After relaxing hair the first time, it is typical to touch-up all new hair growth about every eight weeks, or when the new hair is typically 1/2 to 1 inch long. It's not necessary to relax the older hair because it's already permanently straight from a previous relaxing treatment.
Using a relaxer on already relaxed hair can cause breakage. The chemical composition of relaxers varies by their type. Sodium hydroxide is a main ingredient in lye relaxers. No-lye relaxers include lithium hydroxide, guanidine hydroxide or potassium hydroxide. Some no-lye relaxers, as well as thio relaxers, include ammonium thioglycolate.
Hair relaxer was invented by Garrett Morgan in the early 1900s. Morgan and his wife owned a tailoring shop. He experimented with chemicals that would reduce the friction between sewing machine needles and cloth. After noticing the chemical solution made the hairs of woolen fabric straighter, Morgan tested the product on dog fur, then on his hair. His experiments a success, he started the G.A. Morgan Hair Refining Company to sell his product to black people. Morgan also invented a comb for straightening hair, the traffic signal and a gas mask safety hood used by firefighters and others in situations with dangerous smoke and gas.