may be here remarked, that the fundamental principle on which it proceeds is false; for though fire indeed make bodies expand, it does not increase them in weight; and therefore the sides of the tube, though one of them is expanded by the fire, must still remain in equilibrio; and hence we must conclude, that the causes of these phenomena remain yet to be discovered.

4. Glass is less dilatable by heat than metalline substances, and solid glass sticks are less dilatable than tubes. This was first discovered by Col. Roy, in making experiments in order to reduce barometers to a greater degree of exactness than hath hitherto been found practicable; and since his experiments were made, one of the tubes 18 inches long, being compared with a solid glass rod of the same length, the former was found by a pyrometer to expand four times as much as the other, in a heat approaching to that of boiling oil.—On account of the general quality which glass has of expanding less than metal, M. de Luc recommends it to be used in pendulums: and he says it has also this good quality, that its expansions are always equable, and proportioned to the degrees of heat; a quality which is not to be found in any other substance yet known.

5. Glass appears to be more fit for the condensation of vapours than metallic substances. An open glass filled with water, in the summer time, will gather drops of water on the outside, just as far as the water in the inside reaches; and a person's breath blown on it manifestly moistens it. Glass also becomes moist with dew, when metals do not. See DEW.

6. A drinking glass partly filled with water, and rubbed on the brim with a wet finger, yields musical notes, higher or lower as the glass is more or less full; and will make the liquor frisk and leap. See HARMONICA.

7. Glass is possessed of very great electrical virtues. See ELECTRICITY, passim.

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Materials for glass. Materials for Making of GLASS. The materials whereof glass is made, we have already mentioned to be salt and sand or siliceous earth.

1. The salt here used is procured from a sort of ashes brought from the Levant, called polverine, or rochetta; which ashes are those of a sort of water plant called kali†, cut down in the summer, dried in the sun, and burnt in heaps, either on the ground or on iron grates; the ashes falling into a pit, grow into a hard mass, or

stone, fit for use. It may also be procured from common kelp, or the ashes of the fucus vesiculosus. See KELP.

To extract the salt, these ashes, or pulverine, are powdered and sifted, then put into boiling water, and there kept till one-third of the water be consumed; the whole being stirred up from time to time, that the ashes may incorporate with the fluid, and all its salts be extracted: then the vessel is filled up with new water, and boiled over again, till one half be consumed; what remains is a sort of ley, strongly impregnated with salt. This ley, boiled over again in fresh coppers, thickens in about 24 hours, and shoots its salt; which is to be ladled out, as it shoots, into earthen pans, and thence into wooden vats to drain and dry. This done, it is grossly pounded, and thus put in a sort of oven, called calcar, to dry. It may be added, that there are other plants, besides kali and fucus which yield a salt fit for glass: such are the common way thistle, bramble, hops, wormwood, wood, tobacco, fern, and the whole leguminous tribe, as peas, beans, &c.

Pearl ashes form a leading flux in the manufacture of glass, and mostly supply the place of the Levant ashes, the barillas of Spain, and many other kinds, which were formerly brought here for making both glass and soap.

There are other fluxes used for different kinds of glass, and for various purposes, as calcined lead, nitre, sea salt, borax, arsenic, smiths clinkers, and wood-ashes, containing the earth and lixiviate salts as produced by incineration. With regard to these several fluxes, we may observe, in general, that the more calx of lead, or other metallic earth, enters into the composition of any glass, so much the more fusible, soft, coloured, and dense this glass is, and reciprocally.

The colours given to glass by calces of lead, are shades of yellow: on the other hand, glasses that contain only saline fluxes partake of the properties of salts; they are less heavy, less dense, harder, whiter, more brilliant, and more brittle than the former; and glasses containing both saline and metallic fluxes do also partake of the properties of both these substances. Glasses too saline are easily susceptible of alteration by the action of air and water: especially those in which alkalies prevail; and these are also liable to be injured by acids. Those that contain too much borax and arsenic, though at first they appear very beautiful, quickly tarnish and become opaque when exposed to air. By attending to these properties of different fluxes, phlogistic or saline, the artist may know how to adjust the proportions of these to sand, or powdered flints, for the various kinds of glass. See the article VITRIFICATION.

2. The sand or stone, called by the artists tarso, is the second ingredient in glass, and that which gives it the body and firmness. These stones, Agricola observes, must be such as will fuse; and of these such as are white and transparent are best; so that crystal challenges the precedence of all others.

