Michael Banks on thu 11 nov 99
I commented:
> > The Cd and the Se (another deadly poison BTW), will
> >condense in the kiln walls, flue, etc on cooler surfaces. Cadmium is
> >commercially produced from condensation of flue gases (from smelting zinc
> >ore, Cd having a lower boiling point (767oC) than zinc (907oC).
>
Ray Aldridge wrote:
> This is interesting, because one of the cautions Ron Roy brought up was
> that using cadmium-bearing glazes could result in the contamination of the
> kiln and subsequent contamination of wares fired in the kiln, which in
view
> of what you say here seems pretty plausible. Is there any way this safety
> issue could be addressed-- perhaps by venting the kiln while passing the
> boiling points of cadmium and selenium? If so, this might deal with the
> unencapsulated fraction of the stain, but what about any metal liberated
by
> solution of the zirconium in an active glaze at peak temp?
To me this is not an issue of venting the kiln better, or at crucial
temperatures. Cadmium fume should not be discharged or tolerated under any
circumstances. It is in the same league as mercury vapour. What about the
neighbours, let alone anyone using your kiln room? Choosing a suitable
glaze would seem to be crucial. A starting point for a safety strategy
should be to take cognisance of the manufacturers guidelines and I quote
(from Cerdecs compatibility with glazes information sheet), (Quote):
"Inclusion Pigment Stains
23 416 Zr-Si-Cd-S Yellow /
23 616 Zr-Si-Cd-S Orange /
27 496 Zr-Si-Cd-S Intensive Red /
27 497 Zr-Si-Cd-S Bordeaux Red /
Highly suitable for glazes containing lead, calcium, zinc and boron. less
suitable for glazes rich in alkalis. Full luminous power requires
transparent glazes with high optical refraction. DO NOT MILL THESE STAINS
WITH THE GLAZE."
(Unquote)
In New Zealand; Intensive Red, Bordeaux and Pink TX (271P another
Zr,Si,Cd,Se,S colour) are widely used by potters. The common applications
seem to be painted on top of zircon-opacified glaze before glost firing or
underglaze decoration followed by dipping or spraying of a commercial low
temperature clear glaze, or the potters own clear stoneware glazes.
I've not seen any fading of the stain when applied over zircon opacified
glaze and dissolution of the zircon stain carrier crystals would be unlikely
due to the glaze being already saturated in Zr. The commercial low-fire
clear glaze widely used here (Ferro KMP271D, a low sol calcium lead
borosilicate) also seems compatible. I used ferro 271D for several years
over Pink TX decoration and even light brushstrokes of this colour showed no
sign of colour saturation loss pre and post-fire.
> Michael, in your opinion, what would be the safest type of base glaze for
> these pigments?
If I were using the Cd inclusion stains in or under my own stoneware glaze,
I guess I would initially avoid high levels of the elements highlighted by
Cerdec as causing colour loss (which I interpret to signify zircon-carrier
dissolution), E.g: (from the Cerdec Web Page;
http://www.cerdec.com/products/CadmiumInclusion.html (Quote): "Note that
excessive amounts of barium, boron, calcium, magnesium, tin oxide and other
alkalis can cause the color to take on a browner, grayer, more violet or
more washed-out appearance." (Unquote).
They seem to be saying here that flux-saturation glazes are out. Such glazes
are soft and craze-prone anyway and should not be put on foodware items, let
alone in combination with cadmium stain. BTW I wouldn't personally put Cd
stain on any inside foodware surface anyway. No-one every got poisoned by Cd
red colour on the outside of a food vessel as far as I know (unless it was
coming off on their fingers). Interestingly, Cerdec don't target zinc as a
problem which in moderate levels has a beneficial stabilising effect on
stoneware glazes. I would predict that a well balanced, high silica (SiO2
saturation inhibits zircon dissolution) glaze would be best. As an extra
insurance policy, the glaze could be saturated in Zr on purpose, by adding
small percentages of finely-milled zircon to the point of incipient
clouding at the target maturation point. Testing will establish this, as
will test firing of a number of clear glazes on test tiles with Cd stain
decoration to detect stain dissolution.
>
> Also, do you think this might be partly a problem of scale? In other
> words, is Ron's scenario of more concern in the case of a kiln fully
loaded
> with pots glazed entirely with a high-concentration cadmium glaze, as
> opposed to a kiln in which only a few pots are dabbed with a few accent
> brushstrokes of the pigment, in an optimal glaze?
Of course... More unstable Cd in, more out.
> Let's assume that I was somehow able to devise a Cone 8 glaze, using
> inclusion pigments, which leached cadmium like an underfired Shanghai
> enamel. In fact, let's assume, for the sake of safety, that my glaze
> leaches cadmium at ten times the acceptable level, (which if anyone cares
> is .25 micrograms/ml, for large hollow ware, using the standard leaching
> test as specified by the FDA.)
