Mehndi is the traditional art of henna painting in India and the Middle East. You may see it written as mehandi, mehendi, mendhi, henna, al-henna, and a myriad other names and spellings. In recent times, United States henna artists have come to denote the art with the term "Henna Body Art." All of these words describe the same timeless art form, body painting for festive occasions. However you spell it, mehndi is pronounced meh-hen-di (with a soft, dental d sound like "thee").
The dye used to create the beautiful designs you see on the hands pictured above, is made from dried, ground henna leaves and various other ingredients, which you can investigate further on Google.
Traditional Uses
I specialize in understanding conceptions of henna in India, where it has been used since the 12th century. Many historical documents outline earlier use; for example, it is the Arabic Muslims who brought henna to India, where it has blossomed into its own unique art style. In Indian mehndi, a person applies designs traditionally to a woman's hands and feet. For particularly auspicious occasions, men apply mehndi as well. The most auspicious occasion warranting mehndi artwork is the Indian wedding, where both bride and bridegroom apply henna, as well as several members of the bridal party. Henna on any occasion symbolizes fertility. At the wedding, henna artwork additionally symbolizes the love between husband and wife, and the stain's long-lasting nature symbolizes the enduring nature of their love.
Mehndi came into use because of its cooling therapeutic effect in a hot climate, and, in India, it was also a way for a bride and groom to get to know one another before an arranged marriage. A variety of traditions underlie the use of mehndi, including wedding games and legends. For example, the groom's name is usually written somewhere within the bride's mehndi; if he cannot find his name within the intricate design, the bride is said to have the control in the marriage. Also, a dark mehndi design for both bride and groom signifies that the two will have a strong relationship. Within the past few years, mehndi has become popularized in the West by musicians and Hollywood personalities alike, and is now a quickly rising trend among women and men in world culture.
Henna Dye
Translating the word "henna" literally means "to become queen." The Indian name "mehndi" designates the process, the dye, and the stain of mehndi. To make the dye, henna (mehndi) leaves are dried and finely ground. The powder that results is filtered two or more times through a fine nylon cloth. This process results in removing the coarse fibers from the powder, making what is left finer and easier to use. The artist then mixes this fine powder with an oil (such as eucalyptus, nilgiri, or mehndi oil) and other liquids (lemon, water, or tea), making a thick paste. This paste is applied to the wearer's hand in various designs, which can range from large, thick patterns to Moroccan geometric patterns to traditional Indian paisleys and lace-like drawings. All depends upon the skill of the artist and the style of designs used. A solution of lemon juice and sugar is then applied to the drying mehndi to allow it to remain stuck to the skin and to improve the dying process.
After 2 - 12 hours, during which the mehndi dries, the wearer scrapes the paste off to reveal the designs, which resemble tattoos and last 1-3 weeks on the skin. While the color of the mehndi dye is a deep shade of green, once removed the dye leaves a color varying from light orange to a deep brownish- black. Be sure to keep this in mind as you look at photographs throughout this page. A word of warning before we continue: never use black henna. The black designs you see to the left show skin with the henna paste still applied.
Source: http://www.rupalpinto.com/mehndi/one.html