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

RAW MATERIALS FOR THE MANUFACTURE OF ENAMELS. 25
stand the action of chemicals, potash is used though in the
form of felspar (which contains potassium silicate) and not
as the above-named salt.
B. SODIUM COMPOUNDS.
Common Salt (Sodium Chloride).
This substance is obtained from salt mines, either in the
crude form of rock salt, or in the manufactured (purified)
condition, the former containing gypsum and clay, and being
coloured grey or yellow by compounds of iron. Salt is
not much used in enamel-making, since other sodium com-
pounds better suited to the purpose
rates.
Soda (Sodium Carbonate).
can be obtained at low
This chemical product, which is manufactured in enormous
quantities in special works, is met with in commerce in two
forms : soda crystals and calcined (or anhydrous) soda. The
former kind is in large water-white colourless crystals, which,
however, on prolonged exposure in the air, become covered
with a white incrustation, and finally fall entirely into, the
state of powder, the change being attended with a great loss
in weight. This is due to the evaporation of the water of
crystallisation, though a certain proportion of the latter is
more firmly retained and can only be driven off by cal-
cination.
When soda is heated it melts at a low temperature and
forms a watery flux which continues to evolve aqueous vapour
until the whole of the water of crystallisation has evapor-
ated. It is only when greater heat (strong red heat) is
applied
that the residual water is driven off and the soda is
converted into anhydrous sodium carbonate, which is com-
mercially known as " calcined soda ".
As the anhydrous form will absorb water when brought