In the sixteenth and seventeenth centuries, master bookbinders decorated leather covers with real gold leaf—so delicate it could blow away with a whisper. Heated brass tools pressed each flower, line, and arabesque into the leather by hand.
Centuries later, the finest still glow in museums.
Sources:
Bridwell Library — The Sixteenth Century
Explains how gold tooling spread through Europe in the 1500s and how heated tools pressed gold leaf into leather.
https://bridwell.omeka.net/exhibits/show/sixcenturiesbinding/sixteenth
A Manual of the Art of Bookbinding — James B. Nicholson, 1856
A full historical bookbinding manual describing gold leaf, decorative tools, fine leather bindings, and the skill required to make them.
https://www.gutenberg.org/cache/epub/55056/pg55056-images.html
Guild of Book Workers — 17th-Century Bookbinding Tools
Shows what seventeenth-century binding tools looked like and how historians recreated them from surviving evidence.
https://guildofbookworkers.org/sites/default/files/standards/17th-c.-bookbinding-tools-final.pdf