iandol on sun 18 nov 01
Dear Heidi Haugen,
Seems to be a lot of advice coming in on this one.
But what are we talking about when we say lead is volatilising at Cone 6 =
?? Who provided this information and how valid is that source??
What Compounds of Lead are being used in your friends recipes ??
Is it Galena, commonly used in pre-industrial times and later, dusted =
onto wet clay immediately the pots have been thrown. The chemistry here =
is the oxidation of lead sulphide followed by the combination of that =
oxide with free silica in the clay body.
Is it Lead Carbonate, White Lead, used for generations as a white =
pigment in common household paints (and now replaced by Titanium =
Dioxide). This undergoes thermal decomposition, again giving the oxide =
which melts and fluxes the clay as well as dissolving free silica.
Is it Litharge, PbO, a yellow, water insoluble, crystalline solid which =
melts at 888 deg Celsius.
Is it Red Lead, Pb3O4 which decomposes when heated or Lead dioxide =
PbO2.
Maybe it is as a ceramic frit. The common ones are lead Bi-Silicate and =
Lead Sequi-Silicate, both compounded to be insoluble in water. The =
former is listed by my supplier as a standard low solubility material. =
Someone else may have information about the physical and chemical =
properties of these materials but I would imagine they are relatively =
inert at the temperatures we are dealing with, unless something is =
introduced into the glaze recipe which might lead to a chemical =
reaction. Sodium Chloride or Calcium Fluoride might react with lead =
silicate at the temperatures we are talking about, but I think these =
would be doubtful propositions.
Having given some of the technical information about Metallic Lead in a =
previous post I might as well top that up with some about Lead Monoxide. =
At a pressure of 101.325 kN/sq M (roughly atmospheric pressure) the =
vaporisation temperature is 1477 deg Celsius. At 5 kN/sq M the =
vaporisation temperature is 1212 deg Celsius. So the pressure inside a =
kiln would have to be very low for an appreciable amount of lead oxide =
to evaporate at Cone 6.
Yes, be cautious. But the only way to find things out is to have some =
analyses done on surface chips of kiln refractory. Look at it this way. =
If Lead in one form or another is fuming from the glazes your friend is =
using then eventually they will attack and corrode the refractories =
which line the kiln. Now have you, or has anyone else, ever read or =
heard of a kiln being decommissioned because of this occurring? And =
should this be the case, reheating the kiln several times to discharge =
"Lead" from the kiln seems oxymoronic when it is such a strong melting =
flux at the temperatures you have spoken of.
Without some definitive information derived from a practical =
investigation where samples are analysed, I feel that what you imagine =
may happen is highly improbable in the light of information available to =
me. But yes, be cautious with lead.
Best regards,
Ivor Lewis. Redhill. South Australia
=20
Ron Roy on mon 19 nov 01
The text books I have read that talk about lead say there is no point in
adding it to a glaze to be fired above cone 6 - that must mean there is
none left by then.
Hamer says - PbO - when incorporated in a bisilicate frit is stable to
1200C (cone 6) but - other lead frits become unstable at 1100C and if the
various forms of "raw lead" are used it is distinctly unstable at 1100C.
RR
>But what are we talking about when we say lead is volatilising at Cone 6
>?? Who provided this information and how valid is that source??
>What Compounds of Lead are being used in your friends recipes ??
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Paul Lewing on tue 20 nov 01
on 11/19/01 4:50 PM, Ron Roy at ronroy@TOTAL.NET wrote:
> Hamer says - PbO - when incorporated in a bisilicate frit is stable to
> 1200C (cone 6) but - other lead frits become unstable at 1100C and if the
> various forms of "raw lead" are used it is distinctly unstable at 1100C.
Ron, I think it starts volatilizing sooner than that, even when in the
bisilicate frit form. For a few years I was using a recipe I called Drop
Dead Red, which was a drop-dead gorgeous glossy bright iron red, and had
about 65% lead bislilicate, which was the other reason I named it that.
When I started using it I was firing to cone 5, but I dropped the firing
temperature to cone 4 because it bubbled so much. It still did it sometimes
at cone 4, so I abandoned it. But I only seemed to get bubbling sometimes,
which I never could figure out. In those firings when it did it, it boiled
so violently that it would be all over the tile setter above the glazed
tile, sometimes even fusing them together. Other firings it wouldn't do it
at all, and would come out smooth, glossy and beautiful. So I think it does
start to go off as a gas below cone 6.
Incidentally, a suggestion from Michael Banks to leave out the zinc oxide
that was in the original recipe, did cut down the bubbling, but didn't
eliminate it.
Paul Lewing, Seattle
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