A plan of the furnaces used by the French is given at Fig. 12. of plate C('LXXVI. and a vertical section at Fig. 13, the same letters being used in both. The real furnace A, which contains the two large pots a, a, and two others on the opposite side, is surrounded by four other furnaces or ovens 11, 13, B, Dovhich proceed from the an gles, and are heated by the flame of the same fire passing through flues g, g. The three ovens marked 13, B, B are intended for burning the pots in which the glass is melted and refilled, and also the cisterns into which it is put, to be conveyed to the casting tables. The fourth oven D is intended for heating the frit or material, previous to its being put into the pots to be melted down. The four ovens are all of the same form and dimensions, except that the openings b, b, b into the three first arc larger than the mouth d of the latter : all these openings are closed by doors of bricks fitted in iron frames. The fire is made in the bottom of the great furnace at F., Fig. 13, being includ ed between two sloping walls e, e, which form the benches or seats for the four pots; two pots a, a, being placed up pon each bench, and likewise two cisterns m, m, which are placed at the ends of each bench ; and therefore, in the four angles of the furnace, the form of the arched roof FL: is intended to reverberate the flame clown into and up on the pots, to which it gives the greatest heat, and then passes of by the flues g, g, g, g, Fig. 12. into the four ovens B, B, B, D. The fuel is supplied through arches E at the end of the fire-place, which are of sufficient size to introduce a new pot when necessary ; but when the furnace is at work, these arches are bricked up, except a small open ing at the bottom. On each side of the furnace are three -working holes i, h, h, to admit ladles, by which the glass is put into the pots a, or taken out and transferred to the cis terns in j and, in order to withdraw the cistern from the fire, a door or opening is made in the wall before each, as shewn at 11, Fig. 12. ; and the clotted lines represent a floor ing of iron plate at the level of the bench, upon which the cisterns are received when drawn out of the furnace, which is done by a large pair of forceps, Fig. 14, mounted upon wheels.
The pots are placed in the ovens B. The flues which admit the fire into them, are provided with dampers or sliding doors, which are closed until the pots are placed in the oven. The dampers arc then opened very little at first, to admit the heat gradually, and avoid the danger of crack ing the pots. The cisterns are made and baked in the same manner.
The frit is mixed with the fragments of old glass, which are reduced to powder by heating them in the oven D to redness, and throwing them into cold water. To this mix ture of frit and glass is added the fluxes of nitre or borax, and the composition is baked for some hours in the oven I), until, by the commencement of the vitrification, the ma terials are reduced to a sort of thick paste. This is taken out of the oven in long ladies, carried to the mouth i of the furnace, and put into the great pots a, a, which are al ready heated. Here the great heat vitrifies, and changes the frit into glass. It requires a fusion of 36 or 48 hours
to make fine and clear glass, and sometimes more. If it is found thick or opaque, like porcelain, the heat must he continued for a longer time, m a greater proportion of flux must he added ; borax is the best for this additional flux. if it is fund to he a perfect substance of glass, but colour ed, manganese or the oxide of cobalt is put in. For this purpose, they arc tied up in a thick paper at the end of an iron rod, and put down to the bottom of the pot. A mix ture of arsenic and manganese is used at other times, ac cording to the nature of the colour, which is intended to be corrected.
When the glass is completely vitrified, it is taken out of the pots in ladles, and poured into the cisterns which st nd at the sides of the pots. The cisterns must be introduced into the furnace whilst they are hot, both to save time in heating them in the great furnace, and also to avoid the dan ger of cracking them by a sudden exposure to so great a heat. Fur this purpose a cistern is taken from the oven 13, in which it was baked, by drawing it with hooks to the mouth or door ; and it is then seized by the tongs, Fig. 14, which are a very large pair of pincers, united by a joint pill a, fixed into the axletree of a pair of wheels b, upon which the whole runs. The beaks or jaws d, d, are properly adapted to receive the cistern between them ; and, fur this purpose, the cistern has a groove in each side. The oppo site ends of the tongs have handles c, e, by which the men guide and direct the machine ; and also a small arch g. with pin holes to make fast the tongs, when they have seized the cistern. Having taken hold of the cistern with this ma chine, by two men weighing on the handles e, c, they can readily take up the cistern of the floor of the oven. and by the wheels transport it to the furnace, into which they introduce it through the lower openings. If it is a cistern which has been used before, they take the opportunity of its being hot to clean out the old glass from its sides. The glass remains five or six hours in the cistern until the whole is brought to a white heat. The door at 1, Fig. 12, oppo site the cistern, is then removed, and the cistern with drawn by the pincers, aided by long iron hooks. 13..ing then carried to the casting table, represented in Fig. 15, it is taken up by a crane, and its contents poured out upon the table, upon which it spreads into a thin sheet.
The table AA is strongly framed in wood, and covered over with a thick plate of copper, made very smooth upon the surface. (Fig. 15.) B is a heavy cylindrical roller of copper, which is used to roll over the fluid glass, and flat ten it to the required thickness. To regulate this thick ness, two iron rulers a, a, are laid down upon the table, at such a distance as will include the width of the intended plate ; and the thickness of the rulers is equal to that of the plate, because they bear up the roller to that distance front the surface of the table. The roller B has long pro jecting spindles at each end for two men to roll it along by ; and that it may advance steadily, they are counteracted by two other men, who hold rods c, c, and regulate the advance of the roller.