Tom Buck on sun 14 may 00
Some have discussed a substitution of one form of cobalt colourant for
another, and the common factor cited was 1.5 with two suggesting 2.0
as suitable. When one examines the source of the cobalt compounds a
somewhat different picture emerges.
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 pots. Til later. 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
Dwiggins, Sandra (NCI) on mon 5 jun 00
We did a 2:1 substitution for cobalt carb in a purple matte glaze and came
up with a dark blue instead of the lavendar we should have gotten. The test
also fluxed more since cobalt is a flux. It was also a gloss instead of a
semi-matte as it was supposed to be. But....it was a nice translucent navy
blue! We figured out that it couldn't have possibly been a simple 2 to 1
substitution, and probably closer to what Tom says...1.3 or so.
Sandy
> -----Original Message-----
> From: Tom Buck [SMTP:Tom.Buck@HWCN.ORG]
> Sent: Sunday, May 14, 2000 11:56 PM
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: cobalt substitution
>
> Some have discussed a substitution of one form of cobalt colourant for
> another, and the common factor cited was 1.5 with two suggesting 2.0
> as suitable. When one examines the source of the cobalt compounds a
> somewhat different picture emerges.
> 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 pots. Til later. 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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