Tom Buck on wed 13 sep 00
Rick Hugel:
please let me ease your clearly stated agony...here is what I
wrote recently for local newsletter:
-----------------
How to change cobalt materials
The Cobalt materials we buy have names different from their "real"
chemical names and formula. The Carbonate is not pure carbonate, it is a
mix of carbonate and hydroxide; and the Oxide may well be a mix of two
oxides.
To make Cobalt Carbonate Basic, producers combine soda ash and cobalt
(II) acetate in water solution. The solid that forms is recovered from the
liquid by filtration, dried and shipped to market. It is described as being
red violet crystals that are insoluble in cold water but will decompose in
hot water. When it is heated in a glaze, the Carbonate Basic changes to
CoO, ie, Cobalt (II) Oxide.
If your Carbonate is pale lavender that probably means it also
contains some colour-inert whitish powder (clay? feldspar? flint?). Ask
your supplier for an analysis, or, ask for an answer to this question: how
much of the element, Cobalt, is present. It likely will contain 50% or less
of elemental Cobalt instead of the theoretical 55% (ie, 100% pure carbonate
basic).
Cobalt forms two oxides: Cobalt (II) Oxide, CoO; and Cobalt (III)
Oxide, Co2O3. But the producers likely ship a mixture of both.
Cobalt (II) Oxide is made by "roasting" the Carbonate Basic compound,
and the result is a greyish powder that is quite dense (6.4 g/mL) with very
high melting point (1935 oC). This insoluble material is supplied in two
grades, technical and ceramic. It has several uses aside from pottery.
The other oxide, Co2O3, is a steel-grey or black powder, density
4.8 g/mL, insoluble in water. It changes to CoO at 895 oC. It is made from
other cobalt compounds by heating them at low temperature with excess air.
It is used chiefly as a pigment and an enamel/glaze colourant.
Some makers of Cobalt Oxide ship a material of vague composition, it
is between Cobalt (II) and Cobalt (III) Oxide, and sometimes a formula of
Co3O4 is cited, which is CoO.Co2O3 in equal molar amounts. This oxide mix
arises when cobalt compounds are heated (as above) to form CoO or Co2O3. So
commercial Cobalt Oxide generally contains significant amounts of
CoO.Co2O3; the result is a steel grey to black powder with a density of 6.1
g/mL.
Since neither the Carbonate nor Oxide is pure, the substitution factor
--Oxide to Carbonate-- may range from 1.3 to 1.4 for a similar blue tone.
Hence, we need a test or two to determine which value is right for our
powders. Each of us will choose a preferred hue, obtained by a few tests
using stock on hand, then we use that factor until new cobalt stock
arrives. Then the tweaking process begins again.
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
Rick Hugel on wed 13 sep 00
I have been using a glaze calling for cobalt carbonate which is a beautiful
light sky blue glaze. Well, I ran out of cobalt carbonate, so tried to
mix a batch using cobalt but only one half the carbonate amount. Hmmm.
The color was kind of close - but different. It is not unusable, but not a
substitute for the original. So Cobalt is Cobalt and Cobalt Carbonate is
Cobalt Carbonate and subbing one for the other doesn't appear to work
exactly as one would like it to.
June Perry on wed 13 sep 00
Rick, if you're substituting cobalt oxide for cobalt carb, you'd be closer if
you used about 65% of the amount to get a better match than you got with 50%.
If it calls for 10 grams of cobalt carb, try 6.5 of cobalt oxide.
Regards,
June (just back from vacation!)
David Hendley on thu 14 sep 00
I was hoping that Tom would re-send his excellent comparison
of cobalt oxide and cobalt carbonate. Thanks, Tom.
I would just like to add that, in my experience, cobalt oxide
is prone to making spots of dark blue color in a lighter
blue background.
Like it is a coarser material. I don't know if ball milling would
affect this; never tried it.
For delicate colors, with small percentages of cobalt, I prefer
the carbonate.