>
> But now let's say that I use this dangerous glaze, as I would, for minor
> accents in large bowls. Let's say the glaze covers (and this is a generous
> estimate) 2% of the bowl's inner surface. Let's further posit that
someone
> is going to use this large bowl to store a gallon of salsa in the icebox.
>
> My question is this: is there any reasonable possibility that such a bowl
> would fail a leaching test? I can't see how.
I'd rather be free of the worry by not using Cd stain on the inside in the
first place. But this is a personal character flaw. I like an uncomplicated
life... I don't think that anyone ever expected some Japanese factory
workers to slop 14 times too much U235 solution into a bucket recently. Me,
I'd never touch the stuff (U235).
Michael Banks,
Nelson,
New Zealand
Paul Lewing on fri 12 nov 99
Michael Banks wrote:
> > Michael, in your opinion, what would be the safest type of base glaze for
> > these pigments?
(from the Cerdec Web Page;
> http://www.cerdec.com/products/CadmiumInclusion.html (Quote): "Note that
> excessive amounts of barium, boron, calcium, magnesium, tin oxide and other
> alkalis can cause the color to take on a browner, grayer, more violet or
> more washed-out appearance." (Unquote).
>
> They seem to be saying here that flux-saturation glazes are out.
Now this is really interesting. I have always heard that to get the
brightest colors from these stains, you need a glaze high in calcium.
And in my experience, this has proven true. The best base glaze I've
found for them is Tony Hansen's 5x20 glaze, which has more calcium than
any other recipe I have, about .70 molecular equivalents. The upper
limit according to teh limit formulas I use should be .6. So I don't
know about safe, but I know about color.
I have also tried them in a base I developed that has gobs of barium and
boron, and they're not as bright. They are indeed grayer and more
violet. I've never tried them with tin, but I'd assume that would make
the color paler. I've also never tried them in a high zinc base, but if
you tell me that won't work, I'll for sure try it.
Paul Lewing, Seattle
Joanne Van Bezooyen on fri 12 nov 99
This is a little side thought......
Remember all the people who cooked in their kilns...baking bread, etc.. I can
imagine the dangers.
Michael Banks wrote:
> ----------------------------Original message----------------------------
> I commented:
>
> > > The Cd and the Se (another deadly poison BTW), will
> > >condense in the kiln walls, flue, etc on cooler surfaces. Cadmium is
> > >commercially produced from condensation of flue gases (from smelting zinc
> > >ore, Cd having a lower boiling point (767oC) than zinc (907oC).
> >
> Ray Aldridge wrote:
>
> > This is interesting, because one of the cautions Ron Roy brought up was
> > that using cadmium-bearing glazes could result in the contamination of the
> > kiln and subsequent contamination of wares fired in the kiln, which in
> view
> > of what you say here seems pretty plausible. Is there any way this safety
> > issue could be addressed-- perhaps by venting the kiln while passing the
> > boiling points of cadmium and selenium? If so, this might deal with the
> > unencapsulated fraction of the stain, but what about any metal liberated
> by
> > solution of the zirconium in an active glaze at peak temp?
>
> To me this is not an issue of venting the kiln better, or at crucial
> temperatures. Cadmium fume should not be discharged or tolerated under any
> circumstances. It is in the same league as mercury vapour. What about the
> neighbours, let alone anyone using your kiln room? Choosing a suitable
> glaze would seem to be crucial. A starting point for a safety strategy
> should be to take cognisance of the manufacturers guidelines and I quote
> (from Cerdecs compatibility with glazes information sheet), (Quote):
>
> "Inclusion Pigment Stains
> 23 416 Zr-Si-Cd-S Yellow /
> 23 616 Zr-Si-Cd-S Orange /
> 27 496 Zr-Si-Cd-S Intensive Red /
> 27 497 Zr-Si-Cd-S Bordeaux Red /
> Highly suitable for glazes containing lead, calcium, zinc and boron. less
> suitable for glazes rich in alkalis. Full luminous power requires
> transparent glazes with high optical refraction. DO NOT MILL THESE STAINS
> WITH THE GLAZE."
> (Unquote)
>
> In New Zealand; Intensive Red, Bordeaux and Pink TX (271P another
> Zr,Si,Cd,Se,S colour) are widely used by potters. The common applications
> seem to be painted on top of zircon-opacified glaze before glost firing or
> underglaze decoration followed by dipping or spraying of a commercial low
> temperature clear glaze, or the potters own clear stoneware glazes.
>
> I've not seen any fading of the stain when applied over zircon opacified
> glaze and dissolution of the zircon stain carrier crystals would be unlikely
> due to the glaze being already saturated in Zr. The commercial low-fire
> clear glaze widely used here (Ferro KMP271D, a low sol calcium lead
> borosilicate) also seems compatible. I used ferro 271D for several years
> over Pink TX decoration and even light brushstrokes of this colour showed no
> sign of colour saturation loss pre and post-fire.
>
> > Michael, in your opinion, what would be the safest type of base glaze for
> > these pigments?
