Paul Lewing on wed 8 feb 06
on 2/8/06 8:02 PM, Tom Buck at tom.buck@HWCN.ORG wrote:
> the only way this element (as oxide) may be incorporated (safely?)
> into a glaze is via an "inclusion stain" otherwise "encapsulated stain".
> an inclusion stain seeks to include one component (eg, SeO2 molecules)
> within the crystal structure of zircon (Zirconium silicate).
One thing that most people don't know about cadmium inclusion stains, and
cadmium colors in general, is that the colorant is not cadmium oxide. This
has a lot to do with shy the cadmium colors are so tricky and so unlike any
other colorant in their action.
The red color is actually a combination of two crystals, cadmium sulphate
and cadmium selenate. The selenate is, of course, cadmium and selenium.
The red one is actually the selenate, with the sulphate being the yellow
component of the colorant. Different shades are made by varying the
proportions of the two.
What makes the color so fugitive is that (and here I'd have to consult my
notes to remember which is which) one of these crystals is destroyed by
reduction, the other by oxidation. The cadmium and selenium don't leave
they change to the oxide form. Cadmium oxide is black and selenium is white
(or is it the other way around, I forget) which is why when cadmium reds
disappear you're often left with a gray streaky area.
Paul Lewing, Seattle
Tom Buck on wed 8 feb 06
KL:
my chem dictionary warns that selenium dioxide (SeO2), the most
likely component that might be used in a glaze, would be "highly toxic" by
inhalation, and moderately toxic by ingestion and skin absorption. this
compound sublimes at a low temperature 340-350 oC, and is soluble in water
and alcohol. so it would be a major hazard for the potter using this
compound.
the only way this element (as oxide) may be incorporated (safely?)
into a glaze is via an "inclusion stain" otherwise "encapsulated stain".
an inclusion stain seeks to include one component (eg, SeO2 molecules)
within the crystal structure of zircon (Zirconium silicate). for example,
US Pigment Corp offers inclusion stains made in China. not recommended for
use above Cone 6.
Some supply companies offer high-cost German-made (Cerdec)
encapsulated stains made by a special process, and such stains are
designed for use in industry at high cones (8-10).
These stains are mainly sold to ceramic companies big enough to
perform rigorous leach tests continuously, are not recommended for surface
in contact with food. Most studio potters would be hard-pressed to meet
government standards for testing pots containing these stains in their
glazes.
til later. peace Tom B.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
marci Boskie's Mama =^..^= on fri 10 feb 06
At 11:00 PM 2/9/2006, you wrote:
>The cadmium and selenium don't leave
>they change to the oxide form. Cadmium oxide is black and selenium is white
>(or is it the other way around, I forget) which is why when cadmium reds
>disappear you're often left with a gray streaky area.
>Paul Lewing, Seattle
Hi Paul,
Well that explains why cadmium reds in china paint are so touchy to
fire ! Thank you ! This is something that has been puzzling the china
painting community for a while. We also know that cad colors dont play
well with the other kids on the block so could the reason that you cant
mix cads with other china paints be that it changes the delicate
balance of cad and sel in the cad paints?
Marci Blattenberger Boskie's Mama =^..^=
http://www.marciblattenberger.com
marci@ppio.com
Porcelain Painters International Online http://www.ppio.com
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Paul Lewing on fri 10 feb 06
on 2/10/06 4:49 AM, marci Boskie's Mama =^..^= at marci@PPIO.COM wrote:
>> The cadmium and selenium don't leave
>> they change to the oxide form. Cadmium oxide is black and selenium is white
>> (or is it the other way around, I forget) which is why when cadmium reds
>> disappear you're often left with a gray streaky area.
>> Paul Lewing, Seattle
>
> Hi Paul,
> Well that explains why cadmium reds in china paint are so touchy to
> fire ! Thank you ! This is something that has been puzzling the china
> painting community for a while. We also know that cad colors dont play
> well with the other kids on the block so could the reason that you cant
> mix cads with other china paints be that it changes the delicate
> balance of cad and sel in the cad paints?
>
> Marci Blattenberger
I think what really happens is that the cadmium colors require a very
particular balance of flux oxides to maintain that equilibrium between the
CdS and the CdSe parts of the crystal. Mixing in any other color disrupts
that, because none of the other colors require quite the same flux as the Cd
colors.
Incidentally, this discussion started with cadmium inclusion stains. These
are never used in china paint. For one thing, there's not enough CdS/CdSe
in them, and the very refractory zirconium shell keeps them form melting
properly. The colorant portion of china paints has much more influence on
the chemistry of the melt than in glazes, because the proportion of colorant
is so much higher. This, in turn, is because china paints are almost
entirely lacking in the stabilizer portion of higher fired glazes.
Paul Lewing, Seattle
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