Pat Chesney on tue 20 may 97
Hi all you vapor-type guys.
I am having a problem with my clear liner. On all our cone 10+ firings, in
ox or red., we have a great clear. In salt fire, however, if the vapor hits
the inside of, say a bowl, or if the clear is on the outside of the pot,
then it crazes horribly. If we prevent the vapor from hitting the surface
of the clear, it does well. (or if it is very thin on the outside of the
pot). This glaze does very well in our wood fire kiln-no crazing at all. It
will be matt if it doesn't get a good cone 10 if that helps any.
Donna's Clear
Custer Feldspar 32
Flint 25
Whiting 20
Kaolin 16
Bentonite 1
Estimated thermal expansion (from Hyperglaze)----72.79
Do any of you guys have a clear cone 10 that does good in salt? I can't
handle the terrible crazing. (I don't want to use Barium or frits). I am
going to try the glaze "Leach Clear" for tests next time. Has anyone tested
this in salt?
Leach clear from Clayart database
Flint 30
Whiting 20
Custer feldspar 20
Neph sye 20
EPK 10
Estimated thermal expansion--74.66
Do any of the glaze dudes understand why my glaze crazes with salt? I would
like to understand more. I can run the glaze through Insight and Hyperglaze
to get the formulas and molecular weights and limits and such.
Thanks for the help,
Pat Chesney
Pat-Chesney@easy.com
Waco, Texas
Paul M Wilmoth on wed 21 may 97
Dear Pat
The glaze is mixed with the soda vapor, which is adding a lot of soda to
your previous non crazing glaze. As you know soda has a high coefficient
of expansion and leads to crazing.
You might try salting a little lighter so not as much soda goes into your
pots.
If your body is salting well then it is in a vitrified state and is
rather non-porous, therefore crazing is not as much of a problem.
hope this helps -- Paul W
Gavin Stairs on fri 23 may 97
At 12:03 PM 20/05/97 EDT, you wrote:
>----------------------------Original message----------------------------
>Hi all you vapor-type guys.
>
> I am having a problem with my clear liner. On all our cone 10+ firings, in
>ox or red., we have a great clear. In salt fire, however, if the vapor hits
>the inside of, say a bowl, or if the clear is on the outside of the pot,
>then it crazes horribly. ...
This should not really be a surprise. Salt/soda deposit sodium in the
glaze coat, in one form or another. Sodium imparts a large coeficient of
expansion. Think of nepheline syenite, or soda spar. If you want to be
sure your exposed glaze will not craze, you will have to start with it
quite far towards the shivering/dunting side of the optimal fit.
If you look carefully at old salt glazed ware, you will see that blobs and
thick patches have a tendency to craze. I'm still not sure why salt glazes
work at all, given the obvious mismatch. I suspect it has to do with
thermodynamic disequilibrium, probably in the form of excess chloride ion
in the glaze, but I don't know the mechanism.
If you start out with a well fitting glaze, you probably provide a
convenient solvent into which the salt vapor enters rather quickly. I
would expect quite a lot of sodium in such a case, and consequently, a
rather poor fitting glaze. Perhaps you would have better luck with
underglaze techniques, or slips.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
| |
|
