John Baymore on sat 2 aug 97
------------------
Hi all.
I need a little advice here. I have exhausted my personal knowledge base,
the supplier, and my network of friends for the solution to some
unacceptable levels of S cracking (wet to dry) on pieces thrown with a
SPECIFIC CLAY off the hump. I have a little experience working with clay.
I throw off the hump regularly. I don't have trouble like this with most
clays. Even other products from the same supplier..... even the =
=22same=22
clay with a slight mix variation.
The clay is Sheffield Pottery Supply's 16425 with screened fireclay, also
called Salmon Stone (screened version). I have used the non-screened with
much better results, but I have 3500 lbs of the screened to use at the
moment (long story). Is anyone out there using this specific clay and
throwing off the hump with little S cracking? On to details:
The screened version of the clay uses the same fireclay run through a 30
mesh screen. The unscreened uses the stock 20 mesh from the supplier. I
tested the two variations of the body, and averaged samples I did show
about a 1/2 to 3/4=25 higher shrinkage on the unscreened clay...wet to dry.
The tooth of the screened is tactily smoother even to an untrained person.
This clay is super in all other applications I do. Attachments don't
crack. I can throw large bowls with no cracking problems. Platters and
the like behave themselves nicely. Can fast dry them even. Dinner plates
are fine right out in the open, drying with NO cracks ever. As long as the
clay is on a wheelhead and the floor can be compressed pretty well (or
should I say .........particle alligned =3Cg=3E) it is fine.
The problem is happening with very small forms....... traditional Japanese
teacup forms.... yunomi, that are thrown to about 3 1/2 =22 dia. at the lip
or so. Batch of clay is 6 months old according to the batch data on the
boxes. Large mass of clay is pounded to rough center dry on the wheel, two
hands on either side. Top is lubricated and then centered wet. Clay is
coned up gently then pressed back down to form the ball at the top. Open
and first pull is one movement. Floor (slight curve in bottom...not flat)
is then =22compressed=22 a couple of passes....inside out/outside in
(repeated), as much as you can do off the hump. Bottom is pinched in to
set the location of the cutoff. Two pulls after the opening pull. Light
finish forming is then done, lip is chamois'ed. Gestural throwing, slow
wheel speed, not tight symetrical form, no ribbing. Total work time on the
individual piece is about 1 to 1 1/2 minute.
I throw dry, and use slurry not water, so there is little water involved
here. Never a puddle in the bottom of the form.... not even of slurry.
Pieces are cut off the hump with a cord being allowed to wrap around a
notch scribed in the base at the pinch point and flicked to cut through.
Wheel goes usual American direction. Mass of clay setup on the wheel is 25
lbs. per throwing session, and produces about 35 untrimmed pieces.
Pieces are placed on wareboard to sit til just dry enough to invert onto
the rim. They are then inverted til dry enough to trim. Depending on the
weather....either under plastic or out in the open (no correlation to the
cracking seems to exist on this variable). I trim them pretty wet to
(supposedly) help with S cracking (tried trimming drier....no difference
in cracking). Trimming is done in same direction as throwing. Wheel speed
is slow, and trimming is loose and gestural, not machined. SHARP trimming
tools (kana). Foot ring is not particularly deep..... maybe 1/4 inch, by
maybe 1/4 inch wide. Then the pieces are dried.
I have dried them in the open........ horrendous =25 of them cracking....
over 50=25. I have tried drying them under plastic for a week or
more........ still crack to the tune of 40=25 of production. I have dried
them inverted under plastic. I have turned them over multiple times daily
til dry under plastic. I have dried them upright under plastic. I have
dried them on their sides under plastic (looks interesting..... and a level
wareboard is curucial =3Cg=3E.) Nothing in the drying I have tried has =
helped
appreciably.
One thing does show some difference. The old traditional Japanese
technique for =22bad clay=22 of a short, hard, rapid, slapping the center of
the bottom of the untrimmed pieces when they are just dry enough to invert
decreases the cracking rate, but not enough......maybe down to 30=25 =
cracking
rate. It also is a difficult procedure, and causes some complication in
trimming, since the bottom of the piece is distorted quite a bit.
