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 tents, with an iron shiver between, which diminishes the weight by dividing it, and keeps the tables flat and even.
Of window or table glass there are various sorts, made in different places, for the use of building. Those most known among us are given us by the author of the Builder's Dictionary, as follows:
1. Crown, of which, says Neri, there are two kinds, distinguished by the places where they are wrought; viz. Ratcliff crown glass, which is the best and clearest, and was first made at the Bear garden, on the Bankside, Southwark, but since at Ratcliff: of this there are 24 tables to the case, the tables being of a circular form, about three feet six inches in diameter. The other kind, or Lambeth crown glass, is of a darker colour than the former, and more inclining to green.
The best window or crown glass is made of white sand 60 pounds, of purified pearl ashes 30 pounds, of saltpetre
saltpetre 15 pounds, of borax one pound, and of arsenic half a pound. If the glass should prove yellow, magnesia must be added. A cheaper composition for window glass consists of 60 pounds of white sand, 25 pounds of unpurified pearl ashes, 10 pounds of common salt, 5 pounds of nitre, 2 pounds of arsenic, and one ounce and a half of magnesia. The common or green window glass is composed of 60 pounds of white sand, 30 pounds of unpurified pearl ashes, 10 pounds of common salt, 2 pounds of arsenic, and 2 ounces of magnesia. But a cheaper composition for this purpose consists of 120 pounds of the cheapest white sand, 30 pounds of unpurified pearl ashes, 60 pounds of wood ashes, well burnt and sifted, 20 pounds of common salt, and 5 pounds of arsenic.
2. French glass, called also Normandy glass, and formerly Lorraine glass, because made in those provinces. At present it is made wholly in the nine glass works; five whereof are in the forest of Lyons, four in the county of Eu; the last at Beaumont near Rouen. It is of a thinner kind than our crown glass; and when laid on a piece of white paper, appears of a dirtyish green colour. There are but 25 tables of this to the case.
3. German glass is of two kinds, the white and the green: the first is of a whitish colour, but is subject to those small curved streaks observed in our Newcastle glass, though free from the spots and blemishes thereof. The green, besides its colour, is liable to the same streaks as the white; but both of them are straighter and less warped than our Newcastle glass.
4. Dutch glass is not much unlike our Newcastle glass either in colour or price. It is frequently much warped like that, and the tables are but small.
5. Newcastle glass is that most used in England. It is of an ash colour, and much subject to specks, streaks, and other blemishes; and besides is frequently warped. Leybourn says, there are 45 tables to the case, each containing five superficial feet: some say there are but 35 tables, and six feet in each table.
6. Phial glass is a kind betwixt the flint glass and the common bottle or green glass. The best kind may be prepared with 120 pounds of white sand, 50 pounds of unpurified pearl ashes, 10 pounds of common salt, 5 pounds of arsenic, and 5 ounces of magnesia. The composition for green or common phial glass consists of 120 pounds of the cheapest white sand, 80 pounds of wood ashes well burnt and sifted, 20 pounds of pearl ashes, 15 pounds of common salt, and 1 pound of arsenic.
The common bottle or green is formed of sand of any kind fluxed by the ashes of burnt wood, or of any parts of vegetables; to which may be added the scoria or clinkers of forges. When the softest sand is used, 200 pounds of wood ashes will suffice for 100 pounds of sand, which are to be ground and mixed together. The composition with the clinkers consists of 170 pounds of wood ashes, 100 pounds of sand, and 50 pounds of clinkers or scoria, which are to be ground and mixed together. If the clinkers cannot be ground, they must be broke into small pieces, and mixed with the other matter without any grinding.
III. Working of Plate or Mirror GLASS. 1. The materials of which this glass is made are much the
same as those of other works of glass, viz. an alkali, salt and sand.
The salt, however, should not be that extracted from pulverine or the ashes of the Syrian kali, but that from BARILLA, growing about Alicant in Spain. It is very rare that we can have the barilla pure; the Spaniards in burning the herb make a practice of mixing another herb along with it, which alters its quality; or of adding sand to it to increase the weight, which is easily discovered if the addition be only made after the boiling of the ashes, but next to impossible if made in the boiling. It is from this adulteration that those threads and other defects in plate glass arise. To prepare the salt, they clean it well of all foreign matters; pound or grind it with a kind of mill, and finally sift it pretty fine.
Pearl ashes, properly purified, will furnish the alkali salt requisite for this purpose; but it will be necessary to add borax or common salt, in order to facilitate the fusion, and prevent the glass from stiffening in that degree of heat in which it is to be wrought into plates. For purifying the pearl ashes, dissolve them in four times their weight of boiling water, in a pot of cast iron, always kept clean from rust. Let the solution be removed into a clean tub, and remain there 24 hours or longer. Having decanted the clear part of the fluid from the dregs or sediment, put it again in the iron pot, and evaporate the water till the salts are left perfectly dry. Preserve them in stone jars, well secured from air and moisture.
