blend of ancient and modern techniques.
In one example of calculated ceramic
chemistry, the high iron oxide content in
a liquefied “slip” clay can be utilized to
create various different colors and textures
through methods of temperature variation
and oxidization. Miranda uses two different
types of kilns in her studio at
ShackletonThomas – one gas, and one
electric. When two clay pots with similar
chemical composition are fired in the two
different kilns at different levels of heat
and oxygen exposure, the results can be
drastically different.
Miranda says that the process all starts
with “firing the clay in the kiln at 1000
degrees centigrade. That changes the
core structure in the pot. All of the organic
matter burns away. That stage is called
‘biscuit’ firing. After the ‘biscuit’ firing,
the clay will last in that state for hundreds
of years, and it will never go back to its
original malleable and soft form. After that
firing, we apply our slip clay. The slip clay
used in these two pieces has a very high
level of iron oxide content. Depending on
how we proceed after the biscuit firing, we
have the ability to alter the appearance
of the final piece by adjusting several key
factors. With a gas kiln, we can use what
we call a ‘damper’ to reduce the oxygen
intake of the kiln. When we first fire the pot
in the gas kiln, the fire starts out as a lovely
blue flame. We then gradually turn up the
gas and close the damper down, limiting
the oxygen intake of the kiln. It’s just like
closing down the air intake on a wood
stove. Suddenly, our flame transforms from
a short blue flame into a very long yellow
flame. This process is called ‘reduction’
because we’re reducing the amount of
oxygen. This creates a visible difference
between the two pots. One is exposed to
a high level of oxygen, and the other is
starved of oxygen. By limiting the oxygen
intake, the iron oxide turns back into a
metal because it hasn’t oxidized. When it
is exposed to high levels of oxygen and
higher temperatures in the electric kiln,
the iron oxide becomes inert. Those small
differences change the entire appearance
of the piece. It ends up looking completely
different. We also put a thin layer of liquid
glass over that layer of iron-rich slip, which
becomes our ‘glaze.’ That way, when the
iron that’s in the clay becomes active, the
chemical reaction will color the glaze and
create a beautiful lustrous coating. These
little subtleties are so exciting for potters
to see. It’s like being able to witness a
meteor shower. Every single pot comes
out differently depending on minute and
delicate changes.”
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