Cindy on mon 19 may 97
Hello everybody.
I recently tried throwing a bunch of tea cups off the hump. Most of them
ended up with 'S' cracks in the bottoms. I guess maybe that's because I
couldn't compress them properly since they weren't on the bat. You think?
Any suggestions, etc., would be appreciated. I like throwing them this way,
but not sure how to solve this problem. Thanks,
Cindy Strnad
Rick Swenson on tue 20 may 97
>----------------------------Original message----------------------------
>Hello everybody.
>
>I recently tried throwing a bunch of tea cups off the hump. Most of them
>ended up with 'S' cracks in the bottoms. I guess maybe that's because I
>couldn't compress them properly since they weren't on the bat. You think?
>Any suggestions, etc., would be appreciated. I like throwing them this way,
>but not sure how to solve this problem. Thanks,
>
>Cindy Strnad
----reply--------------
Hi Cindy,
Compression is indeed important in all throwing. Keeping the moisture out
of the bottom of the piece will help too. I find that using firmer (ie
drier) clay for throwing off the hump is helpful. I use a small wood rib
to help compress the bottoms of hump thrown items...and am careful to trim
the items...or at least use a modelling tool to finish the bottom. Drying
carefully is helpful, but compression is the answer.
Practice makes perfect.
Ric
**********************************************************
Ric Swenson, Bennington College, Route 67-A Bennington, Vermont
05201-6001 ph 802 442-5401 x 262 fax x 237 (dedicated line for direct
fax 802 442-6164) email: rswenson@bennington.edu
NOTE: opinions expressed are my own, not necessarily those of my employer.
**********************************************************
Brad Sondahl on tue 20 may 97
Cindy wrote:
> I recently tried throwing a bunch of tea cups off the hump. Most of them ended
Some clays are more susceptable to S cracks than others. But
compression is probably the key. A Japanese potter I talked to
recommended using a wooden rib to press down firmly in the bottom when
throwing. Compressing the leather hard clay when it's upsidedown (as
for footing) can also help. You will notice that most S cracks appear
on the bottom, whether they make it through to the inside, or not.
--
Brad Sondahl
bsondahl@camasnet.com
http://www.camasnet.com/~asondahl
PO Box 96, Nezperce ID 83543
"When it comes to dabbling, I'm just a dilettante..."
Ron Roy on tue 20 may 97
>----------------------------Original message----------------------------
>Hello everybody.
>
>I recently tried throwing a bunch of tea cups off the hump. Most of them
>ended up with 'S' cracks in the bottoms. I guess maybe that's because I
>couldn't compress them properly since they weren't on the bat. You think?
>Any suggestions, etc., would be appreciated. I like throwing them this way,
>but not sure how to solve this problem. Thanks,
>
>Cindy Strnad
Cindy,
You will find a pile of answers in the Archives on this. I'll just add a
few notes that may not have been covered.
When centering that little ball on top of the hump - use enough lubrication
and try not to twist the clay. Twisted, stretched and torn clay is weakened
and will come apart more easily than compressed clay.
The bottoms of your cups are probably also thicker than usual so the
outside clay tends to dry faster. The thicker clay at the bottom lags
behind in the drying - the drier clay around the outside keeps it from
shrinking when it has to.
The combination of not twisting the clay during centering and compressing
the clay around the bottom before cut off combined with slow even drying
before trimming will probably get rid of the cracking.
It would also be helpful, in trying to diagnose this problem, to know the
wet to dry and overall fired shrinkage of your clay.
The Hamer book has the best section on cracking I have ever seen. You find
the crack that looks like yours and they explain how it happened.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
ronroy@astral.magic.ca
Scott P. Ross on tue 20 may 97
after you cut off the cup you just threw off the hump, use no water in
closing up the top of the clay, all those particles are exposedand real
receptive to extra water retention, close it up first then raise your
next amount of clay for the next cup, use as little water as possible,
trim'em well
scott ross, michigan native
Bill Amsterlaw on tue 20 may 97
Hi Cindy:
You wrote:
>> I recently tried throwing a bunch of tea cups off the hump. Most of them
ended up with 'S' cracks in the bottoms. I guess maybe that's because I
couldn't compress them properly... <<
Ways to keep bottom clay compressed while working off the hump:
1. Start out with clay that is
a. not too soft
b. well wedged
2. Before throwing each piece:
a. Use a rib to squeegee the soft stuff off
the entire hump. Leave no soft stuff
near the top.
b. Wet your hands and the hump.
c. Compress the clay at the top of the hump as
forcefully as the clay will allow both down
and coning in. (You should be able to feel
the clay get stiffer under your hands.)
3. Work fast so that the clay does not get
soggy.
4. Last steps in throwing each piece before
cutting off:
a. Sponge any water out of the bottom
b. Run a rib over the bottom
c. Cone-in the clay UNDER the piece
5. Do your cut-off with the wheel slowly turning.
- Bill Amsterlaw (wamster@msn.com)
Plattsburgh, NY
David Hendley on wed 21 may 97
>I recently tried throwing a bunch of tea cups off the hump. Most of them
>ended up with 'S' cracks in the bottoms.
