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 from hyperacidity caused by the influence of sulphur dioxide in humid air, which was particularly abundant due to gas-lit lanterns in earlier times. Sulphur dioxide is converted into sulphuric 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.