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
Carbonate of potash, known by the commercial
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.