Another process that must be carefully controlled to avoid unpleasant results is bleaching. Hair is usually bleached with hydrogen peroxide, a chemical that lightens hair by oxidizing its melanin pigment.
The greater the concentration of hydrogen peroxide and the longer it is left on, the lighter the hair becomes until it approaches the harsh yellow-white of the stereotypical “bleached blonde.” The bleaching process is halted by rinsing the hair with very hot water or a mild acid solution, such as pyruvic acid.
The results achieved by bleaching are also determined by how much ammonium hydroxide, a chemical that activates oxidation, is added to the peroxide solution. Too much of this ammonia compound can produce unattractive reddish tints.
Hair tinting, which is capable of producing a rainbow of natural-looking hues from ginger gold to chestnut brown, also relies on oxidation. Colorants are a complex brew of chemicals. Oxidation is provided by a developer, usually hydrogen peroxide or another oxidizing agent. The developer acts on an intermediate. Intermediates are synthetic compounds, often derived from coal tar, that develop into dyes when they are oxidized.
Other components of hair color include modifiers, compounds that enhance or stabilize certain shades of color; antioxidants, which prevent the intermediates from oxidizing before they are applied to hair; and alkalizers, chemicals that improve the absorption of dye by softening the hair.
Like other hair treatments, coloring does not alter the cells that produce hair or its pigment. Thus, people who tint their hair must touch up their natural roots periodically if they want their growing tresses to retain an even color between complete dye treatments.
In recent years, many women have experimented with semipermanent and temporary dyes. Semipermanent colors use lower concentrations of oxidizing agents and wash out in four to six shampoos. Temporary colors, or rinses, are acid dyes that coat only the surface of the hair and readily wash out in the next shampoo. Because they simply add a tint to the surface of hair, rinses can be used only to produce a darker hair color.