BLUING

(or blackening) is a chemical surface treatment for cast-iron objects that protects against corrosion. A coating technique, it creates a protective layer of iron oxide on the object. The result may appear black or brown, depending on the products used. This effective surface treatment has allowed cast-iron objects to remain in original condition to this day; only small number of objects require a conservator's intervention. (The black appearance on cast iron can also be achieved by treating the surface with a mixture of graphite and wax).

(JMl)