GLASS, a transparent, brittle, factitious body, produced from sand melted in a strong fire with fixed alkaline salts, lead, flasks, &c. till the whole becomes perfectly clear and fine. The word is formed of the Latin gladium, a plaut called by the Greeks ifatis, by the Romans vitrum, by the ancient Britons guadam, and by the English wasad. 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 gladium, 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
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; tho' 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 fictitious 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. Emperor, 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, Aphroditeus, 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: Manifestius est (says Lactantius *), mentem esse, qua per oculos ea que sunt opposita, transpiciat, quasi per sensivas lucente vitro aut 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 annulets of glass, having a narrow perforation and thick rim, denominated by the remaining Britons gleina 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 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 Spainiards. 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, 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 Wernmouth. 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 of 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 glass-men 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.
2 Theory of vitrification 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 eva-
poration; 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 and most ponderous of them all) is dissolvable 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 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 3 the force with which glass balls strike each other be able reckoned 16, that wherewith they recede by virtue of 3 the 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 bells made of annealed 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. Lately, however, some 4 vessels made of such annealed 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.
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 every where 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 glass tubes placed in certain circumstances. When these are laid before a fire in an 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 plane, the fragment, for instance, of coach window-glass, instead 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.
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 makes 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
663.
found
Glas. 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 FLAMMONICA.
7. Glass is possessed of very great electrical virtues. See ELECTRICITY, passim.
Materials for Making of Glass. The materials whereof glass 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 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, and FUCUS.
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. See PEARL-ASHES.
There are other fluxes used for different kinds of glass, and for various purposes, as calcined lead, nitre,
sea-salt, borax, arsenic, smiths elinkers, 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 calc 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 calxes 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 alkalis 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 Ticino, 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 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 a 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-glasses 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.
Furnace 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 CCXX.
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 reverbe-
rates 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, about 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 farosel or room where the glasses are to be blown.
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 linen 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 of fuller's earth, or tobacco-pipe clay, of which earth they also make their melting-pots. In Britain the pots are made of Sturbridge 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 workmen's pots. A small iron-ladle, cased in the same manner, to skim the alkalie 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 tarfo, 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 bolito; set it in pots in the furnace, adding to it a due quantity of magnesia or manganese: when the two are fused, cast the flux into fair water, to clear it of the salt called sanderer; 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, tarfo, 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 flint 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 4 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 salt-petre, 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 facetious 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 boe-eas or apertures: at one of these called the great boe-ea, the furnace is heated, and the pots of frit are at this set in the furnace; two other smaller holes, called bo-earellas, 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 fix 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
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 flat 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 tens, 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 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, 30 pounds of unpurified pearl ashes, 10 pounds of common salt, 2 pounds of arsenic, and two 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.
Glas. 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 those of other works of glass, viz. an alkali as 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 Alicante 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.
Glas. 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 added 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 foder 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 carquasses 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 in 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 punchion 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 introducing the shears into the hole made with the punchion, 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 that 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 carquasses 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 carquass, 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 carquass. The tool already described being introduced 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 carquass or annealing furnace with a small iron raker, and ranged there with a little iron hook. When the carquass 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. Getting or Running of Large Mirror-Glass Plates. The furnace is of a very large dimension, environed with several ovens, or annealing furnaces, called carquasses, 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 rest it. The melting pots are as big as large hogheads, 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
Glaf. 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 glafs 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 pully, 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 EE, which are moveable, retain the fluid matter, and determine the width of the glafs; 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 trusses 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 glafs spread, &c. The whole is inconceivable to such as have not been eye-witnesses of that surprising manufacture.
As full as the cisterns are emptied, they carry them back to the furnace and take fresh ones, which they empty as before. This they continue to do so long as there are any full cisterns; laying as many plates in each carquaille 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 carquailles, 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 glafs. 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 glafs 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 tifers, 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 tiffart;
and continuing their course, do the same for the second. Thus 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 glafs, when taken out of the melting-furnace, needs nothing farther but to be ground, polished, and foliated.
4. Grinding and Polishing of Plate-Glass. Glafs 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-glafs, they lay it horizontally upon a flat stone table made of a very fine-grained free-stone; 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 glafs. Upon this glafs to be ground is laid another rough glafs 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, 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 glafs 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 artists, 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 glafs 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, 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 springs, by constantly bringing the roller back to the same points, facilitates the action of the workman's arm.
