Firescale refers to one or both oxides that are formed when copper bearing alloys are heated in the presence of oxygen. The two main oxides formed are Cuprous Oxide (CuO) and Cupric Oxide (CuO2). Cuprous Oxide usually appears as a black colored oxide that can be dissolved in a pickle. Cupric Oxide is purplish residue that is created within the alloy and is far more difficult to remove.