Janet Kaiser on fri 5 nov 99
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I guess all that I can say has already been said. But in case an odd point =
has
not, here goes:
1. Because the base of a pot undergoes less pressure during throwing and is
worked on less than the walls, it is less aligned in particle structure and
liable to greater shrinkage. Deliberate working on the base or its =
compression
from the outside later in the throwing can help prevent cracks.
2. Cooling too quickly through the inversions of quartz and cristobalite may
cause cracks or dunting. This would be made worse by an excess of silica in =
the
body. A cooling crack or break is usually curved and sharp.
3. Over wetting parts will cause breaks and cracks. E.g. water in the base
during throwing.
4. Stretching the clay when making.
5. Uneven or forced drying... usually the last part to dry cracks.
6. Very uneven wall thickness, especially the bases of pots.
7. Excessive weight on a pot.
8. Over wedging, giving a very short body.
9. Dunting: cracking of pots in the kiln due to drafts of cold air striking =
when
firing or cooling, causing one are to contract more than the rest. Free =
quartz
will increase the risk of dunting at dull read heat and cristobalite at =
lower
temperatures. Biscuit is more likely to dunt than glazed ware and a dense =
body
is more than an open one. Rapid over-all cooling will not necessarily cause
dunting.
Additionally for the attention of the potter with the foot problem (on the
pots):
10. Applying slip to a pot which is either too dry or conversely too plastic
11. The moisture differential of the clay between the pot and the piece/s to=
be
attached.
12. Uneven drying -- feet drying before the pot
13. Making parts from clay which it too hard.
I know Ray Finch once-fires his large plates to minimise the risk, whilst =
other
potters I know set large flat pieces on clay wads so that they will not be
affected by the different cooling rate of the kiln self.
In theory, as long as clean, well prepared clay is made into reasonably
even-walled pots which are allowed to dry naturally will not suffer. Of =
course
=22dried naturally=22 is relative. Live in a desert and maybe drying =
naturally may
be too fast for the clay used.
I also wonder are you turning the large pots? If so, a better designed foot =
ring
may be the solution?
Janet Kaiser
The Chapel of Art: Home of The International Potters' Path
Criccieth LL52 0EA, GB-Wales, UK
WEBSITE: http://www.the-coa.org.uk
EMAIL: postbox=40the-coa.org.uk
If you experience difficulties accessing our web site,
please e-mail me=21 Thank you=21
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