Bleaching Hair
A: Hair bleaching (also called decolorizing) works similarly to regular hair color in that it uses a 'developer' or 'activator' (usually hydrogen peroxide) to soften and raise the cuticle of the hair. However, the similarities end there.
Bleaching is especially useful when you want to go from a darker hair color to a much lighter one. Most hair color can be mixed with higher volume peroxides to provide greater 'lift' of the color, but in order to reach the lightest shades of hair color, you usually need to pre-lighten the hair with a bleaching agent first.
It's important to note that bleaching the hair is a very harsh process and can have devastating results if you aren't careful. I've said it before, but it bears repeating: Normal hair color formulas will go inert after 30 minutes, so it's almost impossible to "over-process" them.
However, hair bleach will remain active as long as it has moisture and the longer it processes, the more damaged the hair will become. Likewise, the lighter you take the hair from its natural color, the more damage will occur, so always use extreme caution when using hair bleach.
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See also:
Hair bleaching problems
Bleaching hair and pregnancy
Hair color level and peroxide developer
Where does the color go when you bleach dark hair?
How can I remove bleach from my hair?