Enamels and enamelling; an introduction to the preparation and ...

30 ENAMELS AND ENAMELLING.
large tubs, a residue is obtained, consisting for the most part
of calcium carbonate together with a small amount of potash
salts, and almost all the ferric oxide present in the wood ashes.
In point of quality this product is comparable with ferrugin-
ous limestone and is therefore of but very low value as a
material for the preparation of enamel.
This alkaline earth, which resembles lime but is more
sparingly soluble, occurs in nature as magnesium carbonate
(magnesite), and magnesium sulphate (Epsom salts); and as
already stated,flkloniite consists of varying proportions of the
nates of lime and magnesia. Magnesium carbonate is
net with as a manufactured product in commerce.
Magnesium Carbonate.
Small quantities of this salt are used in the manufacture
of enamels, to reduce their fusibility ; and freedom from iron
is a particularly important consideration. Good magnesium
carbonate must form a pure white, very light mass, that can
be easily reduced to an impalpable powder by rubbing between
the fingers.
Magnesium Sulphate (Epsom Salts).
This substance appears in the form of colourless, readily
soluble crystals of very bitter taste, and parting with their
water of crystallisation when *heated. It is not very often
used in the preparation of enamels.
LEAD COMPOUNDS.
The silicates of lead being colourless and readily fusible,
compounds of this metal are frequently employed for producing
fusible enamels, the most common forms being the
oxides (litharge and red lead) or the carbonate (white lead).