At Venice they chiefly use a sort of pebble, found in the river Tesino, resembling white marble, and called cuogolo. Indeed Ant. Neri assures us, that all stones which will strike fire with steel, are fit to vitrify; but Dr Morret shows, that there are some exceptions from this

this rule. Flints are admirable; and when calcined, powdered, and seared, make a pure white crystalline metal; but the expence of preparing them makes the masters of our glass-houses sparing of their use. Where proper stones cannot be so conveniently had, sand is used. The best for this purpose is that which is white, small, and shining; examined by the microscope, it appears to be small fragments of rock crystal. For green glass, that which is of a soft texture, and more gritty; it is to be well washed, which is all the preparation it needs. Our glass-houses are furnished with white sand for their crystal glasses from Lynn in Norfolk, and Maidstone in Kent, and with the coarser for green glass from Woolwich.

Some mention a third ingredient in glass, viz. manganese, a kind of pseudo loadstone, dug up in Germany, Italy, and even in Mendip hills in Somersetshire. But the proportion hereof to the rest is very inconsiderable; beside, that it is not used in all glass. Its office is to purge off the natural greenish colour, and give it some other tincture required.

For this purpose it should be chosen of a deep colour, and free from specks of metalline appearance, or a lighter cast; manganese requires to be well calcined in a hot furnace, and then to undergo a thorough levigation. The effect of manganese in destroying the colours of glass, and hence called the soap of glass, is accounted for by M. Montamy, in his Traité des Couleurs pour la Peinture en Email, in the following manner: the manganese destroys the green, olive, and blue colours of glass, by adding to them a purple tinge, and by the mixture producing a blackish brown colour; and as blackness is caused merely by an absorption of the rays of light, the blackish tinge given to the glass by the mixture of colours, prevents the reflection of so many rays, and thus renders the glass less coloured than before. But the black produced by this substance suggests an obvious reason for using it very sparingly in those compositions of glass which are required to be very transparent. Nitre or saltpetre is also used with the same intention; for by destroying, in a certain degree, the inflammable or carbonaceous matter, which gives a strong tinge of yellow to glass prepared with lead as a flux, it serves to free it from this colour; and in saline glasses, nitre is requisite in a smaller proportion to render them sufficiently transparent, as in the case of looking glass and other kinds of plates.

Kinds of Glass. The manufactured glass now in use may be divided into three general kinds; white transparent glass, coloured glass, and common green or bottle glass. Of the first kind there is a great variety; as the flint glass, as it is called with us, and the German crystal glass, which are applied to the same uses; the glass for plates, for mirrors, or looking glasses; the glass for windows and other lights; and the glass for phials and small vessels. And these again differ in the substances employed as fluxes in forming them, as well as in the coarseness or fineness of such as are used for their body. The flint and crystal, mirror and best window glass, not only require such purity in the fluxes, as may render it practicable to free the glass perfectly from all colour; but for the same reason likewise, either the white Lynn sand, calcined flints, or white pebbles, should be used. The others do not demand the same nicety in the choice of the materials;

though the second kind of window glass, and the best kind of phial, will not be so clear as they ought, if either too brown sand or impure salts be suffered to enter into their composition.

Of coloured glass there is a great variety of sorts, differing in their colour or other properties according to the occasions for which they are wanted. The differences in the latter kind depend on the accidental preparation and management of the artists by whom they are manufactured, as will be afterwards explained.

Furnaces for the Making of Glass. In this manufacture there are three sorts of furnaces; one called calcar is for the frit; the second is for working the glass; the third serves to anneal the glass, and is called the leer. See Plate CCXLVII.

The calcar resembles an oven ten feet, long, seven feet broad, and two deep; the fuel, which in Britain is sea coal, is put into a trench on one side of the furnace, and the flame reverberating from the roof upon the frit calcines it. The glass furnace, or working furnace, is round, of three yards diameter, and two high: or thus proportioned. It is divided into three parts, each of which is vaulted. The lower part is properly called the crown, and is made in that form. Its use is to keep a brisk fire, which is never put out. The mouth is called the bocca. There are several holes in the arch of this crown, through which the flame passes into the second vault or partition, and reverberates into the pots filled with the ingredients above mentioned. Round the insides are eight or more pots placed, and piling pots on them. The number of pots is always double that of the boccas or mouths, or of the number of workmen, that each may have one pot refined to work out of, and another for metal to refine in while he works out of the other. Through the working holes the metal is taken out of the pots, and the pots are put into the furnace; and these holes are stopped with moveable covers made of lute and brick, to screen the workmen's eyes from the scorching flames. On each side of the bocca or mouth is a bocarella or little hole, out of which coloured glass or finer metal is taken from the piling pot. Above this oven there is the third oven or leer, above five or six yards long, where the vessels or glass are annealed or cooled: this part consists of a tower, besides the leer, into which the flame ascends from the furnace. The tower has two mouths, through which the glasses are put in with a fork, and set on the floor or bottom: but they are drawn out on iron pans called fraches, through the leer, to cool by degrees; so that they are quite cold by the time they reach the mouth of the leer, which enters the sarosel or room where the glasses are to be stowed.

