I teach a ceramic class at The Salvation Army. I am using Standard low fire slip which I fire to cone 05. The bisque looks white. My problem is when I use a clear glaze and fire it to 05 or 06 it comes out looking yellowish or creamy. I am aware of the formula change. I have tried thinning it with distilled water, and putting on a thinner coat of glaze, and used a different brand of glaze. Nothing I have tried made a difference. My supplier has told me that they have not had anyone else with this problem. Most of the time there is no problem as my class usually use colored glazes. But, there are times when they want an item to be white and since we are a non profit I really don't want to use more expesive white glaze. Is there anyone that can help me?