Bruce Girrell on tue 18 sep 01
> Not your run of the mill s-cracks, no!! I've got circular cracks
> in the bottoms of two of my large jars. They have at least a ten
> inch, if not more, wide bottom. The cracks are not full circles,
> about half way around.
Well, I can't count myself in the Big Dog circle, and once again I don't
have my copy of Hamer and Hamer here, but to me it sounds as if they dried
too quickly. With an object of that diameter, the outside rim can easily dry
faster than the center of the bottom. Once that outer edge has set up hard
there is no way that it can shrink as the inner portion of the bottom tries
to shrink. Eventually the bottom fails in tension. The shape of the crack is
in accordance with this type of failure.
Bruce "my best guess" Girrell
anthony allison on tue 18 sep 01
Hey Chris
I make big pots with wide bottoms, and i think the thing about compressing
the clay is an old wives tale completely. Ususally, a pot will dry on the
exterior walls faster than the bottom which is not exposed to air. On narrow
bottomed pieces the differential doesnt seem to be a major problem, but on
wide bottoms, the stresses in differential shrinkage is enormous. Just as
on a huge platter, you want the center drying faster than the rim. This way
as drying goes along, the clay is compressing upon itself rather than trying
to pull itself apart. I try to keep big pots loosly covered with plastic
till they are firm enough to handle, and i take them outside and turn them
bottom up in the sun, rotating them often, with the aid of a hair dryer, i
get the bottom dryer. The rule of thumb is easy, the closer to the center
of the bottom, the dryer. Once that condition is established, normal air
drying works fine.
With big platters, i use a swing arm light playing over the center of the
piece, up about 16 to 18 inches from the center surface. This gentle warmth
makes the platter dry more from the center outwards, with a bit of care and
watchfulness,(don't want to let the center get too disproportionately dry
than the exterior) (a water spritzer is needed sometimes in very dry air to
keep the Rim damp) it is a great tool as it eliminates cracking while
allowing you to get a piece done in a reasonable amount of time. After this
treatment to a very firm leather hard state, i cover the piece overnight to
let everything even out. Then trim and normal air dry. Never let the lip dry
faster than the rest or lip cracking is almost a certainty.
Hope this helps you avoid a few frustrations. I know the sweat that goes
into a big pot lol.
Warmest regards,
Tony
----- Original Message -----
From: "chris clarke"
To:
Sent: Wednesday, September 19, 2001 1:17 PM
Subject: circular cracks
Not your run of the mill s-cracks, no!! I've got circular cracks in the
bottoms of two of my large jars. They have at least a ten inch, if not
more, wide bottom. The cracks are not full circles, about half way around.
And how I compressed, I stood up and leaned into it I compressed so much (no
replies on proper use of compress, you all know what I mean). And I coned.
Dave Finkelnburg on tue 18 sep 01
Chris,
Sorry to hear about the big pots cracking. That's frustrating!
Of the dozen or so commercial clays I have tested, B-mix is by far the
most difficult to dry without cracking, regardless of shape. Wide forms are
the most difficult forms to dry without cracking. I think you are a victim
of double-jeopardy.
Did you invert your pots when stiff so they dried bottom up? That often
helps.
The cutoff wire you use makes a difference. You don't want the bottom
hanging up in the drying process, right at the start. Sometimes wiring the
piece off again after a few hours can help.
Try looking at a cross section of one of the pots before you boot it.
:-) Ideally the bottom and sides will be similar in thickness.
I hope this helps you think about the problem.
Good potting,
Dave Finkelnburg
From: chris clarke
....I've got circular cracks in the bottoms of two of my large jars. They
have at least a ten inch, if not more, wide bottom. The cracks are not full
circles, about half way around. And how I compressed, I stood up and leaned
into it I compressed so much (no replies on proper use of compress, you all
know what I mean). And I coned.
Oh, and I'm using B-Mix....
iandol on wed 19 sep 01
Dear Chris clarke,
No doubt I will be late with this due to the time differences and =
because I get the Digest. But for what it is worth, F. Hamer(1st Ed.) =
puts this one down to thermal shock or if this occurred during the =
transition form wet to dry, uneven thickness between base and wall =
causing stress due to uneven shrinking as the clay dries. I would =
imagine that your bottoms are thicker than your walls. Had this sort of =
crack once in a large wine jar when I had trimmed the chamfer a bit to =
much between wall and base. Only noticed it when I was testing the =
wooden spigot for leaks. Got this moist feeling down the fly and thought =
I had left the tap in the open position. No! just a big fissure with =
water streaming down the front of my pants.
Cindy Strnad on wed 19 sep 01
Hi, Dave.
1. Add grog. Just dump some in and wedge--maybe 1-2 cups per 25 lbs.
2. Knead and wedge well. After kneading, turn the clay on its end
and shape it into a throwing pug. This changes the direction of the
kneading spiral and may help.
3. Make sure the base is of even thickness. This has been the primary
cause of circle cracks in my experience--a base with varied thickness.
4. Make sure the inner junction between base and wall is not sharp or
thin. This area does tend to get thinner than I want it.
5. Remove your pot from a non-porous bat as soon as possible. Set it on
drywall or, at least, newspaper. Turning pots over is a good idea if they
can take it. If the bottom sags, though, at least have it on a
dry, absorbent surface.
6. As others have said, take care that the bottom is similar in thickness
to the walls of the pot.
7. Dry slowly once the piece reaches leather hard. Your environment will
determine the steps you need to take. If you live in the hot, dry desert,
you may want a damp box (old refrigerator is what I use). If you're
having humid weather, a sheet of plastic may suffice. Remember to
have the pot sitting on an absorbent surface.
8. Pray, repent, make appropriate sacrifices. Fire slowly--both in bisque
and glaze.
Best wishes,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
chris clarke on wed 19 sep 01
Not your run of the mill s-cracks, no!! I've got circular cracks in the =
bottoms of two of my large jars. They have at least a ten inch, if not =
more, wide bottom. The cracks are not full circles, about half way =
around. And how I compressed, I stood up and leaned into it I =
compressed so much (no replies on proper use of compress, you all know =
what I mean). And I coned.
But, damnit, they cracked. Before I kick them around my studio, why =
would they crack like this? Are they grabbing the table and not sliding =
as they shrink?? My foot is poised!!=20
Oh, and I'm using B-Mix, but have had no problems with it cracking =
before. I'm throwing soft, thanks to the "big dogs" on the list. And =
I'm using about twenty five pounds, centering with ease thanks to =
kickboxing. =20
chris, =20
with two hundred pounds of reclaim way past the too dry point because I =
forgot about it.
temecula, california
chris@ccpots.com
www.ccpots.com
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