My 'Parmelee' textbook also mentioned that the SiO 2 has a stabilizing influence and can also reduce the solvent bleaching action of B 2 O 3 on the development of color. I ran another test, increasing the SiO 2 and MgO to lower the expansion. There is no question that I was learning. Well, back at the textbooks, I kept reading about the detrimental effects of zinc on this type of glaze, however, there was no zinc to remove. Then I found another clue.
Few things disagree with magnesia but it was worth a try.
This project really demonstrated that you can learn much when something is unsuccessful. Every time a fired product comes out of the kiln, whether success or failure, you can relate its appearance to the formula and learn something valuable. This page is a chapter from the Magic of Fire book available on this web site.