Red GLASS. A blood-red glàs may be made in the following manner: Put six pounds of glàs of lead, and ten pounds of common glàs, into a pot glazed with white glàs: 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 glàs is become as red as blood; and continue adding one or other of the ingredients till the colour is quite perfect.
A much finer red, however, may be communicated to glàs by means of gold, of which Dr Lewis gives the following account. "The tinging of glàs and enamels by preparations of gold appears to have been first attempted about the beginning of the last century. Libavius, whose works compose a valuable body of the chemical knowledge of his own time, conjectures, in one of his tracts entitled Alchymia, printed in 1606, 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 facitious gems or glàs. Neri, in his Art of Glàs dated 1611, gives a process on this principle, which he says was found to succeed: he directs the gold to be dissolved in aqua regia, the menstruum to be evaporated or drawn off by distillation, more aqua regia added, and the abstraction repeated five or six times: the remaining matter is to be calcined till it becomes purple, and then mixed with a proper quantity of the finest white or crystal glàs. But though this process may be supposed to have sometimes proved successful, it doubtless very often miscarried; inasmuch that the introduction of this desirable colour into the glàs was very little known for many years after.
"Glauber, in the second part of his Philosophical Furnaces published in 1648, gives another method of producing a red colour by gold in a matter which is of the vitreous kind, though not perfect glàs. When powdered flint or sand is well ground with four times its weight of fixt alkaline salt, the mixture melts in a moderately strong fire, and when cold looks like glàs, but on account of its over-proportion of alka-
line salt it runs into a liquid state on being exposed to the air: on adding this liquor to solution of gold in aqua regia, the acid, which held the gold dissolved, unites with the alkali which held the flint dissolved, and the gold and flint precipitate together in form of a yellow powder, which by calcination becomes purple: this powder being mixed with three or four times its weight of the alkaline solution of flint, the mixture dried, and kept melted in a strong fire for an hour, a mass is obtained, of a transparent ruby colour, and of a vitreous appearance; though still soluble in water, or by the moisture of the air, on account of the redundancy of salt.
"Boyle, in his treatise on the Porosity of Bodies, and in the appendix to his Sceptical Chemist published in 1680, mentions an experiment, in which a like colour was introduced into glàs without fusion. A mixture of gold and mercury having been kept in digestion for some months, the fire was at last immoderately increased, inasmuch that the glàs burst with a violent explosion: the lower part of the glàs was found tinged throughout of a transparent red colour, which seemed, he says, to emulate that of a not common ruby.
"About the same time Cassius discovered the precipitation of gold by tin, and that glàs might be tinged of a ruby colour by melting it with this precipitate. I can give no further account of his experiments, having never had the good fortune to meet with his treatise.
"The process was soon after brought to perfection by Kunckel, who says he prepared the ruby glàs in large quantity, and sold it for about forty shillings an ounce; and that he made a chalice of it for the elector of Cologne, weighing no less than 24 pounds, a full inch thick, and of an uniform fine colour throughout. He has nowhere communicated the process he followed, but some useful observations relating to it are dispersed through his writings: he says, that one part of the precipitate by tin is sufficient to give a ruby colour to twelve hundred and eighty parts of glàs, and a sensible redness to upwards of nineteen hundred parts: that the success is by no means constant, and that, after long practice, he still frequently failed: that oftentimes the glàs comes out of the fire colourless as crystal, and receives its ruby colour on being afterwards exposed to a smoky flame, inasmuch that he imagines the discovery of the ruby glàs did not arise from simply melting the gold precipitate with glàs, but from the subsequent softening and working of the glàs in the flame of a lamp, in the use of which Cassius was very conversant: that the addition of nitre and sal ammoniac calls forth the colour, and that the colour produced by sal ammoniac is more beautiful than that by nitre, but quickly disappears on a continuance of the fire.
"Orschal, in a treatise entitled sol fine veste, gives a process, by which he says he obtained a very fine ruby. He directs the purple precipitate, made by tin, to be ground with six times its quantity of Venice glàs in a very fine powder, and this compound to be exquisitely mingled with the flint or vitreous composition to be tinged: his flint consists of equal parts of borax, nitre, and fixt alkaline salt, and four times as much calcined flint as of each of the salts; but in
what proportion the gold precipitate is to be mixed with the fritt, and in what manner the fusion is to be performed, he does not mention. He reports that he had found the muddy matter, obtained in polishing gold by a pumice stone, to impart likewise a ruby colour to glass.
