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

COMPOSITION OF ENAMEL MASSES. 119
itself, a glaze of pure silica would form an ideal enamel for
technical purposes. This being, however, impossible, the silica
has to be mixed with basic substances in order to obtain a
fusible mass.
The most refractory and durable enamels are furnished by
masses consisting entirely of potash and lime, in addition to
silica, since such masses are really nothing more or less than
very refractory glass. As it is the object of every enamel -
maker to produce a constantly uniform article, manufacturers
are strongly recommended, in their own interest, to compound
small quantities of these materials in various proportions, and
thus ascertain by trial smeltings how these mixtures behave
and which is the most suitable for any given purpose.
Masses for ground enamels are mostly composed solely of
silica, borax and felspar, their fusibility increasing in proportion
to the amount of the two last named ; and it is possible,
by varying the quantity of borax, to adjust the fusibility of
the mass within a comparatively wide range.
To decrease the fusibility of any mass, the fused materials
are ground and mixed with a known quantity of finely
levigated clay, the amount of this constituent being so large
that the powdered mass, when applied to the surface to be
enamelled, can only be made to sinter or frit, without actually
fusing. Ground silica or felspar may be used in place of
clay, but in such event the mass will be transparent instead
of opaque, both these bodies consisting of small particles of
transparent material.
To render a mass more fusible it is either mixed with lead
oxide or a large proportion of borax is taken, this modification
being of course made when compounding the mass from the
raw materials.
When the enamels are to be coloured, the addition of lead
oxide must be made with great care, because many pigments
will not develop their colour in presence of this substance. In