Firing Box and Stilts
When firing above a recommended temperature – either on purpose or by
mistake – clay bodies start to melt and fuse or stick to the shelves.
To test clay bars, you’ll need a shallow firing box to protect your
kiln shelves and to make handling and transportation easier. You’ll
also need clay stilts for the warping test.
Using a high-fire clay, make a simple clay box that’s 8 X 10 inches
square with a 1-inch wall as shown at left. Make several boxes at the same time so you
can test multiple bodies in the same firings or just have them on hand
for future use.
Make triangle stilts about the thickness of your pinky and long enough
to span the width of the clay bar. All stilts must be the
You’ll need three clay bars for each body you’re testing. For
consistency and accuracy of results, use the same dimensions for all
your clay bars. While some tests yield correct results regardless of
the bar dimensions, you can always rule out size and dimension as
variables that could cause any irregularities. I make 1/2 inch-thick
bars measuring 2 inches wide by 6 inches long. If your work is thicker,
make bars to match, but don’t exceed 1/2 inches in thickness. After
cutting the bars to size, draw a 10-cm-long line on one of them with a
hash mark at each end (as shown at the top of page). This will be used for a shrinkage test later.
For the tests here, I’ve selected four bodies to test: two white bodies
from Standard (#257 Grolleg Porcelain Cone 8 – 10 and #181 White
Stoneware Cone 6 – 10) and two from Amaco (#29 Brown Stone Earthenware
Cone 06 – 04 and #77 Terra Cotta Clay Cone 5). All four bodies have
different characteristics at the temperatures to which I’m interested
in firing them, and testing several bodies at the same time takes
better advantage of each firing. Remember to write the clay number on
the back of each tile.
Once all bars are cut and firing boxes are built, let them dry to bone
dry. They can be stacked with newspaper layered in between and a board
on top for a little weight to keep them from warping. It’s important
that the firing boxes and clay bars stay flat while drying to assure
accurate test results.
Before the Bisque
At the bone dry stage, visually examine the bars for a color change if
any. Use a ruler to measure in centimeters the shrinkage line to see if
it has changed from its original 10 cm length. Record the results.
For each firing, place the bars side by side in the firing box, and
place one of them on the triangle stilts. The stilts should be placed
about 1/4 to 1/2 inch in from the ends of the bar. If the stilts are too
close, the bar may not warp or slump to its fullest potential. The
shrinkage bar and the other regular bar are simply placed in the bottom
of the box.
While the test firing can be done in any kiln, the results are most
applicable if done in the same kiln used for your work. Indicate in
your notebook or worksheet if it’s an electric or gas kiln, oxidation
or reduction firing, salt, wood, soda, etc. For best results, start
firings at the lowest cone temperature and then progressively refire
the bars at higher cones until the highest desired cone is reached. A
sample for progressive test firings for a cone 10 clay body would be
cone 06, cone 01, cone 6 then finally cone 10.
Clay Bar Tests
The following traditional tests give a good range of basic information
that helps you to better understand your clay body. Keep good records
in your notebook so you’ll have the results for future reference. If
there are other specific qualities, such as glaze fit or color effect,
test for these also.
Visually examine for any warping or sagging movement in the center.
Remove the bar from the stilts, turn it over and place it on a flat
surface so the gap (if any) can be measured. This test
informs you at what temperature the walls may start to warp or deform
or a plate may begin to slump. The information can be applied to the
sculptural or structural applications of the clay body. An exact
percentage for this is not as important as just knowing when the clay
body starts to move.
Measure the length of the line in centimeters on the shrinkage bar, and subtract it from 10 (based on the original 10 cm line).
For example, 10 cm – 8.5 cm = 1.5 cm. An easy way to convert this
result to a percentage is to move the decimal to the right one place,
so 1.5 cm means 15% shrinkage. Knowing the shrinkage rate helps in
determining which glazes will fit the body and even which two bodies
can be used together.
There are two types of tests that can determine the absorption of a
clay body. One is a simple visual test and the other is a weight
calculation. For the visual test, place a few drops of a liquid (like
ink) on the surface of the bar to create a stain. Let it soak in for an
hour then wash off the surface with water. The darker the stain, the
more absorbent the clay. This is not an exact test, but it gives a
quick and useful general result.
The weight calculation test is more specific. Weigh the fired bar on a
gram scale and record the result. Soak the bar in water for
24 hours, pat dry, then weigh again and record the result. Subtract the
first weight (dry bar) from the second weight (water soaked bar) to get
the weight of the absorbed water. Divide the weight of the absorbed
water by the original dry weight and move the decimal two places to the
right to find the absorption rate.
Example: Original bar weight of 4.2 grams is subtracted from soaked bar
weight of 4.6 grams giving you an absorbed water weight of .4 grams.
Divide .4 by 4.2 which equals .095 making absorption 9.5%.
Visually examine a bar to see if there has been a color change. The color change can sometimes be dramatic depending on the cone it
was fired and is important for aesthetic purposes. It can also help you
determine the best glazes to work visually on the surface.
These test bars were fired to Cone 04.
These test bars were fired to Cone 7.
Visually examine the bar to determine if the surface has changed. Run
your fingers across the bar to see if it’s the same, smoother or
coarser. This information is important aesthetically and helps you
determine if it meets your visual and tactile needs.
Using a nail, see if you can scratch the surface to see how hard or
soft it is at the fired temperature. This test helps determine the
surface durability at different temperatures.