"Grummet, who had been operator to Kunckel in making the red glass, published a tract in opposition both to him and Orschal, under the title of Sol non fine vesse; in which he observes, that the furnace ought to be so constructed, that the operator may have full liberty of examining the glass in the fire, and of removing it as soon as it appears to have acquired the proper colour: he says the enamellers obtain a ruby colour, by melting, with a large proportion of Venice glass, the brownish powder precipitated from solution of gold in aqua regia by fixt alkaline salts. But he imagines that the gold is nowise concerned in the production of the colour. Venice glass, and most of the finer colourless kinds of glass, have an addition of manganese, without which it would be very difficult to render them perfectly void of colour: the manganese communicates at first a purplish hue, which on continuing the fire disappears, and at the same time suppresses or discharges any other tinge that the glass may be impregnated with: the addition of a little nitre revives the purplish colour of the manganese; and Grummet is of opinion that the colour with which glass becomes tinged, by the admixture of preparations of gold, is no other than that of the manganese extricated by the nitrous salt which the gold has retained in its precipitation. He affirms, that the same purplish red colour will be obtained on melting Venice glass with an eighth part of nitre, without any gold; that in a hundred repetitions of this experiment, it scarcely fails once; and that neither nitre nor the gold-precipitate were found to give any thing of the admired colour to those kinds of glass which have no manganese in their composition.
"The colours which manganese imparts to glass, it belongs not to this place to examine: but that precipitates of gold will communicate, in certain circumstances, a purplish red colour, I have several times experienced; having myself tinged of this colour fritts composed of calcined flint, nitre and borax, without the addition of manganese or of glasses containing it. Though gold, dissolved in common aqua regia, exhibits its own yellow colour; yet, when the menstruum is separated by fire to a certain point, or when the gold is precipitated by tin, or when it is precipitated by alkaline salts and afterwards moderately heated, or when gold is barely divided by mechanical means into subtile powder, and exposed for some time, in mixture with earthy bodies, to a slight heat, it assumes, in different circumstances, a violet colour, a purple, or a red verging to purple: in a strong fire, these colours vanish, and the gold melts into a mass of its original appearance. All these colours I have introduced into glass by preparations of gold; and I have found them to be nearly as perishable in the fire when the coloured gold-powder was thus diffused through the glass, as when exposed to the fire by itself: when the fire was raised to any great degree, and the glass made to flow thin, there was generally a button of revived gold collected at the bottom.
"A solution of gold in aqua regia being inspissated to dryness in the bottom of a Florence flask, and the heat further increased till the gold resumed its proper colour, the lower part of the glass was by this simple process tinged purplish: pieces of it being exposed to the flame of a lamp, they became in some parts violet coloured, in some of a bright purple, and in others purplish red; and the parts which in one position looked violet or purplish, in another appeared red.
"A colour nearly of the same kind is impressed on glass by gold-leaf in some electrical experiments; a fact which we are obliged to Mr Franklin for the first knowledge of. A narrow strip of gold-leaf being placed between two slips of glass, with both the ends hanging out a little, and the glass well tied round with silk thread, a strong electrical explosion is made to pass through the gold-leaf. On examining the glass, the gold-leaf, he observes, will be found missing in several places, and instead of it a reddish stain on both the glasses, exactly similar on both in the minutest stroke, though sometimes spread a little wider than the breadth of the leaf: the stain appears to have penetrated into the substance of the glass, so as to be protected by it from the action of aqua regia. I have had this experiment several times repeated with plate-glass; and found it tinged, as above described, in some parts violet, in some purplish, and in some reddish: the colours could not be scraped off, and resisted aqua regia and spirit of salt. If the electric explosion is made very strong, the glass commonly flies in pieces, with such force, that it is necessary for the operator to have his face screened from them.
