Page 27 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Corrosion and Environment

CHAPTER J

Things made of copper get covered with copper-rust more quickly
when they are kept rubbed clean than when they are neglected, unless
they are well greased with oil. It is said that the best way of preserv­
ing them is to give them a coating of liquid vegetable pitch.—PLINY
THE E L D E R 1

s

^ 0 trictly speaking, patina and corrosion are different words for
the same surface alteration. Here the term patina will be used to describe a smooth, continuous
layer that preserves detail and shape, while the term corrosion will be used to describe mineral
deposits that do not form a continuous and smooth layer. Surface accretions may represent
a third state, in which soil minerals, textiles, wood, charcoal, and so on, may be bonded to a
surface with copper corrosion products, or even partially replaced by them. Corrosion may be
termed the process of chemical attack of an environment on a material, while patina could be
defined as the accumulation of corrosion products and other materials from the environment.
One person's patina, however, may be another person's corrosion, so a certain ambiguity is
inherent in both words.