Saltglaze

Salt-glaze or Salt glaze pottery is stoneware with a glaze of glossy, translucent and slightly orange-peel-like texture which was formed by throwing common salt into the kiln during the higher temperature part of the firing process. Sodium from the salt reacts with silica in the clay body to form a glassy coating of sodium silicate. The glaze may be colourless or may be coloured various shades of brown (from iron oxide), blue (from cobalt oxide), or purple (from manganese oxide).

The earliest known production of salt glazed stoneware was in the Rhineland of Germany around 1400. Initially, the process was used on earthenware. By the 15th century, small pottery towns of the Westerwald, including Höhr-Grenzhausen, Siegberg, Köln, and Raeren in Flanders, were producing a salt-glazed stoneware. In the 17th century, salt glaze gained popularity in England as well as in Colonial America.Westerwald Pottery was characterized by stamped medallions and the use of a cobalt oxide based colorant for decoration. Production of salt glaze pottery in Westerwald ceased because of environmental considerations in 1983.

In the UK during the 17th century and 18th century high quality salt-glazed stoneware was produced in Derbyshire, Nottinghamshire, London and Staffordshire. Salt glazed pottery was also popular in North America from the early 17th century until the early 19th century, indeed it was the dominant domestic pottery there during the 19th century. Whilst its manufacturer in America increased from the earliest dated production, the 1720s in Yorktown, significant amounts were always imported from Britain.

...
Wikipedia