Fire Clay - High Temperature Mortar IngredientFire Clay, Fireclay, Mortar Clay and or Bricklayers Clay is an organic material that has been dried and turned into a powder form. It is usually a brackish red brown in color or it could be white grey. The texture is like baking flour or baking soda. These powdered clays can be found at most Masonry and Landscape Supply Stores for about $10 per 50lb bag of firing clay. You may find a bag of fire clay at a Big Box hardware store like Home Depot, Lowes