Colouring of Glass. That the colours given to glafs may have their full beauty, it must be observed, that every
every pot when new, and first used, leaves a foulness in the glass from its own earthy parts; so that a coloured glass made in a new pot can never be bright or perfectly fine. For this reason, the larger of these, when new, may be glazed with white glass; but the second time of using the pots lose this foulness. The glazing may be done by reducing the glass to powder, and moistening the inside of the pot with water; while it is yet moist, put in some of the powdered glass, and shake it about, till the whole inner surface of the pot be covered by as much as will adhere to it, in consequence of the moisture. Throw out the redundant part of the powdered glass; and the pot being dry, set it in a furnace sufficiently hot to vitrify the glass adhering to it, and let it continue there some time; after which, care must be taken to let it cool gradually. Those pots which have served for one colour must not be used for another; for the remainder of the old matter will spoil the colour of the new. The colours must be very carefully calcined to a proper degree; for if they are calcined either too much or too little, they never do well; the proper proportion, as to quantity, must also carefully be regarded, and the furnaces must be fed with dry hard wood. And all the processes succeed much the better if the colour be used dividedly, that is, a part of it in the frit, and the rest in the melted metal.
A hard glass, proper for receiving colours, may be prepared by pulverising 12 pounds of the best sand, cleansed by washing in a glass or flint mortar, and mixing seven pounds of pearl-ashes, or any fixed alkaline salt, purified with nitre, one pound of salt-petre, and half a pound of borax, and pounding them together. A glass less hard may be prepared of twelve pounds of white sand cleansed, seven pounds of pearl-ashes purified with salt-petre, one pound of nitre, half a pound of borax, and four ounces of arsenic, prepared as before.
Ametyst coloured. See Purple below, and the article AMETYST.
Balas-colour. Put into a pot crystal frit, thrice washed in water; tinge this with manganese, prepared into a clear purple; to this add alumina caesium, sifted fine, in small quantities, and at several times: this will make the glass grow yellowish, and a little reddish, but not blackish, and always dissipates the manganese. The last time you add manganese give no more of the alumina caesium, unless the colour be too full. Thus will the glass be exactly of the colour of the balas-ruby. See Ruby Glass.
The common black colour. The glass makers take old broken glass of different colours, grind it to powder, and add to it, by different parcels, a sufficient quantity of a mixture of two parts zaffer and one part manganese: when well purified, they work it into vessels, &c.
Glass beads are coloured with manganese only.
Black velvet colour. To give this deep and fine colour to glass, take of crystalline and pulverine frit, of each 20 pounds; of calx of lead and tin four pounds; set all together in a pot in the furnace, well heated; when the glass is formed and pure, take steel well calcined and powdered, scales of iron that fly off from the smith's anvil, of each an equal quantity; powder and mix them well; then put six ounces of this powder to
the above described metal while in fusion; mix the whole thoroughly together, and let them all boil strongly together; then let it stand in fusion 12 hours to purify, and after this work it. It will be a most elegant velvet black. Glas.
There is another way of doing this, which also produces a very fair black. It is this: take a hundred weight of rochetta frit, add to this two pounds of tartar and six pounds of manganese, both in fine powder; mix them well, and put them to the metal while in fusion, at different times, in several parcels; let it stand in fusion after this for four days, and then work it.
A glass perfectly black may also be formed to ten pounds of either of the compositions for hard glass above described, one ounce of zaffer, six drams of manganese, and an equal quantity of iron strongly calcined.
Blue colour. A full blue may be made by adding six drams of zaffer and two drams of manganese to ten pounds of either of the compositions for hard glass, described above. For a very cool or pure blue glass, half an ounce of calcined copper may be used instead of the manganese, and the proportion of zaffer diminished by one half. Glass resembling sapphire may be made with ten pounds of either of the compositions for hard glass, three drams and one scruple of zaffer, and one dram of the calx cassi, or precipitation of gold by tin; or instead of this latter ingredient, two drams and two scruples of manganese. Or a sapphire-coloured glass may be made by mixing with any quantity of the hard glass one eighth of its weight of small. For a beautiful blue glass produced from the calx of regulus of cobalt, see CHEMISTRY, n° 1299.
Venetian brown, with gold spangles, commonly called the philosopher's stone, may be prepared in the following manner: take of the second composition for hard glass above described, and of the composition for palte, of each five pounds, and of highly calcined iron an ounce; mix them well, and fuse them till the iron be perfectly vitrified, and has tinged the glass of a deep transparent yellow brown colour. Powder this glass, and add to it two pounds of powdered glass of antimony; grind them together, and thus mix them well. Take part of this mixture, and rub into it 80 or 100 leaves of the counterfeit leaf gold called Dutch gold; and when the parts of the gold seem sufficiently divided, mix the powder containing it with the other part of the glass. Fuse the whole with a moderate heat till the powder run into a vitreous mass, fit to be wrought into any of the figures or vessels into which it is usually formed; but avoid a perfect liquefaction, because that in a short time destroys the equal diffusion of the spangles, and vitrifies, at least in part, the matter of which they are composed; converting the whole into a kind of transparent olive-coloured glass. This kind of glass is used for a great variety of toys and ornaments with us, who at present procure it from the Venetians.