But the green-glass furnace is square; and at each angle it has an arch for annealing or cooling glasses. The metal is wrought on two opposite sides, and on the other two they have their colours, into which are made linnet holes for the fire to come from the furnace to bake the frit, and to discharge the smoke. Fires are made in the arches to anneal the work, so that the whole process is done in one furnace.

These furnaces must not be of brick, but of hard sandy stones. In France, they build the outside of brick; and the inner part, to bear the fire, is made of a

sort.

sort of fullers earth, or tobacco-pipe clay, of which earth they also make their melting pots. In Britain the pots are made of Stourbridge clay.

Mr Blancourt observes that the worst and roughest work in this art is the changing the pots when they are worn out or cracked. In this case the great working hole must be uncovered; the faulty pot must be taken out with iron hooks and forks, and a new one must be speedily put in its place, through the flames, by the hands only. For this work, the man guards himself with a garment made of skins, in the shape of a pantaloone, that covers him all but his eyes, and is made as wet as possible; the eyes are defended with a proper sort of glass.

Instruments for Making of Glass. The instruments made use of in this work may be reduced to these that follow. A blowing pipe, made of iron, about two feet and a half long, with a wooden handle. An iron rod to take up the glass after it is blown, and to cut off the former. Scissors to cut the glass when it comes off from the first hollow iron. Shears to cut and shape great glasses, &c. An iron ladle with the end of the handle cased with wood, to take the metal out of the refining pot, to put it into the workman's pots. A small iron ladle cased in the same manner, to skim the alkaline salt that swims at top. Shovels, one like a peel, to take up the great glasses; another like a fire-shovel, to feed the furnace with coals. A hooked iron fork, to stir the matter in the pots. An iron rake for the same purpose, and to stir the frit. An iron fork, to change or pull the pots out of the furnace, &c.

Compositions for White and Crystal Glass. 1. To make crystal glass, take of the whitest tarso, pounded small, and scoured as fine as flour, 200 pounds; of the salt of pulverine 130 pounds; mix them together and put them into the furnace called the calcar, first heating it. For an hour keep a moderate fire, and keep stirring the materials with a proper rake, that they may incorporate and calcine together; then increase the fire for five hours; after which take out the matter; which being now sufficiently calcined, is called frit. From the calcar put the frit in a dry place, and cover it up from the dust for three or four months. Now to make the glass or crystal: take of this crystal frit, called also bollito; set it in pots in the furnace, adding to it a due quantity of magnesia or manganese: when the two are fused, cast the fluor into fair water, to clear it of the salt called sandiver; which would otherwise make the crystal obscure and cloudy. This lotion must be repeated again and again, as often as needful, till the crystal be fully purged; or this scum may be taken off by means of proper ladles. Then set it to boil four, five, or six days; which done, see whether it have manganese enough; and if it be yet greenish, add more manganese, at discretion, by little and little at a time, taking care not to overdose it, because the manganese inclines it to a blackish hue. Then let the metal clarify, till it becomes of a clear and shining colour; which done, it is fit to be blown or formed into vessels at pleasure.

2. Flint glass, as it is called by us, is of the same general kind with that which in other places is called crystal glass. It has this name from being originally made with calcined flints, before the use of the white

sand was understood; and retains the name, though no flints are now used in the composition of it. This flint glass differs from the other, in having lead for its flux, and white sand for its body; whereas the fluxes used for the crystal glass are salts or arsenic, and the body consists of calcined flints or white river pebbles, tarso, or such stones. To the white sand and lead a proper proportion of nitre is added, to burn away the phlogiston of the lead, and also a small quantity of magnesia; and in some works they use a proportional quantity of arsenic to aid the fluxing ingredients. The most perfect kind of glass may be made by fusing with a very strong fire 120 pounds of the white sand, 50 pounds of red lead, 40 pounds of the best pearl ashes, 20 pounds of nitre, and five ounces of magnesia. Another composition of flint glass, which is said to come nearer to the kind now made, is the following: 120 pounds of sand, 54 pounds of the best pearl ashes, 36 pounds of red lead, 12 pounds of nitre, and 6 ounces of magnesia. To either of these a pound or two of arsenic may be added, to increase the flux of the composition. A cheaper composition of flint glass may be made with 120 pounds of white sand, 35 pounds of the best pearl ashes, 40 pounds of red lead, 13 pounds of nitre, 6 pounds of arsenic, and four ounces of magnesia; or instead of the arsenic may be substituted 15 pounds of common salt; but this will be more brittle than the other. The cheapest composition for the worst kind of flint glass consists of 120 pounds of white sand, 30 pounds of red lead, 20 pounds of the best pearl ashes, 10 pounds of nitre, 15 pounds of common salt, and six pounds of arsenic. The best German crystal glass is made of 120 pounds of calcined flints or white sand, 70 pounds of the best pearl ashes, 10 pounds of saltpetre, half a pound of arsenic, and five ounces of magnesia. And a cheaper composition is formed of 120 pounds of calcined flints or white sand, 46 pounds of pearl ashes, 7 pounds of nitre, 6 pounds of arsenic, and 5 ounces of magnesia.