Pearl ashes may also be purified in the highest degree, so as to be proper for the manufacture of the most transparent glass, by pulverizing three pounds of the best pearl ashes with six ounces of saltpetre in a glass or marble mortar, till they are well mixed; and then putting part of the mixture into a large crucible, and exposing it in a furnace to a strong heat. When this is red hot, throw in the rest gradually; and when the whole is red hot, pour it out on a moistened stone or marble, and put it into an earthen or clean iron pot, with ten pints of water; heat it over the fire till the salts be entirely melted; let it then stand to cool, and filter it through paper in a pewter cullender. When it is filtered, put the fluid again into the pot, and evaporate the salt to dryness, which will then be as white as snow; the nitre having burnt all the phlogistic matter that remained in the pearl ashes after their former calcination.
As to the sand, it is to be sifted and washed till such time as the water come off very clear; and when it is well dried again, they mix it with the salt, passing the mixture through another sieve. This done, they lay them in the annealing furnace for about two hours; in which time the matter becomes very light and white: in this state they are called frit or fritta; and are to be laid up in a dry clean place, to give them time to incorporate: they lie here for at least a year.
When they would employ this frit, they lay it for some hours in the furnace, adding to some the fragments or shards of old and ill made glasses; taking care first to calcine the shards by heating them red hot in the furnace, and thus calling them into cold water. To the mixture must likewise be added
Glas. ed manganese, to promote the fusion and purification.
The best composition for looking glass plates consists of 60 pounds of white sand cleaned, 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 carquaisse are disposed in a large covered gallery, over against the furnace, as those in the following article, to which the reader is referred.
After the materials are vitrified by the heat of the fire, and the glass is sufficiently refined, the workman dips in his blowing iron, six feet long, and two inches in diameter, sharpened at the end which is put in the mouth, and widened at the other, that the matter may adhere to it. By this means he takes up a small ball of matter, which sticks to the end of the tube by constantly turning it. He then blows into the tube, that the air may swell the annexed ball; and carrying it over a bucket of water, which is placed on a support at the height of about four feet, he sprinkles the end of the tube to which the matter adheres, with water, still turning it, that by this cooling the matter may coalesce with the tube, and be fit for sustaining a greater weight. He dips the tube again into the same pot, and proceeds as before; and dipping it into the pot a third time, he takes it out, loaded with matter, in the shape of a pear, about ten inches in diameter, and a foot long, and cools it at the bucket; at the same time blowing into the tube, and with the assistance of a labourer, giving it a balancing motion, he causes the matter to lengthen; which, by repeating this operation several times, assumes the form of a cylinder, terminating like a ball at the bottom, and in a point at the top. The assistant is then placed on a stool three feet and a half high; and on this stool there are two upright pieces of timber, with a cross beam of the same, for supporting the glass and tube, which are kept in an oblique position by the assistant, that the master workman may with a puncheon set in a wooden handle, and with a mallet, make a hole in the mass: this hole is drilled at the centre of the ball that terminates the cylinder, and is about an inch in diameter.
VOL. IX. Part II.
When the glass is pierced, the defects of it are perceived; if it is tolerably perfect, the workman lays the tube horizontally on a little iron tressel, placed on the support of the aperture of the furnace. Having exposed it to the heat for about half a quarter of an hour, he takes it away, and with a pair of long and broad shears, extremely sharp at the end, widens the glass, by insinuating the shears into the hole made with the puncheon, whilst the assistant, mounted on the stool, turns it round, till at last the opening is so large as to make a perfect cylinder at bottom. When this is done, the workman lays his glass upon the tressel at the mouth of the furnace to heat it: he then gives it to his assistant on the stool, and with large shears cuts the mass of matter up to half its height. There is at the mouth of the furnace an iron tool called pontil, which is now heating, that it may unite and coalesce with the glass just cut, and perform the office which the tube did before it was separated from the glass. This pontil is a piece of iron six feet long, and in the form of a cane or tube, having at the end of it a small iron bar, a foot long, laid equally upon the long one, and making with it a T. This little bar is full of the matter of the glass, about four inches thick. This red hot pontil is presented to the diameter of the glass, which coalesces immediately with the matter round the pontil, so as to support the glass for the following operation. When this is done, they separate the tube from the glass, by striking a few blows with a chisel upon the end of the tube which has been cooled; so that the glass breaks directly, and makes this separation, the tube being discharged of the glass now adhering to the pontil. They next present to the furnace the pontil of the glass, laying it on the tressel to heat, and redden the end of the glass, that the workman may open it with his shears, as he has already opened one end of it, to complete the cylinder; the assistant holding it on his stool as before. For the last time, they put the pontil on the tressel, that the glass may become red hot, and the workman cuts it quite open with his shears, right over against the fore-mentioned cut; this he does as before, taking care that both cuts are in the same line. In the mean time, the man who looks after the carquaisse comes to receive the glass upon an iron shovel two feet and a half long without the handle, and two feet wide, with a small border of an inch and a half to the right and left, and towards the handle of the shovel. Upon this the glass is laid, flattening it a little with a small stick a foot and a half long, so that the cut of the glass is turned upwards. They separate the glass from the pontil, by striking a few gentle blows between the two with a chisel. The glass is then removed to the mouth of the hot carquaisse, where it becomes red hot gradually; the workman, with an iron tool six feet long, and widened at the end in form of a club at cards four inches long, and two inches wide on each side, very flat, and not half an inch thick, gradually lifts up the cut part of the glass to unfold it out of its form of a flattened cylinder, and render it smooth, by turning it down upon the hearth of the carquaisse. The tool already described being insinuated within the cylinder, performs this operation by being pushed hard against all the parts of the glass. When the glass is thus made quite smooth, it is pushed to the bottom of the
carquaille or annealing furnace with a small iron raker, and ranged there with a little iron hook. When the carquaille is full, it is stopped and cemented as in the case of run glasses, and the glass remains there for a fortnight to be annealed; after which time they are taken out to be polished. A workman can make but one glass in an hour, and he works and rests for six hours alternately.
