Many of the cosmetics we use involve interesting chemistry. For example, one popular product sold to darken grey hair uses a substance called lead acetate. The lead reacts slowly with protein in the hair, converting the sulphur it contains to black lead sulphide. Since this reaction takes place slowly, this product darkens hair gradually after several applications.
Hair contains two natural pigments, melanin, which is dark brown, and phaeomelanin, which is yellow or red brown. The colour of hair depends on the relative amounts of these two pigments.
Converting hair to a lighter shade requires bleaching these natural pigments, usually with a solution of hydrogen peroxide. Then a synthetic dye of the chosen colour is applied. Adding dye without bleach merely darkens hair.
Temporary dyes or rinses colour the hair with substances that wash out easily. They use chemicals that don’t penetrate the hair strands, but merely coat the surface. Smaller molecules that can penetrate the hair, produce semi-permanent dyes that last through several washings. Permanent dyes have to be formed inside the hair through the chemical combination of mobile molecules. These are small enough to penetrate the hair before reacting inside it to form larger dye molecules, that remain trapped in the hair.
So next time you dye your hair, be thankful that chemistry gives you some choice.