Fireclays are so named because they are refractory (resistant to heat). Fireclays fuse at about 2732° F and are a buff to brown fired color due to the presence of some iron. A common recipe for making a refractory mortar using Fireclay is: 10: 6: 2: 3 – Sand, Fire Clay, Portland cement, and Lime. Commencement Bay sells Fireclay by the pound or in a 50lb bag.