Jim V Brooks on tue 11 jun 02
Vince, thank you for your suggestion. I will try coils.!!. I have, in
the
past, used silica sand..grog etc. with a high rate of success. However,
the coil idea seems to offer better circulation of the heat...
I can't help but wonder..just how much insulating effect you have from a
thin
layer of sand in a 1800 or 2 K degree kiln. I cant believe that is really
does much insulating.
Maybe. for a very brief moment..a jiffy..there could be some variance.. but
not long enough to have much of an affect. I am really curious about
putting a dry slab on wet coils and dry them with a low heat setting ...and
the potential danger in this.. BUT..I am going to try them (coils) tomorrow
when i load a kiln with a large number of slabs..
Jim in Denton...........
vince pitelka on thu 11 jul 02
> The sand basically seals the underside of the piece and prevents ANY air
> circulation. Because a slab never fits perfectly flat on the shelf, the
air
> circulation is much better without the sand/grog than with it. But of
> course the piece is not evenly supported, and that is not a problem at
all.
I have to correct my own problem here. What I meant to say is "But of
course the piece is not evenly supported, and that is part of the problem."
That's what comes of typing faster than I am thinking.
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
vince pitelka on thu 11 jul 02
The use of a bed of sand or grog under a slab piece has been suggested
numerous times on Clayart, and with some claybodies this might work fine,
but if the objective is to allow air to flow under the piece for even
heating/cooling, then about the last thing you want to do is fire the piece
on a bed of sand or grog, which will very effectively insulate the bottom of
the slab from heating/cooling.
As I have suggested a number of times in the past, the only sure-fire method
of supporting such a piece in the kiln is with a series of parallel coils of
clay rolled to about 3/8" diameter. Use any clay. They will not stick to
the piece, and when they dry during the firing they will crack here and
there and it won't hurt anything. Lay the freshly rolled coils across the
two shelves every two inches or so, all parallel to one another. Do not
criss-cross them, because of course the whole point is to leave the ends of
these "channels" open to allow circulation of air.
Immediately after laying the coils, gently nestle the slab onto them so that
they collapse slightly, in order to insure uniform support of the slab
overall. If you find you are not getting uniform support of the slab with
3/8" coils while still maintaining air spaces beneath the slab, use thicker
coils.
You can start the firing immediatley, but preheat gently for at least four
hours to dry out the coils, and then bisque fire as you normally would. In
the glaze firing, do exactly the same thing with freshly-rolled coils, but
then conduct the early part of the firing just as you would a bisque firing
in order to dry and fire the coils without causing problems.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
vince pitelka on thu 11 jul 02
> I can't help but wonder..just how much insulating effect you have from a
thin
> layer of sand in a 1800 or 2 K degree kiln. I cant believe that is
really
> does much insulating.
Jim -
The sand basically seals the underside of the piece and prevents ANY air
circulation. Because a slab never fits perfectly flat on the shelf, the air
circulation is much better without the sand/grog than with it. But of
course the piece is not evenly supported, and that is not a problem at all.
> Maybe. for a very brief moment..a jiffy..there could be some variance..
but
> not long enough to have much of an affect.
On the contrary, with a low-thermal-shock-resistance body, a bed of sand or
grog on the shelf under a flat slab piece will seal out all air circulation
and is a virtual guarantee of problems.
> I am really curious about
> putting a dry slab on wet coils and dry them with a low heat setting
...and
> the potential danger in this..
If you think this is a problem, than you are WAAAYYYY too cautious and timid
in your firing practices. There is absolutely no danger to this at all.
There is no way it can adversely affect the dry slab, and if you do as I
clearly suggested in the previous post, the coils will dry out without ever
causing any problems. As a matter of general practice, we usually preheat
our toploader electrics overnight with the bottom element on low, and we can
put wet pots in the kiln and bisque fire them the next day.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
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