A glass much harder than any prepared in the common way, may be made by means of borax in the following method: Take four ounces of borax, and an ounce of fine sand; reduce both to a subtile powder, and melt them together in a large close crucible set in a wind furnace, keeping up a strong fire for half an hour; then take out the crucible, and when cold break it, and there will be found at the bottom a pure hard glass capable of cutting common glass like a diamond. This experiment, duly varied, says Dr Shaw, may lead to several useful improvements in the arts of glass, enamels, and factitious gems, and shows an expeditious method of making glass, without any fixed alkali, which has been generally thought an essential ingredient in glass, and it is not yet known whether calcined crystal or other substances being added to this salt instead of sand, it might not make a glass approaching to the nature of a diamond.

There are three principal kinds of glasses, distinguished by the form or manner of working them; viz. I. Round glass, as those of our vessels, phials, drinking glasses, &c. II. Table or window glass, of which there are divers kinds; viz. crown glass, jealous glass, &c. III. Plate glass, or mirror glass.

I. Working or Blowing Round Glass. The working furnace, we have observed, is round, and has six boccas or

or apertures: at one of these, called the great bocca, the furnace is heated, and the pots of frit are at this set in the furnace; two other small holes, called bocarellas, serve to ladle or take out the melted metal, at the end of an iron, to work the glass. At the other holes they put in pots of fusible ingredients, to be prepared, and at last emptied into the lading pot.

There are six pots in each furnace, all made of tobacco-pipe clay, proper to sustain not only the heat of the fire, but also the effect of the pulverine, which penetrates every thing else. There are only two of these pots that work: the rest serve to prepare the matter for them. The fire of the furnace is made and kept up with dry hard wood, cast in without intermission at six apertures.

When the matter contained in the two pots is sufficiently vitrified, they proceed to blow or fashion it. For this purpose the workman dips his blowing pipe into the melting pot; and by turning it about, the metal sticks to the iron more firmly than turpentine. This he repeats four times, at each time rolling the end of his instrument, with the hot metal thereon, on a piece of plate iron; over which is a vessel of water which helps to cool, and so to consolidate and to dispose that matter to bind more firmly with what is to be taken next out of the melting pot. But after he has dipped a fourth time, and the workman perceives there is metal enough on the pipe, he claps his mouth immediately to the other end of it, and blows gently through the iron tube, till the metal lengthens like a bladder about a foot. Then he rolls it on a marble stone a little while to polish it; and blows a second time, by which he brings it to the shape of a globe of about 18 or 20 inches diameter. Every time he blows into the pipe, he removes it quickly to his cheek; otherwise he would be in danger, by often blowing, of drawing the flame into his mouth: and this globe may be flattened by returning it to the fire; and brought into any form by stamp irons, which are always ready. When the glass is thus blown, it is cut off at the collet or neck; which is the narrow part that stuck to the iron. The method of performing this is as follows: the pipe is rested on an iron bar, close by the collet; then a drop of cold water being laid on the collet, it will crack about a quarter of an inch, which, with a slight blow or cut of the shears will immediately separate the collet.

After this is done, the operator dips the iron rod into the melting pot, by which he extracts as much metal as serves to attract the glass he has made, to which he now fixes this rod at the bottom of his work, opposite to the opening made by the breaking of the collet. In this position the glass is carried to the great bocca or mouth of the oven, to be heated and scalded; by which means it is again put into such a soft state, that, by the help of an iron instrument, it can be pierced, opened, and widened, without breaking. But the vessel is not finished till it is returned to the great bocca; where being again heated thoroughly, and turned quickly about with a circular motion, it will open to any size, by the means of the heat and motion.

If there remain any superfluities, they are cut off with the shears; for till the glass is cool, it remains in a soft flexible state. It is therefore taken from the bocca,

and carried to an earthen bench, covered with brands, which are coals extinguished, keeping it turning; because that motion prevents any settling, and preserves an evenness in the face of the glass, where, as it cools, it comes to its consistency; being first cleared from the iron rod by a slight stroke by the hand of the workman.

If the vessel conceived in the workman's mind, and whose body is already made, requires a foot, or a handle, or any other member or decoration, he makes them separately; and now essays to join them with the help of hot metal, which he takes out of the pots with his iron rod: but the glass is not brought to its true hardness till it has passed the leer or annealing oven, described before.

