Feldspar is composed of lithium, potassium or sodium that has been chemically bonded with silica and alumina. Primary clay, or kaolin, is a white, almost translucent, clay that forms when the metals in feldspar have been replaced with water. This level of purity is generally only possible when the feldspar has not been moved from its original location.
Secondary clay forms when primary clay has been mixed with other elements introduced through the weatherization processes, such as rain and wind, that move the feldspar. While these other elements can differ greatly, almost all secondary clays contain iron.
Kaolin, or primary clay, is considered extremely difficult to work. Even expert potters do not always attempt projects in kaolin. It is brittle, temperamental and less malleable, or plastic, than secondary clays. If not fired or dried correctly, a formed piece can easily break. However, because of its purity, it is less likely to crack when dried and fired properly.
Secondary clays vary much more in quality, are more plastic than kaolin and generally easier to work. However, objects made from secondary clay are more likely to crack even when fired and dried correctly.