Linda - I use Amaco undergazes on low firing clay (05-06) all the time. I
have never had a problem. I use the same colors that are giving you problems
too. My kids paint the underglazes on their clay fantasy creatures that have
been bisque fired. When the underglazes are dry, we dip the piece in a big
bucket of clear dipping glaze. Then it is fired again. Maybe you got a bad
batch of underglazes. Has it happened before? Is the kiln cooling down
slowly? It seem strange that it would happen only with certain colors.
We had a total "meltdown" one day. It was really hard to tell the kid -
- - the lessons of life!