Custer Feldspar is made from crushed crystalline rock containing a mixture of aluminum silicates of sodium and potassium. Generally high potash feldspars are employed in bodies and promote vitrification by forming a glassy phase that "cements" more refractory particles together and triggers the formation of mullite from clay mineral. Feldspars are a valuable means of introducing alkalies into a body or glaze. Potash is the most common type of feldspar, So much so that if no other type is indicated it can be assumed potash is called for. Mined in Custer, South Dakota. Larger amounts are available. Please call store for quotes. Please wear protective gear when working with raw materials.