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

RAW MATERIALS FOR THE MANUFACTURE OF ENAMELS. 29
Dolomite.
Many limestones, and more particularly the so-called
dolomites, contain variable quantities of magnesium carbonate.
The effect of this compound is to cause the enamel to become
more refractory in direct proportion to the amount of mag-
nesium present ;
'
in some cases this property is beneficial. As
far as colour is concerned, magnesia has no influence at all
the silicates of the metal being themselves colourless. AL
Chalk.
Chemically speaking, the pulverulent white mineral, chalk,
consists of calcium carbonate. Examined imJer the micro-
scope, it is seen to be made up of the shel^of tiny marine
animals, the organic matter of which has, however, long A-
appeared. For our purposes chalk is an excellent mateKal,
since it is free from iron, and, being in an extremely fine
state of division, readily unites with the other ingredients
during the operauon of smelting.
Calcined mussel shells also constitute a sple^jlid material
for^snamel-making, wherever they are obtainable in large
quantities, as they consist of nothing bu4 ^1f Pure calcium
carbonate.
Argillaceous Limestone.
Certain limestones, kj^wn unoer the name of marl, contain
large proportions of clayand are also generally rather rich in
iron. Even apart from the latter consideration, such lime-
stones cannot be recommended as raw material for enamels,
the presence of clay rendering the product very refractory.
Moreover, the amount of the clay and its behaviour can only
be accurately determined by chemical analysis and trial
smeltings respectively.
Causticising Waste (Kalkdscher).
Where wood ashes are converted into caustic lye, for
soap-making, by treatment with slaked lime and lixiviation in