Doable, but slows down the production rate. It is my current solution as
of right now, though.
It is definately a shrinkage and drying issue.......classic S
cracking........ but I haven't found =22THE KEY=22 to this particular clay =
yet
in this application. Do I have to dry for weeks and weeks under plastic?
In that case, I will stop using it for thrown off the hump work. Can't
wait that long.
So, help me out here. Anyone using this particular clay sucessfully off
the hump without jumping through lots of hoops? What can I do get better
success rates?
Best,
...................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
Louis Katz on sun 3 aug 97
Hi John,
I thorw zillions of three inch bowls with a similar process. Slow drying
will not cure the problem. The problem is the clay, and it is particle
size distribution.
One solution that I have only a little experience with that might help
you is to use a stamp in the middle of the bottom of the pot prior to
trimming to compress the bottom. I don't trim my small pots so this isn't
much help to me. I have used a stick about the size of a new unsharpened
pencil and stamped about 6 times around the center of the bottom of a 2
1/2 " foot. This seemed to work, I thought about making a pattered stamp
to do it all in one action.
If you try this and it works for you let me know.
Louis
New Web site:
Louis Katz
Texas A&M University Corpus Campus
lkatz@falcon.tamucc.edu
NEW WEBSITE:http://maclab.tamucc.edu/lkatz/lkatz/index.html
Jennifer Boyer on sun 3 aug 97
Hi Again John,
I have one word for you: SHEETROCK. Get a carpenter friend to give you
some scraps to cut to the size of your ware boards. Put duct tape around
the edges. Top your ware boards with them when you are throwing off the
hump. Putting pots on sheetrock dries the bottoms out first, eliminating s
cracks. it worked for me. I think the idea came from Jonathan Kaplan.
Tell us how that works for you!
Jennifer in VT
>----------------------------Original message----------------------------
>------------------
>Hi all.
>
>I need a little advice here. I have exhausted my personal knowledge base,
>the supplier, and my network of friends for the solution to some
>unacceptable levels of S cracking (wet to dry) on pieces thrown with a
>SPECIFIC CLAY off the hump. I have a little experience working with clay.
>I throw off the hump regularly. I don't have trouble like this with most
>clays. Even other products from the same supplier..... even the "same"
>clay with a slight mix variation.
>
>The clay is Sheffield Pottery Supply's 16425 with screened fireclay, also
>called Salmon Stone (screened version). I have used the non-screened with
>much better results, but I have 3500 lbs of the screened to use at the
>moment (long story). Is anyone out there using this specific clay and
>throwing off the hump with little S cracking? On to details:
>
>The screened version of the clay uses the same fireclay run through a 30
>mesh screen. The unscreened uses the stock 20 mesh from the supplier. I
>tested the two variations of the body, and averaged samples I did show
>about a 1/2 to 3/4% higher shrinkage on the unscreened clay...wet to dry.
>The tooth of the screened is tactily smoother even to an untrained person.
>
>This clay is super in all other applications I do. Attachments don't
>crack. I can throw large bowls with no cracking problems. Platters and
>the like behave themselves nicely. Can fast dry them even. Dinner plates
>are fine right out in the open, drying with NO cracks ever. As long as the
>clay is on a wheelhead and the floor can be compressed pretty well (or
>should I say .........particle alligned ) it is fine.
>
>The problem is happening with very small forms....... traditional Japanese
>teacup forms.... yunomi, that are thrown to about 3 1/2 " dia. at the lip
>or so. Batch of clay is 6 months old according to the batch data on the
>boxes. Large mass of clay is pounded to rough center dry on the wheel, two
>hands on either side. Top is lubricated and then centered wet. Clay is
>coned up gently then pressed back down to form the ball at the top. Open
>and first pull is one movement. Floor (slight curve in bottom...not flat)
>is then "compressed" a couple of passes....inside out/outside in
>(repeated), as much as you can do off the hump. Bottom is pinched in to
>set the location of the cutoff. Two pulls after the opening pull. Light
>finish forming is then done, lip is chamois'ed. Gestural throwing, slow
>wheel speed, not tight symetrical form, no ribbing. Total work time on the
>individual piece is about 1 to 1 1/2 minute.
