John Sankey on tue 13 nov 07
There have been several comments on this lately that are at odds
with the data I have. Earlier this year I was lucky enough to
persuade a local lab to do X-ray microanalysis and electron
microscope examination of three iron glazes for free. (God knows
how much it would cost commercially! Far too much for me,
anyway.)
An important result:
- Above about 1000C in an electric kiln, Fe2O3 reduces to FeO,
probably to the form FeO.SiO2 which is transparent pale yellow as
a glass. (In a strong reducing atmosphere, this may not be the
case.)
- Fe2O3 can reform again below about 1000C given time, only on
the surface, and only as crystals.
A corollary is that laser (iron toner) decals have to be applied
on the surface, not covered by any glaze, or the iron will go
essentially transparent. Their colour can be black with fairly
fast cooling through 1000-800C, but can take on a reddish colour
with a hold of up to an hour at 950C during the cool.
Another corollary is that glaze expansion should be calculated
using the FeO form, not Fe2O3.
I'm getting excellent results with beldecal.com laser sheets.
See http://sankey.ws/glazeiron.html for more details of the X-ray tests.
--
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Ivor and Olive Lewis on wed 14 nov 07
Dear John Sankey,=20
Greenwood and Earnshaw, "Chemistry of the Elements" is a good modern =
source of information . Has quite a lot about Iron and its oxides.
The Iron Silicate you speak of is related to Olivine, a natural =
Magnesium Iron Silicate (Mg.Fe)2.SiO4, which, in its form of Peridot, is =
a wonderful pale yellow green gem.
At atmospheric pressure the dissociation of Fe3O4 to Fe2O3 is in the =
1350 deg C region. As the partial pressure of oxygen falls so does the =
temperature at which the reaction will happen. To get the F2O3 to FeO =
reaction in motion the partial pressure of oxygen has to be very low, =
even at high glazing temperatures
What does Weyl say about Iron in Glass ?
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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