ART 198 - HISTORY OF WORLD CERAMICS
Salt Glazing was first done in the Rhineland in the 1400's CE, and was an important technical improvement of the time. Salt glazing is a process in which the glaze develops in the kiln rather than through an application of glaze. In this sense, it is similar to the ash glazing process discussed earlier in relation to Chinese high temperature ceramics. In salt glazing, the pottery is placed into the kiln as greenware with no preliminary bisque firing and with no glaze applied. Pots are fired to a temperature of over 2000° F. (stoneware temperatures), at which time raw salt is thrown into the kiln through the spy holes. Often the salt is wrapped into small paper bundles for ease of throwing. At these high temperatures, the salt immediately vaporizes, which means that it is transformed from a solid directly to a gas, bypassing the liquid state. The salt (sodium chloride) disassociates into its constituent ions, sodium and chlorine. The chlorine leaves the kiln with the vented hot air as a poisonous brown gas, and the sodium ions interact with the surface of the stoneware clay. The sodium acts as a flux, and reacts with the silicon and aluminum of the clay, forming a glaze at the surface. So, in a sense, the pots 'self glazes.' Pots go in unglazed, and come out with a thin coating of glaze that often has a somewhat pebbled surface, called 'orange peel' by potters. Advantages of salt glazing are that no preliminary firing or glaze application are necessary, and this saves on fuel costs and labor time. Disadvantages are that colors are limited, usually the brown or gray of the stoneware clay, and kiln damage. The sodium ions are not picky; they attack the kiln bricks (which are made of clay, of course) just as easily as the clay surfaces of the pottery. This means that the entire interior surface of the kiln, including the shelves and supports, become coated with the salt glaze as well. Pots would stick to shelves unless precautions are taken. Usually, small balls of clay are rolled in aluminum hydrate, which is extremely refractory, and these balls are placed between pots and shelves, so that after the firing, the pots can be tapped loose. Just as in ash glazing, salt glazes often build up irregularly on the side of the pot that is closer to the draft, and runs can occur as seen at the bottom of this jug. Salt glazed stoneware pots were excellent and food safe containers for wine, beer, and foods of all kinds.
'Bartmann' jugs such as this one feature the face of a bearded man at the neck. Bartmann means 'bearded man' in German, and is a representation of the Wild Man, a mythical folk creature of Northern European folklore. This jug has applied medallions that contain the coat of arms of Denmark and the Tudor arms with the inscription: Elisabet Die Gracia Regina, Anno 1594. Salt glazing would be practiced in England after the mid 1600's and the English potter John Dwight was well known for his variations of this technique. Salt glazing would find its way to the American colonies with immigrant German and English potters. and a good proportion of early American colonial pottery is salt glazed.
"Bartmann" Jug, Salt Glazed Stoneware,
German Rhineland, 1594 CE