Tarnish is a dull discolouring of a surface be it on metal or stone either by chemical reaction, oxidation or by grime or patina. Tarnishing happens naturally and can be equally welcome and unwelcome. In the production of sculptures, tarnish is often a welcome ingredient in the resulting art experience and is catalysed by various techniques. Everything from acid, manure, to yoghurt may be applied to provide a patinated, aged or weathered effect.
The term ‘tarnished halo’, on the other hand, refers to an angel whose halo is everything but clean, i.e. referring to a person who is less innocent.