iandol on thu 9 may 02
Dear Al Tillis,
I would save these as specimens.
I can suggest three causes.
The first is that your two clays are not sufficiently well blended and =
that there is an demarcation line remaining between the two in the =
fabric of your clay. Because the two clay will have differing linear =
thermal characteristics one will contract more than the other. =
Sufficient tension is set up to initiate a fracture. This would be easy =
to test empirically.
Secondly, it would seem that you may have a misfit between your glaze =
and one, or even both of these clays. Again, this could set up =
sufficient stress on cooling to cause a rupture.
Thirdly, if the glazed ware show cracks that are irregular or jagged, =
like a lightening strike, even though they follow a general direction, =
then you may have a problem associated with Cristobalite.
Others may be able to add to this.
Forget about "Compressing the Walls more often". If you apply a lot of =
pressure you will change the forms you are making. This is because the =
clay will respond by thinning or moving away from where you apply =
pressure.
Concentrate on getting good mixing of your clay and finding a new glaze =
which will better match this mixture of clays. Before you use your clay =
test it for uniformity. This is done by cutting across the pug you are =
blending and slapping one piece onto the bench so that the cut surface =
is vertical to the table top. Look at the effect across the cut surface. =
If there is a series of ripples your clay needs more work. It may be =
necessary to let the clay rest for a week then knead it up again to =
ensure that the moisture distribution is satisfactory.
Have fun,
Ivor Lewis. Redhill, South Australia
Timakia@AOL.COM on fri 10 may 02
Hello Ivor. This was interesting and very true remarks that you came up
with.
I do think though that you might not have understood exactly what I wrote.
Whe saying that the clay particles must be compressed, I meant that one must
make sure that the clay particles fall into place to prevent stress on the
clay.
If your hand is pulling too hard on the claywall and the rotation of your
wheel does not correspond with that, you will stretch the clay too much and
that will cause spiral cracks. By running the walls over a couple of times,
you will make sure that you have a solid wall. It is the same when making a
plate and stretching the clay too hard on the bottom. That will leave you
with a cracked bottom. It is important to learn to balance the hand/
rotation
of the wheel speed. I think this often happens when a potter start gaining
control over clay on the wheel , but does not understand the caracter of
clay exactly. Hope I made myself more understandable.
Also note that when compacting too much, you can also cause cracks, because
then you will start breaking up the magnetic fields between the clay
particles and the clay will become tired.
Take care.
Antoinette.
Antoinette Badenhorst
http://hometown.aol.com/timakia
105 Westwood circle
Saltillo, MS
38866
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