"The preparation of gold which has been principally recommended for tinging glass is Cassius's precipitate by solution of tin. To obtain this precipitate of the due colour, a good deal of care is necessary both in dissolving the tin and in diluting the solutions. A mixture of two parts of aqua fortis and one of spirit of salt, is supposed to be the best menstruum for the tin: into this mixture some fine block-tin, granulated, is to be let fall, grain by grain, waiting till one grain is dissolved before another is dropped in, that the dissolution may go on slowly, without any heat or discharge of fumes. The gold is dissolved in common aqua regia; and a few drops of this solution being mixed with some ounces of pure water, as many drops of the solution of tin are added. If the mixture changes immediately to a clear bright purplish red colour, the due degree of dilution has been hit; if the colour appears dull, a greater quantity of water must be added for the rest of the solutions. After the mixture has deposited its red matter, and become clear, a little more of the tin-solution is to be dropped in, for discovering, and precipitating, any gold that may still remain in it: the liquor being then poured off, the precipitate is washed and dried.
"Kunckel mentions another purple gold-powder, made nearly like that of Neri already mentioned, by inspissating solution of gold to dryness, abstracting from it fresh aqua regia three or four times till the matter looks almost 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
Glas. ly fair, and in this state he directs it to be mixed with a due proportion of Venice glass.
“Hellot describes a preparation which in mixture with Venice glass was found to give a beautiful purple enamel. Equal parts of solution of gold, and of solution of zinc in aqua regia, are mixed together; and a volatile spirit, prepared from sal ammoniac by quicklime, added to the mixture in sufficient quantity to precipitate the two metals. The precipitate is to be gradually heated, till it acquires a violet colour: it does not fulminate, making only a slight dull decrepitation without any of its particles flying about.
“Though a purple, or a red colour approaching to that of the ruby, may by the foregoing means be baked upon 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 a secret.
“I was once, many years ago, fortunate enough to succeed, at a glass-house, in a small pot of glass, of which a salver was blown of a fine ruby red: the tinging matter was the precipitate of gold by tin; the particulars of the process cannot now be recollected. I have since tried the remainder of the same preparation, with common flint glass, with green glass, with various fritts composed of flint, borax, pure fixt alkaline salt, nitre, sal ammoniac. When flint was used, it was several times made red-hot, and quenched in water, to render it more easily pulverizable: both the flint and glasses were powdered in an iron mortar, and the powders well washed with diluted oil of vitriol, to extract such particles of iron as they might have worn off in the trituration; the gold precipitate was ground with the other ingredients, in agate or glass mortars; its proportion was varied from an eighth part to an eight hundredth of the vitreous materials; and the fire was continued, in a wind furnace, from six to thirty hours. All the glasses came out considerably coloured; some of a deep dusky yellow; some of a fine pale transparent yellow; some of a brown colour, greatly resembling that which the glass mentioned below acquired under a muffle; some appeared yellowish or brownish when looked down upon, and of a purple-violet or reddish purple when held between the eye and the light: some had specks and veins of a fine red; no one was either red or purple throughout. Several of these glasses were melted again and again, by themselves, and with the addition of more vitreous matter: some were worked in the flame of a lamp: some were laid in a mixture of powdered charcoal and foot, and made red-hot in a close crucible; and others being laid in the same manner, the fire was increased till they melted. The colours were by these means altered; but did not become uniform, or more approaching to the ruby colour than before: some pieces, which had at first very considerable specks of a ruby lustre, lost them on a repetition of the fusion.
“At the same time that these experiments were tried, the same kinds of vitreous compositions, mixed with different metallic preparations, were exposed to the fire in different parts of the same furnace, and were all found to receive beautiful and uniform colours. To what cause the miscarriage of those with gold was owing; whether the success, in regard to this metal,
is influenced by the quantity of the matter, by the unsteadiness of the heat in a small furnace, by the fusibility of the vitreous composition, by the metallic matter being ground with the ingredients before their exposure to the fire, or added to them in fusion, by the continuance of the fire, by the fluid matter being kept unmoved or stirred with an iron rod, by the crucible being covered or open, or other like circumstances; or whether the admixture of a little manganese, though gold will certainly give a ruby colour without it, does not contribute to secure the success; I have not yet discovered. The proportion of the gold precipitate to the vitreous matter is perhaps of principal importance. Solution of gold, as we have seen already, produces no redness with tin unless diluted with a very large quantity of water, in which circumstance the whole mixture acquires that beautiful colour which we here want to transfer from the watery fluid into fluid glass. It should seem, therefore, that the quantity of gold precipitate, for communicating the admired colour to a certain volume of glass, ought to be the same with that which communicated a like colour to an equal volume of water in the precipitation: a quantity extremely minute, and much less than that employed in any of my experiments.
