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fwd: re: cone 6 glossy black?

updated wed 3 feb 99


John Hesselberth on tue 2 feb 99


I've tested a very similar glaze at Alfred.

Cone 6 Metallic Black

Custer 79
Whiting 5.5
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:

>----------------------------Original message----------------------------
>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.
>>Glossy Black
>>Kona F-4 79
>>Whiting 5
>>EPK 5
>>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.
>Ron Roy
>93 Pegasus Trail
>Scarborough, Ontario
>Canada M1G 3N8
>Tel: 416-439-2621
>Fax: 416-438-7849
>Web page:

John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: web site:

"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer