Tom Buck on thu 14 feb 02
I keep wondering why this 2 to 1 ratio for cobalt materials pops up on
Clayart every so often. It shouldn't be so. Not that it matters much
because the blue colour you want is decided by your eyes, not some magic
formula. If your supplier isn't cheating on you by diluting the cobalt
material with say silica or other similar filler, then 2 cobalt carbonate
basic will give a much stronger blue than 1 cobalt oxide.
I did some chemical literature searching a year or two ago, and
then wrote the following for my Guild's newsletter (did I post it? not
sure.) But here it is now:
-----------------
Ask Tom, coloumn on technical matters
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.
(Copyright February 2002 by Tom Buck. All rights reserved including
electronic forms and hardcopy forms. Please respect my copyright and
refrain from making copies for others.)
---------------
I hope you get good results with 1.3 carbonate for 1.0 oxide. If you don't
you are being sold a low-grade material loaded with filler.
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
mariko cruse on thu 14 feb 02
Hello Tom,
I read your Cobalt story. Thank you.
How come cobalt oxides are not affected by redox reaction(in reduction
firing), although the valence varies a lot among the oxides? MC
----- Original Message -----
From: "Tom Buck"
To:
Sent: Thursday, February 14, 2002 12:47 AM
Subject: cobalt materials
> I keep wondering why this 2 to 1 ratio for cobalt materials pops up on
> Clayart every so often. It shouldn't be so. Not that it matters much
> because the blue colour you want is decided by your eyes, not some magic
> formula. If your supplier isn't cheating on you by diluting the cobalt
> material with say silica or other similar filler, then 2 cobalt carbonate
> basic will give a much stronger blue than 1 cobalt oxide.
> I did some chemical literature searching a year or two ago, and
> then wrote the following for my Guild's newsletter (did I post it? not
> sure.) But here it is now:
>
> -----------------
> Ask Tom, coloumn on technical matters
>
> 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.
> (Copyright February 2002 by Tom Buck. All rights reserved including
> electronic forms and hardcopy forms. Please respect my copyright and
> refrain from making copies for others.)
> ---------------
>
> I hope you get good results with 1.3 carbonate for 1.0 oxide. If you don't
> you are being sold a low-grade material loaded with filler.
> 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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mariko cruse on sat 16 feb 02
Dear Ivor,
I wish I could fix this situation - I can read Clayart Discussions, but =
I can not be sure if my message goes out. Not only that, I can not =
receive messages if they are sent to me off List, because my receiving =
address is in trouble at the moment, so the AT&T Broad band says.
I am ashamed of myself being so computer-illiterate, but I don't know =
what to do at the moment. But, Thank God! I can read you all on the =
Clayart. Mariko
Michael Banks on sun 17 feb 02
It seems established in the literature that the transmission colour of
cobalt ions is due to their coordination number in relation to oxygen atoms
(or other groups). Both red (6-fold coordination) and blue colouration
(4-fold coordination) is possible from the divalent (Co2+) ion. The cobalt
III (Co3+) ion produces red to pink colours also, being of a radius (smaller
than Co2+) that fits into 6-fold coordination exclusively.
I expect that Mariko is asking why cobalt II (Co2+) does not re-oxidize on
slow cooling in a glaze to cobalt III (Co3+), as does iron in a saturated
red iron glaze, in which Fe2+ re-oxidizes to Fe3+.
My suggestion to this question, is that unlike iron, cobalt has a stronger
tendancy to occupy 4-fold (tetrahedral) coordination positions than 6-fold
(octahedral) sites, and the tetrahedral sites favour the divalent (Co2+) ion
over the trivalent. So, even below cooling below the stability temperature
of Co3O4 (which contains both Co2+ and Co3+) cobalt II is still the most
favoured ion because tetrahedral sites are abundant in silicate glass and
octahedral sites are not. I put this down as an example of crystal field
stabilisation energies over-riding ionic potential and electronegativity
forces in the filling of stable sites in silicate glass.
As a footnote, the violet and purple colours possible with cobalt in some
glazes (e.g. magnesia matts) are due to Co2+ ions filling both octahedral (6
fold) and tetrahedral (4-fold) sites, their being a blending of red and blue
colours. The octahderal sites are provided not by the glass, but by
pyroxene crystallites.
Michael Banks,
Nelson, New Zealand
----- Original Message -----
Ivor Lewis wrote:
>The Di-Valent Cobalt Ion is stable at high temperatures but the
>Tri-Valiant Ion is unstable. Co3O4 decomposes to CoO at 895
>deg Celsius. If it were possible to get Co+++ in a glaze it might
>give a bright red. This might be possible with some low temp
>enamels formulations.
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