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The clay has to pull into the center from the outer edge. If it catches on
anything, it may crack in drying. Making sure the slab can move so that it
can travel towards its center is the best way to tackle the problem.
May I suggest:
1. Leave on canvas or burlap and cover with plastic. Take care with the
edges and make
sure they are covered the longest amount of time. If you could copy
the idea behind the layer of
grog we place on the shelf under large pots when firing, you have a
winner. I was wondering
if you could have small diameter rollers under the slab so they
would roll easily as they shrink in.
You can get those at the hobby shops. I believe the slab rolling
tables are built
with this mind.
2. If your clay is just not cooperating, maybe another type would do or
add a percentage of grog.
Check with your clay supplier who should know something about the
body you are using.
3. Don't add bentonite to the clay body. Remember it shrinks the most of
all the clays. Also gains
the most in size when you add water to the dry.