Clay and Glazes
We use several types of clay at The Potter’s House. Most of our forms are made from a very smooth white high-fire clay body that is in the porcelain family. We also use a porcelain clay that we have mixed ourselves for several years that needs to age for one to two years. We got the recipe from Jonathan Leach, an English third-generation wood-fire potter. We use the porcelain for our reds and celadons.
We are familiar with glaze experimentation. Hardly ever do we fire a kiln load without several glaze test pieces in it.
Our glazes would for the most part fit into the category of feldspathic glazes because of the high percentage of feldspar we use in each glaze. We mix each glaze by dry weight and liquid measure.
The glaze is applied after it has been bisque fired to 1830ºF. Bisque firing renders the clay body strong enough to withstand the glazing processes, and porous enough to accept the glaze. We have learned over the years that each glaze requires different modes of applications. We apply many of our glazes by dipping the vessels in vats of liquid glazes. Some of our special glace effects are achieved by dipping parts the vessel in 2 or 3 different glazes, overlapping them in sequence. We paint some glazes on using brushes, others are sprayed. Others, such as the blue rims of the bluebonnet pottery, are airbrushed.
**What is a glaze**?
In simple terms, a glaze is a thin layer of glass that has been fused onto the clay body. It is composed of 3 parts:
Silica—the glass maker which melts at temperatures ranges from 1710º to 3110º F
A flux—lowers the melting point of silica
Alumina—increases the viscosity of the glaze and stabilizes it onto the pot (sort of like flour is to gravy)
Different percentages of these components will give the glaze its matte, satin or gloss finish. Oxides are what give the glazes their color. In our shop we use cobalt, chrome, copper rutile and 3 different types of iron—black, yellow and red. We have chosen not to use lead or barium. Although they create beautiful hues of color, they are highly toxic to the potters.