Bleaching Retouch and Hair Breakage
A: It's very true. The bleaching process of hair lightening results from the combination of hydrogen peroxide developer with the alkali of bleaching agents releasing nascent oxygen, which is what affects the pigment molecules, lightening the hair color by turning the colored melanin into oxymelanin (colorless).
When a retouch service is performed, care is taken to prevent overlap, because the effect of bleaching (and therefore the damage) is cumulative. Obviously, the primary goal is to avoid creating a demarcation line where the color is lighter than the previously colored hair or the newly lightened new growth.
Yet, an equally significant concern is having a segment of the hair shaft that has been doubly processed and doubly affected by the chemical stresses of the lightening process. Depending on the condition of the hair at the start and the level of care received by the previously lightened hair, you could end up with hair that is significantly more fragile at the point where the bleaching processes overlapped.
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See also:
The hair bleaching process
Is it ok to re-bleach my entire head?
Is it safe to use a home bleaching kit to bleach re-growth?
Where does the color go when you bleach hair?