Sam Hoffman on sun 28 nov 04
I have been thinking about Tony's lusterous glazes and some of Hideaki=20=
Miyamura's work (http://www.miyamurastudio.com) and thought it might be=20=
appropriate to post this small article about Islamic ceramics. Again,=20=
the point is driven home: technology can often be a potter's friend,=20
but is certainly not the deciding factor of the creation of great art!
** NANOTECHNOLOGY THE OLD-FASHIONED WAY
With very few electron microscopes at their disposal, Islamic
ceramists of the 9th to 12th centuries had to rely on trial-and-error
to develop their nanotechnological glazing techniques, but the
iridescent and color-change effects of polychrome luster pottery are
proof that they got it right. When illuminated by white light, luster
ware changes color as the angle of observation moves toward grazing
incidence, that is, nearly parallel with the surface being observed.
Light of successively shorter wavelength, moving from blue to red, is
diffracted or absorbed as the angle changes, and we now know that the
material cause of this iridescence (diffraction) and color shift
(absorption/diffusion) -- whether observed in Islamic pottery, opals,
butterfly wings, or CD-ROMs - is the periodic array of particles with
specific optical properties. The more regular the array the more
pronounced the effect and the wider the range of color. Using a
non-destructive technique of observation known as Raman scattering,
CNRS researchers examined a series of luster ware objects excavated
in Syria and Egypt and discovered glazes composed of many layers of
differing composition. Combining experimental results with ancient
reports by potters describing their techniques, the team was able to
establish the chemical/optical story of what happens. Luster
ceramists decorated the surface of previously lead-oxide-glazed
pottery with mixtures of clay, ochre, silver and copper salts, and
vinegar sediment, heated the object to dark red (500-600=B0C) and then
cooled it in a controlled atmosphere. The clay and ochre slow the
evaporation of the vinegar's acetic acid, giving it the chance to
partially dissolve the lead-oxide-based glaze (lead oxide is readily
dissolved by acetic acid at 80-100=B0C). The upper regions of the glaze
become porous, lead acetate forms on the surface and, as the
temperature continues to rise, silver and copper salts melt and enter
the meso-porosity created by the acetic acid. A pronounced presence
of oxygen during heating probably contributed to luster ware's
special surface as it encouraged the combustion of acetate residues,
which in turn could have raised the object's surface temperature to
over 1100=B0C. Varying the composition of the initial mixtures would
therefore have had an effect on surface temperature and allowed the
ceramist to vary the final luster color across different areas of the
object's surface. And all this without a spectrometer! (CNRS
Communiqu=E9, March 22, 2004)
Anyone who has seen examples of this pottery know how breathtaking it=20
can be. MUCH different than the gaudy raku luster glazes. Wish we had=20=
more examples to see in museums on the west coast.
-Sam
S.L. Hoffman Pottery
Corvallis, Oregon
www.samhoffman.com
Mike Gordon on mon 29 nov 04
On Nov 28, 2004, at 4:47 PM, Sam Hoffman wrote:
> I have been thinking about Tony's lusterous glazes and some of Hideaki
> Miyamura's work (http://www.miyamurastudio.com) and thought it might
> be appropriate to post this small article about Islamic ceramics.
> Again, the point is driven home: technology can often be a potter's
> friend, but is certainly not the deciding factor of the creation of
> great art!
>
> Hideaki Miyamura's work is truly beautiful. But when I tried to
> enlarge them for a closer look, I got a msg. saying this URL is
> unavailable. Did anyone else have this problem? Mike Gordon
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