Cosmetics were known thousands of years ago in ancient Egypt, where lead compounds were applied as foundation or eye makeup. A hair-dye recipe first described in Greco-Roman times involves applying a paste of lead oxide and calcium hydroxide, or lime, to graying and fair locks. As in the famed Grecian Formula--also a lead-based recipe--repeated applications darken the hair as much as desired. Researchers knew the lead-lime mix reacts with sulfur present in hair's keratin proteins to form crystals of lead sulfide, also called galena.
To figure out just how tiny and penetrating those galena crystals are, a team of investigators from the Center for Research and Restoration of the Museums of France (C2RMF), L'Oral Research and Argonne National Laboratory soaked blond hairs in a solution of the Greco-Roman chemicals for up to three days. Once the strands were sufficiently darkened the team examined them under a special electron microscope capable of imaging thin sections from a relatively thick sample. They saw that the strands were shot through with lead-sulfide crystals averaging 4.8 nanometers in size--about the same as the so-called quantum dots studied by researchers today. The crystals formed strings down the length of the hair fiber. Judging from the spacing of these strings and chemical changes to the hair, the crystals apparently grow among the sulfur-rich amino acids that surround the hair's keratin microfibers, the group reports in the September Nano Letters. "It is remarkable that the composition and supramolecular organization of keratins can control PbS [lead-sulfide] nanocrystal growth inside a hair," the group writes.
Philippe Walter of the C2RMF, lead author of the report, says he has not studied what happens to hair dyed with Grecian Formula, but he suspects nanocrystals are also at work. Such lead-based dyes don't seem to be harmful, he notes, because they have trouble penetrating the skin.