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Clay
The clay
that I use is a high fired stoneware or porcelain fired in a gas
downdraft kiln to a cone 10 using a gas reduction fire of 2380 degrees Fahrenheit. This is the
most desirable of all firings due to the depth of colors, it is also
the most unpredictable of firings and can be very difficult to
reproduce the same effects. The two types of clay bodies that I use
are dense enough that the clay particles fuse together at 2300-2400
degrees Fahrenheit (hence the name "stoneware"). These
clays are water-tight and very durable which make wonderful
utilitarian pieces. Stoneware clays are great for baking, as they
transmit heat evenly through the clay and retain the heat for an
even and consistent bake.
Bisque Firing
When completely air-dried slowly over several days, the pottery is bisque fired at a relatively
low temperature, around 1830 degrees Fahrenheit, to make the clay ready for glazing. Bisque firing removes still more moisture
out of the clay and makes the clay firm enough to handle, yet porous enough to absorb the liquid glaze. Bisque firing
temperatures are similar to temperatures used for "low-fire" pottery, such as red clay garden pots and bricks. After bisque
firing, stoneware and porcelain are ready to be glazed.
Glazing
This is the most complicated and difficult part of the production of
my pottery. Glazes are not pigments like paints that allow you to
select a color by looking at the raw glaze. Glazes are refined
chemicals, usually mineral oxides, that are mixed in a chemical
formula that will allow the glaze to "flux" (melt) at a
certain temperature, much like glass. Chemicals added to glazes to
add color to the pot include copper, cobalt, iron, and rutile. Glaze
formulas are often "proprietary," meaning a potter who has
developed a certain color or look over years of experimentation will
not share that information with other potters.
Each layer of glaze thickens
the glaze on the pot. If glazes are applied too thickly, the glazes
will melt and frequently flow more than planned during the glaze
firing. When this happens, the glazes will run right off the pot and
onto the shelf, and as the kiln cools, the pot is stuck to the shelf
and in most cases both the pot and the expensive kiln shelf are
ruined. This makes for a very expensive mistake.
Each glaze base is a powder
which is mixed with water, until the desired density or thickness is
obtained. This density is measured regularly during each glazing
process, to ensure that the glaze will be the right thickness when
applied to the pots. Pots can be dipped in the glaze, or glaze can
be poured over the pot, or the glaze can be brushed or sprayed onto
the pot - all these techniques produce different effects.
Glaze firing
Glaze firing can be done in either an electric kiln or a gas kiln.
Depending on which type of kiln is used, the difference in the
appearance of a glazed pot can be quite astounding. Electric kilns
require oxygen to be present through the entire firing process
(oxidation firing). Gas kilns allow the potter to control the oxygen
level that mixes with the natural gas during the firing. Reducing
the air flow at designated intervals for a short period of time
(reduction firing) changes the atmosphere in the kiln and affects
the glaze chemicals, producing the vivid "copper reds" and
the soft, deep blues, lavenders, purples, and greens.
The glaze firing takes 3 days
to complete. On the first day the kiln is loaded. That night the
kiln is "candled" (allowed to sit at a very low temperature
to finish the drying process). On the second day, the kiln is fired, usually to
temperatures between 2350 and 2400 degrees Fahrenheit, a process
that normally takes 10 to 12 hours, after which the kiln is left to
cool very slowly for 24 to 48 hours or until the inner tem is below
300 degrees before opening. On the last day, the kiln is slowly opened over a
period of 3 to 4 hours, as the pots continue to cool to handling
temperatures.
The pots are finally removed
from the kiln the afternoon of the third day. Pots that are cooled
too rapidly will shatter. Once pots are removed from the kiln and
brought back into the studio, the unglazed edges are sanded, and the
pots are ready for sale.
Care of your Stoneware
To use in your oven, place the
piece in a *cold* oven. Do not use on a burner of your stove. The biggest thing is to avoid thermal shock and let the
stoneware heat up with the oven. This stoneware is safe for microwave, dishwasher, oven and freezer [but return to
room temperature before reheating].
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