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Custer Feldspar is a potash feldspar.
Feldspar is a large group of minerals which have decomposed from granite and igneous rocks and are thus allied to clay. The main group consists of the alumino-silicates of potassium, sodium, calcium, and, more rarely, barium. The ideal formulas, never quite achieved in nature are:
Potash Feldspar, Orthoclase Microline, K2
Soda Feldspar, Albite, Na2
Lime Feldspar, Anorthite, CaO.Al2
Feldspars are the most useful minerals in ceramics, apart from clays. Used as a primary flux above 1200°C (2192°F) and a secondary flux in earthenware. - p. 108, Illustrated Dictionary of Practical Pottery, 3rd Ed. by Robert Fournier