Michael Banks on wed 21 nov 01
Hi Ivor,
I've mainly seen the effects of volatilisation of green chromium oxide in
electric kilns under purely oxidising conditions and I understand that
transition metal carbonyl complexes dissociate at less than bisc
temperatures. So invoking carbon monoxide interaction in this case would
seem unlikely. I've not noticed chromium green flashing whilst bisc firing
(900-1000 degrees) pots with chromium green-bearing slips/ underglaze
decoration.
No, I don't have any vapour/liquid partition-coefficient data for lead in
molten aluminosilicate solutions and I doubt
whether this information (if any exists) would be particularly useful given:
(a) the difficulty of compiling these data for a range of glaze compositions
(noting that many fluxes/compounds destabilise lead) &
(b) that available limit formulae - derived from 400 years of empirical
observation, clearly indicate lead is a non-starter above 1180.
I think that Occam's Razor might apply here. The simplest explanation
being; that the kinetic energy of lead atoms in most glazes exceeds their
tenuous attraction for molten silicate (i.e: they attain escape velocity),
over about 1180oC.
Michael
----- Original Message -----
Ivor Lewis wrote (abridged):
The fact that blushing can occur on free clay adjacent to glazes indicates
that there is a lot going on in a kiln which we know little about. And as
you say, the movement of Chromium sesquioxide is easily traced by the strong
colour it induces on contaminated surfaces. But are you sure this is due to
evaporation and not some other process, especially under conditions of high
concentrations of Carbon Monoxide.
In saying that lead compounds are notoriously volatile due to their high
vapour pressures, do you have values for the Lead silicates, or rates of
loss in micro/milligrams per K per Min. over a range of temperatures?
Best regards,
Ivor Lewis. Redhill, South Australia
| |
|
