patination

Patination is the development of a surface film on a metal or other material through chemical and environmental action, resulting in a stabilized, colored layer known as patina. The patina acts as a barrier to further corrosion and is valued for its aesthetic and historical cues. The term is most commonly applied to copper and copper alloys, but can also describe aging on iron, aluminum, and other metals.

Natural patination occurs when copper, bronze, or brass is exposed to air and moisture, forming a complex sequence of copper oxides, carbonates, sulfates, and sulfides. The familiar green patina on architectural copper or bronze is largely copper carbonate minerals such as malachite and azurite. Iron tends to form reddish-brown rust, zinc may develop pale gray-blue films. The rate and hue depend on climate, pollutants, alloy composition, and surface details.

Artificial patination can be accelerated or altered by controlled chemical treatments, heat, or electrochemical methods to produce specific colors and textures. Artisans and conservators use patination to achieve or replicate historic finishes on sculptures, ornaments, and architectural metalwork. The process is designed to be stable and sometimes to provide protective properties, although aggressive treatments can damage underlying metal.

In conservation, patina is often considered part of a material's historical record; decisions about cleaning or removing patina are made on a case-by-case basis. Modern practices strive to balance preservation, aesthetic value, and historical integrity.