The so-called red decay or red rot is a phenomenon that occurs mainly in libraries with very old vegetable-tanned leather book bindings. The tanned leather suffers by hyperacidity caused by the influence of sulphur dioxide in humid air, which was abundant in particular due to gas lit lanterns in earlier times. Sulphur dioxide is converted into sulphur acid and this attacks the leather and pulverizes the surface. Such damages are irreversible.
Red decay - Red Rot
Red decay, seen in the DLM - German Leather Museum in Offenbach.