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

38 ENAMELS AND ENAMELLING.
BONE ASH.
In the case of enamels which are merely required to be
translucent, without being brilliantly white, bone ash may be
used with advantage. This substance, prepared by calcining
bones in presence of air and then grinding and elutriating the
residue, forms a greyish- white powder, and chiefly consists of
tri-basic calcium phosphate together with calcium and mag-
nesium phosphate.
For our purpose the calcium phosphate is the principal
constituent, which substance, when pure, is a perfectly infusible
white crystalline mass. A large proportion of the bone ash
now met with in commerce comes from South America, the
meat extract factories at Fray-Bentos in Uruguay employing
the bones of the slaughtered cattle for heating the furnaces.
Bone ash of dark grey colour contains an admixture of carbon
from the added fuel, but this substance can easily be got rid
of by adding to the enamel mass a little saltpetre, which, during
the process of smelting, gives oft' oxygen and consumes the
carbon.
SUBSIDIARY INGREDIENTS.
This category comprises ail substances added to the white
fundamental mass of the enamel with the object of modifying
its properties in any way. They may be subdivided into
three classes according to the purposes which they are intended
to serve :
1. Fluxes, to increase or diminish the fusibility of the mass.
2. Pigments, to colour the enamel.
3. Decolorants, for modifying or totally destroying the
colour of the fundamental mass.