Two other nonoxidations-albeit permanent-dyes deserve mention: henna and metallic dyes. Henna, in use for thousands of years, is a plant extract. Part of its appeal rests on its being a safe, natural substance. Ground leaves and stems are applied to your scalp in the form of a pack, which is actually a paste made with hot water. The pack is left in place until the desired color is attained. However, the only color that can be obtained with pure henna is orange-red. To achieve any other colors, henna must be combined with petroleum-derivative dyes. When combined with other dyes, however, any advantages to using a product containing henna are lost.
Metallic dyes, which usually contain lead acetate, are also called “progressive” dyes. They require daily application to achieve darkening of your hair. The lead reacts with the sulfur in your hair to produce the color. These products have enjoyed particular popularity among men who prefer to comb their color in. Metallic dyes usually result in intense, somewhat artificial colors. Furthermore, hair treated in this way can be difficult to permanently wave.
Permanent waving and hair straightening are two other ways to give your hair something that nature did not. Permanent waving or cold waving as it is also known is a process for making straight hair wavy or curly. Hair straightening as its name indicates is a technique for doing just the reverse, making naturally curly or wooly hair straight. The object of chemical reactions in these two processes is basically the same: to break the stronger link ages between the protein molecules in your hair and reform them at new angles to give the desired effect. As a rule it is more difficult to permanently wave fine, limp, brittle, wiry, or previously damaged hair. For short hair, permanent need to be repeated about once every three months, for longer hair about once every six months.