Talc supplies SiO2, MgO, CaO, and small amounts of Na2O, Al2O3, Fe2O3, and MnO. TX talc is reported to be less gassy than NY talc. Talc is a magnesium silicate mineral with theoretical composition 3MgO 4SiO2 H2O. Rocks made of talc are also known as Soapstone or Steatite. The talc used by potters (tremolitic talc) is usually a mixture of the minerals Tremolite (2CaO 5MgO 8SiO2 H2O), Serpentine (3MgO 2SiO2 2H2O), and Talc (3MgO 4SiO2 H2O), in an approximately 2:1:1 ratio by weight. Talc is formed by a complex metamorphic process from quartzite and dolomite. The process occurs in several stages; each stage produces a later mineral in the sequence. New York talc is mined by R.T. Vanderbilt Co., Inc. in St. Lawrence County, NY. This talc was formed in the Precambrian (over 600 million years ago). Talc is a common flux in low-fire clay bodies, especially white clay bodies, and is used as a flux in stoneware glazes.
(courtesy of glazy.org)
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