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refiring a crystalline

updated mon 26 sep 05


Tom Buck on sun 25 sep 05

Kent W:
like most clay things -- a crystalline glaze refire may improve
a pot or it may make it worse. it depends.

The softening point of many Cone 6 glazes (thanks to Ron Roy) is between
620 - 740 oC (x1.8 for oF). Cone 10 glossy glazes will soften somewhat
higher but below 1000 oC. Above this transition temperature the glaze will
start to flow. Crystalline glazes usually have low alumina so they flow

Should you refire a Xtalline glaze to Cone 04 (1060oC/1940oF) the
glaze will be quite fluid and flow noticeably. Xtal glazes often contain
Boron-3 Oxide (aka borate of some kind), and B2O3+flux oxide (KNaO) melts
below 1000 oC.

so what is achieved by refiring to C04, and soaking for say 30-60
minutes? you might salvage a Xtal glaze that was fired too fast. but you
may also shed an already thin glaze coat, and not obtain the zinc
silicate macro-crystals you expect.

you decide: put a refire with runoff cup in with raw-ware in a
bisque firing and see what occurs. Or make some more shards.

later. peace Tom B

Tom Buck ) -- primary address.
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tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
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