Talbott on mon 8 dec 97
"S" cracks are a real problem in pots that are thrown off the hump...
Probably has something to do with the lack of compression at the pot's
bottom while the pot is being thrown. At least that is what we have
experienced. ....Marshall
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Pranton on tue 9 dec 97
I have compressed my heart out, both from the outsides of the bottom and on
the bottom from above. I was told years ago that I torque the clay too much
when coning and centering on the wheel. I try to use a lighter hand nowadays
and it's better, but still lose one in ten pieces.
I think I need to mellow out even more and not attack the hump in the
beginning. I should prob do a little meditation first and tell the clay it's
my friend before starting. Seriously, I do think most of my problem is still
torquing and I'm working on it. I always remember the story I heard years ago
about a potter who could throw without hardly using any water, he just had the
feel down right. I don't ever expect to reach that plane, just improve myself
here and there along the way.
Lynne Antone
Olympia WA
Louis Katz on wed 10 dec 97
If you are trying to cut down the torque on the uncentered ball of clay,
try patting it on center. Slap the uncentered ball of clay with your
hand(s) moving you hand to the same spot with each slap.keep working the
lump until it is near centered and near the shap you need. Some clays
hardly S crack at all, you might want to try changing your formula.
Louis
Michael McDowell on wed 10 dec 97
------------------
Hi again gang,
Eric asks about how to get same sizes for pieces thrown off the hump.
Jennifer responds that with practice one learns to raise the same amount of
clay off the hump before opening and that will do it, Then Marshall chimes
in that in his experience there are too frequent s-cracks in pieces thrown
off the hump. That's what I've seen on this thread so far of late.
I would concur with Jennifer that one helps to limit the range of sizes
considerably by making a distinct knob of clay above the rest of the hump,
the size of which you gauge by feel.How do you know what size starting knob
will make the right size finished piece? If you want all your pieces to be
pretty much a particular size, then start with a guess at the right size
knob and measure the result with your calipers or some sort of template.
Keep that up till you zero in on the right amount. Of course, to be
consistent every step from the knob on should feel the same. No substitute
for practice here. It helps to not be too particular about exact size
measure. Throw fifty or a hundred pieces. You can make sub-groupings of
same size pieces after the fact if you want to present them as sets.
S-cracks are another matter. Practice alone will not eliminate them if you
don't have a workable hypothesis for their cause and cure. I think that
Marshall is on the right track with suggesting that it has something to do
with a lack of compression, as potters use that term. But it sounds as if
he's abandoned the practice of throwing of the hump altogether because he's
found no cure for it. Rest assured, those who would like to master the
technique, the s-crack problem can be beaten back to reasonable
proportions. How?
Start examining your throwing process right at the start. When you are
managing the whole hump of clay, bringing it in to rough center and
bringing up the individual knobs. If the mound is forming a cup at the top
when you bring it up you are creating a sort of vortex in the center -
s-crack city. Learn to move the clay in the mound with pressure directed
through to the heart of the mass, not raking up the sides. Many successful
hump throwers insist that the knob should not be raised from the clay at
all. Rather they lower the rest of the mass around it. This approach leaves
a relatively large amount of unusable clay spread out on the wheelhead at
the end of the hump, but hey, it works for them. The next step that seems
to have some impact on the amount of s-cracking is the opening. Just as
with pieces thrown on bats or the wheelhead, you should make some effort to
throw or rake back in to the center after opening. The last part of the
throwing process that will impact s-cracking is cutting the piece off the
hump. This should be done with a slow wire while the wheel is spinning
slowly. This will insure that as the wire drags through the clay it is
pushing back in toward the center. The cut off is especially important if
you are not planning to trim the pieces later. I was never able to reduce
my s-cracks below about five percent when I was making mugs off the hump
that I didn't trim the bottoms. But I learned from observation which pieces
would develop the cracks. It is invariably the thicker bottomed ones that
will crack. I found that if I could distinguish where to cut the piece well
enough to insure that the center of the bottom was thinner than the
surrounding walls, the problem almost disappeared. But hey, nobody's
perfect, and I didn't want to cut the bottom out of the piece, so my error
factor on bottom thickness accounted for my loss to s-cracks. It's been
years since I was willing to turn out untrimmed bottoms as the standard of
my work. It's much easier to control the relative thickness of your bottoms
if you are trimming, and if you are trimming while the clay at the bottom
is still pretty damp, it won't matter much how thick it was when you cut it
off.
Well, that's enough of that for this morning.
Happy humping...
Michael McDowell
Whatcom County, WA USA
Paul Lewing on wed 10 dec 97
My experience with throwing off the hump, which I used to do quite a
lot of back in the days when I made round stuff, was that the sole
determiner of whether a pot would have an S crack in it was the
thickness of the bottom. If it was thin enough there was no crack. I
used to use a twisted monofilament cutoff wire, and if I could see a
ripple go through the bottom when I ran the wire under the bottom with
the wheel turning, there would be no crack. This was on stuff like
mugs that I left untrimmed with just the horse-eye pattern on the
bottom. By the way, that is quite thin, if you can see the wire go
through- less than an eighth of an inch.
Dave Shaner used another trick on the bottom of his tea bowls thrown
off the hump. After he trimmed them, he stuck on a tiny ball of wet
clay in the center of the bottom, then mushed it down with about one
turn of the wheel so he ended up with this kind of spiral point that
looked like it was the result of really fast and loose trimming. Never
cracked.
Paul Lewing, Seattle
Kris Baum on thu 11 dec 97
Michael McDowell wrote ...
> The next step that seems to have some impact on the amount of s-cracking is th
>as with pieces thrown on bats or the wheelhead, you should make some
>effort 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
===============================================
Kris Baum, Shubunkin Pottery
mailto:shubunki@erols.com
===============================================
Joyce M. Gearhart on thu 11 dec 97
Hi Lynne,
Are you throwing with a kick or with an electric wheel?
I have two wheels, both a kick and an electric. A friend of mine who
uses only an electric came over one day to throw with me. As I threw on
the kick and she on the electric, I noticed she had the speed of the
wheel a too fast, causing twisting in her pot walls, the same result as
too much torque. As soon as she slowed the wheel down a little, her
pots were much better.
When the wheel spins too fast, the pressure exerted by a hand movement
is exacted on a small portion of the pot making it uneven. Perhaps I
haven't explained this too well but if your using an electric, it's
worth a try.
Having first learned to throw on a kick wheel, I learned that very high
speeds are not needed and, are sometimes detrimental to throwing.
Good luck,
Joyce, in Bethesda, Maryland, painting the front door magenta (tried
true red but looked a tad Republican for me) and staying chilled to the
bone while it's off the hinges.
Eleanora Eden on fri 12 dec 97
When I used to throw off the hump I made and bisqued a set of mushroom-like
thingies upon which to invert a thrown shape after it had been trimmed and
with the porcelain pestle from my mortar and pestle press once firmly in
the middle of the bottom. No more s cracks.
Eleanora
Eleanora Eden 802 869-2003
Paradise Hill
Bellows Falls, VT 05101 eden@sover.net
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