Cracking and Wheel Throwning
you ever had trouble with cracks developing in the base of your
wheel thrown work? This problem can be avoided by following a few
Cracks develop for two main reasons: uneven drying and uneven compression
in the throwing process.
OK, so you put your wheel thrown work in a drying cupboard. You
even additionally cover the work with plastic sheeting, trying to
ensure even drying. But if you are throwing on a bat, no air reaches
the bottom of a platter, vessel or other thrown object. What happens?
The top dries a bit slower. As the top dries, it shrinks and pulls
together, cracking the slightly wetter and therefor less shrunk
surface towards the bottom. Once a crack has developed, it is near
to impossible to get rid of, even by turning (trimming). One way
to avoid this might be to turn over the work halfway in between
the drying process, but this is not usually feasible. So what to
do? How to get that bottom drying at a similar rate? The answer:
throw your work on plaster bats. (How to make plaster bats will
be the topic of a feature soon to come.)
The bat will need to be moistened, otherwise the clay will pop
off too soon and throwing will become an impossibility. After throwing
your work, the piece can be lifted off while still on the bat, and
the bat, work and all, can be placed in your normal drying area.
After a few hours, depending on moisture content of the bat, the
work will pop off all by itself, and may even be ready for turning
that same day.
- Avoid uneven compression when wheel throwing
Cracks may not appear until in the firing itself, often even until
a second high temperature firing, especially in fine clays like
porcelain or porcelaineous stoneware. Rougher clays like raku are
generally not so prone to cracking, unless you really treat them
badly, as the grog tends to absorb a lot of tension. For finer clays,
the best technique to get a consistently compressed clay throughout
is to center the clay on the wheelhead, then cut off the clay with
a cutting wire. Invert the clay and recenter 'upside down'. This
ensures that the clay will be quite compressed at the bottom as
well as at the top, thus avoiding 's-cracks'.
Uneven firing can also cause cracking. If one side of a pot gets
more heat than another, it may shrink more rapidly, causing tenion,
which could lead to cracks developing on cooling.
Combined, these techniques should be able to cure most cracking
problems with wheel thrown work.