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stability of low fire glazes

updated thu 13 jul 00


John Hesselberth on wed 12 jul 00

Hi Everyone,

I have just completed testing several low fire glazes using the vinegar
test followed by leach testing for the ones that passed the vinegar test.

Overall I would offer these observations. Stable glazes can certainly
be made in the low fire framework; however I believe the options on glaze
surface and color intensity will be more limited than they are in the
cones 6 and 10 ranges. If you are experimenting with glazes in this
temperature range you would do best if you make sure they have plenty of
silica -- probably 2.5 or more. You will then have to add enough boron
and/or sodium and potassium to get good melting. Be particularly careful
if you are using copper as a colorant. As with high fire glazes, it is
the most difficult colorant to keep in a glaze.

Recipes and test results for the three glazes that I tested which are
relatively stable are now posted on my web site (URL below). They are
Potter's Pallette Alkaline Green, a Black Mat by Chappell and a Cobalt
Blue by Gail Kendall.

I also tested 2 that failed the vinegar test. These two were as unstable
as any glazes I have ever seen. I guess my advice to functional low fire
potters would be to be very cautious and make sure you test the glazes
you are using. The failure rate seems to be higher than on high fire

As always, I remain willing to post any glaze you have leach tested so
the results can be shared with the potter community as a whole. Just
send them to me.

John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: web site:

"Pots, like other forms of art, are human expressions: pleasure, pain or
indifference before them depends upon their natures, and their natures
are inevitably projections of the minds of their creators." Bernard
Leach, A Potter's Book.