Why Marble Looks Like Skin

Most opaque materials settle the question of light at their surface. A ray arrives, bounces, and what reaches the eye has never been inside the material at all. Marble does not work this way, and the difference explains why sculptors kept returning to it long after harder and cheaper stones were available. Marble is a mass of calcite crystals grown tightly against one another: tightly enough that the stone looks solid, not tightly enough to keep light out. A ray striking a polished marble surface passes in, meets the boundaries between crystals, scatters among them, and emerges again a short distance from where it entered. The light that finally reaches the eye has traveled inside the stone. The visible consequence is that marble has no hard optical edge. Where a shadow falls across a bronze figure it begins at a line, because the metal returns light from its surface and from nowhere else. Across marble the same shadow softens over a band, since light entering on the lit side re-emerges slightly into the dark one. Living tissue behaves in the same way. Light passes into the outer layer of skin, scatters in what lies beneath, and leaves at a point other than where it arrived. A marble portrait bust and a painted wooden one may be carved to identical dimensions and lit identically. Only one of them handles light the way a face does.