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

128 ENAMELS AND ENAMELLING.
Only, when bone ash is used, it becomes necessary, in order to
obtain a pure white enamel, to add to the mass a little saltpetre
for the purpose of oxidising the carbon present, which is the
case of the grey colour of the ash. Lead oxide for producing
opacity in enamels is rightly going more and more out of use,
for though, when added to the fused and re-ground mass for
fusible cover enamels, lead oxide can by careful firing be pre-
served in an unfused and uncombined condition and therefore
causes the mass to appear white, the colour is always tinged
with yellow from the oxide and is never pure white unless
an addition of smalt be made, which greatly contributes to
the compensation of the colour. Moreover, the use of lead
is inadvisable in enamels for technical purposes, the main
object in which case is to obtain a product capable
of with-
standing chemicals, whereas a simple experiment will show
}iow easily enamels containing lead oxide can be corroded by
such agencies.
Thus if an egg be beaten up in a vessel lined with a cover
enamel containing lead oxide, and left to stand in it for a
day, a brownish-black stain of lead sulphide will be found at
the place where the egg has come into contact with the
enamel, the small quantity of sulphuretted hydrogen liberated
from the egg being capable of modifying the lead oxide in the
manner described. Again, when weak vinegar
is boiled in
a similarly enamelled vessel for a short time, the enamel will
lose its lustre and become rough, while an appreciable amount
of lead will be detected in solution in the vinegar.
Consequently, as lead is a very poisonous metal, buyers of
enamelled utensils are perfectly justified in refusing ware
coated with lead oxide enamels. Less objection applies to the
presence of lead silicate as a constituent of the vitreous matter
of the cover enamel, lead oxide being less readily dissolved
out of this combination.
It must therefore be regarded as a welcome advance in the