[Clayart] Cobalt carcinogen

johanna at demaine.org johanna at demaine.org
Tue Nov 8 15:38:38 EST 2016

Quoting ronroy at ca.inter.net:

> Hi Doug,
> This is not aimed at you - I was just waiting to tell this story  
> about the subject.
> This is an unusual example but I'll bet you could find examples  
> close to this without too much trouble.
> I was buying some supplies at a local supplier and the owner asked  
> me into his office to show me a mug. It was glazed with a barium  
> matte coloured with cobalt - an intense blue - inside and out.
> About half way up from the bottom inside the glaze was white - pure  
> white - no blue!
> The owner of the mug had left it overnight half filled with  
> neo-citron - a cold relief remedy. A powder mixed with water sold in  
> Canada - maybe in the US as well?
> All the cobalt had been leached out of the glaze along with some of  
> the Barium and whatever else was in the glaze.
> Imagine how the owner felt about the potter who made that mug -  
> especially if they also had the current information about cobalt  
> being a carcinogen? Imagine how that person now felt about hand made  
> pottery.
> At best it's an excellent argument to limit colour in liner glazes  
> especially if  there is little interest in glaze durability on the  
> part of the maker.
> RR
> Quoting Douglas Fur <23drb50 at gmail.com>:
>> I looked at the report. What they are concerned about seems to be water
>> soluble cobalt compounds which might release cobalt metal ions inside the
>> body. I didn't see any density level or threshold level.
>> Practically speaking we use cobalt oxide in very small amounts not cobalt
>> metal. Cobalt oxide is rated as insoluble or slightly soluble in water. If
>> you've used cobalt to mark glaze tests you may have noticed a sheen on the
>> surface after it is fired. This is where the CoO has reacted with the SiO2
>> to form cobalt silicate. I couldn't find a reference to the water
>> solubility of cobalt silicate but my sense is that if cobalt is so reactive
>> with silica  that it fuses with the silica in a clay body it's not likely
>> very soluble in water.
>> Since we don't use  cobalt compounds noted for solubility in water and we
>> use relatively small amounts of cobalt compounds which, once fired they
>> seem to be firmly bound in the glaze, I don't think we need to be concerned
>> about cobalt release from our pots.
>> Maybe in a situation where cobalt decoration is painted on the glaze
>> surface you might have a local condition where the glaze is overloaded but
>> even then it seems like you would more likely have insoluble cobalt oxide
>> than a soluble cobalt compound.
>> This has more speculation than facts so my summary would be don't panic but
>> get more facts.
>> Duff
>> Seola Creek
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> Ron Roy
> ronroy at ca.inter.net
> Web page ronroy.net

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