>
>I throw dry, and use slurry not water, so there is little water involved
>here. Never a puddle in the bottom of the form.... not even of slurry.
>Pieces are cut off the hump with a cord being allowed to wrap around a
>notch scribed in the base at the pinch point and flicked to cut through.
>Wheel goes usual American direction. Mass of clay setup on the wheel is 25
>lbs. per throwing session, and produces about 35 untrimmed pieces.
>
>Pieces are placed on wareboard to sit til just dry enough to invert onto
>the rim. They are then inverted til dry enough to trim. Depending on the
>weather....either under plastic or out in the open (no correlation to the
>cracking seems to exist on this variable). I trim them pretty wet to
>(supposedly) help with S cracking (tried trimming drier....no difference
>in cracking). Trimming is done in same direction as throwing. Wheel speed
>is slow, and trimming is loose and gestural, not machined. SHARP trimming
>tools (kana). Foot ring is not particularly deep..... maybe 1/4 inch, by
>maybe 1/4 inch wide. Then the pieces are dried.
>
>I have dried them in the open........ horrendous % of them cracking....
>over 50%. I have tried drying them under plastic for a week or
>more........ still crack to the tune of 40% of production. I have dried
>them inverted under plastic. I have turned them over multiple times daily
>til dry under plastic. I have dried them upright under plastic. I have
>dried them on their sides under plastic (looks interesting..... and a level
>wareboard is curucial .) Nothing in the drying I have tried has helped
>appreciably.
>
>One thing does show some difference. The old traditional Japanese
>technique for "bad clay" of a short, hard, rapid, slapping the center of
>the bottom of the untrimmed pieces when they are just dry enough to invert
>decreases the cracking rate, but not enough......maybe down to 30% cracking
>rate. It also is a difficult procedure, and causes some complication in
>trimming, since the bottom of the piece is distorted quite a bit.
>
>Doable, but slows down the production rate. It is my current solution as
>of right now, though.
>
>
>It is definately a shrinkage and drying issue.......classic S
>cracking........ but I haven't found "THE KEY" to this particular clay yet
>in this application. Do I have to dry for weeks and weeks under plastic?
>In that case, I will stop using it for thrown off the hump work. Can't
>wait that long.
>
>So, help me out here. Anyone using this particular clay sucessfully off
>the hump without jumping through lots of hoops? What can I do get better
>success rates?
>
>Best,
>
>..................john
>
>John Baymore
>River Bend Pottery
>22 Riverbend Way
>Wilton, NH 03086 USA
>
>603-654-2752
>JBaymore@Compuserve.com
~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*
Jennifer Boyer
Thistle Hill Pottery
Montpelier, Vt. 05602
jboyer@plainfield.bypass.com
Debby Grant on mon 4 aug 97
Hi John,
I just got back from the lake and read your post with interest. Last spring
I tried a sample of the same clay body from Sheffield and threw 8 punch
cup size pots off the hump for glaze testing, as well as some large bowls
not off the hump. Of the 8 test pots only one had an S crack and of course
I didn't care because it was only for a glaze test. I also took no special
care in drying, in fact, dried them very fast after trimming, so I can't
imagine what is going on. However, I have one suggestion for you that
might work if you have a kick wheel or reversable electric. Try trimming
clockwise instead of counter clockwise so as to unwind the clay. And
when you are finished trimming use a rib to compress the bottom and
smooth it out.
Hope to see you in September at our next OK group meeting. Try to
get up to Sunapee. The "Living With Crafts" exhibit is spectacular this
year.
Best, Debby Grant
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