Caroline and Hedley Saunders on mon 14 sep 98
Hi Fabienne
On the recent glaze course at Bath Potters Supplies we made up 30 test
stoneware glazes with different proportions of silica, alumina and fluxes:
tests 1 - 5 had 75% silica and alumina from 25% - 5%,
tests 6 - 10 had 70% silica and alumina from 25% - 5%,
tests 11 - 15 had 65% silica and alumina from 25% - 5%,
tests 16 - 20 had 60% silica and alumina from 25% - 5%,
tests 21 - 25 had 55% silica and alumina from 25% - 5%,
tests 26 - 30 had 50% silica and alumina from 25% - 5%.
We tried out these glazes on around 15 different bodies some fired in
oxidation atmospheres some reduction (cone 8 electric, cone 9 gas). When we
compared the results we found the shiniest glazes came out with 15% alumina.
The shiniest of all was 15% alumina and 60% silica, whatever the body,
whatever the firing method. The glaze with 20% alumina and 60% silica was a
lovely satin with the slight excess of alumina staying in the suspension.
We plotted our tests on a triaxial graph and this showed just how critical
the ratio of silica to alumina is. I have done quite a lot of reading about
glaze but it does not compare with seeing and touching!
Mike Bailey took our course and he and David Hewitt (a Clayart subscriber
too) have a booklet called Understanding Your Glaze Tests & How Computers
Can Help The Potter, this shows similar results on triaxial graphs and is
available from BPS
BPS website: http://www.bathpotters.demon.co.uk
Hope this helps
Caroline
Xdelphin@eclipse.co.uk
Remove the X from the address (there to keep spam spiders at bay)
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