Ruth Ballou on fri 23 apr 99
Earl,
The purple/magnesium relationship is also dependent on a low alumina
content, somewhere around .2 to .3 moles. Cobalt will turn blue as the
alumina increases. Silica content does not have an impact on the color.
Crawling is a problem with high magnesia glazes, when the source is light
magnesium carbonate. Try to use dolomite or talc as the source for at least
part of the MgO. Some magnesium carbonate will help keep the glaze in
suspension.
A possible starting point:
46 G-200 Spar
13 Dolomite
6 Magnesium Carbonate
5 Whiting
30 Flint
1 - 2 Cobalt Carbonate
This is a ^10 glaze, so do a line blend with a boron frit, like Ferro 3134
to bring the melt down. And/or you can try subbing Neph Sye for part of the
spar.
I can't make any predictions about the safety of this glaze, so I advise
you to have the final glaze tested.
Ruth Ballou
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