carrie jacobson on sun 29 mar 98
A question from a novice: I made a porcelin bowl. While it was
leather-hard, I took straight cobalt oxide, put a brush in it, brushed it
in a couple of bands around the bowl, then dribbled it down the inside of
the bowl. After the bowl was bisqued, I glazed it with a semi-opaque white
cone six glaze.
This bowl was one of the ones that went into a kiln that fired to cone who
knows what. Most everything melted, but this bowl did not.
I ended up with a very, very hard bowl (vitrified?) with gorgeous cobalt
patterning and bands on the inside and out.
My question is: Am I in danger using this as tableware? I have read lots of
discussion on this list lately about leaching, and about cobalt being one
of the most dangerous metals. Advice or comment from someone who knows what
they're doing would be great.
Thanks,
Carrie
Carrie Jacobson
Pawcatuck, CT
mailto:jacobson@brainiac.com
Gavin Stairs on tue 31 mar 98
Hi Carrie,
Since I posted the latest piece of misinformation on Cobalt, I'd better
respond to this.
Cobalt is not the worst: I wrote that lead and cobalt were super bad guys.
I MEANT to write lead and cadmium, these being the two with legislative
proscriptions against them.
That said, no-one should be complacent about ANY glaze constituents
leaching into food. Cobalt is not the worst, but it cannot be said to be
innocuous.
To test for leaching of cobalt, fill the bowl with vinegar and leave for a
whole day. Then pour out the vinegar into a clear glass bowl, and look
carefully for a) discoloration of the bowl, and b) any color cast or
cloudiness in the vinegar. This test will show if your bowl is badly
susceptible to leaching, but it won't tell you much more than that. If it
fails the test (any color change or cloudiness), it is definitely not
suitable for use with wet food contact. To tell more, you will have to
have the leachate tested for cobalt at a lab, or better yet, send the bowl
for test at a lab.
If you have badly overfired the bowl, in all liklihood the overglaze has
been through a very runny stage. In that case, the cobalt will have
mixed/diffused through the overglaze, and it cannot be considered to be
enclosed by the overglaze. You should consider the pot to be glazed with a
cobalt glaze. If you have any doubt about its safety, you should not use
it with wet food contact.
You seem to find the bowl attractive. Consider using it for some display
purpose, or for dry foods. If it is seriously overfired, it would probably
not be suitable for use as an ordinary kitchen bowl anyway.
My mate has a set of blue bowls made in Mexico during the seventies or
eighties. She says she was assured that they were lead free. I have my
doubts, since by the eighties most Mexican potteries knew enough to at
least DENY that they used lead, if they wanted to sell pottery. So these
bowls have, in my mind, at least two strikes against them: probably lead
glazed, with a cobalt glaze/overglaze, and who knows how well fired? And
yet we use them. Not every day, and we don't use them as storage vessels.
The risk to us of using these bowls in the way we do is minimal. If I put
dry food in a bowl, or even hot soup, drink or eat from it, wash it and put
it away for a week, how much less risk is there in this that if I put
tomato sauce in it, store it in the fridge for a week (ok, this is an
extreme case), pull it out and eat from it? The second case gives me
something like 100 to 10,000 times the exposure to whatever the glaze
contains. I could conceivably use the bowl for a lifetime without
incurring that much exposure.
So, if this were my bowl, I would use it with care and consideration, but I
would not, on any account sell it or give it away, knowing that others
might incurr much more risk than I through carelessness or ignorance.
Gavin
At 01:40 PM 3/29/98 EST, Carrie wrote:
....
>My question is: Am I in danger using this as tableware? I have read lots of
>discussion on this list lately about leaching, and about cobalt being one
>of the most dangerous metals. ...
Sharon Pollock-De Luzio on tue 2 mar 99
> Diane Woloshyn wrote: asking about cobalt carb vs. cobalt oxide.
>
> Many thanks to Tom Buck. Just when I thought I knew alot....
>
> Anyway, Diane:
>
> What I've always done when I have to make cobalt substitutions is figure that
> part cobalt oxide = 1.5 parts cobalt carb. It's probably not exact but it's
> simple and it's always worked for me.
>
> Good luck,
>
> Sharon in rainy RI
Sharon Pollock-De Luzio on tue 2 mar 99
> Diane Woloshyn wrote: asking about cobalt carb vs. cobalt oxide.
>
> Many thanks to Tom Buck. Just when I thought I knew alot....
>
> Anyway, Diane:
>
> What I've always done when I have to make cobalt substitutions is figure that
> part cobalt oxide = 1.5 parts cobalt carb. It's probably not exact but it's
> simple and it's always worked for me.
>
> Good luck,
>
> Sharon in rainy RI
Fay & Ralph Loewenthal on thu 4 mar 99
One has to check with the manufacturer of the
Cobalt Oxide and Cobalt Carbonate to know what
percentage pure cobalt there is in each. Here in
South Africa, the manufacturer who supplies me,
has told me that there is about 90% pure cobalt in
the oxide and about 45% in the carbonate. So that
makes the ratio 2 to 1, carbonate to oxide. If your
supplier does not know, insist that they find out for
you. This applies to all oxides and carbonates so
that you can have good basis for your calculations.
Hope this helps Ralph in PE SA.
Wade Blocker on mon 2 oct 00
Anji,
cobalt oxide is black. I would assume that you are dealing with the same
thing in two different bottles.Mia in still hot ABQ
Alyss Dorese on wed 16 jul 03
Re Cobalt.........
It is > > from the Feds so you know it is true....>>>>>
This is the best laugh I had today.
Alyss Dorese
Palm Springs, CA
Ivor and Olive Lewis on sat 5 jul 08
Speaking to my Chemical supplier a couple of weeks ago I raised the
issue of Cobalt oxide. Bear in mind this would be analytical grade,
usually better than four nines purity. I was quoted $Au 55.00 plus
sales tax for 100 gram vials. Current exchange rate is $Au= $Us 0.95.
Just to stir the pot. Gasoline is rising through $Au 1.65 per Litre.
Approx equal to $Us 7.00 per gallon.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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