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Metal Clay Connections
Issue
No. 9, Fall 2008
Garnets have been one of the mainstays for natural stones that can be fired in
place. True, they tend to darken (especially Rhodolite Garnets), but many of
us have had great success in firing them. Unfortunately, some people have experienced
garnets having a metallic sheen after firing. The first reports I had of this
were always in conjunction with stones fired with cork clay and I hypothesized
that something in the cork clay (possibly iron) was causing the problem. Now,
further reports find that this is not be the only problem with firing garnet.
Tests by Yvonne Padilla and Kevin Whitmore of Rio Grande, as reported by Lora
Hart on the MetalClay email
list, show a greater instability in these types of garnet than previously
thought—the exact composition of individual stones may be the dominant
factor whether the stones remain clear or gain a metallic sheen when fired.
The discussion on the
MetalClay list further reveals that the problem has more to do
with the chemical composition of Pyrope-Almandine garnets [(Fe,Mg)3Al2Si3012].
While Pyrope garnet contains Magnesium, garnets of the Almandine
family contain Iron. Rhodolite garnet is an intermediate form
of garnet in the Pyrope-Almandine series and contains varying
amounts of Iron. Zora Monster hypothesizes that the iron content
of the stone is the culprit. When heated the chemical bonds may
weaken and break, allowing the iron to form bonds with oxygen
and create Hematite (Fe2O3) on the surface of the stone. This
suggests that torch firing or firing in a kiln with a reduction
atmosphere (without oxygen) may minimize the likelihood of this
happening.
On a happier note, Spessartite,
Grossular, and Andradite Garnets (including yellow, gold, green,
Tsavorite, Hessonite, and Demantoid garnets) don’t contain
iron in their chemical composition (unless it is an impurity)
and shouldn’t have this problem. However, I did once have
a gold garnet become muddy when fired for an extended time at
1650 (cork clay may have been a factor), so it is probably a
good idea to fire all garnets at the lowest practical temperature
and test them before firing them in place in silver clay.
To read the full discussion,
go to the MetalClay
Yahoo Group's discussion.
Additional Source: Simon & Schuster’s Guide to Gems and Precious
Stones, New York, 1986.
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