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

120 ENAMELS AND ENAMELLING.
such event it is easy to increase the fusibility of the product
by the aid of borax.
The effect of magnesia is to render the enamel more readily
distributable over the surface to which it is applied, and the
use of this substance is therefore particularly advisable when
preparing enamels desired to distribute uniformly
on small
articles.
A very important point in the preparation of all enamels
is the so-called decoloration by means of oxidising bodies,
added in order to convert ferrous oxide into ferric oxide, and
thereby diminish the pigmentary power exerted by the
former. It is evident that the sole object accomplished by
this treatment is the production of enamels with a yellow
tinge, but not perfectly colourless, the sole means of effecting
this latter desideratum being the use of perfectly pure, and
therefore costly, raw materials.
There is, however, a simple means whereby enamel masses
from somewhat impure materials can be obtained of a fine
white colour, and this is by the addition of very small quanti-
ties of fine smalt to the mass. Like all cobalt preparations,
smalt is capable of imparting a deep blue colour to glass, but
the shade in this case is complementary to the yellow tinge
produced by ferric oxide, and therefore the one counteracts
the other, the result being to leave the enamel quite white.
Of course this mutual destruction of colour only occurs
when the right proportion of the two pigmentary bodies is hit
upon ;
for if an insufficient amount of smalt be taken, the
yellow from the iron is only
weakened and still remains
apparent, whilst an excess of smalt imparts a bluish (milkwhite)
tinge to the mass.
The expense of decolorising enamels in this way is a
negligible factor a minute quantity of smalt sufficing for a
large batch of enamel mass. All that is necessary
is to
accurately determine, by several tentative experiments, how