Talc

Talc is a hydrated magnesium silicate (Mg3Si4O10(OH)2), the softest mineral on the Mohs hardness scale at just 1. Known for its slippery, soapy feel, talc has been used for thousands of years as a lubricant, cosmetic powder, and ceramic ingredient.

In glazes, talc supplies magnesium oxide (MgO), the flux responsible for the characteristic buttery satin surfaces of “magnesia matte” glazes. MgO is a soft, slow flux at high temperatures that produces smooth, silky finishes beloved for functional pottery and tea ware.

Talc’s biggest role is in low and mid-fire clay bodies, where it dramatically reduces thermal expansion and prevents crazing. Classic American low-fire white earthenware bodies often contain 40 to 60 percent talc, which allows commercial glazes to fit the body without cracking. Talc also contributes to the whiteness and workability of these bodies.

Historically, Chinese potters have used talc in porcelain and celadon bodies since the Song Dynasty, and Japanese traditions use talc-rich clays for raku and tea ware.

Typical usage: 5 to 20 percent in satin matte glazes. 20 to 50 percent in low-fire earthenware bodies. 2 to 8 percent in stoneware and porcelain bodies for whiteness and thermal expansion control.

Note on talc colors: Talc can shift colorants. Cobalt in talc-rich glazes often moves from pure blue toward purple. Copper may dull. Iron becomes more muted.