patina: one construct, many systems

In geomorphology patina, most familiar as rock or desert varnish, is a thin surface coating that builds up on exposed rock over long spans, enriched in clay, manganese, and iron and, as Dorn and Oberlander argued, partly microbial in origin. Its layering records environmental history and is used to gauge how long a surface has been exposed. The referent is a slowly accreted coating on rock.[1]

Unit

The thin accreted surface coating on exposed rock

Recovery means

Not applicable

Measured by

Characterised by thickness, manganese and iron enrichment, and microlamination used for relative dating

In the conservation of metals patina is the surface layer, often green or brown, that forms on copper and bronze as the metal corrodes and reacts with its surroundings over time. Prized aesthetically yet sometimes a sign of active decay, it is a comparable slowly formed surface skin, here grown by corrosion of an alloy rather than deposited on rock.[2]

Unit

The corrosion-formed surface layer on copper or bronze

Recovery means

Not applicable

Measured by

Identified by its mineralogy, such as copper carbonates or chlorides, and whether it is stable or actively corroding