Michael Banks on tue 9 nov 99
In answer to a couple of queries from Ray Aldridge:
> Flue? Does this mean that unencapsulated cadmium vaporizes from a
high-temp
> glaze?
The cadmium compound enclosed in the zircon carrier is cadmium sulpho
selenide, a suphosalt. Se-S sulphosalts have similar characteristics to
sulphides and are essentially immiscible in alumino-silicate melts. I would
expect the CdSeS pigment inclusions to instantly separate from the glaze
melt and volatilise, if liberated from the zircon carrier crystals at
stoneware temperatures. This would be analogous to firing the older
conventional (low earthenware) cadmium pigments to twice their recommended
stability temperature. 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).
>
> >
> >A salutary tale here: I one mixed up some Cd encapsulated stain with
alkali
> >frit, water and some clay in a jar. A couple of months later I opened
the
> >jar and Whew!, got a putrid whiff of very nasty smelling gas which I
would
> >bet money had a lot of sulphur, selenium and maybe even cadmium in it.
This
> >I take to be evidence that even alkaline water at room temp can release
the
> >Cd stain. So maybe here's a mechanism after all Ray.
> >
> I wonder if this might be due to the thought Ron Roy posted, which is that
> the stains might not be pure-- that some unencapsulated cadmium and
> selenium might be found in the stain as it comes from the manufacturer. (I
> noticed that on the manufacturers' site, they mentioned that a strong
smell
> might be detected the first time a container of pigment is opened.)
Yes, Ron may be right. Peering closely at the photomicrograph on Cerdecs
site ( http://www.cerdec.com/products/CadmiumInclusion.html ), I think I
can see a couple of small grains of pigment outside of the zircon crystals,
although the JPEG resolution is quite low.
> However, if this is true, does this contamination remain in the glaze at
> high temps?
No, I would expect it to leave as vapour, for reasons listed above. As for
thermal shock or tensional cracking of the zircon carrier, my microscope
observations of natural zircon in igneous rocks lead me to believe that
zircon has a high tensile strength, because cracking seems uncommon in
natural crystals. The widespread occurrence of natural euhedral zircon
(perfect crystalline form) in beachsands is supporting evidence that this is
not a brittle mineral at all. I tend to be more concerned that the zircon
stain carrier can be significantly dissolved by some high alkaline stoneware
glazes.
Michael Banks,
Nelson,
New Zealand
Ray Aldridge on wed 10 nov 99
At 01:00 PM 11/9/99 EST, Michael wrote:
>----------------------------Original message----------------------------
Michael, thanks-- extremely helpful information. I hope I can draw on your
knowledge a little more.
> 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).
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?
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?
>>
(snip)
I tend to be more concerned that the zircon
>stain carrier can be significantly dissolved by some high alkaline stoneware
>glazes.
>
Michael, in your opinion, what would be the safest type of base glaze for
these pigments?
Thanks again for your patience-- I appreciate it very much. I know some
folks are tired of the discussion, but while I'd find these pigments
extremely useful in my work, I want to be as sure as possible that I'm
producing safe ware. Because of the techniques I use, it's really not
feasible for me to test every possible glaze combination for leaching. I
currently use six slips and six major glazes, layering these freely. Four
of the slips are quite active and have significant effect on the overlying
glazes, and some pots might have two layers of slip and three layers of
glaze. And firing temps may vary by as much as 2 cones, depending on
placement. Add to that the differences engendered by the order in which
the slips and glazes are laid down, and the possibilities become an
unmanageable number, at $30 a pop (testing for cadmium and selenium.)
Because of this, I use nothing but the most innocuous ingredients in my
glazes.
A couple of posters have said that in the United States, manufacturers are
required by law to test their products for leaching, and if that's true,
then I might as well abandon the idea of using inclusion pigments. But no
one has been able to point me toward the actual regulation, and I haven't
been able to find it on my own.
The reasons I might consider using these pigments without rigorous testing
are based on what may be a faulty analysis of the potential problems, and
I'd really be grateful for a critique of my thinking, which goes like this:
I read hundreds of Import Alerts issued by the FDA pertaining to ceramic
wares impounded for excessive leaching. It's clear that most if not all of
these wares are low-fired wares, or wares which have been decorated with
enamels in a secondary low firing. Most of these wares were detained for
excessive lead leaching alone, but the few examples that also showed
cadmium leaching were invariably accompanied by high levels of lead,
leading me to believe that they were lead-based enamels. (And I should
note here that it's apparently entirely feasible to make a glaze with raw
cadmium pigments that does not leach excessive cadmium.)
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.
Ray
Aldridge Porcelain and Stoneware
http://www.goodpots.com
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