Medieval “pocket Bibles,” which proliferated in 12th century France, England, Italy, and Spain, were small and portable, a technological breakthrough at the time. Extremely thin animal skin pages made the books possible, and manuscript experts have long debated how medieval craftsmen produced such thin sheets. One hint lay in contemporary references to abortivum, or “uterine vellum.” Perhaps, one hypothesis went, the thin parchment was made from the skin of stillborn calves, or from small animals like rabbits. Some researchers had their doubts: Thousands of the books were produced, which would have taken a lot of stillborn calves, and small animal skins aren’t thought to be tough enough for parchment purposes. A competing hypothesis was that the thinner pages were a technological development, perhaps achieved by splitting skins apart or grinding and pounding them extremely thin. Now, in a study published online today in the Proceedings of the National Academy of Sciences, scientists have investigated the mystery using protein analysis. First they had to find a nondestructive way to gather samples from centuries-old manuscripts. Their solution: ordinary rectangular art erasers made from polyvinyl chloride, or PVC, a common conservation tool used to remove dirt from old books without damaging the pages. The same effect that attracts your hair to a balloon when you rub it on your head pulled enough collagen onto eraser “crumbs” to yield results in a mass spectrometer, a machine that determines the type of chemicals in a sample. The results show cow, goat, and sheepskin were all used, not just cow, suggesting that the thin vellum was a question of time-consuming craftsmanship rather than reliance on fetal calfskin.