A-Z of Ceramics - U is for Underglaze
Underglaze is the opposite of overglaze. It refers to ceramic decoration that lies under the glaze, which forms a protective layer. The technique only became possible in Europe when biscuit firing was introduced for fine earthenwares and porcelain (see B is for Biscuit). Only a few pigments could withstand the heat of the glaze firing.
The toughest and most reliable underglaze colour proved to be cobalt blue, used by the Chinese potters from the 14th century. In England, blue painting on porcelain was soon largely replaced by transfer printing. This was less skilled work, and required less labour, so provoked a strike among worried factory decorators in 1770. Happily, the technique was then applied to cheaper earthenwares with such success that the pottery industry rapidly expanded.
The decoration on 'Tortoiseshell ware' is also under-the-glaze, produced either by applying metallic oxides with a sponge or brush directly onto the biscuit body, or perhaps with clay slips stained with oxide colours.