Coloring the surface of metal by applying concentrated heat from a blowtorch or oxyacetylene torch. Oxidation gives the metal a blue tinge. Also spelled blueing. There is a chemical solution available which also blues metal. [Can you tell us precisely what that is called?]Example: England, Royal Workshops, Greenwich, Armor of George Clifford, Third Earl of Cumberland, c. 1580-85, steel, etched, blued, and gilded, height 69 1/2 inches (176.5 cm), weight 60 pounds (27.2 kg), Metropolitan Museum of Art, NY. See arms & armor.Also see patina.