“I have lately been favoured with some pieces of glass, in greatest part colourless, with one or two large red spots, several small streaks of violet, and some of a light brownish yellow. The person from whom I received them informs me, “that he had found, that in a heat not very strong, under a muffle, the glass becomes of an opaque brown, and, “if then polished, appears variegated like a fine “pebble.” I exposed a colourless piece to the flame of a lamp, impelled by a blow-pipe; and on working it about, sometimes in the smoke, and sometimes in the flame, found it change to a true ruby-red, perfectly transparent, and free from veins of any other colour. Another piece, kept for two hours under a close muffle, in such a heat as made it just soft enough to bend and receive an impression, became on the surface green; brown, and pale yellow in different parts, greatly resembling the coat of some pebbles: in this state, looked through against the sun, it appeared of a beautiful ruby-colour, and on breaking it, the internal part was found throughout of an uniform dark-red when looked down upon, and of the ruby-red when placed between the eye and the light. A large piece being continued under the muffle for four hours, its figure was found scarcely altered, the coat was much thicker, and beautifully veined with various colours, which were all lost in a glorious red when the piece was viewed between the light.
“All I have been able to learn in regard to the preparation of this glass is, that the quantity made at once is about six cwt.; that the tinging matter is mixed with the vitreous materials before they are put into the melting-pot, the mixture being brought to the glass-house in subs; that the matter is not stirred in fusion; and that it is kept no longer in the fire than is necessary for perfecting the glass, which, as soon as fine, is cast into a kind of bricks. Some imagine that this glass has no mixture of calx of lead, of which a large proportion is used in the composition of the-
the common flint-glass, and that the principal vitreifying ingredient is nitre: others judge it to be composed of the same materials as the common sort; its weight seeming to be a proof of its containing lead; for it is found to be nearly of the same specific gravity with flint-glass, which is greater than that of the glasses made without lead in the proportion of above six to five. This point we have determined in a more satisfactory manner: 400 grains of the glass, made red-hot and quenched in water, were reduced into powder, and mingled with about twice as much black flux and a little alkaline salt: the mixture being melted in a crucible, and the vessel suffered to cool, a lump of metal was found at the bottom, weighing 90 grains. The metal appeared to be somewhat stiffer than pure lead, and experiments convinced me that it contained some tin and a little gold."—The same author, however, afterwards informs us, that he has reason to believe, that his experiments would have succeeded better if the solution of gold, instead of its precipitate, had been mixed with the materials.
Yellow Glass. It is necessary to remark in glass-making, that the crystal glass made with salt that has an admixture of tartar will never receive the true gold yellow, though it will all other colours: for yellow glass, therefore, a salt must be prepared from pulverine, or pot-ashes alone, to make the glass.
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 CXL. fig. 1.
The calcar resembles an oven ten feet long, seven feet broad, and two deep: the fuel, which in Britain is sea-coal, is put into a trench on one side of the furnace; and the flame reverberating from the roof upon the frit calcines it. The glass-furnace, or working furnace, is round, of three yards diameter, and two high; or thus proportioned. It is divided into three parts, each of which is vaulted. The lower part is properly called the crown, and is made in that form. Its use is to keep a brisk fire, which is never put out. The mouth is called the bocca. There are several holes in the arch of this crown, through which the flame passes into the second vault or partition, and reverberates into the parts 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. Thro' 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, thro' the leer, to cool by degrees; so that they are quite cold by the time they reach the mouth of the leer, which enters the farosel, or room where the glasses are to be flowed.
But the green glass-furnace is square; and at each angle it has an arch for annealing or cooling glasses. The metal is wrought on two opposite sides, and on the other two they have their colours, into which are made linnet holes, for the fire to come from the furnace to bake the frit, and to discharge the smoke. Fires are made in the arches to anneal the work, so that the whole process is done in one furnace.
