Like parchment, leather is made from animal skins. Fundamentally, leather consists of a network of fibers made up of proteins, the most important of which is collagen. There are many different types of leather, and making leather is a complex process, for which different manufacturers have different methods.
Perhaps the most well-known leather damage is red rot, the process in which the leather fibers break down and turn into a red powder. In this process, sulphur dioxide is absorbed from the atmosphere and converted into sulphuric acid through oxidation. The sulphuric acid catalyzes acid hydrolysis, breaks down the molecular chains of the collagen, and weakens the leather.
Internal factors related to the leather manufacturing process also contribute to leather decay. Remnants of sulphur compounds used during the manufacturing process remain even after the leather is washed and convert over time into sulphuric acid. Traces of metals from the rinsing water also remain behind, catalyzing the conversion of sulphur dioxide into sulphuric acid, which leads to acid hydrolysis.
In addition, some vegetable tannins used during the tanning process seem to cause leather to decay more quickly. Finally, marbled decorations applied to book bindings in the 17th and 18th centuries can leave black corroded spots on the binding or damage the grain of the leather.