II. Working or blowing of Window or Table Glass. The method of working round glass, or vessels of any sort, is in every particular applicable to the working of window or table glass, till the blowing iron has been dipped the fourth time. But then instead of rounding it, the workman blows, and so manages the metal upon the iron plate, that it extends two or three feet in the form of a cylinder. This cylinder is put again to the fire, and blown a second time, and is thus repeated till it is extended to the dimensions required, the side to which the pipe is fixed diminishing gradually till it ends in a pyramidal form; so that, to bring both ends nearly to the same diameter, while the glass is thus flexible, he adds a little hot metal to the end opposite the pipe, and draws it out with a pair of iron pincers, and immediately cuts off the same end with the help of a little cold water as before.

The cylinder being now open at one end, is carried back to the bocca; and there, by the help of cold water, it is cut about eight or ten inches from the iron pipe or rod; and the whole length at another place, by which also it is cut off from the iron rod. Then it is heated gradually on an earthen table, by which it opens in length; while the workman, with an iron tool, alternately lowers and raises the two halves of the cylinder; which at last will open like a sheet of paper, and fall into the same form in which it serves for use; in which it is preserved by heating it over again, cooling it on a table of copper, and hardening it 24 hours in the annealing furnace, to which it is carried upon forks. In this furnace an hundred tables of glass may lie at a time, without injury to each other, by separating them into tents, with an iron shiver between, which diminishes the weight by dividing it, and keeps the tables flat and even.

Of window or table glass there are various sorts, made in different places, for the use of building. Those most known among us are given us by the author of the Builder's Dictionary, as follows:

1. Crown, of which, says Neri, there are two kinds, distinguished by the places where they are wrought; viz. Ratcliff crown glass, which is the best and clearest, and was first made at the Bear garden, on the Bankside, Southwark, but since at Ratcliff: of this there are 24 tables to the case, the tables being of a circular form, about three feet six inches in diameter. The other kind, or Lambeth crown glass, is of a darker colour than the former, and more inclining to green.

The best window or crown glass is made of white sand 60 pounds, of purified pearl ashes 30 pounds, of saltpetre

saltpetre 15 pounds, of borax one pound, and of arsenic half a pound. If the glass should prove yellow, magnesia must be added. A cheaper composition for window glass consists of 60 pounds of white sand, 25 pounds of unpurified pearl ashes, 10 pounds of common salt, 5 pounds of nitre, 2 pounds of arsenic, and one ounce and a half of magnesia. The common or green window glass is composed of 60 pounds of white sand, 60 pounds of unpurified pearl ashes, 10 pounds of common salt, 2 pounds of arsenic, and 2 ounces of magnesia. But a cheaper composition for this purpose consists of 120 pounds of the cheapest white sand, 30 pounds of unpurified pearl ashes, 60 pounds of wood ashes, well burnt and sifted, 20 pounds of common salt, and 5 pounds of arsenic.

2. French glass, called also Normandy glass, and formerly Lorraine glass, because made in those provinces. At present it is made wholly in the nine glass works; five whereof are in the forest of Lyons, four in the county of Eu; the last at Beaumont near Rouen. It is of a thinner kind than our crown glass; and when laid on a piece of white paper, appears of a dirtyish green colour. There are but 25 tables of this to the case.

3. German glass is of two kinds, the white and the green: the first is of a whitish colour, but is subject to those small curved streaks observed in our Newcastle glass, though free from the spots and blemishes thereof. The green, besides its colour, is liable to the same streaks as the white; but both of them are straighter and less warped than our Newcastle glass.

4. Dutch glass is not much unlike our Newcastle glass either in colour or price. It is frequently much warped like that, and the tables are but small.

5. Newcastle glass is that most used in England. It is of an ash colour, and much subject to specks, streaks, and other blemishes; and besides is frequently warped. Leybourn says, there are 45 tables to the case, each containing five superficial feet: some say there are but 35 tables, and six feet in each table.

6. Phial glass is a kind betwixt the flint glass and the common bottle or green glass. The best kind may be prepared with 120 pounds of white sand, 50 pounds of unpurified pearl ashes, 10 pounds of common salt, 5 pounds of arsenic, and 5 ounces of magnesia. The composition for green or common phial glass consists of 120 pounds of the cheapest white sand, 80 pounds of wood ashes well burnt and sifted, 20 pounds of pearl ashes, 15 pounds of common salt, and 1 pound of arsenic.

The common bottle or green is formed of sand of any kind fluxed by the ashes of burnt wood, or of any parts of vegetables; to which may be added the scoria or clinkers of forges. When the softest sand is used, 200 pounds of wood ashes will suffice for 100 pounds of sand, which are to be ground and mixed together. The composition with the clinkers consists of 170 pounds of wood ashes, 100 pounds of sand, and 50 pounds of clinkers or scoria, which are to be ground and mixed together. If the clinkers cannot be ground, they must be broke into small pieces, and mixed with the other matter without any grinding.

III. Working of Plate or Mirror Glass. 1. The materials of which this glass is made are much the

same as those of other works of glass, viz. an alkali, salt and sand.