>
> If I were using the Cd inclusion stains in or under my own stoneware glaze,
> I guess I would initially avoid high levels of the elements highlighted by
> Cerdec as causing colour loss (which I interpret to signify zircon-carrier
> dissolution), E.g: (from the Cerdec Web Page;
> http://www.cerdec.com/products/CadmiumInclusion.html (Quote): "Note that
> excessive amounts of barium, boron, calcium, magnesium, tin oxide and other
> alkalis can cause the color to take on a browner, grayer, more violet or
> more washed-out appearance." (Unquote).
>
> They seem to be saying here that flux-saturation glazes are out. Such glazes
> are soft and craze-prone anyway and should not be put on foodware items, let
> alone in combination with cadmium stain. BTW I wouldn't personally put Cd
> stain on any inside foodware surface anyway. No-one every got poisoned by Cd
> red colour on the outside of a food vessel as far as I know (unless it was
> coming off on their fingers). Interestingly, Cerdec don't target zinc as a
> problem which in moderate levels has a beneficial stabilising effect on
> stoneware glazes. I would predict that a well balanced, high silica (SiO2
> saturation inhibits zircon dissolution) glaze would be best. As an extra
> insurance policy, the glaze could be saturated in Zr on purpose, by adding
> small percentages of finely-milled zircon to the point of incipient
> clouding at the target maturation point. Testing will establish this, as
> will test firing of a number of clear glazes on test tiles with Cd stain
> decoration to detect stain dissolution.
> >
> > Also, do you think this might be partly a problem of scale? In other
> > words, is Ron's scenario of more concern in the case of a kiln fully
> loaded
> > with pots glazed entirely with a high-concentration cadmium glaze, as
> > opposed to a kiln in which only a few pots are dabbed with a few accent
> > brushstrokes of the pigment, in an optimal glaze?
>
> Of course... More unstable Cd in, more out.
>
> > Let's assume that I was somehow able to devise a Cone 8 glaze, using
> > inclusion pigments, which leached cadmium like an underfired Shanghai
> > enamel. In fact, let's assume, for the sake of safety, that my glaze
> > leaches cadmium at ten times the acceptable level, (which if anyone cares
> > is .25 micrograms/ml, for large hollow ware, using the standard leaching
> > test as specified by the FDA.)
> >
> > But now let's say that I use this dangerous glaze, as I would, for minor
> > accents in large bowls. Let's say the glaze covers (and this is a generous
> > estimate) 2% of the bowl's inner surface. Let's further posit that
> someone
> > is going to use this large bowl to store a gallon of salsa in the icebox.
> >
> > My question is this: is there any reasonable possibility that such a bowl
> > would fail a leaching test? I can't see how.
>
> I'd rather be free of the worry by not using Cd stain on the inside in the
> first place. But this is a personal character flaw. I like an uncomplicated
> life... I don't think that anyone ever expected some Japanese factory
> workers to slop 14 times too much U235 solution into a bucket recently. Me,
> I'd never touch the stuff (U235).
>
> Michael Banks,
> Nelson,
> New Zealand
Ray Aldridge on sat 13 nov 99
At 11:55 AM 11/12/99 EST, Paul Lewing wrote:
>----------------------------Original message----------------------------
>Michael Banks wrote:
>
>> > Michael, in your opinion, what would be the safest type of base glaze for
>> > these pigments?
>
>(from the Cerdec Web Page;
>> http://www.cerdec.com/products/CadmiumInclusion.html (Quote): "Note that
>> excessive amounts of barium, boron, calcium, magnesium, tin oxide and other
>> alkalis can cause the color to take on a browner, grayer, more violet or
>> more washed-out appearance." (Unquote).
>>
>> They seem to be saying here that flux-saturation glazes are out.
>
>Now this is really interesting. I have always heard that to get the
>brightest colors from these stains, you need a glaze high in calcium.
>And in my experience, this has proven true. The best base glaze I've
>found for them is Tony Hansen's 5x20 glaze, which has more calcium than
>any other recipe I have, about .70 molecular equivalents. The upper
>limit according to teh limit formulas I use should be .6. So I don't
>know about safe, but I know about color.
>I have also tried them in a base I developed that has gobs of barium and
>boron, and they're not as bright. They are indeed grayer and more
>violet. I've never tried them with tin, but I'd assume that would make
>the color paler. I've also never tried them in a high zinc base, but if
>you tell me that won't work, I'll for sure try it.
Good information, and particularly useful for me. Many of my glazes are
outside the limits for calcium, because I'm trying to develop some
variation in the surface quality of the glazes, and by using a fairly
active titania-bearing slip under a high-calcium glaze, crystal-seeding
often gets me what I want (or so I theorize.) So I've been somewhat
discouraged by Michael's thought (as I understood him) that high-calcium
glazes might not be suitable carriers for encapsulated cadmium. On the
other hand, Michael has implied that brightness, or at least saturation of
color might be a direct indicator of glaze suitability, on the theory that
less of the zircon carrier has been dissolved and more of the colorant has
been retained in a safe form.
Ray
Aldridge Porcelain and Stoneware
http://www.goodpots.com
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