Chalcedony. A mixture of several ingredients with the common matter of glass, will make it represent the semi-opake gems, the jaspers, agates, chalcedonies, &c. The way of making these seems to be the same with the method of making marbled paper, by several colours dissolved in several liquors, which are such as will
Glas. will not readily mix with one another when put into water, before they are cast upon the paper which is to be coloured. There are several ways of making these variously coloured glasses, but the best is the following.
Dissolve four ounces of fine leaf silver in a glass vessel in strong aquafortis; stop up the vessel, and set it aside.—In another vessel, dissolve five ounces of quicksilver in a pound of aquafortis, and set this aside.—In another glass vessel, dissolve in a pound of aquafortis three ounces of fine silver, first calcined in this manner: amalgamate the silver with mercury, mix the amalgam with twice its weight of common salt well purified, put the mixture in an open fire in a crucible, that the mercury may fly off, and the silver be left in form of powder. Mix this powder with an equal quantity of common salt well purified, and calcine this for six hours in a strong fire; when cold, wash off the salt by repeated boilings in common water, and then put the silver into the aquafortis. Set this solution also aside.—In another vessel, dissolve in a pound of aquafortis three ounces of sal ammoniac; pour off the solution, and dissolve in it a quarter of an ounce of gold. Set this also aside.—In another vessel, dissolve three ounces of sal ammoniac in a pound of aquafortis; then put into the solution cinnabar, crocus martis, ultramarine, and ferretto of Spain, of each half an ounce. Set this also aside.—In another vessel, dissolve in a pound of aquafortis three ounces of sal ammoniac; then put into it crocus martis made with vinegar, calcined tin, zaffer, and cinnabar, of each half an ounce; let each of these be powdered very fine, and put gently into the aquafortis. Set this also aside.—In another vessel, dissolve three ounces of sal ammoniac in a pound of aquafortis, and add to it brass calcined with brimstone, brass thrice calcined, manganese, and scales of iron which fall from the smith's anvil, of each half an ounce; let each be well powdered, and put gently into the vessel. Then set this also aside.—In another vessel, dissolve two ounces of sal ammoniac in a pound of aquafortis, and put to it verdigrease an ounce, red lead, crude antimony, and the caput mortuum of vitriol, of each half an ounce; put these well powdered leisurely into the vessel, and set this also aside.—In another vessel, dissolve two ounces of sal ammoniac in a pound of aquafortis, and add orpiment, white arsenic, painters lake, of each half an ounce.
Keep the above nine vessels in a moderate heat for 15 days, shaking them well at times. After this pour all the matters from these vessels into one large vessel, well luted at its bottom; let this stand six days, shaking it at times; and then set it in a very gentle heat, and evaporate all the liquor, and there will remain a powder of a purplish green.
When this is to be wrought, put into a pot very clear metal, made of broken crystalline and white glass that has been used; for with the virgin-frit, or such as has never been wrought, the chalcidony can never be made, as the colours do not stick to it, but are consumed by the frit. To every pot of 20 pounds of this metal put two or three ounces of this powder at three several times; incorporate the powder well with the glass; and let it remain an hour between each time of putting in the powders. After all are in, let it stand 24 hours; then let the glass be well mixed, and
take an assay of it, which will be found of a yellowish blue; return this many times into the furnace; when it begins to grow cold, it will show many waves of different colours very beautifully. Then take tartar eight ounces, foot of the chimney two ounces, crocus martis made with brimstone, half an ounce; let these be well powdered and mixed, and put them by degrees into the glass at six times, waiting a little while between each putting in. When the whole is put in, let the glass boil and fettle for 24 hours; then make a little glass body of it; which put in the furnace many times, and see if the glass be enough, and whether it have on the outside veins of blue, green, red, yellow, and other colours, and have, beside these veins, waves like those of the chalcidonies, jaspers, and oriental agates, and if the body kept within looks as red as fire.
When it is found to answer thus, it is perfect, and may be worked into toys and vessels, which will always be beautifully variegated: these must be well annealed, which adds much to the beauty of their veins. Masses of this may be polished at the lapidary's wheel as natural stones, and appear very beautiful. If in the working the matter grow transparent, the work must be stopped, and more tartar, foot, and crocus martis must be put to it, which will give it again the necessary body and opacity, without which it does not show the colours well.
Chrysolite colour may be made of ten pounds of either of the compositions for hard glass described above, and six drams of calcined iron.
