Potassium Feldspar

Potassium Feldspar, chemically a potassium aluminosilicate (KAlSi3O8), is one of the oldest and most essential materials in ceramic glaze making. It belongs to the feldspar family, a group of rock-forming minerals that make up nearly 60 percent of the Earth’s crust.

In glazes, potassium feldspar performs three jobs at once: it fluxes (lowers the melting temperature), it stabilizes (contributes alumina to prevent the glaze from running off the pot), and it forms glass (contributes silica). This triple role makes it the single most important material in high-fire stoneware and porcelain glazes.

Potters have used feldspar-rich rocks for over 2,000 years, with Chinese potters using local feldspathic rocks as early as the Song Dynasty to produce the first true porcelains. In the West, feldspar became central to glaze chemistry during the industrial revolution.

Typical usage: 20 to 50 percent in cone 6 to 10 glazes. In classic high-fire limestone glazes, feldspar often makes up 40 to 50 percent of the recipe. It also appears in clay bodies at 15 to 30 percent to lower vitrification temperature.

Western equivalents: Custer Feldspar, G-200, Mahavir.