Kaolins are used in carpet backings, paper coatings, make-ups and are even additives in some foods. So basically, we are subject to trying to make the best of products that are not mined for us. The skill is in knowing how best to react and change when necessary. A. A glaze must fit just right over the clay body. Too tight and crazing or shivering results, too loose and your glaze could crawl.
Every clay body shrinks at a different percentage, as do glazes.
Glaze is also affected by the ingredients in the clay bodies themselves. High iron bodies will almost always interfere with lighter glazes, giving off-colors or deepening darker colors. Combine this with the fact that materials sometimes change and it leaves you with a sense of wonder. Some clays will have elements in them that are heavier and will tend to settle to the bottom. A simple solution is to add just a little common Epsom Salts into your throwing water.