Red cornelian colour may be formed by adding one pound of glass of antimony, two ounces of the calcined vitriol called scarlet ochre, and one dram of manganese or magnesia, to two pounds of either of the compositions for hard glass. The glass of antimony and magnesia are first fused with the other glass, and then powdered and ground with the scarlet ochre: the whole mixture is afterwards fused with a gentle heat till all the ingredients are incorporated. A glass resembling the white cornelian may be made of two pounds of either of the compositions for hard glass, and two drams of yellow ochre well washed, and one ounce of calcined bones: grind them together, and fuse them with a gentle heat.
Emerald colour. See Green below.
Garnet colour. To give this colour to glass, the workmen take the following method. They take equal quantities of crystal and rochetta frit, and to every hundred weight of this mixture they add a pound of manganese and an ounce of prepared zaffer: these are to be powdered separately, then mixed and added by degrees to the frit while in the furnace. Great care is to be taken to mix the manganese and zaffer very perfectly; and when the matter has stood 24 hours in fusion, it may be worked.
Glass of this kind may be made by adding one pound of glass of antimony, one dram of manganese, and the same quantity of the precipitate of gold by tin, to two pounds of either of the compositions for hard glass; or the precipitate of gold may be omitted, if the quantities of the glass of antimony and manganese be doubled.
Gold colour. This colour may be produced by taking ten pounds of either of the compositions for hard glass,
glass, omitting the saltpetre; and for every pound adding an ounce of calcined borax, or, if this quantity doth not render the glass sufficiently fusible, two ounces; ten ounces of red tartar of the deepest colour; two ounces of magnesia; and two drams of charcoal of fallow, or any other soft kind. Precipitates of silver baked on glass will stain it yellow, and likewise give a yellow colour on being mixed and melted with 40 or 50 times their weight of vitreous compositions: the precipitate from aquafortis by fixed alkali seems to answer best. Yellow glasses may also be obtained with certain preparations of iron, particularly with Prussian blue. But Dr Lewis observes, that the colour does not constantly succeed, nor approach to the high colour of gold, with silver or with iron. The nearest imitations of gold which he has been able to produce have been effected with antimony and lead. Equal parts of the glass of antimony, of flint calcined and powdered, and of minium, formed a glass of a high yellow; and with two parts of glass of antimony, two of minium, and three of powdered flint, the colour approached still more to that of gold. The last composition exhibited a multitude of small sparkles interspersed throughout its whole substance, which gave it a beautiful appearance in the mass, but were really imperfections, owing to air-bubbles.
Neri directs, for a gold-yellow colour, one part of red tartar and the same quantity of manganese, to be mixed with a hundred parts of frit. But Kunckel observes, that these proportions are faulty; that one part, or one and a quarter, of manganese, is sufficient for a hundred of frit; but that six parts of tartar are hardly enough, unless the tartar is of a dark red colour, almost blackish; and that he found it expedient to add to the tartar about a fourth of its weight of powdered charcoal. He adds, that the glass swells up very much in melting, and that it must be left unstirred, and worked as it stands in fusion. Mr Samuel More, in repeating and varying this process in order to render the colour more perfect, found that the manganese is entirely unessential to the gold colour; and that the tartar is no otherwise of use than in virtue of the coaly matter to which it is in part reduced by the fire, the phlogiston or inflammable part of the coal appearing in several experiments to be the direct tinging substance. Mr Pot also observes, that common coals give a yellow colour to glass; that different coaly matters differ in their tinging power; that caput mortuum of foot and lamp-black answer better than common charcoal; and that the sparkling coal, which remains in the retort after the rectification of the thick empyreumatic animal oils, is one of the most active of these preparations. This preparation, he says, powdered, and then burnt again a little in a close vessel, is excellent for tinging glass, and gives yellow, brown, reddish, or blackish colours, according to its quantity; but the frit must not be very hard of fusion, for in this case the strong fire will destroy the colouring substance before the glass melts: and he has found the following compositions to be nearly the best; viz. sand two parts, alkali three parts; or sand two, alkali three, calcined borax one; or sand two, alkali two, calcined borax one: and though saltpetre is hardly used at all, or very sparingly, for yellow glasses, as it too much volatilizes the colouring substance; yet here for the most
part a certain proportion of it, easily determined by trial, is very necessary; for without it the concentrated colouring matter is apt to make the glass too dark, and even of an opaque pitchy blackness. It does not certainly appear that there is any material diversity in the effects of different coals, the difference being probably owing to the different quantities of the inflammable matter which they contain; so that a little more shall be required of one kind than of another for producing the same degree of colour in the glass. Nor does the softness or fusibility of the frit appear to be in any respect necessary.
Gold-coloured spangles may be diffused through the substance of glass, by mixing the yellow tals with powdered glass, and bringing the mixture into fusion.
