Works of art on aged paper such as Antiquarian Maps, Prints and Engravings can be damaged by light, extreme or fluctuating temperature, humidity, pollution, pests and poor handling.
Foxing
Brown spots caused by bacteria or mould which grow on acidic paper in a humid environment or when metallic particles collect in the paper during manufacture
Burn
Mount Burn appears when the print paper touches unpurified wood pulp within the mountboard. Brown Air Burn marks (brown streaks) can form through gaps in old wooden backs. Both can be removed without trace.
Example
The illustrated print was badly stained by age and water damage. The frame had flaked and was beyond economical repair.