Furnaces for the making 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 FURNACE.
The calcar A (Plate XCVII. fig. 1.) resembles an oven ten feet long, seven broad, and two deep: the fuel, which in England 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 B, 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 C is properly called the crown,
and is made in that form. Its use is to keep a brisk fire of coal and wood, which is never put out. The mouth of it is called the bocca. There are several holes in the arch of this crown, through which the flame passes into the second vault, or partition, and reverberates into the pots filled with the ingredients above mentioned. Round the insides are eight or more pots placed, and piling pots on them. The number of pots is always double that of the boccas D, 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 workmens 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, is annealed, or cooled: this part consists of a tower, besides the leer F, 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 flowed.
But the green glass furnace is square; and at each angle it has an arch for annealling, 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.
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 workmens 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 tar-pentine. This he repeats four times, at each time rolling the end of his instrument, with the hot metal thereon, on a piece of iron G, 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 H, and blows gently through the iron tube, till the metal lengthens like a bladder about a foot. Then he rolls it on a marble stone I, a little while, to polish it, and blows a second time, by which he brings it to the shape of a globe of about eighteen or twenty 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 K, 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 it 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. And by this means we come to learn the cause why the edge of all bowls and glasses, &c. are thicker than the other parts of the same glasses; because in the turning it about in the heat, the edge thickens; and the glass being as it were doubled in that part, the circumference appears like a selvage.
If there remains any superfluities, they are cut off with the shears L; 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 separate; and now assays 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 dpt 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 pinchers, 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 twenty-four 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 fifty 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 G, 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 3500 l. 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 250 l. 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; and A is the man breaking the frit for that purpose. 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. H is the bocca, or mouth of the furnace; K 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 marked I, and placed upon a carriage with four wheels marked L, by two men P P. This carriage has no middle piece; so that when it has brought the cistern to the casting-table M, they slip off the bottom of the cistern, and out rushes a torrent of flaming matter O, upon the table: this matter is confined to certain dimensions by the iron rulers N, N, N, which are moveable, retain the fluid matter, and determine the width of the glass; while a man R, with the roller Q 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 ten days. After this, the glass needs only 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 stucco: for otherwise the force of the workmen, or the motion of the wheel with which they grind it, would move it about.
This stone-table is supported by a strong frame A, made of wood, with a ledge quite round its edges, rising about two inches higher than the glass. Upon this glass to be ground, is laid another rough glass not above half so big, and so loose as to slide upon it; but cemented
mented 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 weight laid upon it, to promote the grinding, or frisure, 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 word advanceth, till at last they must pour in the powder of smalt. As the upper or incumbent glass polishes, and grows smoother, it must be taken away, and another from time to time put in its place.
This engine is called a mill by the artists, and is used only in the largest size 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 care of 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.