Green. This colour may be imparted to glass by adding three ounces of copper precipitated from aquafortis, and two drams of precipitated iron to nine pounds of either of the compositions for hard glass. The finest method of giving this beautiful colour to glass is this: Take five pounds of crystalline metal that has been passed several times through water, and the same quantity of the common white metal of pulverine, four pounds of common pulverine frit, and three pounds of red lead; mix the red-lead well with the frit, and then put all into a pot in a furnace. In a few hours the whole mass will be well purified: then cast the whole into water, and separate and take out the lead; then return the metal into the pot, and let it stand a day longer in fusion; then put in the powder of the residuum of the vitriol of copper, and a very little crocus martis, there will be produced a most lively and elegant green, scarce inferior to that of the oriental emerald. There are many ways of giving a green to glass, but all are greatly inferior to this.—To make a sea-green, the finest crystalline glass only must be used, and no manganese must be added at first to the metal. The crystal frit must be melted thus alone; and the salt, which swims like oil on its top, must be taken off with an iron ladle very carefully. Then to a pot of twenty pound of this metal add six ounces of calcined brails, and a fourth part of the quantity of powdered zaffer: this powder must be well mixed, and put into the glass at three times; it will make the metal swell at first, and all must be thoroughly mixed in the pot. After it has stood in fusion three hours, take out a little for a proof: if it be too pale, add more of the powder. Twenty-four hours after the mixing the powder the whole will be ready to work; but must be well stirred together from the bottom, lest the colour should be deepest there, and the metal at the top less coloured, or even quite colourless. Some use for this purpose half crystal frit and half rochetta frit, but the colour is much the finest when all crystal frit is used.
Lapis lazuli colour. See Lapis Lazuli.
Opal colour. See Opal.
Purple of a deep and bright colour may be produced by adding to ten pounds of either of the compositions for hard glass, above described, six drams of zaffer and one dram of gold precipitated by tin; or to the same quantity of either composition one ounce of manganese and half an ounce of zaffer. The colour of amethyst may be imitated in this way.
Red. A blood-red glass may be made in the following
lowing manner: Put six pounds of glass of lead, and ten pounds of common glass, into a pot glazed with white glass. When the whole is boiled and refined, add by small quantities, and at small distances of time, copper calcined to a redness as much as on repeated proofs is found sufficient: then add tartar in powder by small quantities at a time, till the glass is become as red as blood; and continue adding one or other of the ingredients till the colour is quite perfect.
Ruby. The way to give the true fine red of the ruby, with a fair transparency, to glass, is as follows: Calcine in earthen vessels gold dissolved in aqua-regia; the menstruum being evaporated by distillation, more aqua-regia added, and the abstraction repeated five or six times, till it becomes a red powder. This operation will require many days in a hot furnace. When the powder is of a proper colour, take it out: and when it is to be used, melt the finest crystal glass, and purify it by often calling it into water; and then add, by small quantities, enough of this red powder to give it the true colour of a ruby, with an elegant and perfect transparency.
The process of tinging glass and enamels by preparations of gold was first attempted about the beginning of the last century. Libavius, in one of his tracts intitled Alchymia, printed in 1606, conjectures that the colour of the ruby proceeds from gold, and that gold dissolved and brought to redness might be made to communicate a like colour to factitious gems and glass. On this principle Neri, in his Art of Glass, dated in 1611, gives the process above recited. Glauber in 1648 published a method of producing a red colour by gold, in a matter which is of the vitreous kind, though not perfect glass. For this purpose he ground powdered flint or sand with four times its weight of fixed alkaline salt: this mixture melts in a moderately strong fire, and when cold looks like glass, but exposed to the air runs into a liquid state. On adding this liquor to solution of gold in aqua-regia, the gold and flint precipitate together in form of a yellow powder, which by calcination becomes purple. By mixing this powder with three or four times its weight of the alkaline solution of flint, drying the mixture, and melting it in a strong fire for an hour, a mass is obtained of a transparent ruby colour, and of a vitreous appearance; which nevertheless is soluble in water, or by the moisture of the air, on account of the redundancy of the salt. The honourable Mr Boyle, in a work published in 1680, mentions an experiment in which a like colour was introduced into glass without fusion; for having kept a mixture of gold and mercury in digestion for some months, the fire was at last moderately increased, so that the glass burst with a violent explosion; and the lower part of the glass was found tinged throughout of a transparent red colour, hardly to be equalled by that of rubies.
About the same time Cassius is said to have discovered the precipitation of gold by tin, and that glass might be tinged of a ruby colour by melting it with this precipitate; though he does not appear, says Dr Lewis, from his treatise De Auro, to have been the discoverer of either. He describes the preparation of the precipitate and its use; but gives no account of the manner of employing it, only that he says one
dram of gold duly prepared will tinge ten pounds of glass.
