These make documents go brown and brittle. Older papers, made of clean linen rag, are chemically stable and should last indefinitely. Old inks, however, are sometimes so acid that they burn through paper. Contact with water, typically under a leaking roof or pipe, can lead to rapid and irreversible rotting. Storage in a damp atmosphere, such as is found in cellars, leads to the slower growth of moulds, which can stain paper, remove its strength, and cause it to crumble.
Wetting can make parchment cockle or go hard, brittle and transparent.
Documents which have become wet have to be dried promptly, but carefully. Rapid fluctuations in humidity can cause differential expansion and contraction which can distort or break the bindings of books and cause writing or illumination to come away from parchment. For these reasons the temperature and humidity in our strongrooms are carefully controlled. The effects of wear and tear are self evident, but material already weakened by chemical decay or mould is much more easily damaged by bad handling. Mouldy or damp paper is more prone to attack by insects, such as silver fish.