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crystal glazes going bad -- and bottles changing color!

updated thu 22 aug 02


Fara Shimbo on wed 21 aug 02

Hi Brad, et al.,

I usually mix my glazes in batches of less than 500 grams
(the pieces I make are generally small). Many of my glazes are no good
after about a year, some after six months. The iron glazes will produce
beautiful aventurines, but no crystals.

I just started thinking of something else that happens that may or may
not be a related phenomenon: the glaze that touches the sides of the
bottle changes color.

Glazes containing both cobalt and titanium get a layer of mustard
yellow-brown (the color of cobalt titanate which is what I suspect this
is) on the glass. This can develop in some glazes in as little as a
week. It doesn't happen in titanium-free glazes. The glazes work well
for at least a few months (around here blue sells really well and I
almost never have blue glazes sitting around in jars very long).

With copper glazes, black and then (sometimes) red copper oxides develop
in the glaze against the glass within a matter of hours. You can almost
watch it happen if you put the bottle in the sun. This happens in all
the copper glazes.

Glazes containing silver turn pink! I have no idea what this compound
could be.

Iron and manganese glazes show no changes; but cobalt, copper and silver
glazes will do this no matter how small the concentration of colorant.

Any clue what might be causing this?

Fara Shimbo, Master Crystallière, Certified Public Nuisance
Shimbo Pottery, Box 41, Hygiene, Colorado, USA 80533 ^^^