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is this glaze foodsafe?

updated thu 31 oct 96 on wed 2 oct 96

I am using the following cone 6 recipe, which makes a gorgeous, shiny glaze.
It contains only 5 percent barium, so I am hoping it is foodsafe. Does anyone
Ruth Jacobson

Deep Shiny Blue
Custer Feldspar 44
Colemanite 20
Whiting 4
Barium Carb 5
Flint 25
Copper Carb 6
Cobalt Oxide .9
Lithium Carb 4

Kerr - M. Christine on thu 3 oct 96

did you try subbing strontium carb for the barium carb? youll need about
3.8 % SrCO3 to get the same equivalent of Sr as Ba

Ron Roy on fri 4 oct 96

Hi Ruth,

I have checked this glaze using 2 different limit formulas, one with the
B2O3 included with fluxes and one with it out.

With the B2O3 included in unity it is somewhat short of Alumina so I would
say there is a chance of the BaO leaching - even more so with all that
copper - which increases solubility in glazes.

The normal way of checking with B2O3 out of flux unity gives a better
picture but again I don't know what effect the copper will have.

There is a simple answer - replace the BaO with SrO - three quarters the
amount of the barium (3.75 Strontium carb.)

An other option is to have the glaze tested. Test results can vary
according to placement in a kiln so it would be wise to submit several
samples to see if lower or higher fired glaze are the same.

Because CaO, BaO and SrO have similar effects on glazes you could also try
upping the whiting to 8 and leaving out the barium - the glaze calculates
out to almost the same formula - this would be the least expensive of all

Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849