What happens when you bleach hair?
Hair bleach works through a process called oxidation. It is this chemical process that allows the product to actually lighten your hair, and it is hydrogen peroxide in the form of developer which produces the oxidation.
Now, developer is already mostly water with conditioning agents and other nourishing ingredients added to it. Only a small percentage of the solution is actually hydrogen peroxide. In most cases, this will be at a concentration of 3 - 9 percent peroxide (10 - 30 vol), with the most abundant ingredient in developer actually being water.
This means that when you apply bleach to your hair, you are wetting your hair because of the water in the developer. In other words, hair is always wet when you bleach it, but it's the amount of water that matters in this case.
If your hair is already wet, this adds water into the bleach mixture, diluting it down significantly. Bleach that is added to wet hair becomes much weaker than it would have been otherwise and this can present a problem if you need greater lightening power.
When applying bleach in this way, the wet hair is also beneficial because the shampoo that is added to the bleach is able to penetrate the hair a lot more effectively if the hair is slightly damp, speeding up the stripping process and better removing stubborn dye for correction.
If you have to correct or soften a bad dye job, it's perfectly fine to bleach wet hair, but if you need decent lightening and more reliable results, it's always better to apply bleach to dry hair.
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Can you bleach wet hair? Certainly; but it's not the best idea if you need the best lightening effect and results. Have another question about bleach? Leave a comment for tailored advice...