[Clayart] Cobalt carbonate toxicity

David Woof woofpots at hotmail.com
Sat Jan 30 19:59:57 EST 2016

Hi John,
I knew it was an oldie glaze, but time and travel at times loses the original name.
Way back "then" the order of the day for a good black was to over load with metallic oxides and adjust for the consequent over fluxing.    
I inherited a 5 gal. bucket of Glossy Black with the recipe written on the side.  It looked to be unstable for leaching but finding a suitable use for a free bucket of glaze is a thrifty pastime so I played with it to see what value I could find as a glaze for exteriors and non food and drink surfaces.  It is indeed "glossy" and as i said before, interesting in the chemistry, alchemy, of glaze layering effects and surprises.

Just leave it out of the mug, bowl, and pickle dish!!!
It would be interesting if you have time to locate the old leach test results.


Subject: Re: [Clayart] Cobalt carbonate toxicity
From: jjhesselberth at gmail.com
Date: Sat, 23 Jan 2016 12:19:50 -0500
CC: woofpots at hotmail.com
To: clayart at lists.clayartworld.com

Hi David,
An interesting comparison. Two very different base glazes and two very different colorant combinations both give a glossy black—though they interact differently with other glazes.
This Glossy Black recipe looked familiar so I looked through my recipe database. I found it called Metallic Black which (to my knowledge) was first published in Making Pottery without a Wheel by F. Carlton Ball in 1965.
I looked at the unity formula for Glossy/Metallic Black and it is way out of range for a cone 6 glaze because it has too much silica and alumina and too little boron to thoroughly melt. It must be glossy because the copper is fluxing it enough to get some level of melting. I agree with you that I would certainly not use Glossy Black on food surfaces. I actually did leach test this glaze when I was doing the research to try to understand glaze stability. Indeed it leached badly although I don’t have my notebook in front of me to be able to quote exact numbers.
The cobalt/iron combination we use in Licorice is a much better way to get a good black. Both of those colorants are relatively soluble in a molten base glaze that is itself in the stable range.

On Jan 23, 2016, at 9:22 AM, David Woof via Clayart <clayart at lists.clayartworld.com> wrote:Case in point:  a "Glossy Black" glaze calls for Copper carb. 4.1 %,  Manganese diOx 4.1 %,  Cobalt carb. 2.0 %.Ron and John's black #2 (licorice) asks for a simple 2.0 % cobalt, and 9.0 % Red Iron Oxide to get a very stable, rich and pleasing black.Interesting to me is that I use the Glossy Black, on outsides of mugs, over splashed/spritzed/poured with a "Frosty White Crystal" to get an attractive blue on a black field.  Conversely, I get another pleasing blue when layering Ron's black over the Frosty White Crystal but not in the reverse layering as with the Glossy Black.The 78.9 % Custer and 4.9 % Epk in the Glossy Black would seem to give near 42% silica and 20 % Alumina (give or take a few points) but the classic combined overload of copper and manganese, combined with the Cobalt, give me pause and cause for stability concerns re using the Glossy Black as a mug liner.I have no such concerns regarding stability of John's and Ron's."Glossy Black"   Custer78.9 %,  Gerstley B 10.7 %,  Whiting 5.5%,  Epk 4.9%^^^^Copper carb 4.1%/ Mang. DiOx 4.1 %/ Cobalt carb. 2.1%"RR Black"  3134 26.0%,  Silica 26.0%,  Custer 22.0 %,  Epk 17.0%,  Talc 5.0 %,  Whiting 4.0% ^^^Cobalt Carb.2.0%/Red Iron Oxide 9.0%.Misneach,Woof*************************************************************

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