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

PIGMENTS. 57
RED PIGMENTS.
Gold Oxide.
This oxide is the sole oxide known which is capable of
colouring glass red when employed by itself, and this yields a
purple red which will be dealt with later on. Owing to its costly
nature the use of this pigment is of course confined to the
finest enamels, the ordinary red, in all its various shades, being
produced by means of a special preparation ferric aluminate.
Ferric Aluminate.
This product is now so largely used in the enamel industry
as to be manufactured on an extensive scale, so that crucibles
have to be discarded, and use made of special furnaces or ovens
for fusing the mass.
The ordinary method of preparation consists in mixing
20 to 25 parts of green vitriol (ferrous sulphate) with 10 parts
of aluminium sulphate (the commercial "
sulphate of alumina,"
met with in the form of greyish-white quadratic blocks and
largely used by dyers).
The mixture is at first gently heated
in tiat iron pans to expel the water of crystallisation, the
completion of which task is evidenced by the change of colour
from the original green to a white or yellowish shade.
The hot mixture is then shovelled out of the pans on to
the floor of an oven, where it is heated until of a uniform red
colour. The attainment of the proper temperature, however,
entails the exercise of some skill ; since if heated too strongly
a part of the ferric oxide is reduced to the ferrous state, and
the enamel in which the preparation is used will be coloured
black or green in place of red. To get the proper result, the
mass in the ovens must be frequently raked over and stirred
up, a sample being
drawn from time to time and examined
for colour after having been cooled down rapidly. So long
as the sample is hot it appears quite black, but turns red on
cooling. Special precautions which will be referred to