The layman can relate to patina when it is defined as a mellowing of tone or texture acquired by aging and use in furniture, leather, or paintings. In sculpture, it is a film or incrustation that forms on copper or bronze after a certain amount of weathering and as the result of the oxidation of the copper contained within bronze. When green, it is known as aerugo or verdigris. Patinas are often made to occur in the foundry upon the sculptor's request by special treatments that duplicate the green copper carbonates and hydated oxides of natural bronze patinas. Rarer bluish and reddish patinas can also be effected. A patina is normally a kind of protection, which tends to retard further corrosion considerably. However sometimes a malignant type of corrosion known as bronze disease occurs. The process whereby a patina is either naturally acquired or artificially induced is known as patination. Some sculptors imitate the patination process on non-metallic sculpture with the use of oils, waxes and pigments: i.e., shoe polish on plaster.