This process was soon after brought to perfection by Kunckel; who says, that one part of the precipitate is sufficient to give a ruby colour to 1280 parts of glass, and a sensible redness to upwards of 1900 parts; but that the success is by no means constant. Kunckel also mentions a purple-gold powder, resembling that of Neri; which he obtained by inspissating solution of gold to dryness; abstracting from it fresh aqua-regia three or four times, till the matter appears like oil; then precipitating with strong alkaline ley, and washing the precipitate with water. By dissolving this powder in spirit of salt and precipitating again, it becomes, he says, extremely fair; and in this state he directs it to be mixed with a due proportion of Venice glass.
Orschal, in a treatise intitled Sol sine Veste, gives the following process for producing a very fine ruby. He directs the purple precipitate made by tin to be ground with six times its quantity of Venice glass into a very fine powder, and this compound to be very carefully mingled with the frit or vitreous composition to be tinged. His frit consists of equal parts of borax, nitre, and fixed alkaline salt, and four times as much calcined flint as of each of the salts; but he gives no directions as to the proportion of the gold precipitate or mode of fusion. Hellot describes a preparation, which, mixed with Venice glass, was found to give a beautiful purple enamel. This preparation consists of equal parts of solution of gold and of solution of zinc in aqua-regia mixed together, with the addition of a volatile salt prepared from sal ammoniac by quicklime, in sufficient quantity to precipitate the two metals. The precipitate is then gradually heated till it acquires a violet colour. However, though a purple or red colour, approaching to that of ruby, may, by the methods above recited, be baked on glass or enamels, and introduced into the mass by fusion, the way of equally diffusing such a colour through a quantity of fluid glass is still, says Dr Lewis, a secret. The following process for making the ruby glass was communicated to Dr Lewis by an artist, who ascribed it to Kunckel. The gold is directed to be dissolved in a mixture of one part of spirit of salt and three of aquafortis, and the tin in a mixture of one part of the former of these acids with two of the latter. The solution of gold being properly diluted with water, the solution of tin is added, and the mixture left to stand till the purple matter has settled to the bottom. The colourless liquor is then poured off, and the purple sediment, while moist and not very thick, is thoroughly mixed with powdered flint or sand. This mixture is well ground with powdered nitre, tartar, borax, and arsenic, and the compound melted with a suitable fire. The proportions of the ingredients are 2560 parts of sand, 384 of nitre, 240 of tartar, 240 of borax, 28 of arsenic, 5 of tin, and 5 of gold.
Topaz colour. Glass resembling this stone may be made by pulverizing ten pounds of either of the compositions for hard glass with an equal quantity of the gold-coloured glass, and fusing them together.
White opake and semitransparent glass may be made of ten pounds of either of the compositions for hard glass and one pound of well calcined horn, ivory, or bone;
or an opaque whiteness may be given to glass by adding one pound of very white arsenic to ten pounds of flint glass. Let them be well powdered and mixed by grinding them together, and then fused with a moderate heat till they are thoroughly incorporated. A glass of this kind is made in large quantities at a manufacture near London; and used not only for different kinds of vessels, but as a white ground for enamel in dial-plates and snuff-boxes, which do not require finishing with much fire, because it becomes very white and fusible with a moderate heat.
Yellow. See Gold colour above.
Painting in GLASS. The ancient manner of painting in glass was very simple: it consisted in the mere arrangement of pieces of glass of different colours in some sort of symmetry, and constituted what is now called Mosaic work. See MOSAIC.
In process of time they came to attempt more regular designs, and also to represent figures heightened with all their shades: yet they proceeded no farther than the contours of the figures in black with water-colours, and hatching the draperies after the same manner on glasses of the colour of the object they designed to paint. For the carnation, they used glass of a bright red colour; and upon this they drew the principal lineaments of the face, &c. with black.
At length, the taste for this sort of painting improving considerably, and the art being found applicable to the adorning of churches, basilicas, &c. they found out means of incorporating the colours in the glass itself, by heating them in the fire to a proper degree; having first laid on the colours. A French painter at Marcellies is said to have given the first notion of this improvement, upon going to Rome under the pontificate of Julius II.; but Albert Durer and Lucas of Leyden were the first that carried it to any height.
This art, however, has frequently met with much interruption, and sometimes been almost totally lost; of which Mr Walpole gives us the following account, in his Anecdotes of Painting in England.
"The first interruption given to it was by the reformation, which banished the art out of churches; yet it was in some measure kept up in the equesterions of the nobility and gentry in the windows of their seats. Towards the end of queen Elizabeth's reign it was omitted even there; yet the practice did not entirely cease. The chapel of our Lady at Warwick was ornamented anew by Robert Dudley earl of Leicester, and his countess, and the cipher of the glass painter's name yet remains, with the date 1574: and in some of the chapels at Oxford the art again appears, dating itself in 1622, by the hand of no contemptible master.
