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white underglaze problem

updated mon 2 mar 98


Foresthrt on sun 1 mar 98

>undergalze can get quite tricky with
>glaze fit.

I'm having trouble with an old jar of white "cover coat" underglaze bubbling
up under the clear glaze in firing. It seems to be doing this under each of
several different clears. The bubbles are small- like mustard seeds at their
largest, mostly quite small, but some have burst open and leave little pits or
jaggedness, and the others look like little warts or pimples. It only happens
where the glaze goes over the white- none of the other underglazes are doing
it. I thought at first that it was due to a reaction between one of the
underglazes where it was layered under the white, but I've changed the
combination and it isn't any better. Seems to be the white/glaze combo that is
the trouble.

I'm going to get a new jar of the underglaze and see if that's any better (the
old one could be easily 15 years old, possibly the formula has been improved?
Am thinking of switching to a white slip sort of thing instead- but how do I
make it fit the bisque and be really opaque? Or, is there a particular brand
of white underglaze that is known for being trouble free? Or a glaze that
someone could recommend? I'm firing to cone 6, and this happens on a buff
stoneware and on a dark brown clay so far.

Mary Klotz