In most cases, rust stains are brownish or yellowish in colour and are caused by long-term contact between metal with the parchment.

Foxing stains are brownish dots or spots that usually occur in clusters.

Fluctuations in humidity and temperature, together with sulphur and nitrogen dioxide in the environment, result in the degradation of parchment. The chemical reactions (hydrolysis and oxidation) that cause degradation can be significantly accelerated by the impact of metal inserts containing metal compounds and iron gall ink. Foxing stains are localised and easy to discern.