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cobalt color change

updated wed 11 oct 00


Chris Schafale on tue 10 oct 00

Getting back to mixing glazes after the summer, I was surprised to
notice that my cobalt carbonate supply looks different. It was a
very fine, light purple powder. Now it is somewhat clumpy and the
color is more pinkish with a slightly brownish tinge. I assume that
this is a result of moisture (in my very humid basement studio), but
wonder if anyone else has observed this, and whether it could
change the way the material behaves in a glaze. Presumably, the
additional moisture would reduce the coloring intensity of a given
weight of material (since some of what I'm weighing now is water
and not cobalt). Anything else?

Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)

Cindy Strnad on tue 10 oct 00

Hi, Chris.

I dunno--my cobalt carbonate has always arrived pink rather than purple, but
not brownish or clumpy. Maybe yours is stronger, and that's why it's purple.
You could put it in a bisqued (but not glazed) bowl, cover it with a bisqued
saucer or something to protect it from wind, birds, dust--whatever, and set
it on top of the kiln for a firing or two. That should dry it out, but
you'll still want to de-clump it, I expect.

I store my stuff in the basement, too, and I have a really damp basement.
Everything's in plastic bins of one size or another. Very nice, and a little
expenditure at the local discount store sure makes it nicer to work with
glaze tests.

Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730

Tom Buck on tue 10 oct 00

Cobalt carbonate is a misnomer, the stuff actually is
2CoCO3.3Co[OH]2.H2O or cobalt(II) carbonate, basic. The crystals are when
"pure" red violet in colour. But the hue is detuned by adding some other
material that may release (or takeup) mositure. That you are seeing some
red crystals indicate mositure loss rather than increase.
However, since cobalt compounds are used in glazes in a relatively
small amount, 1-2wt% is usually the maximum, it really won't matter much
if the stuff has lost or increased by even 10%. Say, eg, you mix 5 kg of
a glaze with 1% cobalt "carb" added....that is 50 grams added to 5000
grams. So you might get 45 grams or 55 grams in the glaze mix, and I doubt
whether you will notice any colour change. Generally, the variation in
cobalt compounds over time requires that you always do a test of a new
supply of this colourant, and adjust the amount added to suit your eye.
good tests. bfn. peace. tom b.

Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada