GLASS, a transparent, brittle, facitious body, produced from sand melted in a strong fire with fixed alkaline salts, lead, flags, &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 Cæsar, 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 facitious matter we are speaking of came to be called glass; as having always something 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 facitious 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, Arilotle, 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 exegitaverat: 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 vitree camere, 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: Manifestius est (says Laetantius*), mentem esse, quæ per oculos ea quæ sunt opposita, De opif. transpiciat, quasi per fenestras lucente vitro aut speculari Dei, cap. 5. 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 annulets 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 annulets by the druids†. It can† See As scarcely be questioned that the Britons were sufficiently gaimen well versed in the manufacture of glass, to form out Oxen 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 annulets, 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
Glas. 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, appear 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 Wercemouth. 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 1670, 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 Theory of almost totally in the dark. In general, it seems to be vitrification uncertain. 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 stones. 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 has
Glafs has no other effect than either to melt or dissipate it in vapour.
The other properties of glafs are very remarkable, some of which follow:
3 Remarkable properties of glafs. 1. It is one of the most elastic bodies in nature. If the force with which glafs balls strike each other be reckoned 16, that wherewith they recede by virtue of their elasticity will be nearly 15.
4 Surprising fragility of unannealed glafs. 2. When glafs is suddenly cooled, it becomes exceedingly brittle; and this brittleness is sometimes attended with very surprising phenomena. Hollow balls made of unannealed glafs, 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 glafs 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 glafs. 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 glafs only from the height of three inches, in about two seconds the glafs 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.
5 Attempts to account for it. 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 glafs, 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 glafs extremely small, ought to cause an impression on the glafs 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 glafs, 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 glafs is filled; and that the motions of this fluid, when once excited in a particular part of the glafs, 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 glafs 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 glafs 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.
6 A most remarkable phenomenon also is produced in Rotation of glafs tubes placed in certain circumstances. When these glafs tubes are laid before a fire in a horizontal position, having before a 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 glafs pane, the fragment, for instance, of coach window-glass, in-
read
Glas. 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 475.
§ 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 equilibrium; and hence we must conclude, that the causes of these phenomena remain yet to be discovered.
4. Glas is less dilatable by heat than metalline substances, and solid glas 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 glas 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 glas 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. Glas appears to be more fit for the condensation of vapours than metalline substances. An open glas 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. Glas also becomes moist with dew, when metals do not. See DEW.
6. A drinking glas partly filled with water, and rubbed on the brim with a wet finger, yields musical notes, higher or lower as the glas is more or less full; and will make the liquor frisk and leap. See HARMONICA.
7. Glas is possessed of very great electrical virtues. See ELECTRICITY, passim.
8
Materials for Making of GLASS. The materials whereof glas is made, we have already mentioned to be salt and sand or stones.
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 glas: 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 glas, 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 glas and soap.
There are other fluxes used for different kinds of glas, 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 glas, so much the more fusible, soft, coloured, and dense this glas is, and reciprocally.
The colours given to glas by calces of lead, are shades of yellow: on the other hand, glases 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 glases containing both saline and metallic fluxes do also partake of the properties of both these substances. Glases 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 glas. See the article VITRIFICATION.
2. The sand or stone, called by the artists tarso, is the second ingredient in glas, 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 Tefino, 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
† See Sal-fola, Botany Index.
Glas. this rule. Flints are admirable; and when calcined, powdered, and scarred, 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 phlogiston which gives a strong tinge of yellow to glass prepared with lead as a flux, it serves to free it from this coloured tinge; 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 clearness 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. Glas.
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 frachies, 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 farosel or room where the glasses are to be flowed.
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 fort.
Glas. Gl. fort 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.