Salt glazing developed by accident in the 13/14century in the Rhine region of Germany.
By this time in Europe there was not a great deal of wood available for potters to fire the kilns, and coal isn’t found in that region. So potters used any form of wood they could get. It is thought that barrels and boxes used to salt fish were broken up to fire the kilns. This area on the Rhine was also an important trading and fish region.
Another factor that came into the equation was that the potters in the area were firing at a temperature higher than other areas in Europe. They were producing vitrified pots for liquid transport such as oil and wine, and a high temperature is needed for the salt glaze to form in the kiln.
Over a hundred or so years it was realised that introducing salt directly into the kiln would give a glazed, a water tight surface that was ideal for the storage and transport of oils, wine etc. Previously this “glazed” surface was at times random dependent on the residual salt in the wood/barrel/boxes used to fire the kiln.
All clays contain silica, in porcelain the proportion is quite high. Silica melts to form a glass that gives the translucent characteristic of porcelain and of course for windows!
When sodium chloride [salt] is thrown into the kiln at a time when the silica is melting, usually around 1300C, the sodium, as a vapour, attacks the silica and forms a sodium-silicate glass – technically similar to normal window glass! The vapour builds a layer of glass, and like dew on a foggy morning, if it builds sufficiently it starts to run,hence these runs are one of the most obvious characteristics of a salt glazed pot.
A salt glaze is clear and transparent and colorants and oxides are applied to the pot prior to firing to give colours and tones.
It should be mentioned that, as the kiln is made from clay bricks, the kiln also gets glazed. This leads to a very high maintenance regime and a very shortened life of the kiln. A salt kiln can only be used for salt glazing as the residual salt in the bricks vaporises off at each firing.
The atmosphere is so severe normal high temperature glazes simply don’t survive. Special glazes and oxide blends are needed for a salt firing.