Crystal glazes are a relatively new phenomenon.
There are not many examples before the early 20th century. The
early examples were discovered by accident without an understanding
of what happened. The ceramic industry
in
Europe and in America started experimenting
with the glaze in the late 1800's but
decided
that the glaze was not practical economically
because of several factors which were
difficult
and time consuming. It was not until
the
1980's, when electronics started making
fully
programmable, automatic kilns possible,
that
crystal glazed ware started to become
more
common. Even today, however, most potters
do not want to deal with the difficulties
involved in producing these glazes.
Crystal glazes require an intricate long
cooling schedule. They run off the
pot and
need special containers to collect
the running
glaze so that it does not ruin the
kiln shelves. It is impossible to repeat something again.
Each piece is unique. Contrary to some ideas,
we do not put crystals in the glaze, although
we do sometimes for effect. The crystals
actually form in the glaze in a chemical
reaction during cooling and grow from small
nuclei created during the melting process
when silica and zinc come together to form
zinc-silicate. I fire the crystal glazed
ware to approximately 2340 degrees F. and
then hold the temperature in the kiln on
cooling between 2000 F. and 1830 F. for approximately
3 to 5 hours depending on the glaze.
Each glaze composition, together with the
firing schedule and glaze thickness, makes
different forms and colors of crystals. I
use cobalt oxide, nickel oxide, iron oxide,
copper oxide, and manganese oxide for different
colored crystals.
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