Michael Banks on wed 7 jun 00
Ivor,
Perhaps two examples can help to illustrate the efficacy of these two models
in explaining the behaviour of some glaze components:
1. The fact that tin and zirconium make good opacifiers in glazes, may be
due to the fact that both elements are known to exhibit high crystal field
stabilisation energies. The crystal field stabilisation energy of a
transition element varies from high (e.g. zirconium) to low (e.g. zinc) and
controls the potential of transition metals to occupy octahedral sites in
crystals. Zircon crystals (zirconium silicate) crystallise early out of
natural magmas at high temperatures and resist dissolution in glazes to a
great extent. Tin is also a high field strength element and behaves
similarly to zircon, even though the melting point of tin oxide is less than
for zinc oxide. Zinc is a low field strength element however and dissolves
readily in glaze, because it's ability to stay put in crystal form is weak.
Zinc oxide is thus a poor opacifier. Titanium has intermediate crystal
field strength and titanium dioxide has only moderate ability to resist
dissolution in a glaze, but a proportion of it crystallises out of solution
during glaze cooling.
2. The quasicrystalline model regards silicate melts -not as oxide soup, but
to consist of free moving mineral component molecules, such as feldspar,
pyroxenes, nesosilicates, some oxides etc. These molecules vary
considerably in size and complexity and are essentially ultra-small
crystals. But their crystalline nature is too small to be detected by x-ray
methods etc, hence the term "quasicrystal". This concept is quite useful to
explain the nature of many glazes. For example, feldspathic glazes are
viscous and "fat". This can be visualised as due to the fact that feldspar
molecules (feldspar quasicrystals) are large clunky structures which do not
easily flow around each other. Add other elements which favour smaller,
compact mineral quasicrystals (excess alkalis, calcium, magnesia, boron,
zinc etc) and the glaze becomes very much more fluid. Adding sources of
alumina to a lime, or alkali rich glaze though, has the opposite
effect -again favouring large, viscosity promoting feldspar quasicrystals.
Hope this helps,
Michael,
in NZ
Ivor Lewis wrote:
----- Original Message -----
Snip:
any more.--- crystal field theory and the quasicrystalline model>
I suppose the best way to become one is to be educated in one of the
institutions which was responsible for the Renaissance movement. And to read
Classics.
Right Michael, put these post renaissance ideas into words we simple people
can understand and relate it to the stuff which runs down the pot and onto
the shelf.
Best regards, and thanks for some thought provoking information
Craig Martell on mon 12 jun 00
Hi:
Thanks for this post Michael. I've been working on an irridescent shino
glaze that has very high surface reflection and I've been working on some
crystal stuff to change the light refraction in selected places on any
given piece. Your post has given me some other directions and
considerations concerning my crystal experiments.
By itself, the glaze is almost as irridescent as copper raku stuff. The
colorant is Iron though. When I use titanium slips under this glaze, I get
a rainbow prism effect which produces blues, voilets, pinks that are very
subtle. So I am encouraged to press on.
Thanks for some great info.
Craig Martell in Oregon
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