John Hesselberth on tue 2 feb 99
I've tested a very similar glaze at Alfred.
Cone 6 Metallic Black
Gerstley Borate 10.7
Copper Carbonate 4
Manganese Dioxide 4
Cobalt Carbonate 3
Custer instead of F-4 so less sodium and 3 vs. 2 cobalt. Results:
20.65 ppm Cu
3.12 ppm Co
0.73 ppm Mn
Not the highest numbers I've seen, but I don't like to see copper that
high. Cobalt is also higher than on most cobalt containing glazes I've
tested. I have no basis to judge what is high or low for Mn--this is the
only glaze I've tested for Mn. Also it is one of the few glazes I have
tested where I saw visible etching of the surface from exposure to the
The F-4 containing glaze would probably be worse with its higher sodium
content. This is not one I would use for food. It is a beautiful
metallic glossy black, though. John Hesselberth
Ron Roy wrote:
>I just calculated this glaze out of curiosity - it has a good supply of
>alumina and silica so the temtation would be to assume it is durable. There
>is a fly in the ointment however - well actually two - there is a lot of
>sodium and a lot of copper - both of which promote leaching.
>I sure would like to know if this glaze is releasing any copper, cobalt or
>manganese into acidic food.
>It is going to craze on a lot of bodies as well and that increases the area
>that the acid gets to work on.
>>This is one of our regular studio glazes.
>>Kona F-4 79
>>Gerstley borate 11
>>Cobalt carbonate 2
>>Manganese dioxide 4
>>Copper carbonate 4
>>It's a wonderful shiny black, but waxy where really thick. Hope it
>>works for you.
>93 Pegasus Trail
>Canada M1G 3N8
>Web page: http://digitalfire.com/education/people/ronroy.htm
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