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crystal glazes going bad

updated wed 21 aug 02


Fara Shimbo on mon 19 aug 02

Hi, Judy,

I think the reason crystalline glazes go bad after a while isn't that
they're so high in zinc and low in alumina, but that they're so
extremely alkaline. The components of many of them are highly reactive
even at room temperature and may combine to produce compounds which
inhibit crystallization (very refractory silicates and such -- or
perhaps they act as catalysts; alas I'm not sure how it all works).
I've noticed that any of my recipes containing lithium compounds are
notorious for this; after three months they are nearly useless for
crystals (although still nice glazes).

I have noticed that crystalline glazes in plastic jars go bad much more
quickly than those in glass jars.

Another problem may be that the zinc oxide becomes so hydrated that
there tends to be less of it per application of glaze (or that you put
on less glaze overall)? Makes me think I ought to take one of my
non-working glazes and calcine it in my test kiln to see what happens.

Your idea about dating jars is an EXCELLENT one!

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

Brad Sondahl on tue 20 aug 02

I mix crystalline glazes by the 10,000 gram batch, and have not had the
problems people are talking about. The batch usually lasts a month or
more before I've depleted it, sometimes 3 months in winter. My glazes
use Frit P-25, which is high in sodium. On the other hand, my recipes
also include more kaolin than many do. Successful crystalline glazing
has many variables, so it may be easy to be misled as to what causes or
fails to cause crystallization. My own experience says that thickness
of glaze, and final temperature achieved are the critical ones.
Brad Sondahl
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