Craig Martell on sun 1 mar 98
At 11:34 AM 2/28/98 EST, Numo Jaeger wrote:
>Does anyone know the level of leachability with cobalt carbonate fired at
>cone 10 on a stoneware claybody? Would this surface be food safe with out a
>glaze one the surface?
Hi:
I would think that cobalt on a cone 10 body without a glaze would be at it's
highest level of leaching for that temp. What you have in this case is a
heavy metal oxide in a fairly free state and unattached to any silica except
what it is contacting on the surface of the pot. No, it would not be food
safe and this is the worst possible case scenario. Don't even think about it!!
regards, Craig Martell-Oregon
Gracedart on tue 3 mar 98
Craig,
Since I use cobalt in a lot glazes of glazes I was concerned about your
comments of cobalt leaching out at ^10. Am I correct to think that at the
lower (^06-^6) temps the oxide is not as apt to leach out ???? Is it accurate
to say any oxide (copper, iron) oxidized at high temperatures become this
metal that will leach out in clay dishware?
Grace
Gracedart on tue 3 mar 98
Craig,
Its Grace again...I just read some key words>>"not under glaze", therefore
bound to silica...I assume if the metal is bound with certain chemicals
they're not harmful? Is it only silica metal must bind to not be free agents
and dangerous in pottery for food?
Grace ( this is why all potters need a good course in chemistry for potters !)
Marin Ceramic Supply on wed 4 mar 98
Numo:
As far as I know the FDA has never established limits for cobalt
leachability. Nor have I ever heard of anyone routinely testing for it.
However, cobalt in various forms is an extremely common colorant for
dinnerware that is currently and has throughtout history been on the
market. Cobalt-enhanced commercial dinnerware is produced at all different
temperatures and cobalt can be found in glazes, underglazes, and onglazes.
It is commonly used with hotel china that is subject to as much regulatory
scrutiny as any ware. Most hotel china is fired between cone 8 and 11. If
your concern relates to the form of cobalt that you are using, i.e. cobalt
carbonate, it is inconsequential because what is left in the glaze is
cobalt oxide. CoCO3 (cobalt carbonate)becomes CoO (cobalt oxide) +
CO2(carbon dioxide). If are interested in having your ware tested or would
like to know how testing is done, let us know.
Bill Palmer
Technical Support Manager
Marin Ceramic Supply
112 Ohio Ave.
Richomond, CA 94804
510-236-1492
email: ceramic@slip.net
web: www.slip.net/~ceramic
At 11:34 AM 2/28/98 EST, you wrote:
>----------------------------Original message----------------------------
>Hello all,
>
>Does anyone know the level of leachability with cobalt carbonate fired at
>cone 10 on a stoneware claybody? Would this surface be food safe with out a
>glaze one the surface?
>
>Numo Jaeger
>
>Studio One Art Center
>365 45th Street
>Oakland, CA 94606
>
>
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