[Clayart] Cobalt carbonate toxicity

Robert Harris robertgharris at gmail.com
Mon Dec 14 18:30:53 EST 2015

Teri - I'm a little surprised by the Cobalt Carbonate toxicity claim,
although Cobalt is somewhat toxic (but not nearly as much as barium,or
manganese, let alone mercury, or cadmium or a myriad of other heavy
metals), it is not generally considered a problem for potters (mostly based
on the fact we use such small amounts).

In terms of your specific question however (and this is true for many heavy
metals), is that cobalt carbonate, particularly in the presence of a little
acid, can become (very) slightly soluble and can then be easily assimilated
into the body. When we use it in a glaze bucket, the slightly soluble
cobalt compounds can be absorbed through the skin, or we may get a little
in our mouth and absorb it that way.

In a glassy matrix (i.e. a glaze) the cobalt is very much locked in. It
cannot be dissolved out by acids (in contrast to copper, which in high
amounts and in poorly formulated glazes can leach quite significantly).
Cobalt glasses are very stable. As long as you use a nice stable base glaze
(the definition of which can be found a myriad of places), then you have
absolutely no worries about cobalt in your functional ware.


On Mon, Dec 14, 2015 at 11:24 AM, Teri Lee via Clayart <
clayart at lists.clayartworld.com> wrote:

> Hello,
> I've been reading about Cobalt Carbonate toxicity in John Britt's excellent
> book on high fire glazes. As someone who never took chemistry in school,
> I'm a bit confused if cobalt carbonate is mainly dangerous in its "raw"
> form, but after being mixed in a glaze and fired to cone 10, it is no
> longer toxic. I'm especially concerned about using cobalt carbonate in
> functional ware. Thank you.
> Teri Lee
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> Clayart at lists.clayartworld.com
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