What is terra sigilatta?
Terra sigilatta (or terra sig. to cool potters) is a slip made of extremely fine clay particles. The terra sig is applied to the “leather-hard” greenware, or even bisqued pot and when mostly dry can be burnished (or polished) with a stick, smooth pebble, back of a spoon, or whatever works. The resulting surface is rich and shiny. Burnishing the surface was important in many cultures because the resulting surface made the pot more waterproof when fired to low temperatures. This is because the burnishing lines up the flat clay mineral particles along the surface of the pot. The alignment minimizes the porosity of the surface. Burnished pots must be fired to only very low temperatures to maintain the shiny surface because the clay minerals begin to recrystalize at higher temperatures and the structure is lost leading to a dull surface.
