Salt Glazed Chimney Pots
Salt Glazing is a very old technique for glazing ware or even bricks. You typically see it on old sewer pipes, the pieces on top of old brick walls and those old fancy paving brick used for sidewalks. The colors of salt glazed chimney pots range (even on an individual piece) and run from tan to brown to dark black to purple/black.
The “Wood Fired” method is as follows:
The kiln is taken up to temperature (about 1850F) with fuel (wood). Then salt is introduced into the red hot kiln. What happens is the NaCl dissociates and becomes a gas inside the kiln. The Na (soda) combines with the clays on the surface of the clay chimney pots to form a thin layer of glaze. It is similar to the composition used in common window glass. At these temperatures, when the salt is introduced, there is not enough oxygen in the kiln (this is usually done on purpose) and when there is not enough oxygen, these really nice dark black/purples/dark browns form on the clay chimney pots. It is called “reducing” conditions.