These furnaces must not be of brick, but of hard sandy stones. In France, they build the outside of brick; and the inner part, to bear the fire, is made of a 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 alkaline salt that swims at top. Shovels, one like a peel, to take up the great glasses; another, like a fire-shovel, to feed the furnace with coals. A hooked iron fork, to stir the matter in the pots. An iron rake for the same purpose, and to stir the frit. An iron fork, to change or pull the pots out of the furnace, &c.
Working or Blowing Round Glass. The tools thus provided, 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 thro' 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
Glas. 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.
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.
This 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 an invention of the Sieur Abraham Thevart, in France, about the year 1688, of casting or running large plates of glass in the following manner.
Casting or Running of Large Looking-Glass Plates. The furnace (fig. 2.) 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 refit 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 heat of this furnace is so intense, that a bar of iron laid at the mouth thereof becomes red hot in less than half a minute. The materials in these pots are the same as described before. When the furnace is red-hot, these materials are put in at three different times, because that helps the fusion; and in twenty-four hours they are vitrified, refined, settled, and fit for casting. A is the bocca, or mouth of the furnace; B is the cistern that conveys the liquid glass it receives out of the melting-pots in the furnace to the casting table. These cisterns are filled in the furnace, and remain therein six hours after they are filled; and then are hooked out by the means of a large iron chain, guided by a pulley, placed upon a carriage with four wheels marked C, by two men. This carriage has no middle piece; so that when it has brought the cistern to the casting-table D, they slip off the bottom of the cistern, and out rushes a torrent of flaming matter upon the table: this matter is confined to certain dimensions by the iron rulers EE, which are moveable, retain the fluid matter, and determine the width of the glass; while a man, with the roller F resting on the edge of the iron rulers, reduceth it as it cools, to an equal thickness, which is done in the space of a minute. This table is supported on a wood-
Glas. en 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 ten days. After this, the glass needs only to be ground, polished, and foliated for use.
Grinding and Polishing of Plate-Glass. Glass is made transparent by fire; but it receives its lustre by the skill and labour of the grinder and polisher, the former of whom takes it rough out of the hands of the maker.
In order to grind plate-glass, they lay it horizontally upon a flat stone table (fig. 3.) made of a very fine-grained free-stone; and for its greater security they plaster it down with lime or flucco; for otherwise the force of the workmen, or the motion of the wheel with which they grind it, would move it about.
This stone table is supported by a strong frame A, made of wood, with a ledge quite round its edges, rising about two inches higher than the glass. Upon this glass to be ground, is laid another rough glass not above half so big, and so loose as to slide upon it; but cemented to a wooden plank, to guard it from the injury it must otherwise receive from the scraping of the wheel to which this plank is fastened, and from the weights laid upon it to promote the grinding or triture of the glasses. The whole is covered with a wheel, B, 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 advance, till at last they must pour in the powder of smalt. As the upper or incumbent glass polishes and grows smoother, it must be taken away, and another from time to time put in its place.
This engine is called a mill by the 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 glass to an exact plainness, it is turned over to the polisher; who, with the fine powder of tripoli-stone, or emery, brings it to a perfect evenness and lustre. The instrument made use of in this branch is a board, c. c., furnished with a felt, and a small roller, which the workman moves by means of a double handle at both ends. The artist in working this roller, is assisted with a wooden hoop or spring, to the end of which it is fixed: for the spring, by constantly bringing the roller back to the same points, facilitates the action of the workman's arm.
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.
But in time, 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.
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 escheutheons 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 of 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 pinxit, 1637. At Christ-church, Isaac Oliver, aged 84, 1700. Window in Merton-chapel, William Price, 1700. Windows at Queen's New-college, and Maudlin, 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 Hagley; but soon broke. A little after him,
Glas. 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 Afciootti, 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 Afciootti 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, Paterfon, 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; 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 oker, 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, scarcely 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 shew other kind 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 heightnings, 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,
low, 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 glasse, 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 glasse 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 glasse, the design whereof is to secure the painted glasse 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 glasse 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 compleated. 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 essays, 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.
GLASS of Antimony. See CHEMISTRY, n° 454.
GLASS of Lead. See GLAZING.