What is Slag?

Legal Definition
Slag is the glass-like by-product left over after a desired metal has been separated (i.e., smelted) from its raw ore. Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals. While slags are generally used to remove waste in metal smelting, they can also serve other purposes, such as assisting in the temperature control of the smelting, and minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal.
-- Wikipedia
Legal Definition
The waste produced from processing metal ores. It floats to the surface and contains silicates, sulphates and phosphates of calcium. Called tailings.