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glazes containing cobalt carb.

updated fri 31 jan 97


David Donica on tue 21 jan 97

Would like any info/input anyone has about glazes containing cobalt carb.
being food safe? I'm new to this list and really appreciate all the info-
I had the pleasure of briefly meeting Mr. Roy at NCECA in Diego 5 years
ago- you're still a wealth of knowledge!!!
Maggie Shepard
FireWorks Pottery
Mt. Shasta, Calif.

Ron Roy on wed 22 jan 97

Hi Maggie,

My books indicate cobalt is not much of a danger but you don't want to be
sprinkling it on your cereal. Send me your glaze recipe and the cone you
fire to and I will give you my opinion about the integrity of your glaze.

Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849

David Donica on thu 23 jan 97

The cobalt-containing glaze I had questions about is called Gloss Blue, I
got it about 15 years ago from the studio where I first was introduced to
clay. I volunteered to mix glazes and in looking up the recipie, this is
what I had (^10R):
Feldspar: 2175 43.5
China Clay: 100 2
Silica: 1362.5 27.25
Whiting 137.5 2.75
Colemanite: 450 9
Dolo: 450 9
Zinc: 100 2
Barium: 225 4.5
Tin: 125 2.5%
Cobalt: 50 1%

If you add up all but the last two ingredients, you get 5,000 gms so I
divided everything by 50 to bring it to 100 which yields the second column.
I then took what percentage of 5000 was the tin and cobalt for those two
percentages in the second column. Is this correct?
Then there's the oops when I noticed the glaze contained Barium, which I
know is a no no for food items- is Barium contraindicated in any amount?
And alas, the question of what can I substitute how much with apologies if
this is redundant. Lastly, what would be a good text for a glaze neophyte
such as myself- I am intrigued by Robin Hoppers book, The Ceramic Spectrum
which I haven't really reviewed- any comments? Many thanks.
Maggie Shepard
FireWorks Pottery
Mt. Shasta, Calif.

Patsy Catsos on fri 24 jan 97

I was reading a dog food label at the supermarket yesterday and I was
surprised to see that cobalt carbonate was listed among the ingredients! I
would guess that its not toxic in tiny amounts.

Patsy Catsos
Maine, USA