Henna produces Lawsone, a red-orange dye molecule that is mainly concentrated in the leaves of henna, especially in the petioles. This molecule has an affinity or attraction for bonding with protein, and has been used for many purposes such as dyeing protein-rich substances such as skin, hair, fingernails, leather, silk and wool.
Henna paste, made from the crushed leaves of the henna plant, is used for making temporary tattoos. Uncrushed henna leaves do not stain the skin since lawsone molecules have not been released from the henna leaf. Smashed henna leaves are typically mixed with a mildly acidic liquid such as lemon juice or tea which helps to release the lawsone from the leaves during the crushing process.
A thick paste is formed and used to apply intricate, detailed body designs. A few drops of essential oils like eucalyptus, cajeput, lavender or tea tree are added to henna paste. These oils contain monoterpene alcohols that help in improving the staining characteristics.
Our skin is made up of a number of cell layers. The outermost layer of the skin is called stratum corneum. It helps keep away dirt and other infectious agents from the lower layers of the skin. This layer of skin is thick on certain parts of the body like palms and foot soles (especially the heels) and may be thinner on other parts like the ear skin. The stratum corneum is made up of keratin which also makes up for fingernails and hair. When henna is applied on your skin or hair, the lawsone molecule is small enough to penetrate the skin cell. It enters the columns of skin cells and does not bloat or spread out like a drop of ink would on a tissue paper. Thus, the stains remain sharp and clear, till the complete exfoliation of the upper layer of skin. Henna stains darkest on the cells that are in close contact with the dye and the skin cells farthest from the dye have lighter shades.
The colour of the stain also depends on individual skin type and the amount of time that henna was allowed to stay on the skin.