Vince Pitelka on mon 18 sep 06
Snail wrote:
> I agree with you that a bed of sand is problematical,
> but I find that a very thin dusting of sand is entirely
> adequate to allow movement between the clay and
> the shelf, while keeping the heating and cooling
> conditions similar to a bare shelf. The sand grains
> act like little ball bearings, and it doesn't take much
> to get a real benefit.
Snail -
That's a good point. Large sculptural pieces will benefit from a very light
dusting of sand or grog on the shelf, and it does exactly as you say - acts
as tiny ball bearings. But with large flat slab pieces, firing them flat on
the shelf with our without sand is about the same thing. The shelf itself
insulates the center underside of the flat piece from changes in
temperature, so the center of the piece heats up slower in the firing and
cools down slower in the cooling, which can cause differential shrinkage and
resulting cracking. The degree of susceptibility of course depends on the
particular claybody and the rate of heating/cooling, but I have found that
the most reliable way to avoid such problems is to fire any at-risk pieces
on wadding. I've been doing this for decades.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Snail Scott on tue 19 sep 06
On Sep 18, 2006, at 7:56 PM, Vince Pitelka wrote:
> Snail wrote:
>> I agree with you that a bed of sand is problematical,
>> but I find that a very thin dusting of sand is entirely
>> adequate...
> Vince wrote:
> ...Large sculptural pieces will benefit from a very light
> dusting of sand or grog on the shelf, and it does exactly as you say -
> acts
> as tiny ball bearings. But with large flat slab pieces, firing them
> flat on
> the shelf with our without sand is about the same thing...
You are right, Vince; I'd missed that we were discussing
flat slabs, not just slab-made forms. Putting spacers
between the slab and the shelf does reduce firing
cracks. Heat retention with a slab in full contact with the
shelf can be a real issue.
-Snail
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