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

SUBSTANCES ADDED TO PRODUCE OPACITY. 37
but when more heat is applied this is converted into stannic
oxide, a change indicated by the whitening of the pellicle.
This skin is now removed by means of a hook, whereupon the
underlying surface of metal forms another pellicle
of oxide
directly it comes in contact with the air. When the tempera-
ture is sufficiently high, the process of oxidation goes on very
rapidly, so that a large quantity of tin can be converted into
oxide in a short time.
The operation may also be accelerated by employing an
alloy of tin and lead, the following proportions :
Tin . . . .82
Lead . . . . 18
per cent.
being highly adapted for the purpose. Of course the amount
of lead present must be borne in mind when compounding the
enamel mass, and the amount of lead oxide be proportionally
reduced. An alloy of the foregoing composition oxidises more
readily than pure tin, and furnishes a yellow-looking mass, the
colour of which is due to the lead oxide present. When the
enamel mass is fused, the lead compound combines with silica
and the pure tin oxide renders the product opaque.
Crude tin ash always contains a certain amount of unaltered
tin and stannous oxide. The simplest method of eliminating
these impurities is by moistening the mass with fuming nitric
acid and leaving it to stand for a while, the action of the acid
being
to convert both substances into stannic oxide. This
method, however, is applicable only when pure tin has been
employed, as in the case of tin-lead alloys the nitric acid dis-
solves the resulting lead oxide.
In the latter event, therefore, the product must be purified
by sedimental washing (elutriation), by which treatment the
the final traces of stannous
particles of metal are left behind ;
oxide in the product are then converted into stannic oxide by
calcining the dried mass in thin layers.