The sculpture known as the Boston Green Head is little more than 4 inches high, its nose has been brutally smashed off, and the remains of a supporting pillar at the back indicate that it was once part of a larger standing or kneeling figure. But what’s left is a likeness of mesmerizing clarity: a thin-lipped, sad-eyed man whose world-weariness shines through careful modeling of the obdurate sandstone known as graywacke. Archaeologists who discovered the Green Head in 1857 at Saqqara, the burial ground for the ancient capital of Memphis, realized that this was a portrait of an Egyptian priest, most likely from the fourth...