Aging leather presents a variety of issues. This leather cover (right) has been decorated with sprinkled acid, causing the leather to darken in areas exposed to the acid. The acid has eaten away the leather in affected areas and will continue to do so.
Torn and Missing Leather
In some cases, leather that in extant areas appears to be in good condition, is nevertheless torn away or partially missing. Hundreds of years of use, and unknown physical trauma, can damage even strong leather.
Weak, Thin Leather
Weak, thin leather can simply break at stress areas like the joint of the book. This is likely to happen to a 19th or 20th century volume, since leather produced since the mid-19th century tends to have been thinner and produced with damaging tanning agents or acidic dyes, and therefore weaker than older leather.
This 19th century leather cover is weak, cracked and exhibits the phenomenon known as "red rot" that manifests in a powdery red layer of rotted leather under the cracking, paper-thin surface. This could be caused by use of sulfuric acid in the dyeing process.
Leather in Good Condition
"Rotted" 19th century leather can be contrasted with this resilient leather covering a 15th century manuscript. With the help of conservation treatment in the 20th century, and despite minor damage from insects, the leather binding of this manuscript is in beautiful condition. Leather in this era tended to be thick, including the strong fibers below the surface that later leathers lack, and processed without harsh acidic dyes.