making the Compound Glasses, to cleanse it from chalk and other impurities, by which it loses about one fifth of its original bulk, requires about eight waters; it is afterwards passed through a heated arch, called a calker, and thoroughly dried, or burnt, at the discretion of the operator; and then sifted through a lawn sieve, to separate the larger grains and pieces of coal or coke, and burn off the vegetable matter.

Carbonate of potash, known by the commercial term of pearlash, is imported from British America, and from Russia: it contains, besides carbonate of potash, sulphates and muriates of potash, and other impurities, which will not enter into the composition of Flint Glass. These may be separated, by dissolving the potash in warm water; when cold, the impurities finally sink to the bottom of the vessel, and the lye containing the refined carbonate of potash is drawn off by a syphon, and evaporated to dryness.

This process is, however, rarely practiced, as pure carbonate of potash is now obtained from the alkaline residuum of nitric acid—viz., sulphate of potash. Alkali makers at Birmingham and Liverpool supply Flint Glass-makers with refined pearlash made from residuum or caustic potash, so as to save the trouble of refining potash on the Glass-maker's premises.

Litharge is usually produced at Newcastle, in furnacing pig-lead, in order to extract the silver from it. Red lead is made also from furnaced pig-lead to obtain oxygen from the atmosphere; after which it is ground in water, and dried to an impalpable powder. Litharge has but one atom of oxygen; red lead has two, or a double dose of oxygen, and has less power than litharge by about two percent., which must be allowed for in the mixing.