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 sulfur dioxide in humid air, which was favored in particular by the gas illumination in earlier times. The sulfur dioxide is converted into sulfuric acid. This attacks the leather and pulverize the surface. The damage is irreversible.
Red declay - Red Rot
Red declay, seen in the DLM - German Leather Museum in Offenbach.
Additional information
a kotori web solution