Michael Banks on mon 14 jan 02
David,
I think the iron oxides are a bit tricky, because unlike the other colouring
oxides (with possible exception of cobalt), the trivalent oxide (fFe2O3,
haematite, red iron oxide, RIO), apparently has eutectic relationships with
silica and the alkalis.
So, whilst acting as an anti-flux in most situations, at high concentrations
RIO in alkali saturated glazes can break the rules and act as a flux. I have
a feeling that finely-divided Fe2O3 (in conjunction with alkali) is an
natural active flux in red earthenware clay. But, notwithstanding all
this, in actual fact RIO probably does not directly act as a flux itself,
but does so only after reacting with something else (by eutectic relations,
or reduction) first, and being converted to divalent iron in the process.
RIO has a high thermal expansion, which one would suppose could initiate
crazing, but on the other hand, RIO crystals crowding oxidised,
saturated-iron, tessho and kaki type glazes, are often in the form of
matted, platy crystals of specular haematite (aka specularite), which act to
reinforce and increase the flexibility of the glass.
The black Fe(II+III) oxide (Fe3O4, magnetite, iron spinel), is the most
common iron oxide found in nature and is a likewise tricky beast, being
neutral in some glazes and reactive in others. It is a hybrid oxide,
containing equal numbers of divalent (ferrous) and trivalent (ferric) ions.
It is quite stable at high temperatures, even surviving some reduction to an
extent, and it is possibly the main iron-mineral colouring tenmoku-type
black glazes (though most texts ascribe this to be FeO). I'm not sure what
it's effect on crazing is, but it's crystal form is equant octahedra.
The black Fe(II) oxide (FeO, wustite) is quite rare on Earth, requiring very
low oxygen activities at high temperature to produce it (only occurring in
meteorites, steelmills and a couple of other places). (Apparently the
humble potters kiln can generate more reduction than the World's volcanoes).
Wustite is so uncommon that my mineralogy textbooks have little data
concerning it and I know not it's expansion. Ferrous iron is the (green to
blue ion) colouring celadons. It acts as a strong flux in glazes (and clay
bodies too -as anyone who has fired a red terracotta pot in reduction by
accident soon finds out). FeO reacts with silica to produce Fe2SiO2
(fayalite, green olivine, peridot) which melts at ~1200 oC.
Michael,
Nelson,
NZ
----- Original Message -----
From: David Hewitt wrote:
> Just one question. You mention a number of the colouring oxides but not
> iron oxide in any of its forms. Could I prevail on you to say how the
> various forms of iron oxide might be expected to react in respect of
> fluxing and crazing.
>
> David
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