You may have seen some mounts which have a brown bevel cut edge, this would have originally been a cream colour but the core of the mountboard quickly turns brown. This is caused by the lignin (tree sap) in the board fibres. Lignin is the binder which holds wood and paper fibres together. Over time lignin becomes highly acidic. This process is accelerated by heat and light. This acid then causes the board to weaken and discolour, and the acid can soon migrate from the mountboard and cause damage to the artwork. The second photo shows the damage that this acid migration has caused to the paper it surrounded.
