What’s causing the white bloom on my red clay pieces?

The white bloom that sometimes develops on the surface of unfired red clay bodies is as a result of soluble salt deposits in the clay migrating to the surface, this is known as efflorescence or lime bloom.  The same issue can sometimes be seen on external brickwork. Thicker ware cross section and slower drying promote this process. If possible, dry the ware more quickly; although we often warn against rapid drying which can cause stress in the pot, it is possible to dry pottery very fast as long as the heat is applied evenly. A thinner cross section will help.  When the piece is bone dry any efflorescence can be cleaned by gently rubbing the surface with coarse dish sponge.  In severe case a masonry cleaner can be use after the firing. 

Incidentally, the same process of salt deposits causes the toasted effect seen with Original Craft Crank 153-1114. The calcium combines with the iron content at higher temperatures. Toasting can therefore be encouraged in Original Craft Crank by thicker cross section of ware, slower drying and higher firing.