For Halloween tomorrow night, many ghouls and goblins may change their hair color – whether it rinses right out or stays until the hair grows out is a matter of chemistry.
If your local vampire wants to have black hair or your favorite witch wants to become Curella de Vil for Halloween, dyeing their hair is an alternative to wearing a wig. The most popular option for costume parties is a temporary hair dye. These substances coat the outside of the hair with color, and since they do not penetrate into the hair shaft, the color washes out after a shampoo or two. A longer term color change requires a bit more chemistry.
In changing the color of hair permanently, there are two main components to the process. The first ingredient is the developer or oxidizing agent, which is generally hydrogen peroxide and which removes the natural color of the hair. The greater the desired “lift” or lightening in the hair color, the higher the concentration of hydrogen peroxide is used. The bleach reacts more significantly with the dark eumelanin pigment responsible for brown and black colors than with the phaeomelanin protein that provides the lighter colors, so the hair often retains blond or reddish tints rather than being completely colorless after the process is completed.
The second ingredient of the process is ammonia, which helps to catalyze the bleaching process and also helps to open the cuticle so that the new color can penetrate into the cortex of the hair and bind permanently. When the bleaching and coloring process is complete, conditioners are used to close the hair cuticle and lock the new color inside.
Since both hydrogen peroxide and ammonia tend to damage hair leaving it brittle, less drastic chemical processes are often alternatives to produce color that lasts for a shorter period of time. For semi-permanent color that does not require lightening, no bleach or ammonia is used. Instead, the dye contains small molecules that penetrate the cuticle and can be retained for a week or two before slipping out again.
More information is available at http://www.salonweb.com/pro/color.htm