Butch Welch on sun 7 dec 03
Randy,
A lot has been written on this subject in the past. A lot of theory's on
technique, and I am sure a little bit of each will contribute to success.
One thing I would suggest to try that appeared to work for me in the last
sets I did was to slow down the drying of the bottom of the pot or bowl. I
covered them up with plastic and let it dry much slower. I am not an
expert, or full time potter. Good luck, regards, Butch
Randy McCall on sun 7 dec 03
I switched from Standard 112 to Desert Buff and now get a lot less cracks in
the bottoms of ware, but still get about 10% I have to throw away due to
small s type cracks in the bottoms of pieces. Is this the best I can do or
is it my throwing technique?
Randy
Chris Schafale on mon 8 dec 03
You can do better. I started out using 112 and used have S-cracks in about
half my pots. I switched to Highwater Speckled Brownstone and, like you,
saw a dramatic reduction, but still had some losses. I tried many things, but
the ones that I continue to do are the following (and I never have s-cracks
now, except occasionally when throwing off the hump).
1) compress the bottom well
2) don't use plastic or other non-absorbent bats
3) get the pots off the bats as soon as practical, and put them on wareboards
made of sheetrock, or plywood with a couple of sheets of newsprint over it
Of these, I think that getting the bottoms to dry more expeditiously is the
most important.
Good luck.
Chris
On 7 Dec 2003 at 18:21, Randy McCall wrote:
> I switched from Standard 112 to Desert Buff and now get a lot less cracks in
> the bottoms of ware, but still get about 10% I have to throw away due to
> small s type cracks in the bottoms of pieces. Is this the best I can do or
> is it my throwing technique?
>
>
> Randy
>
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Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
Carol Tripp on mon 8 dec 03
Look to your clay. I've used Tucker's since I started and the ONLY time I
have ever had S-cracks is when throwing off the hump before I read how to do
it on Clayart. Otherwise, the clay was good enough to cover my
learner-driver techniques. (But don't forget to tidy up water, compress and
dry at appropriate speeds.)
Best regards,
Carol
Dubai, UAE
>On 7 Dec 2003 at 18:21, Randy McCall wrote:
>
> > I switched from Standard 112 to Desert Buff and now get a lot less
>cracks in
> > the bottoms of ware, but still get about 10% I have to throw away due to
> > small s type cracks in the bottoms of pieces. Is this the best I can do
>or
> > is it my throwing technique?
> >
>
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Michael Wendt on mon 8 dec 03
Randy,
For pots thrown on the bat, I think 10% loss rate is very high. I lose 1-2%,
probably due to timing on drying since I leave the pots out to dry until
they can be inverted and the bottom burnished with a stainless steel rib.
If you lose 10% of the hump pots, improved technique can reduce this
number. I suspect a lot of "S" cracking comes in applying too much inward
force when coning up the hump to set the base size.
What appears to happen is the beginnings of a crack form when the clay is
torqued so severely that it cavitates ( forms small torn places) which are
the nucleation points for the "S" crack. Bottom compression inside and
outside can heal most of these cracks since they are under a high enough
vacuum to rejoin solidly.
Either use a rib or the tip of your finger to create a high pressure area to
recompress the bottom after you blank out the form.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
Earl Krueger on mon 8 dec 03
Randy,
I have been using a clay that likes to S crack but have eliminated most
of it by compressing the bottom. Yesterday a bowl cracked, big time,
and upon inspection I found that the bottom was considerably thicker
than the sides.
In this case I believe the sides dried and became rigid so as the
bottom continued to shrink there was no flexibility and a crack was the
only way for the clay to relieve the stress.
Earl...
Bothell, WA, USA
Susan Setley on mon 8 dec 03
You should not get s-cracks. Make sure you are compressing the bottom very,
very well. Are you throwing off the hump? You could still put it on the wheel
and compress the bottom.
Make sure the bottom and the sides are about the same thicknes, and don't
rush the drying.
Are you separating it from the bat with a wire? Do you dry it on a bat?
Ron Roy on wed 10 dec 03
By all means compress bottoms - don't twist the ball while centering any
more than you have to (the s in the crack is because of this part) and dry
slow enough to have the clay all the same dryness before you trim.
I am also sure the reason some clays are more of a problem than others has
to do with plasticiy - if it's too high or too low.
If anyone who is throwing off the hump has any shrinkage (wet to dry and
over all to glaze temp) info on the clays they use would they post it to
see if that is a link to the problem.
RR
>You can do better. I started out using 112 and used have S-cracks in about
>half my pots. I switched to Highwater Speckled Brownstone and, like you,
>saw a dramatic reduction, but still had some losses. I tried many things, but
>the ones that I continue to do are the following (and I never have s-cracks
>now, except occasionally when throwing off the hump).
>
>1) compress the bottom well
>2) don't use plastic or other non-absorbent bats
>3) get the pots off the bats as soon as practical, and put them on wareboards
>made of sheetrock, or plywood with a couple of sheets of newsprint over it
>
>Of these, I think that getting the bottoms to dry more expeditiously is the
>most important.
>
>Good luck.
>
>Chris
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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