The term “salt glaze” got its name from the process of using salt in the kiln. Because of the involvement of salt, which naturally contains sodium, the process is also called sodium glazing or sodium firing, while the pieces created by this method are sometimes called “saltware”. Due to the heat, the salt decomposes and vaporizes. After that, it combines with alumina and silica from the clay of the pottery.
However, the salt glaze doesn’t stick only to the pottery, and it adheres to everything in the kiln. Thus, the pottery needs to have the bottom covered with salt vapor resistant materials, or be placed on stilts, thus avoiding any contact with the surface. This action happens everywhere, besides the inside parts of the wares, unless they have big openings and are hollow.
But even in those cases it can easily be avoided by coating the insides in a glaze prior to firing it in the kiln. This process creates a very durable and glossy surface on the pieces, however it is still banned in many areas, as the process is followed by the creation of several poisonous gasses.