Debby Grant on sat 18 oct 97
This is an answer to several questions about my last post and I thought
the whole list might benefit. Let me start at the beginning. About a
year ago the glaze Berry Rust was posted on the list. I don't know who
originated it so I can't give credit where credit is due. I made a test
and loved it. Then I made a batch and the results were wonderful.
However, after sitting for a few weeks the glaze started to clump in
the bucket and had to be resieved several times. Then it thickened
and I had to add water. Worst of all it lost its vibrancy in the fire
and started spitting onto the kiln shelves.
Now I have to tell you that the last time I cracked a chemistry book
was almost 50 years ago. What I know about glazes I have gotten
through observation, experience, intuition and a few good pottery
books. One thing I do know is to avoid soluble materials in a glaze,
such as sodium and boron, unless you intend to make up the glaze
fresh each time. There is no question that the alkaline earths promote
great color responses in a glaze but you have to use them in an
insoluble form such as feldspar for sodium and Gerstley borate for
boron. The trouble is that Gerstley is somewhat variable and can be
slighgtly soluble, but this glaze was behaving like it was very soluble,
not like the other Gerstly borate glazes I've used.
So I inquired of this wonderful list and got some great responses from
potters like Ron Roy, Tom Buck and Craig Martell who are far more
knowledgeable about glazes than I am. Tom Buck suggested that I
dry out the glaze and reconstitute it using 1% epsom salts added to
the water. I did that and found great improvement. Craig Martell
went to the trouble of recalculating the glaze substituting Ferro Frit
3134. He tested it and said it came out fine, so I'm going to try that
next.
The original formula and Craig's recalculation are as follows:
BERRY RUST - ^7 - 10 - oxidation (I fire to a full ^ 9 bend)
Original Formula Craig's Formula
Silica 27.27 25.13
EPK 18.18 22.69
Neph Sy 18.18 8.61
Dolomite 9.09 0
Gerstley Borate 9.09 Frit #3134 19.93
Talc 9.09 14.67
Bone Ash 9.09 8.97
Red Iron Oxide 9.09 8.59
I hope this answers everyone's questions. I have only one request.
Please don't start that silly thread again about "Ghastly Borate".
Debby Grant in NH
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