iandol on sat 27 jul 02
Dear John Post,
As I read Tom's original post, all he asked for was a cone 3 base glaze, =
adding, as a rider, that the Watts formula might be a good place to =
start. Sort of seeding the inspiration. I would have omitted that, =
considering it better to leave the problem in a more general context of =
designing original glazes to mature at any temperature.
The origins or the Watts formula is an interesting story. It drawn from =
a period of Ceramic History when there was a lot of research being done =
into the replacement of lead glazes with what we might call Zinc fluxed =
Bristol glazes. As far as I know this example was the lowest melting =
point composition in an extensive array of glazes which were tested.
Your proposal confirms my opinion that glazes which are generated as =
hypothetical constructs will not mature unless they include Boric oxide =
in some form or another.
I agree with Tom that Segar or Unity formulae are great tools for =
enabling comparisons to be made or ideas to be shared when we do not use =
identical materials.
I suspect your example should have it's alumina reduced to ensure a =
glossy transparent sample. Such a large proportion of Kaolin may take =
some time to dissolve to give a transparent result. Another point to =
consider is that without the Boron Frit and the Zinc oxide it seems =
remarkably like something which will mature at cone 8 or higher.
A pointer for others who are interested in the concept of Bristol Glazes =
is to omit or reduce Zinc oxide, which is a refractory material which =
may not dissolve completely thereby giving rise to a milky opalescence, =
and substitute a high Zinc Frit.
I would be interested to know why you chose Boric oxide as the compound =
responsible for the higher than expected CoE when other commentators =
cite this as being a material which reduces the problems associated with =
that factor.
Thanks for contributing to this thread.
Best regards,
Ivor Lewis
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