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When you dye your hair, the color changes as the result of a chemical process reflected in a series of reactions between hydrogen peroxide, ammonia and the molecules in your hair. For hair to take on a new, permanent color, the outer layer of the hair shaft (the cuticle) must be exposed to allow the molecules of new color to be deposited and bonded to the hair. Once the outer layer is open, the coloring agents react with the inner portion of the hair (the cortex). Most modern permanent hair coloring kits use a 2-step process to remove the original color while simultaneously depositing the new one.
- Hydrogen peroxide, or H2O2, is used to activate the color development. When mixed in the coloring bottle with ammonia, the amount of peroxide in the solution will determine how much of the original color is left on the hair shaft while the new color is being deposited. This process is called "oxidation," as the oxygen in the peroxide breaks the chemical bonds in hair, altering the pigment in the cortex and releasing sulfur, which accounts for the hair coloring process' distinctive aroma. The higher the concentration of peroxide in the developer, the greater the amount of sulfur removed from the hair and the stronger the odor.
- The relatively high pH of ammonia causes it to soften, swell and open the cuticle of the hair. This allows the hair color to penetrate deep into the hair shaft. It also acts as a catalyst, speeding up the reaction between the permanent hair color and the hydrogen peroxide
- Intermediaries are ingredients that aid in the coloring process while moisturizing and conditioning the hair. As the original hair color is removed, the new color becomes chemically bonded to the inner core of the hair. The alcohols and conditioners present in the hair coloring close the cuticle after the color has entered, sealing and protecting the new color. The intermediaries then cause the color molecules to expand in size so they cannot be washed out.
