Michael Wendt on fri 22 nov 02
I am the culprit who suggested that fast firing can cause warping. I have
strong evidence that it is so because we fire from red heat to cone 10 in as
little as three and one half hours. I fire Olympic 28" updrafts souped up to
run this fast with low mass "C" brick rings backed with Kaowool. Due to the
small size of the kilns, firing at this pace requires large pieces to be
loaded symmetric to the center axis to prevent warping even if they are
perfectly bedded on a setter medium. My thermal gradient tests show why this
is. Near vitrification, a body begins to change size and if one side is
nearer a rapidly changing heat source like a burner, that portion of the
piece will be soft first and try to shrink, but the rest of the piece lags
behind by a small amount.
That lag causes the hot portion to be placed in tension which stretches it
slightly, and then when the rest of the piece matures, we see a "pointer"
stick out where the hot spot was. Tall narrow pieces like goblets show this
same effect.
Solutions for small kilns like mine include centering large pieces in the
load well bedded on sound supports that are truly flat, ringing them with
smaller pieces to act as shielding and in critical cases, using insulating
fire brick to create saggars around very sensitive pieces. It also helps to
run a thermal gradient series on the clay body to determine when the
maturation shrinkage starts to become large and slow the firing down prior
to that temperature and hold at final temperature long enough to assure that
there is little or no differential from edge to center of the load.
Cooling is also critical because cold air can warp pieces too, so shutting
the kiln up tightly at the end seems to reduce defects from warping as well.
Cold air leaking in like a jet stream can cause similar defects if you drift
out of reduction into oxidation and your kiln is not very air tight.
Obviously, the other causes of warping mentioned by Vince and others also
come into play, but Vince, RR and others have mentioned "clay memory" as a
mechanism that causes warping during firing. I set up a series of
experiments to test the hypothesis that clay bent and restraightened
remembers the bending and returns to the bent shape. My clay, at least, has
amnesia... it remembers nothing. That doesn't mean that other people's clay
doesn't have memory though. You can try these tests yourself to see if your
clay has memory or not:
TEST 1: a wet cup 3.5" in diameter and 4" tall was bent into an oval shape,
the eccentricity (about 20%) was measured with a digital caliper and then it
was bent back to round, dried in still air in a damp box along with a
control piece, bisqued and then fired to cone 10. To eliminate other firing
variables, the test piece was fired in the center of the load and set next
to the control piece made at the same time which was not altered. After
firing, the two showed the same degree of roundness. I tried three corner
forming with the same result.
TEST 2: Per RR's suggestion, I made bowls 6" in diameter. One set was made
8" first and collared and compressed down to 6". The second set was made 4"
in diameter and stretched out to 6" without compression on the lip. After
firing cone 10, both sets of bowls were exactly the same diameter within the
tolerances of measurement.
TEST 3: A potter from England wrote me to say that spouts twist because of
clay memory. I made spouts and left them as vases, squared up the bottoms so
they would sit flat during firing and used a square to make vertical lines
on all four sides of the spouts while they were wet. After firing to cone
10, the vertical lines were still straight and vertical.
Possible Explanations: I believe that my clay does not behave like regular
clay because it is only 38% clay, the rest is nonplastics and because it is
mixed in a deairing ribbon mixer under a vacuum for extended time periods. I
suspect that air voids are in part responsible for clay memory as described
by virtue of the balloon effect.
Think of blowing up a balloon. When the balloon is small, you have very
little working surface for the air pressure your lungs exert to overcome the
small resistance the rubber has to stretching. As the balloon gets bigger,
it gets easier to blow up even though the rubber is now stretched very tight
because you have more working area due to the increased diameter. With clay,
there are small air voids even in deaired pugged clay which are either
compressed or expanded depending on the action of the potter. One side of a
tile will be stretched and the other compressed if the tile is bent at some
time during handling. This changes the size of the air voids within the two
surfaces and may be what sets up the internal stress that shows up later as
warping. In essence, we create a density differential across the tile if air
is present in the clay body and dense materials are stiffer than spongy
materials as a rule so they resist shrinkage at a slightly different rate
during firing and show up as warping.
We need to do experiments that are measured and reproducible and that get at
the heart of these phenomena. Only then will we be able to control things
like warping better.
Wendt Pottery
2729 Clearwater Avenue
Lewiston, Idaho 83501
1-208-746-3724
wendtpottery.com
Vince wrote:
3) clay memory from forming and drying, leaving stress zones in the
wares which "relieve" themselves in the firing, causing serious warping.
The importance of the latter cannot be overstated. There are so many things
people do to leave undesirable clay memory in the wares - stresses which are
stored in the rigid unfired clay, but which cause the clay to move around
once it becomes pyroplastic in the firing.
Best wishes -
- Vince
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