Michael McDowell on sat 13 dec 97
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I caught this message in the list digest last night
Michael McDowell wrote ...
=3E The next step that seems to have some impact on the amount of s-cracking
is th
=3Eas with pieces thrown on bats or the wheelhead, you should make some
=3Eeffort to throw or rake back in to the center after opening.
Michael -
Your comments were wonderfully helpful, but I didn't understand the
concept cited above. Would you mind describing what you mean by
throwing or raking back in to the center? Thanks
I'll give it my best try to reword this or explain it so that it makes more
sense to those who don't know what I'm referring to here. In potter's
jargon, this maneuver is called =22compressing=22 the bottom. It is jargon, =
not
a dictionary use of the term. What we are actually achieving is a superior
alignment of the clay particles to minimize shrinkage across the bottom,
not any actual compaction of the mass. After opening, and especially when
throwing off the hump, the particles of clay (Imagine them shaped like tiny
disks) are aligned randomly, or even tending to be aligned more often with
their long axis perpendicular to the plane of the bottom of the form we are
making. Slight downward pressure against the clay at the bottom with the
wheel spinning at normal throwing speed and movement of that pressure
across the bottom in toward the center has the effect of knocking those
particles down flat so that they are paralell to the line of the bottom.
With their long axes in line with the bottom, there are fewer spaces
between particles across the bottom, hence less shrinkage in drying as the
water molecules leave and clay particles move together to fill the empty
spaces between them.
I hope this makes it clear. I know that some have expressed the opinion
that =22compression is a myth=22, but I think that opinion is just due to a
failure to recognize that as potters use the term it has a different
meaning than the standard dictionary definition. It does work, call it what
you will=21
Michael McDowell
Whatcom County, WA
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