Joseph Herbert on fri 21 aug 98
Autumn Downey wants to fire rocks as adjuncts to her glazes.
One thing mentioned, fool s gold or Iron Pyrite, is a famous self-roasting ore
of iron. This rock will burn and produce a kiln full of sulfur, sulfur
dioxide around the kiln, and a lump of iron where the glittery crystals used
to be. Probably not a good choice. Any rock containing a carbonate mineral,
like limestone, should not be fired this way. Other rocks, regardless of
their composition, present problems if they contain any water. The words
shatter and crack may be insufficient to describe how some rocks come apart.
Explode comes to mind. Finally, there are several rock-forming minerals that
change significantly at kiln temperatures. This can be an emission of gas
when a mineral looses water or carbon dioxide, again the possibility of
explosion, or it can be melting. Remember most glaze "chemicals" are ground
up rock forming minerals.
You should test each of the rocks you want to use by test firing in a closed
container. If you can identify a source of identical rocks, trinity ceramics
sells feldspar crystals, for example, you can test one and then rely on the
results. If you are using found rocks, the reliability of one test applied to
a second rock may not be too reliable. At the very least you might want to
slowly heat a batch of rocks to 300 degrees F. and use the ones that survive.
I wish a more than normal amount of good luck, it will be needed.
Joseph Herbert
JJHerb@aol.com
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