The salt, however, should not be that extracted from pulverine or the ashes of the Syrian kali, but that from BARILLA, growing about Alicant in Spain. It is very rare that we can have the barilla pure; the Spaniards in burning the herb make a practice of mixing another herb along with it, which alters its quality; or of adding sand to it to increase the weight, which is easily discovered if the addition be only made after the boiling of the ashes, but next to impossible if made in the boiling. It is from this adulteration that those threads and other defects in plate glass arise. To prepare the salt, they clean it well of all foreign matters; pound or grind it with a kind of mill, and finally sift it pretty fine.

Pearl ashes, properly purified, will furnish the alkali salt requisite for this purpose; but it will be necessary to add borax or common salt, in order to facilitate the fusion, and prevent the glass from stiffening in that degree of heat in which it is to be wrought into plates. For purifying the pearl ashes, dissolve them in four times their weight of boiling water, in a pot of cast iron, always kept clean from rust. Let the solution be removed into a clean tub, and remain there 24 hours or longer. Having decanted the clear part of the fluid from the dregs or sediment, put it again in the iron pot, and evaporate the water till the salts are left perfectly dry. Preserve them in stone jars, well secured from air and moisture.

Pearl ashes may also be purified in the highest degree, so as to be proper for the manufacture of the most transparent glass, by pulverizing three pounds of the best pearl ashes with six ounces of saltpetre in a glass or marble mortar, till they are well mixed; and then putting part of the mixture into a large crucible, and exposing it in a furnace to a strong heat. When this is red hot, throw in the rest gradually; and when the whole is red hot, pour it out on a moistened stone or marble, and put it into an earthen or clean iron pot, with ten pints of water; heat it over the fire till the salts be entirely melted; let it then stand to cool, and filter it through paper in a pewter cullender. When it is filtered, put the fluid again into the pot, and evaporate the salt to dryness, which will then be as white as snow; the nitre having burnt all the phlogistic matter that remained in the pearl ashes after their former calcination.

As to the sand, it is to be sifted and washed till such time as the water come off very clear; and when it is well dried again, they mix it with the salt, passing the mixture through another sieve. This done, they lay them in the annealing furnace for about two hours; in which time the matter becomes very light and white: in this state they are called frit or fritta; and are to be laid up in a dry clean place, to give them time to incorporate: they lie here for at least a year.

When they would employ this frit, they lay it for some hours in the furnace, adding to some the fragments or shards of old and ill made glasses; taking care first to calcine the shards by heating them red hot in the furnace, and thus casting them into cold water. To the mixture must likewise be add-

ed manganese, to promote the fusion and purification.

The best composition for looking glass plates consists of 60 pounds of white sand cleansed, 25 pounds of purified pearl ashes, 15 pounds of saltpetre, and 7 pounds of borax. If a yellow tinge should affect the glass, a small proportion of magnesia, mixed with an equal quantity of arsenic, should be added. An ounce of the magnesia may be first tried; and if this proves insufficient, the quantity should be increased.

A cheaper composition for looking glass plate consists of 60 pounds of the white sand, 20 pounds of pearl ashes, 10 pounds of common salt, 7 pounds of nitre, 2 pounds of arsenic, and 1 pound of borax. The matter of which the glasses are made at the famous manufacture of St Gobin in France, is a composition of solder and of a very white sand, which are carefully cleaned of all heterogeneous bodies; afterwards washed for several times, and dried so as to be pulverized in a mill, consisting of many pestles, which are moved by horses. When this is done, the sand is sifted through silk sieves and dried.

The matter thus far prepared is equally fit for plate glass, to be formed either for blowing or by casting.

The largest glasses at St Gobin are run; the middle sized and small ones are blown.

2. Blowing the plates. The workhouses, furnaces, &c. used in the making of this kind of plate glass, are the same, except that they are smaller, and that the carquaisse are disposed in a large covered gallery, over against the furnace, as those in the following article, to which the reader is referred.

After the materials are vitrified by the heat of the fire, and the glass is sufficiently refined, the workman dips in his blowing iron, six feet long, and two inches in diameter, sharpened at the end which is put in the mouth, and widened at the other, that the matter may adhere to it. By this means he takes up a small ball of matter, which sticks to the end of the tube by constantly turning it. He then blows into the tube, that the air may swell the annexed ball; and carrying it over a bucket of water, which is placed on a support at the height of about four feet, he sprinkles the end of the tube to which the matter adheres, with water, still turning it, that by this cooling the matter may coalesce with the tube, and be fit for sustaining a greater weight. He dips the tube again into the same pot, and proceeds as before; and dipping it into the pot a third time, he takes it out, loaded with matter, in the shape of a pear, about ten inches in diameter, and a foot long, and cools it at the bucket; at the same time blowing into the tube, and with the assistance of a labourer, giving it a balancing motion, he causes the matter to lengthen; which, by repeating this operation several times, assumes the form of a cylinder, terminating like a ball at the bottom, and in a point at the top. The assistant is then placed on a stool three feet and a half high; and on this stool there are two upright pieces of timber, with a cross beam of the same, for supporting the glass and tube, which are kept in an oblique position by the assistant, that the master workman may with a puncheon set in a wooden handle, and with a mallet, make a hole in the mass: this hole is drilled at the centre of the ball that terminates the cylinder, and is about an inch in diameter.

