Linda - I am using the Amoco underglazes on low fire white clay. I have
never had a problem. My kids are base coating the fantasy animals with a solid
color. When dry, they add patterns, etc. with another color. My school system
orders a huge bulk order of clay for all schools, so I do not know the
specifics of the clay. It comes from Campbell's Ceramics in Richmond, Virginia.
Occasionally, if I run low on clay, I've bought low fire white clay from a local
studio and have had no problems with it either. I feel confident that it is not
the Amaco underglazes. I usually order my underglazes from Sax. Trying a new
clay might be a good idea.