Opaque glass is made by the addition of the oxide of tin, and produces that beautiful imitation of enamel which is so much admired. Dials for watches and clocks are thus made.
Bottle-glass is made of soap-boilers' waste and river sand, or sand and lime with clay and salt, mixed, evaporated, and fritted. Common window-glass, of 2 soap waste, 1 kelp, and 1 sand. Super window-glass, 25 sand, 12 sulphate of soda, or Glauber's salt, 4 carbonate of lime, or lime unburnt, and 1 of charcoal ; or 2 purified sand, 3 strong kelp. Plate, or sodaie glass, is sand 100, sub-carbonate of soda 55, unslaked limo 9, nitre 4, and powdered glass 60. The product is three fourths. Flint, or litharge glass, is 10 fine sand, 6 red lead, 3 pearlash, a half part oxide of manganese.
Grinding and polishing give plate-glass a fine lustre. The grinder takes it rough out of the bands of the caster, and, * lug it upon a stone table, to which it is fixed with stucco, he lays another rough glass, half the size of the former, upon it. To the smaller glass a plank is fast ened, by means of stucco, and to the whole a wheel, made of hard, light wood, about six inches in diameter, by thepul ling of which from side to side, and from end to end, of the glass, a constant at trition is kept up ; and, by allowing water and fine sand to pass between the plates, the whole is very finely polished; but, to give the finishing polish, powder of smalt is used. As the upper glass grows smoother, it is taken away, and a rougher one substituted in its stead ; and so on till the work is done. Except in the very largest plates, the workmen polish their glass by means of a plank, having four wooden handles to move it ; and to this plank a plate of glass is ce mented, as above.
Various ornamental forms are given to the surface of glass vessels by metallic moulds. The mould is usually of cop per, with the figure cut on its inside, and opens with hinges to permit the glass to be taken out. The mould is filled by a workman, who blows fluid glass into its top. The chilling of the glass, when it comes in contact with the mould, im pairs its ductility, and prevents the im pression of the figure from being sharp. Some moulds, however, are made in parts, which can be suddenly brought together on the inside and outside of the glass vessel, and produce specimens nearly equal to cut glass.
Cut so called,. is produced by grinding the surface with small wheels of stone, metal, or wood. The glass is held to the surface of the wheels. The first cutting is with wheels of stone ; then with iron, covered with sharp sand or emery ; and, finally, with brush wheels, covered with putty. A small stream of water is kept continually running on the glass, to prevent the friction from excit ing too much heat.
Glass may be ground on any coarse grained stone, with sand, or emery and water. Flat pieces of glass may be di vided in any shape, by making a notch with a file, and carrying a piece of hot charcoal before the line in which it is in tended the fracture shouldproceed. The charcoal must be kept alive with the breath, and the progress humored by experience. Tubes, &c., are cut with a file all round, and then broken.