"Sheepskin is a marvelously durable medium, though it has to be treated with some care. Whereas ink soaks into the fibers on paper, on sheepskin it stays on the surface, rather like chalk on a blackboard, and so can be rubbed away comparatively easily. "Sixteenth-century paper was of good quality, too," he went on. "It was made of rags and was virtually acid free, so it has lasted very well."