What's that stuff? Henna

I first met Ruby Bansal at school when we were nine years old. Through high school chemistry, college life, and budding science careers in different parts of the U.S., we kept in close contact.

In September, I attended her wedding. In accordance with her family's Indian tradition, Ruby's hands were adorned with mehendi-designs drawn on the body using henna. The night before the wedding, Deepal Vora, a Boston-based henna artist born in Mumbai, India, decorated Ruby and also her sister Neeta and me. Vora applied the henna as a paste by squeezing it out of a small plastic bag through a tiny hole and drawing the pattern freehand. Each of her designs included a version of a peacock-a motif, she told us, that means good luck.

As Vora expertly put tiny swirls and dots of the green-brown henna paste on my hands, I felt a cooling sensation and noticed a smell like that of fresh-cut grass and spinach. As a rule of thumb, the longer the henna stays on the skin, the longer and darker the stain will be. So to keep the paste from flaking off too quickly, Vora swabbed the patterns with a mixture of lemon juice and sugar. The next morning, I scraped the dried paste off with the back of a butter knife, revealing auburn-colored mehendi. The stains darkened to coffee color the day after the wedding.

Henna paste is prepared from the green leaves of Lawsonia inermis, a small tree that grows in warm, arid regions of the world such as India, Pakistan, and Northern Africa. Numerous artifacts found in Middle Eastern and Mediterranean countries, dating back to 1400 B.C., show women with henna patterns on their hands. The earliest writing on an artifact that refers to the specific use of henna as an adornment for a bride or a woman's special occasion is an inscription on a tablet from about 2100 B.C. found in northwest Syria, says Catherine Cartwright-Jones, a Kent State University Ph.D. student who is carrying out research on the culture and geography of henna.

The use of henna is not a religious practice, she says. It is commonly used as a hair dye and, increasingly in the Western world, for body art. The material can also reduce dandruff, kill ringworm and head lice, act as a sunscreen, and, in some preparations, be used to rustproof the hulls of metal ships. In India, especially in desert areas where the temperatures are extremely high, henna was first used not to decorate the body but to cool it, Vora says.

Henna's characteristic staining properties stem from the compound 2-hydroxy-1,4-naphthoquinone, also known as lawsone, hennotannic acid, or natural orange 6. Henna leaves contain up to 5% by weight of the compound, which, in its pure form, is a yellow-orange powder that does not dissolve in water.

Henna paste is prepared by mixing crushed dry henna leaves with a mild acidic ingredient. Vora uses lemon juice. The acid releases the lawsone from the plant. Various oils and herbs may also be added to enhance the scent of the paste.

At room temperature, it normally takes about a day for the acid to activate the dye and three days for the paste to lose its staining capabilities. The process is faster in hotter environments.

Lawsone dye infuses skin, hair, and porous surfaces but does not permanently or chemically alter them. The dye molecules, which are about the same size as amino acid molecules, migrate from the henna paste into the stratum corneum-that is, the outermost layer of the skin, explains biophysicist Boyan Bonev at the University of Nottingham, in England. The dye penetrates down through the stratum and does not spread like ink on blotting paper. As a result, the stains initially appear darkest on hands and feet because they have thicker strata cornea than other parts of the body.

Various physiological factors, such as skin type and temperature, hormone levels, and stress affect the appearance of mehendi on people. After the dried paste is scraped off the skin, air oxidation or perspiration can further darken the stain over the next 48 hours.

Although henna may appear as different shades, depending on these physiological factors, true henna is a red dye, Vora says. Other color products, marketed as henna products, may not actually contain henna. "Black henna," for example, can contain up to 40% of a synthetic black hair dye called p-phenylenediamine.

As I traveled home from Ruby's wedding, my hennaed hands attracted the attention of many people, including complete strangers. Some recognized my mehendi with compliments; others asked if my hands were tattooed. Tattoo ink has a different composition and is injected into the dermal layer of skin.

My henna disappeared before I finished this story, and Ruby's henna wore off during her honeymoon. But as with many other memorable times in our friendship, we have pictures.