If you have too much clay in a glaze recipe, you might have issues with your glaze crawling during the firing. Crawling is where the glaze pulls away from the clay body due to a combination of shrinkage, poor adhesion and high surface tension.
Most of our glaze materials come to us in their very basic, unprocessed form. They are dug out of the ground, impurities may or may not be removed, they are ground into a fine powder, bagged and shipped to our suppliers. Working with these minerals in their raw state poses some health risks.