Clennell on thu 12 feb 98
In my haste to clear all the collected Clayart messages I deleted one
asking if I had ever altered our green/celedon to a blue glaze. the answer
is yes. We have a beauty blue from this glaze by changing the oxide dept to
1% iron and .25% cobalt silicate. Because cobalt is so expensive in Canada
$54 for 500 grams we buy cobalt silicate from Pottery Supply House for $39
for 500 grams.If you were buying with american dollars you could get it
pretty cheap. Their number is 1-800-465-8544.
Cobalt silicate I think is just cobalt carb that has been cut with silica
to make you think you're getting a deal.. It's like when you were a
teenager and you stole some of your dads whiskey and topped the bottle up
with water. They add silica to the cobalt so you think you are getting
alot for a cheap price. The advantage though is that it does cut the
strength of the cobalt.
I think the secret to dispersing the cobalt throughout the glaze is either
ball milling or seiving. We mix the glaze (without the oxide) then add
some mixed glaze to the measured out oxides /put them in the blender then
add to the glaze and put through a 150 mesh screen twice.
I have seen peoples celedons and cobalt glazes that have just been put thru
a 80 mesh screen and they have little brown spots in the celedon or blue
spots in the cobalt glaze. The oxides have not been dispersed throughout
the glaze properly. Cobalt is an extremely strong colourant and I think
care in blending it throughout the glaze is important.
I bet you could cut your cobalt carb by 50% with silica and have what I buy
as cobalt silicate.
A well known Canadian potter Harlan House with celedons and chuns to die
for, has been rumoured to use the liquid from magic markers that have
cobalt in the ink to colour his glazes. Fact or fiction?
cheers,
tony
June Perry on fri 13 feb 98
Dear Tony:
Harlan uses a tube of blue water color pigment added to a bucket of glaze for
his blues. At least that was what he was doing in the 70's.
You would have to test because I don't know which blue he used or how large
his glaze batch was.
Have fun. Maybe you can share your results with us. I recently bought a couple
of tubes of blue, but am in the middle of typing a manuscript for my husbands
book, so I won't be playing with glaze testing for a while.
Warm regards,
June
Gavin Stairs on fri 13 feb 98
At 08:00 AM 2/12/98 EST, Tony wrote:
....
>Cobalt silicate I think is just cobalt carb that has been cut with silica
>to make you think you're getting a deal..
....
I did a little cursory check on the net to see what's what. Cobalt
Carbonate is a raw material, and as a mineral has a chemical formula like
x(CoCO3).y(Co(OH)2).z(H2O), where the x, y and z may vary. The Co content
lies in the range 45-47%, from the source I found. Chemically pure CoCO3
is about 63% Co. In that form, it is quite expensive, since it has been
refined. Cobalt Silicate is a manufactured product, a Potassium-Cobalt
glass. It is one of the first man-made pigments, called smalt, colour deep
blue. The Co content is about 6-16%. So, you get about 3 to 8 times as
much Co with cobalt carbonate as from cobalt silicate, depending on
sources. If cobalt is what you are looking for, it may be false economy to
buy cobalt silicate. Of course, as the silicate, it is already a frit, and
has some advantages, like superior dispersal in the glaze, easier weighing
and measuring, and lower loi, which means fewer problems in pinholing, etc.
See Tony Hansen's materials glossary for some more stuff about various
forms of Cobalt for the ceramist.
I think much of the cobalt comes from mines in Zaire. I imagine virtually
all commercial cobalt will come with a rough analysis for cobalt content,
and with lots of traces of other stuff, including copper and iron, for
example.
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