Such was the method formerly made use of for blowing plate glass, looking glasses, &c.; but the workmen, by this method, could never exceed 50 inches in length, and a proportional breadth, because what were larger were always found to warp, which prevented them from reflecting the objects regularly, and wanted substance to bear the necessary grinding. These imperfections have been remedied by the following invention of the Sieur Abraham Thevart, in France, about the year 1688.
3. Casting or Running of Large Mirror GLASS Plates. The furnace is of a very large dimension, environed with several ovens, or annealing furnaces, called carquailles, 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 materials in these pots are the same as described before. When the furnace is red hot, these materials are put in at three different times, because that helps the fusion; and in 24 hours they are vitrified, refined, settled, and fit for casting. A is the bocca, or mouth of the furnace; B is the cistern that conveys the liquid glass it receives out of the melting pots in the furnace to the casting table. These cisterns are filled in the furnace, and remain therein six hours after they are filled; and then are hooked out by the means of a large iron chain, guided by a pulley, placed upon a carriage with four wheels marked C, by two men. This carriage has no middle piece; so that when it has brought the cistern to the casting table D, they flip 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 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 glass spread, &c. The whole is inconceivable to such as have not been eye witnesses of that surprising manufacture.
As fast as the cisterns are emptied, they carry them back to the furnace and take fresh ones, which they empty as before. Thus they continue to do so long as
there are any full cisterns; laying as many plates in each carquaille as it will hold, and stopping them up with doors of baked earth, and every clink 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 glass. The workmen work six hours, and are then relieved by others.
When the pots are emptied, they take them out, as well as the cisterns, to scrape off what glass remains, which otherwise would grow green by continuance of fire, and spoil the glasses. They are not filled again in less than 36 hours; so that they put the matter into the furnace, and begin to run it every 54 hours.
The manner of heating the large furnaces is very singular; the two 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. This they hold without interruption for six hours successively; after which they are relieved by others, &c. It is surprising that two such small pieces of wood, and which are consumed in an instant, should keep the furnace to the proper degree of heat; which is such, that a large bar of iron, laid at one of the mouths of the furnace, becomes red hot in less than half a minute.
The glass, when taken out of the melting furnace, needs nothing farther but to be ground, polished, and foliated.
4. Grinding and Polishing of Plate GLASS. Glass is made transparent by fire; but it receives its lustre by the skill and labour of the grinder and polisher; the former of whom takes it rough out of the hands of the maker.
In order to grind plate glass, they lay it horizontally upon a flat stone table made of a very fine grained freestone; and for its greater security they plaster it down with lime or stucco; for otherwise the force of the workmen, or the motion of the wheel with which they grind it, would move it about.
This stone table is supported by a strong frame A, made of wood, with a ledge quite round its edges, rising about two inches higher than the glass. Upon this glass to be ground is laid another rough glass not above half so big, and so loose as to slide upon it; but cemented to a wooden plank, to guard it from the injury it must otherwise receive from the scraping of the wheel to which this plank is fastened, and from the weights laid upon it to promote the grinding or triture of the glasses. The whole is covered with a wheel B, made
Glass. made of hard light wood, about six inches in diameter, by pulling of which backwards and forwards alternately, and sometimes turning it round, the workmen, who always stand opposite to each other, produce a constant attrition between the two glasses, and bring them to what degree of smoothness they please, by first pouring in water and coarse sand; after that, a finer sort of sand, as the work advances, 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, cc, 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.