George VanOstrand on sun 13 oct 96
I like to throw lids with a knob, upright, off the hump. This
leaves a fairly thick center and I am usually getting an "S" crack on the
bottom side. Does anyone have a suggestion about how to avoid this?
George VanOstrand
gvanostr@julian.uwo.ca
London, Ontario
John Guerin on mon 14 oct 96
The "S" crack is occuring because the outside edge of the lid is drying
faster than the center because it is thinner. Then when the thicker center
continues to dry (and Shrink) it pulls itself apart because it is held in the
hard, dry shell of the outer edge that is rigid. There are two possible
solutions. One is to hollow out the handle from the inside so it is not so
thick, thus allowing it to dry faster (along with the thinner outer edge).
The other solution would be to wrap a strip of plastic ( or saranwrap) around
the thinner outer edge to slow the drying time down so that it dries at the
same time as the thicker center part with the knob attached. Good
Luck----John.
Doug Gray on mon 14 oct 96
George
It has been my experience that "S" cracks come from:
a) not enough compression of the clay
b) letting water stand in the bottom as the pot dries
Since you are throwing these off the hump and right side up, I would
quess that it is a compression problem. This is usually the case with
any thing off the hump. Make sure you center well, and run a rib over
the piece several times before you cut it off.
Hope this helps.
Doug Gray
Alpine, TX - two hours from Mexico
dgray@sul-ross-1.sulross.edu
Jim Connell or set clayart mail on mon 14 oct 96
WINTHROP UNIVERSITY Electronic Mail Message
Date: 14-Oct-1996 08:11am EDT
From: James Connell
CONNELLJ
Dept: Art and Design
Tel No: 323-2126
TO: SMTP%"CLAYART@lsv.uky.edu" ( _SMTP%"CLAYART@lsv.uky.edu" )
Subject: RE: Cracked lids from the hump
"S" cracks from "off the hump" throwing has always puzzled me as well until I
saw a Korean video and finally understood what was meant by compressing the
bottom of the pot. After throwing and completeing your form simply collar the
base under the pot and squeeze, narrowing the column and lifting the pot (or
lid). Then cut off the piece as usual. It is as simple as this and has taken
all the frustration out of throwing off the hump for me.
Jim
Toni Martens on mon 14 oct 96
>> I like to throw lids with a knob, upright, off the hump. This
leaves a fairly thick center and I am usually getting an "S" crack on
the bottom side. Does anyone have a suggestion about how to avoid
this? George VanOstrand gvanostr@julian.uwo.ca
George,
When both your vessel and lid are leather hard,centre the vessel on
the wheel, place the lid upside down on the pot, attach with small
coils at the rim, and then trim the inside of the lid.You can also
hollow out the knob in this way.Your lid will be thinner and less
prone to cracking.
Hope this helps.Let me know if you need further info.
Toni M in Durban feeling penitant over silly Math letter.
Dave and Pat Eitel on mon 14 oct 96
>----------------------------Original message----------------------------
>
> I like to throw lids with a knob, upright, off the hump. This
>leaves a fairly thick center and I am usually getting an "S" crack on the
>bottom side. Does anyone have a suggestion about how to avoid this?
>
>George VanOstrand
>gvanostr@julian.uwo.ca
>London, Ontario
George--My feeling is the s cracks are caused by the difference in
thickness between the center of the lid and the rest of it. You might just
want to put up with them if they aren't too awful. Or, you might consider
adding the knobs, or trimming clay away from inside the lid to get a more
uniform thickness.
Later...Dave
Dave Eitel
Cedar Creek Pottery
Cedarburg, WI
daveitel@execpc.com
http://www.digivis.com/CedarCreek/home.html
ZALT@aol.com on tue 15 oct 96
When throwing off the hump you have to compress the clay at the center of the
piece. I suggest you get Robin Hopper`s book "Functional Pottery" It will
give you some good Info.
Terrance F. Lazaroff,
St Hubert, QC, Canada
PJLewing@aol.com on tue 15 oct 96
To avoid those cracks, trim the bottoms of the lids and trim them fairly
thin. You can invert the lid in its jar as a chuck, but a faster way if you
have lots of lids to trim is to make a trimming chuck out of thick foam
rubber with a hole in the center to accomodate the knob. And when you have
trimmed them thin enough, compress the center of the bottom of the lid with a
wooden tool.
Hope that does it.
Paul Lewing, Seattle
Vince Pitelka on tue 15 oct 96
George -
Doug Gray has it right, but I do not think that going over the lid with a
rib will do any good. many years ago I tried making lids as you describe,
and could not come up with anything to eliminate the S-cracks. The problem
is that when there is a knob on the top of the lid, and the "mushroom stem"
which supports the lid underneath, it is impossible to adequately compress
the clay inbetween. It will partially depend on the claybody. High
shrinkage claybodies are of course much more prone to S-cracks. After lots
of experiments I gave up on this kind of lid. Instead, I throw them off the
hump upside down as a shallow bowl. When leather hard I center and trim
them with the Giffen Grip, and while still centered I score and slip the
center, add a small lump of clay, and throw a knob. It's very fast, because
throwing the lids off the hump as upside-down bowls is so quick. This gives
a very good lid with never an S-crack.
- Vince
Vince Pitelka - vpitelka@Dekalb.Net
Phone - home 615/597-5376, work 615/597-6801
Appalachian Center for Crafts, Smithville TN 37166
Leonard Smith on tue 15 oct 96
I won't go into a long discussion about the reasons for the 'all in one'
lids S cracking but I will add my solution to the problem.
As already mentioned it is important to squeeve the clay beneath the lid to
get a little extra compression, Once cut off and in the early stages of
drying, not hard leather, I turn the underside of the lid and then tap the
base onto a bench, not too hard but hard enough to get just that little
extra compression. I usually then turn them to fit at the normal leather
hard stage
Best wishes
Leo
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Leonard Smith ARCADIA STUDIOS Email: smithl@ozemail.com.au
10 Marrakesh Place
Arcadia, 2159, NSW, Australia. Phone + 61 2 9653 2507
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