>Cindy Strnad
> You got lots of good advice today. Throwing technique is very
important. You have to think about every subtile step in the process. Here's
a tip that will stop the S-cracks, but its better if you don't have to use
it. (work on technique first).
When the teacup is leather-hard invert it over a dowel of the
appropriate diameter (slightly smaller than interior diameter). Put another
dowel on the outside bottom and pound it. This will compress the clay
between the 2 dowels. As someone else pointed out, some clays are more prone
to S-cracks, and this extra step will help if you have a particularly
stubborn clay.
David Hendley
Maydelle, Texas
Pierre Brayford on wed 21 may 97
Cindy wrote:
>
> ----------------------------Original message----------------------------
> Hello everybody.
>
> I recently tried throwing a bunch of tea cups off the hump. Most of them
> ended up with 'S' cracks in the bottoms. ......
> Cindy Strnad
I've been interested in the replies ... compression of the base is I
suspect the most important. Nobody has mentioned using an absorbent
board to put the lifted off pots on.
When throwing any pots that I lift off I use pieces of absorbent board
as ware boards - this speeds up the drying of the base considerably.
The pots can be transferred to other boards quite quickly if necessary.
The board is supplied as as a fire proof board for use in the building
trade. It comes in several thicknesses the largest available here (UK)
being 12mm it is fairly easy to damage and not cheap to buy (only
available here in 8' x 4' sheets!) but I wouldn't be without it. Can
also be used for drying clay, slabs etc.,
Pierre
Ric Swenson on thu 22 may 97
I had almost forgotten this old trick, which you might have some success
with..in preventing s cracks. If the bottom if wetter than the side
walls...to slow the drying process, COAT the FOOT with wax resist.
Hope this might help.
Ric Swenson
>----------------------------Original message----------------------------
> >I recently tried throwing a bunch of tea cups off the hump. Most of them
>>ended up with 'S' cracks in the bottoms.
>>Cindy Strnad
>> You got lots of good advice today. Throwing technique is very
>important. You have to think about every subtile step in the process. Here's
>a tip that will stop the S-cracks, but its better if you don't have to use
>it. (work on technique first).
> When the teacup is leather-hard invert it over a dowel of the
>appropriate diameter (slightly smaller than interior diameter). Put another
>dowel on the outside bottom and pound it. This will compress the clay
>between the 2 dowels. As someone else pointed out, some clays are more prone
>to S-cracks, and this extra step will help if you have a particularly
>stubborn clay.
>David Hendley
>Maydelle, Texas
The Shelfords on sat 31 may 97
Ric Swenson wrote (ages ago - sorry if this is a fly-blown subject):
>I had almost forgotten this old trick, which you might have some success
>with..in preventing s cracks. If the bottom if wetter than the side
>walls...to slow the drying process, COAT the FOOT with wax resist.
I would have thought you would want to slow the drying on the sides, so that
the wetter foot could catch up - equalizing and all that stuff. But maybe
there is a more subtle advantage to slowing the foot drying even more? Or
am I reading this wrong? (This is awfully early in the morning - our cat
started catching imaginary mice among our toes at 5:15, and I couldn't get
back to sleep...)
- Veronica
____________________________________________________________________________
Veronica Shelford
e-mail: shelford@island.net
s-mail: P.O. Box 6-15
Thetis Island, BC V0R 2Y0
Tel: (250) 246-1509
____________________________________________________________________________
Ric Swenson on sun 1 jun 97
Hi there group,
The method I described coats the entire foot...ie entire bottom portion of
the piece....the part usually trimmed....with wax resist, acrylic medium,
floor wax or "what have you", that will burn off in the bisque and not
give you toxic odors. ( remember to always ventilate firing kilns!)
Going up the sides of the piece couldn't hurt. Slowing the rate of drying
seems to help my clay body. I dry pieces up-side-down usually and coat
only the outside foot area IF I suspect the hump thrown pieces might be
subject to the dreaded "SSSSSSSScrrracccckkks !" These thrown pieces are
usually tea bowls...small jars...small lids....etc...
.............
.............Results may vary with our body, tax and license extra, your
mileage may vary, not available in some states, etc...
Regards and happy potting !
Ric Swenson
>----------------------------Original message----------------------------
>Ric Swenson wrote (ages ago - sorry if this is a fly-blown subject):
>
>>I had almost forgotten this old trick, which you might have some success
>>with..in preventing s cracks. If the bottom if wetter than the side
>>walls...to slow the drying process, COAT the FOOT with wax resist.
>
>I would have thought you would want to slow the drying on the sides, so that
>the wetter foot could catch up - equalizing and all that stuff. But maybe
>there is a more subtle advantage to slowing the foot drying even more? Or
>am I reading this wrong? (This is awfully early in the morning - our cat
>started catching imaginary mice among our toes at 5:15, and I couldn't get
>back to sleep...)
>
>- Veronica
>____________________________________________________________________________
>Veronica Shelford
>e-mail: shelford@island.net
>s-mail: P.O. Box 6-15
> Thetis Island, BC V0R 2Y0
>Tel: (250) 246-1509
>____________________________________________________________________________
Ric Swenson, Bennington College, Route 67-A, Bennington, VT 05201-6001 ph
802 442-5401 vox 262 fax 237 direct fax line 802 442-6164 or
email rswenson@bennington.edu
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