Furnace waste is a term used to describe the by-products of metal smelting and other industrial processes. SLAG is one of the most common forms of furnace waste, which is composed of non-metallic materials such as silicates, oxides, and sulfides. SLAG is usually created when molten metal is poured into a mold and the impurities are left behind. It is usually gray or black in color and can be used in construction materials, fertilizer, and other applications.