"I could supply even this gap of 48 years by many dates on Flemish glass; but nobody ever supposed that the secret was lost so early as the reign of James I. and that it has not perished since will be evident from the following series, reaching to the present hour.
"The portraits in the windows of the library at All Souls, Oxford. In the chapel at Queen's College there are twelve windows dated 1518. P. C. a cipher on the painted glass in the chapel at Warwick, 1574. The windows at Wadham-college: the drawing pretty good, and the colours fine, by Bernard Van Linge, 1622. In the chapel at Lincoln's Inn, a window, with the name Bernard, 1623. This was
probably the preceding Van Linge. In the church of St Leonard, Shoreditch, two windows by Baptista Sutton, 1634. The windows in the chapel at University-college, Hen. Giles pint, 1687. At Christ-church, Isaac Oliver, aged 84, 1700. Window in Merton-chapel, William Price, 1700. Windows at Queen's New-college, and Maunlin, by William Price, the son, now living, whose colours are fine, whose drawing is good, and whose taste in ornaments and mosaic is far superior to any of his predecessors; is equal to the antique, to the good Italian masters, and only surpassed by his own singular modesty.
"It may not be unwelcome to the curious reader to see some anecdotes of the revival of taste for painted glass in England. Price, as we have said, was the only painter in that style for many years in England. Afterwards one Rowell, a plumber at Reading, did some things, particularly for the late Henry earl of Pembroke; but Rowell's colours soon vanished. At last he found out a very durable and beautiful red; but he died in a year or two, and the secret with him. A man at Birmingham began the same art in 1756 or 1757, and fitted up a window for Lord Lyttelton, in the church of Hagely; but soon broke. A little after him, one Peckitt at York began the same business, and has made good proficiency. A few lovers of that art collected some dispersed panes from ancient buildings, particularly the late Lord Cobham, who erected a Gothic temple at Stowe, and filled it with arms of the old nobility, &c. About the year 1753, one Asciotti, an Italian, who had married a Flemish woman, brought a parcel of painted glass from Flanders, and sold it for a few guineas to the honourable Mr Bateman, of Old Windsor. Upon that I sent Asciotti again to Flanders, who brought me 450 pieces, for which, including the expence of his journey, I paid him 36 guineas. His wife made more journeys for the same purpose; and sold her cargoes to one Palmer, a glazier in St Martin's-lane, who immediately raised the price to one, two, or five guineas for a single piece, and fitted up entire windows with them, and with mosaics of plain glass of different colours. In 1761, Paterson, an auctioneer at Essex-house in the Strand, exhibited the two first auctions of painted glass, imported in like manner from Flanders. All this manufacture consisted in rounds of scripture-stories, stained in black and yellow, or in small figures of black and white; birds and flowers in colours, and Flemish coats of arms."
The colours used in painting or staining of glass are very different from those used in painting either in water or oil colours.
For black, take scales of iron, one ounce; scales of copper, one ounce; jet, half an ounce: reduce them to powder, and mix them. For blue, take powder of blue, one pound; sal nitre, half a pound; mix them and grind them well together. For carnation, take red chalk, eight ounces; iron scales, and litharge of silver, of each two ounces; gum arabic, half an ounce; dissolve in water; grind all together for half an hour as stiff as you can; then put it in a glass and stir it well, and let it stand to settle fourteen days. For green, take red lead, one pound; scales of copper, one pound; and flint, five pounds: divide them into three parts; and add to them as much sal nitre as put
put them into a crucible, and melt them with a strong fire; and when it is cold, powder it, and grind it on a porphyry. For gold colour, take silver, an ounce; antimony, half an ounce; melt them in a crucible; then pound the mass to powder, and grind it on a copper plate; add to it yellow ochre, or brick-dust calcined again, fifteen ounces; and grind them well together with water. For purple, take minium, one pound; brown stone, one pound; white flint, five pounds: divide them into three parts, and add to them as much sal nitre as one of the parts; calcine, melt, and grind it as you did the green. For red, take jet, four ounces; litharge of silver, two ounces; red chalk, one ounce; powder them fine, and mix them. For white, take jet, two parts; white flint, ground on a glass very fine, one part; mix them. For yellow, take Spanish brown, ten parts; leaf-silver, one part; antimony, half a part; put all into a crucible, and calcine them well.
In the windows of ancient churches, &c. there are to be seen the most beautiful and vivid colours imaginable, which far exceed any of those used by the moderns, not so much because the secret of making those colours is entirely lost, as that the moderns will not go to the charge of them, nor be at the necessary pains, by reason that this sort of painting is not now so much in esteem as formerly. Those beautiful works which were made in the glass-houses were of two kinds.
