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two underglaze problems

updated mon 2 mar 98


Berry Silverman on sun 1 mar 98

I am experiencing two opposite, but equally annoying, problems with
reactions between underglaze and glaze.

I decorate platters with underglaze -- 3-4 coats applied to greenware
for intense color -- then bisque to 04, dip in clear leadfree glaze,
then fire to 06.

One color which is causing a problem is a deep purple underglaze (Mayco
UG 94). This color seems to absorb the glaze more than the other colors,
and as a result the clear glaze becomes milky from lots of tiny bubbles.
I have used this color and glaze without problem for over a year --
until about two weeks ago.

The second problem is a coral underglaze (Colorobbia deep coral engobe)
which seems to repel or resist absorbing the same glaze. So after
dipping it requires lots of touch-up and often seems glaze starved after
firing. All other colors fire beautifully with the glaze -- bright
strong colors with a clear transparent smooth finish.

Is there something I can/should add to the purple to decrease its glaze
absorption? Is there anything I can try with the coral to increase its
absorption of the glaze? Any other tricks or techniques would be
appreciated. The frustration of too much glaze and too little glaze on
the same piece -- in the middle of perfect color and gloss -- is driving
me to distraction.