Brenda Z on tue 2 apr 02
In December I asked the list for help with problems the we are having
formulating our studio glazes at the Potters' Guild where I am a member. We
were very grateful for all of the helpful advice that we received from the list.
I promised all those who gave suggestions, that I would let you know how our
testing went. We are not finished yet, but I thought that I would give an
update on the results of our first round of testing.
The problem was that when we heard of the demise of Gerstley Borate, we decided
it was a good time to change our studio glazes. After testing a lot of
different base glazes last year, we had settled on Tony Hansen's base glaze with
colourants. As suggested on the Digitalfire web site, we added 10% superpax to
get a white glaze. Unfortunately this white glaze crawls wherever it meets any
of our coloured glazes (the ones that use the T.H. base).
The recipe we were using is:
Wollastonite 20
FRIT 3134 20
E.P.K 20
Silica 20
Potash Feldspar 20
Bentonite 2
Superpax 10
All of our tests were done using Tucker's Smooth White clay, fired to cone 6.
These are the suggestions that we have tried so far:
1) Using less superpax in the glaze
We tried using 3, 5 and 7 percent superpax in the glaze. These tests looked
beige on the smooth white clay.
2) Using other opacifiers in the glaze
The only other opacifier that we could find at our local supplier was tin. We
tried the tin at 3, 5, 7 and 10 percent. We had one test with a lovely pink,
but none of the results gave us a nice white.
3) Thinning of the glazes
The Tony Hansen glazes do seem to work much better when they are thinner, but we
still had crawling where layers of glaze overlap.
4) Calcining the EPK
This has given us the best results so far.
For the next round we plan to do more (and larger) tests using the recipe with
the calcined EPK. We will also continue to explore some of the other
suggestions that we received:
- removing the bentonite (which we are using because the glaze was very powdery
for our co-operative studio situation)
- decanting the glaze and adding back the water again
- we still plan to test the revised recipe that was offered by Ron Roy
Thanks again to everyone for their help,
Brenda Z.
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