High
school ceramics students are clay prospecting. They not only made
pottery from the clay. They built a kiln and wood fired the pots in the
kiln. Eric Good Kaufmann, their teacher, is an accomplished potter and teaches art at Bethany Christian High School, Goshen, Indiana, USA. Mr. Kaufmann is an alumnus, class of '97, Goshen College.
PROCESS to rework good clay that becomes too dry to use. Processing self-dug clay follows below and is shown in the photos to the left on this page.
- Clay
must be totally dry. There is no need to break up dry clay.
PRECAUTIONS:
Be sure it has no plaster chips in it - plaster causes pop-outs when bisque
fired. Leather hard clay or moist clay does not slake well because it
is not porous like dry clay is. Instruct students to handle the dry
clay without making dust. A lot of airborne dust is not healthy to breath.
see health hazards page
- Flood clay with clear water (or drop it into water) in
something like garbage cans. Use enough water so clay is totally under
water.
- Never stir it. Stirring clogs up the porosity and
prevents good slaking (soaking to mush).
- In a few days,
even huge chunks of dry clay will slake to mush. Go to step 6 below.
SELF-DUG CLAY VARIATION
If you dig clay yourself, it often has impurities that need to be removed. Most
kids love to help with this and there are few better learning
experiences. If you are a teacher, invite students to bring in samples
for testing. If it works well, ask them to bring more.
- Let the clay become totally dry.
- Slake it as described in 3 above.
- When it is all soft and mushy, stir it until it is a slip. I use a mixer on an electric drill or a
blunger. Add water if needed to liquefy it.
- Pour the slip through ordinary window screen available at any building supply store.
- The
screening removes stones, roots, and other trash that causes trouble.
The chief culprit is limestone. Limestone, like plaster, pieces cause
pots to break after firing.
- When the clay has settled and turned
to mush, remove extra water from top. Dip water off or siphon it off.
- Spread the mush a few inches thick on clean
dry porous surfaces. I use, dry plaster, clean concrete, canvas, denim,
etc. Smooth the top to avoid getting small dry pieces on the
surface.
- If you want it to dry faster, use a fan and/or set it all on a wire rack to allow air under it.
- When it is nearly dry
enough,
I make coils as thick as my arm and set them around like big arches (a
foot
tall) and they are ready to wedge and use in 24 hours or less.
This
clay can be stored forever in an airtight plastic.
In ancient China, potters stored moist clay in caves for
the next generation to improve the plasticity of the clay. If it is to be stored long-term, double wrap it.
Double wrapping in plastic bags from the supermarket works. Students
can bring in hundreds of these.
notes on digging clay
WHERE IS CLAY? - Check stream banks,
construction sites, roadway cuts, and any place that gets slippery
after a
rain and sticky as it starts to dry. When dry, it is nearly
rock-hard. Many of us can find clay under the topsoil in our back yards.
PLASTICITY
- Some clay is too sandy and some is too sticky. When I prospect I look
for clay that can be rolled between my hands into a pencil thick coil
of soft clay and wrapped around my finger without cracking. If the coil
cracks, it may be too sandy or its clay particles may be too large.
Sticky clay tends to be cling to my hands too much. It will often have
severe drying shrinkage and tend to crack during drying. Potters often blend several clays to get the right properties. See photos on left.
Commercial
clays
can be added to balance the mix. Commercial ball clay adds plasticity
(so it is
less apt to crack when bending it). On the other hand, crude
ground fireclay, china clay (kaolin) fine sand, and/or grog reduce
plasticity (make it less sticky and shrink less). Do a web search of
"ceramic chemicals and clay" for sources of commercially available clay
types near you. See photos on left.
IMPURITIES -
Most common clay contains impurities,
often in the form of iron oxide, sand, roots, and other debris. Troublesome impurities can be removed by
making a thin slip. The sand settles to the bottom first. Allow the
sand to settle a short time. Then decant the clay water (the good slip
from the top down to the sand) and discard
the sand in the bottom. Allow the clay (slip) to settle and process it
as
described in the 9 steps above.
Iron impurities are very common and not easily
removed. Iron
gives it the reddish brown color when fired and causes the clay to melt more
easily. It may not work for stoneware,
but most common clays are fine for earthenware. Most of it will fire to cone 05 without problems.
GOOD USES OF IMPURE
CLAY - Potters who make high fire stoneware sometimes add small amounts
of impure local clay to their clay body to add character and blemishes.
I regularly add some common brick clay to add character to my pottery.
Color and iron spots look more natural and give a warmer feeling.
Stoneware potters also
use local clay as a source of glaze material. These "slip glazes" have
been used for thousands of years for lining jugs and traditional
crockery.
Outdoor FIRING BASICS top of page
Responsible Adult Supervision is Required
Never leave an outdoor fire unattended
Never fire if there is any chance of a wildfire
Have emergency fire quenching equipment on hand
Leave the site cleaner than you found it
Obey all laws and codes
WHAT IS FIRING?
Clay becomes pottery at temperatures over 1,000 degrees F (the
beginning of glowing red heat - about 540 C). Common tribal earthenware
is fired to about 1,400 degrees F (760 C). Heat removes the molecular
water in the clay. This heat converts clay molecules to pottery, sculpture, brick molecules that do not dissolve in water. Glazed pottery and modern brick is
fired in kilns to temperatures ranging from 1,800 F to 2,400 F. Most
common clays start to deform and melt if they are fired higher than
about 1,900 F. Modern toilets are fired to about 2,400 F.
In tribal settings it is common to use an outdoor bonfire type of
firing that is fueled with enough wood kindling under the pottery to
exceed red glowing heat during the burn. The stack of pottery is above
the wood and the pottery stack is covered with a thick layer of natural
material like tall grass, animal dung, etc. to hold the heat in. This
insulating layer also burns toward the end of the firing. In many cases
this insulation-fuel layer is covered with a thin crust shell. This
shell can be made of a clay/sand/straw or grass mixture. One air hole
is provided at the top of the mound and several openings are provided
around the bottom so the wood burns with enough gusto that the clay
gets red hot. The openings around the bottom provide a place to ignite
the wood and allow adequate combustion air to enter. The top opening
needs to be large enough to allow rapid air flow to enter at the bottom
and small enough so the heat is contained.
BREAKAGE
Experiment and learn. If pots break it may mean they are too thick or the clay needs some opener. Sand is an opener. It allows the moisture to steam to escape at the early stages of heating. Steam pressure is what breaks most pots.
Modern electric kilns have a very prolonged heating stage at 200 degrees F. This is just below the point at which moisture turns to steam. The clay gets totally dry before it hits the steam forming temperature. This prevents the clay explosions that often happen when clay is heated to too rapidly.
If you do not have a kiln, it may work to use a kitchen oven set to 190 degrees F. Pots can be totally dried by "baking" below the boiling temperature of water for several hours in a kitchen oven just before firing them in a primitive no-kiln firing.
