Apparently, you cannot fire low-firing clays with high-firing glazes at high firing temperatures.
Who knew? Actually, a little voice inside my head said, "This ain't gonna work." And looking back, I think the guy at the supply house knew. When we bought high-firing glazes he sent us home with the appropriate clay. By mistake, I later bought the wrong clay. Not only did the glazing turn out poorly, but they all cracked as well. Bummer, huh? I had been very excited about these three pieces. Ah well, I'll try them again.