In some, the colour was diffused through the whole substance of the glass. In others, which were the more common, the colour was only on one side, scarce penetrating within the substance above one-third of a line; though this was more or less according to the nature of the colour, the yellow being always found to enter the deepest. These last, though not so strong and beautiful as the former, were of more advantage to the workmen, by reason that on the same glass, though already coloured, they could show other kinds of colours where there was occasion to embroider draperies, enrich them with foliages, or represent other ornaments of gold, silver, &c.
In order to this, they made use of emery, grinding or wearing down the surface of the glass till such time as they were got through the colour to the clear glass. This done, they applied the proper colours on the other side of the glass. By these means, the new colours were hindered from running and mixing with the former, when they exposed the glasses to the fire, as will appear hereafter.
When indeed the ornaments were to appear white, the glass was only bared of its colour with emery, without tinging the place with any colour at all; and this was the manner by which they wrought their lights and heightenings on all kinds of colour.
The first thing to be done, in order to paint or stain glass, in the modern way, is to design, and even colour the whole subject on paper. Then they choose such pieces of glass as are clear, even, and smooth, and proper to receive the several parts; and proceed to distribute the design itself, or papers it is drawn on, into pieces suitable to those of the glass; always taking care that the glasses may join in the contours of the figures and the folds of the draperies; that the carnations, and other finer parts, may not be impaired by
the lead with which the pieces are to be joined together. The distribution being made, they mark all the glasses as well as papers, that they may be known again: which done, applying every part of the design upon the glass intended for it, they copy or transfer the design upon this glass with the black colour diluted in gum-water, by tracing and following all the lines and strokes as they appear through the glass with the point of a pencil.
When these strokes are well dried, which will happen in about two days, the work being only in black and white, they give a slight wash over with urine, gum arabic, and a little black; and repeat it several times, according as the shades are desired to be heightened; with this precaution, never to apply a new wash till the former is sufficiently dried.
This done, the lights and risings are given by rubbing off the colour in the respective places with a wooden point, or the handle of the pencil.
As to the other colours above mentioned, they are used with gum-water, much as in painting in miniature; taking care to apply them lightly, for fear of effacing the outlines of the design; or even, for the greater security, to apply them on the other side; especially yellow, which is very pernicious to the other colours, by blending therewith. And here too, as in pieces of black and white, particular regard must always be had not to lay colour on colour, or lay on a new lay, till such time as the former are well dried.
It may be added, that the yellow is the only colour that penetrates through the glass, and incorporates therewith by the fire; the rest, and particularly the blue, which is very difficult to use, remaining on the surface, or at least entering very little. When the painting of all the pieces is finished, they are carried to the furnace or oven to anneal or bake the colours.
The furnace here used is small, built of brick, from 18 to 30 inches square. At six inches from the bottom is an aperture to put in the fuel and maintain the fire. Over this aperture is a grate made of three square bars of iron, which traverse the furnace, and divide it into two parts. Two inches above this partition is another little aperture, through which they take out pieces to examine how the coction goes forward. On the grate is placed a square earthen pan, six or seven inches deep, and five or six inches less every way than the perimeter of the furnace. On the one side hereof is a little aperture, through which to make trials, placed directly opposite to that of the furnaces destined for the same end. In this pan are the pieces of glass to be placed in the following manner: First, the bottom of the pan is covered with three strata or layers of quicklime pulverised; those strata being separated by two others of old broken glass, the design whereof is to secure the painted glass from the too intense heat of the fire. This done, the glasses are laid horizontally on the last or uppermost layer of lime.
The first row of glass they cover over with a layer of the same powder an inch deep; and over this they lay another range of glasses, and thus alternately till the pan is quite full; taking care that the whole heap always end with a layer of the lime-powder.
The pan being thus prepared, they cover up the
furnace with tiles, on a square table of earthen ware, closely luted all round; only leaving five little apertures, one at each corner, and another in the middle, to serve as chimneys. Things thus disposed, there remains nothing but to give the fire to the work. The fire for the first two hours must be very moderate, and must be increased in proportion as the coction advances, for the space of ten or twelve hours; in which time it is usually completed. At last the fire, which at first was charcoal, is to be of dry wood, so that the flame covers the whole pan, and even issues out at the chimneys.
During the last hours, they make effays, from time to time, by taking out pieces laid for the purpose through the little aperture of the furnace and pan, to see whether the yellow be perfect, and the other colours in good order. When the annealing is thought sufficient, they proceed with great haste to extinguish the fire, which otherwise would soon burn the colours, and break the glasses.