CoE of the glass carrier for
paints is a distraction.
Paint has been applied to glass
and fired for at least seven centuries – long before CoE measurement. The
earliest enamels were intensely coloured glass powders applied to depressions
in the base metal (iron, gold, copper, brass, etc) and heated. More detailed images began to be created when
the powers were mixed with a liquid binder and painted on either in a single,
or multiple layers onto glass and metals.
Silver stain became popular in
the 16th century and has continued since. This is a different way of colouring the
glass, as the colour does not laminate with the surface, but is chemically
combined with the glass. Various silver
salts produce different colours and vary in intensity at different
temperatures. This can provide a variety
of effects at fusing temperatures where it “metalises”, providing ambers and
blues.
CoE in relation to paint does
not matter.
The amount of paint is
miniscule in relation to the mass of glass to which it is applied, and so any
incompatibility would not have sufficient strength to break the glass. If the
paint’s glass carrier was too incompatible, it would come off instead of
breaking the glass, in any case.
The composition of the fusing
glass paints is largely unknown, although commonly supposed to be powdered glass
frit. Some may be the same as enamels used in metal enamelling. Some others may
be the same as the on-glaze ceramic colours. They all have glass as the carrier
of the colour. Still, the amounts of
glass involved are very small and compatibility is not a concern.
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