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iron in oxidation =3d?windows-1252?q?=3d96_?=3d the difference par=

updated fri 17 dec 10


Neon-Cat on thu 16 dec 10

ticle size makes in low-fired work: two photo examples

Hi there all! :: To illustrate the effect of particle size on iron
colors I have put up two examples of work out of the kiln today. One
is of an iron dark-violet to maroon-colored terra sigillata and the
other is of an iron orange-red terra sig. Both were fired to 1652 F or
cone 010 (900 C).

In the greenware photo the vase on the right has had applied terra
sigillata made from OM-4 ball clay with the addition of the art
pigment Caput Mortuum (Mars Violet) to supply the iron as Fe2O3
(hematite). The large particle size of the hematite, from 1 to 5
micrometers (microns) grain size, is responsible for the blue-red to
purple color to violet-purple hue. Kate Helwig of the Canadian
Conservation Institute and a well-known pioneer in iron pigment
research, has done extensive work, as have others, on this
relationship between iron colors in pigments and the particle size of
the iron involved. In unfired work or when used as art pigment, Caput
Mortuum derives a metallic luster associated with the hexagonal
plate-like micaceous (non-crystalline) structure characteristic of the
specular form of hematite used to make this pigment. Unfired, the
burnished ware had quite a nice shine.

Particles of size 0.1 to 0.2 microns and smaller appear bright red. To
illustrate this I chose an art pigment derived from synthetic iron
molecule (color code PG8) from a Terre Verte art pigment (vessel on
the left in the photos). In this synthetic azomethine pigment a single
Fe3+ ion is joined by counter-ion ligands (in this case NO3-) to
carbon-containing organic molecular segments. They decompose and the
ferric ions combine with oxygen during firing to leave us with very
small particles of Fe2O3, our red iron oxide (hematite):

2Fe3+ + 3O2- ---> Fe2O3

In its entirety the synthetic azomethine probably has a particle size
between 0.5 to 0.1 micrometers =3D96 the Fe3+ ion itself has an effective
ionic radius of 0.000068 micrometers and a Fe2O3 particle created from
two of the Fe3+ ions will have a particle size of about 0.01
micrometers =3D96 or something very tiny for sure. Aggregation of the
newly created hematite particles may occur during firing to some
extent. Mixed with a little bit of terra sigillata made from OM-4 ball
clay the Fe3+ ions in the organic metal-complex were well dispersed.
The terra sigillata made from this chelated iron-green pigment fired
to a lovely, smooth, uniform, bright orange-red.

Anyway, you can clearly see the difference particle size makes when
firing large vs. small hematite particles here:
(first 2 photos, fired and unfired)
tMarian Gooding
Neon-Cat Ceramics