Glazing and Firing Pottery
Once a clay piece is
built and comes off the wheel it needs to sit and dry until it's leather hard,
meaning it's still a little damp, but can be handled without changing its form.
At this point, you can trim off any excess clay and carve details into the
piece. Pottery at this stage, called greenware,
is very fragile and needs to be handled with care.
The next step is to
put the piece into the kiln for the first round of firing, called a bisque firing. The purpose of this
initial firing is to turn your pottery into ceramic material. The firing
process is measured in cones, a standard unit of measurement that accounts for
time and temperature. It's important that the temperature rises slowly and
cools slowly. Failure to do so could cause the piece to burst, putting you back
at square one. Many kilns have programmable cone settings to help prevent this.
After the bisque
firing, you want the piece to be strong enough that it doesn't fall apart
during glazing, but porous enough to accept the glaze. This allows it to bake
without completely drying out. The temperature of a bisque firing typically
ranges between 1700 and 1900 degrees Fahrenheit (around 926 to 1038 Celsius).
This is the equivalent of 05 to 04 cones. If you went to a craft store where
you can paint your own pottery, you'd be decorating the product of a bisque
firing, or bisqueware.
Now comes the fun
part -- you get to decorate or color your work of art by painting or glazing.
Painting is pretty straightforward -- all you need is acrylic paint and your
imagination.
Glazes consist of silica, fluxes and
aluminum oxide. Silica is
the structural material for the glaze and if you heat it high enough it can
turn to glass. Its melting temperature is too high for ceramic kilns, so silica
is combined with fluxes,
substances that prevent oxidation, to lower the melting point. Aluminum oxide
is used as a stiffening agent, allowing the glaze to adhere to the surface of a
bowl or vase without run off. Glazes get their colors from a wide variety of
mineral oxides. Glazes often require multiple coats and a lot of patience to
get them just right. When that time comes and the piece is dry, you're ready
for the glaze firing, where the pottery is heated to maturity.
Kilns: The three most common types of kilns are electric, gas and wood. Electric kilns are probably the most common type of kiln used in ceramics. They're comparatively inexpensive, and small ones can plug directly into a 120-Volt wall socket, making them accessible to small pottery operations.
