The behaviour of metal objects depends on their materials and construction – they may be made of one solid metal or a thin layer of one metal overlying another.
The main causes of damage to church plate are:
Airborne gases and moisture: a reaction with oxygen in the air causes all metals apart from gold to lose their lustre and darken if they are not kept polished:
- Silver and gold do not react with oxygen at normal temperatures. However, other metals, often present in gold and silver objects as alloys, are affected by high moisture levels. Crusty green deposits can form where the silver contains a high copper content.
- Silver tarnishes (gradually discolours to brown and black) because of sulphur compounds in the air from the burning of fossil fuels and other industrial activity.
- Copper corrodes to form green crusts or powdery spots.
- Brass, an alloy of copper and zinc, is sensitive to damp and develops a brown tarnish and green copper corrosion products.
- Pewter, an alloy of tin and lead, is particularly vulnerable to corrosion by organic acids, for example in wooden boxes, especially where conditions are damp. It develops a darkened surface or patina where left unpolished.
Dust can contain chemical contaminants and can also hold moisture on metal surfaces which causes them to corrode, even under appropriate conditions.
Frequent handling: the salts and grease from fingerprints can corrode silver and copper surfaces.
Frequent cleaning can wear away engraving, gilding and silvering, particularly if there is abrasive dust on the surface. Vigorous wiping of the communion vessels can also distort their shape.