When the glass is pierced, the defects of it are perceived; if it is tolerably perfect, the workman lays the tube horizontally on a little iron tressel, placed on the support of the aperture of the furnace. Having exposed it to the heat for about half a quarter of an hour, he takes it away, and with a pair of long and broad shears, extremely sharp at the end, widens the glass, by insinuating the shears into the hole made with the puncheon, whilst the assistant, mounted on the stool, turns it round, till at last the opening is so large as to make a perfect cylinder at bottom. When this is done, the workman lays his glass upon the tressels at the mouth of the furnace to heat it: he then gives it to his assistant on the stool, and with large shears cuts the mass of matter up to half its height. There is at the mouth of the furnace an iron tool called pontil, which is now heating, that it may unite and coalesce with the glass just cut, and perform the office which the tube did before it was separated from the glass. This pontil is a piece of iron six feet long, and in the form of a cane or tube, having at the end of it a small iron bar, a foot long, laid equally upon the long one, and making with it a T. This little bar is full of the matter of the glass, about four inches thick. This red hot pontil is presented to the diameter of the glass, which coalesces immediately with the matter round the pontil, so as to support the glass for the following operation. When this is done, they separate the tube from the glass, by striking a few blows with a chisel upon the end of the tube which has been cooled; so that the glass breaks directly, and makes this separation, the tube being discharged of the glass now adhering to the pontil. They next present to the furnace the pontil of the glass, laying it on the tressel to heat, and redden the end of the glass, that the workman may open it with his shears, as he has already opened one end of it, to complete the cylinder; the assistant holding it on his stool as before. For the last time, they put the pontil on the tressel, that the glass may become red hot, and the workman cuts it quite open with his shears, right over against the fore-mentioned cut; this he does as before, taking care that both cuts are in the same line. In the mean time, the man who looks after the carquaisse comes to receive the glass upon an iron shovel two feet and a half long without the handle, and two feet wide, with a small border of an inch and a half to the right and left, and towards the handle of the shovel. Upon this the glass is laid, flattening it a little with a small stick a foot and a half long, so that the cut of the glass is turned upwards. They separate the glass from the pontil, by striking a few gentle blows between the two with a chisel. The glass is then removed to the mouth of the hot carquaisse, where it becomes red hot gradually; the workman, with an iron tool six feet long, and widened at the end in form of a club at cards four inches long, and two inches wide on each side, very flat, and not half an inch thick, gradually lifts up the cut part of the glass to unfold it out of its form of a flattened cylinder, and render it smooth, by turning it down upon the hearth of the carquaisse. The tool already described being insinuated within the cylinder, performs this operation by being pushed hard against all the parts of the glass. When the glass is thus made quite smooth, it is pushed to the bottom of the

carquaisse or annealing furnace with a small iron raker, and ranged there with a little iron hook. When the carquaisse is full, it is stopped and cemented as in the case of run glasses, and the glass remains there for a fortnight to be annealed; after which time they are taken out to be polished. A workman can make but one glass in an hour, and he works and rests for six hours alternately.

Such was the method formerly made use of for blowing plate glass, looking glasses, &c.; but the workmen, by this method, could never exceed 50 inches in length, and a proportional breadth, because what were larger were always found to warp, which prevented them from reflecting the objects regularly, and wanted substance to bear the necessary grinding. These imperfections have been remedied by the following invention of the Sieur Abraham Thevart, in France, about the year 1688.

3. Casting or Running of Large Mirror Glass Plates. The furnace is of a very large dimension, environed with several ovens, or annealing furnaces, called carquaisse, besides others for making of frit and calcining old pieces of glass. This furnace, before it is fit to run glass, costs 3500l. It seldom lasts above three years, and even in that time it must be refitted every six months. It takes six months to rebuild it, and three months to refit it. The melting pots are as big as large hogsheads, and contain about 2000 weight of metal. If one of them bursts in the furnace, the loss of the matter and time amounts to 250l. The materials in these pots are the same as described before. When the furnace is red hot, these materials are put in at three different times, because that helps the fusion; and in 24 hours they are vitrified, refined, settled, and fit for casting. A is the bocca, or mouth of the furnace; B is the cistern that conveys the liquid glass it receives out of the melting pots in the furnace to the casting table. These cisterns are filled in the furnace, and remain therein six hours after they are filled; and then are hooked out by the means of a large iron chain, guided by a pulley, placed upon a carriage with four wheels marked C, by two men. This carriage has no middle piece; so that when it has brought the cistern to the casting table D, they slip off the bottom of the cistern, and out rushes a torrent of flaming matter upon the table: this matter is confined to certain dimensions by the iron rulers F E, which are moveable, retain the fluid matter, and determine the width of the glass; while a man, with the roller F resting on the edge of the iron rulers, reduceth it as it cools to an equal thickness, which is done in the space of a minute. This table is supported on a wooden frame, with trustles for the convenience of moving to the annealing furnace; into which, strewed with sand, the new plate is shoved, where it will harden in about 10 days.

