Red decay
From www.leather-dictionary.com - The Leather Dictionary
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 favored in particular by the gas illumination in earlier times. The sulphur dioxide is converted into sulphur acid. This attacks the leather and pulverize the surface. The damage is irreversible.
Red decay - Red Rot
Red decay, seen in the DLM - German Leather Museum in Offenbach.
Additional information
a kotori web solution