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?
Fa
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Fara Shimbo, Master Crystallière, Certified Public Nuisance
Shimbo Pottery, Box 41, Hygiene, Colorado, USA 80533
www.shimbopottery.com ^^^ www.crystalline-ceramics.info
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