Knowing how hair dyes work will help you decide how to best take care of your hair coloring needs.
About Your Hair
Every strand of hair is made up of three parts—the medulla, the cortex, and the cuticle. The cuticle is the outside layer of the hair shaft, and the cortex is the inner layer between the cuticle and the medulla (the core). At the root, hairs enter the skin at their roots, which sit in a follicle underneath the skin surface. Hair growth begins with rapid cell division at the base of the follicle. As the cells divide and increase in number, the other cells are pushed up and out, thus lengthening the strand of hair. Hair is made up of dead cells except at the very root where cells are dividing.
Hair color comes in two main forms—permanent and temporary. Temporary dies do not penetrate the hair cuticle, but rather lay on top of the shaft and stain the outside of the hair. They may leave a slight tint in your hair even after they have washed out, depending on how light your hair is to begin with. Temporary dyes last only a few weeks.
Permanent dyes contain two main chemicals that make the color cling to your hair. The chemicals are hydrogen peroxide and ammonia. Hydrogen peroxide removes sulfur from the hair, which causes it to harden and lose weight. The chemical starts the color-forming process and helps the color stay in the hair longer. Ammonia separates the cuticle and allows the hair color to penetrate the cortex of the hair strand. It also helps the hydrogen peroxide react with the dye color. Both chemicals are used in varying amounts depending on the type of hair dye.
Different Types of Dyes
Permanent color comes in three different types: semi-permanent, demi-permanent, and permanent. The dyes all work at different strengths, but each has the same basic process. The ammonia causes the cuticle to swell, which allows the dye molecules to enter the hair shaft and react with the cortex. The size of the color molecule dictates how long the permanent dye will last. The molecules will eventually fall out, resulting in a color fading, but the natural pigmentation will be permanently altered. The only way to return to your natural color at this point is to wait for it all to grow back.