What is most surprising throughout the whole of this operation, is the quickness and address wherewith such massy cisterns, filled with a flaming matter, are taken out of the furnace, conveyed to the table, and poured therein, the glass spread, &c. The whole is inconceivable to such as have not been eye witnesses of that surprising manufacture.

As fast as the cisterns are emptied, they carry them back to the furnace and take fresh ones, which they empty as before. Thus they continue to do so long as

there are any full cisterns; laying as many plates in each carquaisse as it will hold, and stopping them up with doors of baked earth, and every chink with cement, as soon as they are full, to let them anneal, and cool again, which requires about 14 days.

The first running being dispatched, they prepare another, by filling the cisterns anew from the matter in the pots; and after the second, a third; and even a fourth time, till the melting pots are quite empty.

The cisterns at each running should remain at least six hours in the furnace to whiten; and when the first annealing furnace is full, the casting table is to be carried to another. It need not here be observed, that the carquaisse, or annealing furnaces, must first have been heated to the degree proper for them. It may be observed, that the oven full, or the quantity of matter, commonly prepared, supplies the running of 18 glasses, which is performed in 18 hours, being an hour for each glass. The workmen work six hours, and are then relieved by others.

When the pots are emptied, they take them out, as well as the cisterns, to scrape off what glass remains, which otherwise would grow green by continuance of fire, and spoil the glasses. They are not filled again in less than 36 hours; so that they put the matter into the furnace, and begin to run it every 54 hours.

The manner of heating the large furnaces is very singular; the two tisors, or persons employed for that purpose, in their shirts, run swiftly round the furnace without making the least stop: as they run along, they take two billets, or pieces of wood, which are cut for the purpose: these they throw into the first tissant; and continuing their course, do the same for the second. This they hold without interruption for six hours successively; after which they are relieved by others, &c. It is surprising that two such small pieces of wood, and which are consumed in an instant, should keep the furnace to the proper degree of heat; which is such that a large bar of iron, laid at one of the mouths of the furnace, becomes red hot in less than half a minute.

The glass, when taken out of the melting furnace, needs nothing farther but to be ground, polished, and foliated.

4. Grinding and Polishing of Plate Glass. Glass is made transparent by fire; but it receives its lustre by the skill and labour of the grinder and polisher; the former of whom takes it rough out of the hands of the maker.

In order to grind plate glass, they lay it horizontally upon a flat stone table made of a very fine grained freestone; and for its greater security they plaster it down with lime or stucco; for otherwise the force of the workmen, or the motion of the wheel with which they grind it, would move it about.

This stone table is supported by a strong frame A, made of wood, with a ledge quite round its edges, rising about two inches higher than the glass. Upon this glass to be ground is laid another rough glass not above half so big, and so loose as to slide upon it; but cemented to a wooden plank, to guard it from the injury it must otherwise receive from the scraping of the wheel to which this plank is fastened, and from the weights laid upon it to promote the grinding or triture of the glasses. The whole is covered with a wheel B,

Glass. made of hard light wood, about six inches in diameter, by pulling of which backwards and forwards alternately, and sometimes turning it round, the workmen, who always stand opposite to each other, produce a constant attrition between the two glasses, and bring them to what degree of smoothness they please, by first pouring in water and coarse sand; after that, a finer sort of sand, as the work advanceth, till at last they must pour in the powder of smalt. As the upper or incumbent glass polishes and grows smoother, it must be taken away, and another from time to time put in its place.

This engine is called a mill by the artist, and is used only in the largest sized glasses; for in the grinding of the lesser glasses, they are content to work without a wheel, and to have only four wooden handles fastened to the four corners of the stone which loads the upper plank, by which they work it about.

When the grinder has done his part, who finds it very difficult to bring the glass to an exact plainness, it is turned over to the polisher; who, with the fine powder of tripoli stone or emery, brings it to a perfect evenness and lustre. The instrument made use of in this branch is a board, c c, furnished with a felt, and a small roller, which the workman moves by means of a double handle at both ends. The artist, in working this roller, is assisted with a wooden hoop or spring, to the end of which it is fixed: for the spring, by constantly bringing the roller back to the same points, facilitates the action of the workman's arm.