Henna, Lawsonia inermis, contains a red-orange pigment, lawsone, the molecule of which is also known as hennotannic acid. When henna leaves are crushed in an acidic medium and applied to skin, the lawsone molecules migrate from the henna paste, traverse the outermost layer of the skin, Stratum corneum and stain the skin. Prolonged applications of henna result in diffusion of the pigment deeply into the skin.

Chemically, the molecule of lawsone is 2-hydroxy-1, 4-naphthoquinone. Industrial classifications also describe lawsone as Natural Orange 6 and C.I. 75480. The name and molecular structure (see picture) of lawsone show its congeniality to naphthalene. In lawsone, two oxygen atoms are attached to the naphthalene carbons at positions 1 and 4 to form 1,4-naphoquinone and a hydroxyl (–OH) group is present at position 2. Its molecule contains 10 carbons, 6 hydrogens and 3 oxygens (C10H6O3), giving a total molecular weight of 174.16 atomic units of mass. Pure lawsone is an orange powder, insoluble in water, with a melting point higher than 192ºC and optical absorption maximum of 452 nm.

(All physical data on lawsone are taken from the Aldrich Chemical Database.)

The size of the lawsone molecule, approximately 6.5x5x1.3 Å, compares to that of naphthalene. It is slightly bigger than the amino acids, the building blocks of proteins, and is slightly smaller than that of the sugar (sucrose) molecule. This is rather small in comparison to the size of biologically important molecules like proteins, containing typically hundreds or thousands of amino acids or even to phospholipids (8x8x40Å), which are major constituents of the membranes of cells and are also present in the top layer of skin Stratum corneum.

Henna stains on skin and hair darken during the first 48 hours after paste removal. If keratin is saturated with lawsone, under certain conditions the darkening can be dramatic.

If silk is stained with henna, and the cloth is heated by ironing, the stain will become dark brown. If palm or sole skin is saturated with henna, steam will darken the henna to dark brown or near black. If a henna-stained palm is treated with dilute ammonia or another alkaline, the stain will darken to brown-black or greenish black.