Thanks to Susan for posting about the esoteric art of fore-edge painting. I had never heard of it before and I am fascinated by it. It involves fanning the pages of a book open, painting an image on the edge, and then gilding or marbling the straightened edge so you can't see the painting when the book is closed. For a great explanation of the technique with videos and images, see the website of fore-edge artist Martin Frost. Wikipedia also has a short article on the history of fore-edge painting.
via Pages Turned
