The 16th century source, "The Three Books of the Potter's Art" by Cipriano Piccolpasso of Castel Durante, is a detailed how-to guide for setting up a maiolica workshop. The 16th century author describes how clay was located, gathered and purified. Clay in Renaissance Italy was obtained from rivers and pits. Potters clay could be found wherever the soil was white and contained "genga". According to Piccolpasso, heavy red clay was used for making tiles and white light clay was used for making vessels. In order to be usable for maiolica, dug clay had to be white in color. If the dug clay was too "blue", it would not take the white tin glaze. However, if the clay was obtained from a river, blue clay was considered good. 

Renaissance maiolica was extensively traded throughout Europe. Because Italian potters emigrated to other countries and to other pottery centers, visual means of identifying painted pieces of maiolica are not always accurate. Maiolica scholars can use a chemical analysis of the materials in maiolica vessels to determine the city of origin of a particular piece. 

Clays come from the weathering of rocks and can contain a variety of minerals. Scholarly studies have shown that differences in the chemical composition of clays can be detected for distances as close as 10 km. The British Museum has done analysis of ceramics known to represent main centers of maiolica production. These studies help to determine the origin of archaeological tin-glazed pottery.

Italian clays are naturally calcareous (containing calcium carbonite). In northern Europe, red iron rich clays were mixed in appropriate proportions with white clay rich in calcite. In England, white clays were found at Boyton on the Suffolk coast. In Belgium, white clays were found at Tournai. Maiolica produced in London used local red or gray clay mixed with white clay imported from Norwich. 

Clays higher in calcium than iron, fire to a buff or cream color. Bisque wares of calcareous clays tend to be pink in color and are lighter in color after the second firing. Nearly all quality Renaissance maiolica was painted on buff, off white, or pinkish clay vessels.

Bibliography:

David Gaimster, Maiolica in the North: The Archaeology of Tin-Glazed Earthenware in North-west Europe c. 1500-1600, British Museum Occasional Paper Number 122, The British Museum, 1999.

 Ronald Lightbown and Alan Caiger-Smith, translators. The Three Books of the Potter's Art, Scolar Press, 1980.