Dave Finkelnburg on wed 10 dec 03
Randy and others pondering this problem,
Bottom cracks are a problem for most beginning clay artists. They
certainly were for me, and I haven't solved all the problems they cause yet!
Virtually all failures which are cracks across the bottom of a piece and
start from the center are caused by irregular drying. If you keep this
fundamental thought in mind, you can usually figure out a solution for the
cracking problem wherever you find it.
If you want a trimmed foot on a pot, you intentionally throw the foot
thick. The pot sits on the foot while the walls and especially the rim lose
moisture, shrink and stiffen. By the time you can invert the pot, the rim
has shrunk measurably, but the center of the foot is still about as moist as
it was when thrown and has not shrunk at all. In a dry climate, even a
plaster bat does not alter this situation much.
If you draw a force diagram of the partially dry pot you have to
conclude that as the clay at the rim shrinks and moves in horizontally, the
clay at the foot has to move out. Any clay can take a little of this. Most
clay can't take much, though. Many potters have observed this phenomenon,
hence the adage, "The last part to dry is the first part to crack."
So what's one to do?
While throwing you can press down on the area prone to crack as the
wheel is turning. With that process you fill in holes, really micro-tears
or rips, caused by centering or opening clay that is too dry for the speed
it is being deformed.
Another approach is to make the ware with a potter's (thin, flat) foot.
Another is to invert the pot as soon as possible, so the foot can dry
and shrink along with the rim. On wide ware, like platters, or with rapidly
drying, open clay bodies like porcelains, cover the rim with plastic as soon
as it is firm, to stop the rim from drying further until the foot can catch
up in the drying process.
Trimming the foot as soon as it is workable, so the foot is similar in
thickness to the walls and rim also helps. Once the foot is trimmed it can
begin drying at the same rate as the rest of the pot. Burnishing the
trimmed foot, or compressing it with a stamp or the thumbs, especially in
the center, also helps.
Good potting!
Dave Finkelnburg, watching fine snowflakes fall onto the jaunty
juncos and one elegant towhee as they scratch for food on the ground under
the bird feeder in our mountain valley in Idaho
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