For dark colors, I use the oxide because, on a pure cobalt coloring
power basis, it is usually cheaper.
--
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com/
----- Original Message -----
From: Tom Buck
To:
Sent: Wednesday, September 13, 2000 10:36 PM
Subject: Re: Cobalt and Cobalt Carbonate
| Rick Hugel:
| please let me ease your clearly stated agony...here is what I
| wrote recently for local newsletter:
|
| -----------------
| How to change cobalt materials
|
Rick Hugel on thu 14 sep 00
Well, Tom, I sure do want to shake your hand and thank you for the time and
effort put into posting the below . I have a note file at the studio into
which I put info garnered here and there about the various materials used
in the clay world. Your paper is about the most comprehensive I have seen
on the subject of Cobalt and its Carbonate relative - easy to read, easy to
understand(even though I've only read it once so far), and without a doubt
very well graduated in building a rounded coverage of the subject. I hope
everyone on this list hard copies it and adds it to their files. AND I
don't want to forget to thank the other good folk who assisted with their
comments, suggestions which were gladly received.
Rick@the typhoon is gone and the Autumn moon is really beautiful.
>Rick Hugel:
> please let me ease your clearly stated agony...here is what I
>wrote recently for local newsletter:
>
>-----------------
>How to change cobalt materials
>
> The Cobalt materials we buy have names different from their "real"
>chemical names and formula. The Carbonate is not pure carbonate, it is a
>mix of carbonate and hydroxide; and the Oxide may well be a mix of two
>oxides.
> To make Cobalt Carbonate Basic, producers combine soda ash and cobalt
>(II) acetate in water solution. The solid that forms is recovered from the
>liquid by filtration, dried and shipped to market. It is described as being
>red violet crystals that are insoluble in cold water but will decompose in
>hot water. When it is heated in a glaze, the Carbonate Basic changes to
>CoO, ie, Cobalt (II) Oxide.
> If your Carbonate is pale lavender that probably means it also
>contains some colour-inert whitish powder (clay? feldspar? flint?). Ask
>your supplier for an analysis, or, ask for an answer to this question: how
>much of the element, Cobalt, is present. It likely will contain 50% or less
>of elemental Cobalt instead of the theoretical 55% (ie, 100% pure carbonate
>basic).
> Cobalt forms two oxides: Cobalt (II) Oxide, CoO; and Cobalt (III)
>Oxide, Co2O3. But the producers likely ship a mixture of both.
> Cobalt (II) Oxide is made by "roasting" the Carbonate Basic compound,
>and the result is a greyish powder that is quite dense (6.4 g/mL) with very
>high melting point (1935 oC). This insoluble material is supplied in two
>grades, technical and ceramic. It has several uses aside from pottery.
> The other oxide, Co2O3, is a steel-grey or black powder, density
>4.8 g/mL, insoluble in water. It changes to CoO at 895 oC. It is made from
>other cobalt compounds by heating them at low temperature with excess air.
>It is used chiefly as a pigment and an enamel/glaze colourant.
> Some makers of Cobalt Oxide ship a material of vague composition, it
>is between Cobalt (II) and Cobalt (III) Oxide, and sometimes a formula of
>Co3O4 is cited, which is CoO.Co2O3 in equal molar amounts. This oxide mix
>arises when cobalt compounds are heated (as above) to form CoO or Co2O3. So
>commercial Cobalt Oxide generally contains significant amounts of
>CoO.Co2O3; the result is a steel grey to black powder with a density of 6.1
>g/mL.
> Since neither the Carbonate nor Oxide is pure, the substitution factor
>--Oxide to Carbonate-- may range from 1.3 to 1.4 for a similar blue tone.
>Hence, we need a test or two to determine which value is right for our
>powders. Each of us will choose a preferred hue, obtained by a few tests
>using stock on hand, then we use that factor until new cobalt stock
>arrives. Then the tweaking process begins again.
>
>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
>
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