Photographs on metal supports are subject to corrosion and degradation caused by oxidation of the metal. The surface of a daguerreotype is susceptible to tarnish. Tarnish on daguerreotypes is composed of silver sulphide, silver oxide and silver chloride. Tarnish may appear at the edge of the brass window mat opening or over the entire plate. It can be characterized by a series of interference colours and/or pale grey, blue, green, brown or black.