Vince Pitelka on mon 18 sep 06
I wrote:
"Putting such a piece on a bed of sand or grog is never a good idea, because
it insulates the piece from changes in heating and cooling, and can cause it
to crack."
Carole Fox responded:
"Can you explain why the wadding coils wouldn't do the same thing?"
Carole -
I should have been clearer in my description. The objective is to roll out
coils of clay, and then lay them in straight, parallel rows, so that the
spaces are open at the ends. That way, heat and atmosphere can circulate
beneath the piece. It is important that you nestle the piece gently onto
the coils so that they do not collapse completely. That would defeat the
purpose.
The reason for using wadding rather than plain clay is that wadding can be
heated as fast as you would heat any glaze firing, with no chance of the
wadding exploding. The 50% flint allows steam to escape very easily.
- 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/
Vince Pitelka on tue 19 sep 06
Carol Fox wrote:
> At first, my understanding was that a piece with any height to it, (like a
> serving dish, would need to be insulated from the shelf ( to prevent the
> extreme temperature difference that would occur between the rim and the
> bottom of the piece) and that the wadding coils would do that. Also, if a
> piece was flat, like a tile, it would not need insulation from the shelf
> because all parts of it were in contact with the shelf ( so it would heat
> up
> and cool slowly, no?) I thought, that in the case of something flat, using
> the firebrick dust would be helpful to keep the clay from getting caught
> on
> irregularities in the shelf as it moved in the firing.
> Now, after reading your post, I see I still don't understand. Please be
> patient with me.Your statement below is what confuses me.
Carole -
What makes you think that the shelf heats up perfectly evenly? But that's
really beside the point. Unless you are using Advancers, the shelf is much
thicker than the ware, and essentially insulates it from heating and from
cooling. So, if you are firing a large slab piece flat on the shelf, it
heats up around the outside faster than in the center, and the difference
can be extreme. And it cools off faster around the outside while still
remaining very hot in the center. As I said, depending on the type of
claybody, and the rate of heating/cooling, this can be a serious issue and
can cause flat slab pieces to crack, either across the center, or from the
edge towards the center. Obviously, if you elevate the piece off the
surface with wadding, the heat and atmosphere can circulate around the
piece, causing it to heat and cool much more evenly. Same is true of a
large bowl or platter - exactly the same phenomenon takes place - the rim
will heat up faster than the center, and cool off faster. So, cracking can
be a serious issue there as well, again depending on the claybody and the
firing/cooling rate. The safest thing to do with large slab pieces, bowls,
and platters is to fire them on wadding. With flat slab pieces I use
parallel rows of wadding rolled as coils. For large bowls and platters I
use a sunburst pattern of coils radiating out from the center.
I hope this clarifies things -
- 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/
Carole Fox on tue 19 sep 06
Vince-
At first, my understanding was that a piece with any height to it, (like a
serving dish, would need to be insulated from the shelf ( to prevent the
extreme temperature difference that would occur between the rim and the
bottom of the piece) and that the wadding coils would do that. Also, if a
piece was flat, like a tile, it would not need insulation from the shelf
because all parts of it were in contact with the shelf ( so it would heat up
and cool slowly, no?) I thought, that in the case of something flat, using
the firebrick dust would be helpful to keep the clay from getting caught on
irregularities in the shelf as it moved in the firing.
Now, after reading your post, I see I still don't understand. Please be
patient with me.Your statement below is what confuses me.
>I wrote:
> "Putting such a piece on a bed of sand or grog is never a good idea,
> because
> it insulates the piece from changes in heating and cooling, and can cause
> it
> to crack."
Can you explain? Thank you!
Carole Fox
Silver Fox Pottery
Elkton, MD
silverfoxpottery@comcast.net
Bob Masta on thu 21 sep 06
Instead of wadding, how about some pre-made (dried and
possibly fired) little clay bars? You can make a bunch of
these by slicing up a slab. Keep a little box of them near
the kiln. You can make an instant "tile rack" by setting
the tile on 2 bars, then putting 2 bars across the ends to
space up the thickness of the tile and repeating. For a
big platter, you could scatter them in radial patterns as
Vince suggests.
Best regards,
Bob Masta
potsATdaqartaDOTcom
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