which it is again heated, cooled on a copper-table, hardened 24 hours in the annealing furnace, and afterwards preserved for use.
Window-glass is divided into various sorts, the principal of which are: 1. Crown-glass, which is the clearest and most expensive. The best window-glass is made of white sand, 60lbs.; purified pearl-ashes, 30lbs.; salt-petre, 15lbs.; borax, 1lb.; and of half a pound of arsenic. These materials are melted in the manner before mentioned, and if the glass should assume a dusky yellowish hue, a sufficient quantity of manganese must be added to remove that defect.
2. Newcastle-glass, which is most commonly used in England: it is of an ash-colour, and frequently speckled, streaked, and otherwise blemished. Its preparation consists of 60lbs. of white sand, 25lbs. of unpurified pearl-ashes, 10lbs. of common salt, 2lbs. of arsenic, and 2 oz. of manganese.
3. Phial-glass is an intermediate kind between flint and the common bottle or green-glass. The better sort is made of 120lbs. of white sand, 50lbs. of unpurified pearl-ashes, 10lbs. of common salt, 5lbs. of arsenic, and 5 oz. of manganese. The composition for green or common phial-glass consists of 120lbs. of the cheapest white sand; 80lbs. of wood-ashes well burnt and sifted, 20lbs. of pearl-ashes, 15lbs. of common salt, and 1lb. of arsenic.
4. The common bottle, or green glass, is prepared from any kind of sand fused with wood-ashes, to which may be added the clinkers of forges.
Plate-glass is the last and most valuable kind, and is thus called, from its being cast in plates or large sheets: it is almost exclusively employed for mirrors or looking glasses, and for the windows of carriages.—It is composed of 60lbs. of white sand cleansed; 25lbs. of purified pearl-ashes; 15lbs. of salt-petre, and 7lbs. of borax; and, if a yellow tinge should appear in the glass, a small quantity of manganese and arsenic are added, in equal proportions.
Plate-glass was formerly blown, but that method having been found very inconvenient, casting was invented; namely, the liquid metal is conveyed from the furnace to a large table, on which it is poured, and all excrescences, or bubbles, are immediately removed by a roller that is swiftly passed over it. It is then annealed in the manner already referred to.
The last process is that of grinding, which is performed by certain machinery, that is not generally known. In Britain, it is practised to the greatest perfection by Bohemians.
The colouring of glass with various shades, is an art known only to a few persons, and as it is not an object of domestic economy, we shall only notice a patent granted in February 1778, to Mr. John Kent Tarrant, for his invention of painting, spangling, gilding, and silvering glass.—This is effected by applying the painting to the back of the glass, so that it may appear on the front, when finished: the colours are to be prepared with oil or varnish. Those parts which are intended to be gilt, must be previously traced on the glass, and when perfectly dry, the gold-leaf is to be applied: a similar method is to be followed for silvering. With respect to spangling, the patentee directs this process to be performed
