Page:The American Cyclopædia (1879) Volume VIII.djvu/27

 GLASS 19 cylinders are occasionally blown 77 in. in length, requiring about 38 Ibs. of glass. The largest sizes are only produced by the most skilful workmen. The relative antiquity of the two processes of making crown and sheet glass is involved in no little obscurity. ^ The cylindrical process is the only one mentioned by Theophilus, who is supposed to have lived in the 12th century, and this method was long retained by the Venetians and the Bo- hemians, as being best adapted to the pro- duction of their colored glasses on account of the uniformity of thickness and of color se- cured. But in the north of Germany, France, and England, it fell into disuse, and the rotary principle prevailed exclusively. Subsequently the latter was abandoned on the continent, but held its supremacy in England, where crown glass was used for houses of the better class, while the use of sheet glass was limited to infe- rior dwellings. In 1832 the improved process of making cylinder glass was introduced into England from France, and subsequently the improved method above mentioned of polish- ing the sheets was adopted. The cylindrical method is the one now in general use in Eng- land, much of the glass being known in com- merce as patent plate. The building or fac- tory for the manufacture of plate glass is gen- erally of very large size. That of the British plate-glass works at Ravenhead, where it is called the foundery, is 339 ft. long by 155 wide; while the famous halle of St. Gobain in France is 174 by 120 ft. In the centre is the square melting furnace, with openings on two parallel sides for working purposes, while along two sides of the great building are arranged anneal- ing ovens, which are sometimes 30 by 20 ft. in order to receive the immense plates that are to be annealed. Two kinds of pots are used, the ordinary one, open at the top, for melting the glass, and cisterns or cuvettes, in which the molten glass is carried to the casting table. In France the cuvette is usually of a quadrangular form, with a groove in each of its sides, or, as in the case of the larger cisterns, in two paral- lel sides, in which the tongs or iron frame are fitted when the cuvette is moved. Between each two pots in the furnace are placed, ac- cording to their size, one or more cuvettes. In some establishments the cuvette is not now used, the metal being poured from the pot in which it is melted on to the casting table. In France 16 hours are allowed for the melting, and the same time for the metal to remain in the cuvettes ; but the latter term is often ex- tended in order that the aeriform bubbles may escape and the excess of soda become vola- tilized. Toward the last the temperature is allowed to fall, and the glass then acquires the slight degree of viscidity suitable for casting. The molten glass is transferred from the pots into the adjacent cuvettes by means of wrought- iron ladles with long handles. When the glass is in the proper condition to be cast, the "tongs carriage," consisting of two powerful bars of iron united like two scissors blades, and resting upon two wheels, is pushed into the opening made in the furnace, and the cuvette is clamp- ed in the quadrant formed at the extremity of the tongs, two workmen manipulating the handles at the other extremity. The cistern thus taken from the furnace, while filled with molten glass, is placed on another carriage and quickly conveyed to the casting table. This consists of a massive slab, usually of cast iron, supported by a frame, and generally placed at the mouth of the annealing oven. At the Thames works in England the casting plate is 20 ft. long, 11 ft. broad, and 7 in. thick. For- merly these tables were of bronze, and the great slab of St. Gobain of this alloy weighed 50,000 Ibs. ; but cast iron was found less liable to crack, and is now generally used for this purpose. On each side of the table are ribs or bars of metal, which keep the glass within prop- er limits, and by their height determine the thickness of the plate. A copper or bronze cylinder about a foot in diameter, resting upon these bars, extends across the table. After being heated by hot coals placed upon it, FIG. 15. Casting Table. the table is carefully cleaned preparatory to casting. The cistern containing the melted glass is raised from the carriage on which it was brought from the furnace by means of a crane, its outside carefully cleaned, and the glass skimmed with a copper sabre. The cu- vette is now swung round over the table, over which a roller covered with cloth is drawn to remove all impurities, and the molten glass poured out in front of the cylinder, which be- ing rolled from one extremity of the table to the other spreads out the glass in a sheet of uniform breadth and thickness. The ope- ration is a beautiful one from the brilliancy of the great surface of melted glass, and the variety of colors exhibited upon it after the passage of the roller. While the plate is still red hot about two inches of its end is turned up like a flange, against which an iron rake-like instrument is placed, and the plate is thrust for- ward into the annealing oven, the temperature of which is that of dull redness. Another plate is now immediately cast upon the hot table, and the annealing oven when filled is closed and left for about five days to cool. The pro- cess of casting is done so systematically and with such despatch in a well regulated estab-