The process of coloring your hair, whether at home or at the salon, involves chemical processes which deposit color into the hair strands. Understanding how these processes work is essential to maintaining healthy hair.
About three-quarters of women dye their hair regularly, according to a 2017 report by Statista and a 2008 study by Clairol. But how does hair coloring work? Let’s look at the science of how the color gets into your hair and stays there.
Where Does Natural Hair Color Come From?
Hair gets its natural color from melanin pigments that are in the cortex of each hair strand. There are two types of hair pigments, eumelanin and pheomelanin. The darker shades like black and brown are caused by eumelanin. Pheomelanin produces lighter shades like pale yellow and blonde. Red hair is produced by a nearly equal amount of eumelanin and pheomelanin in the hair.
White hair is caused by the absence of melanin pigments. Either hair melanin is no longer being produced, or it cannot be moved into the hair cells. Gray hair is the result of the missing pigments (white hair) and the remaining pigments (eumelanin and pheomelanin). Your specific hair color is determined by the ratio of each pigment in your hair, which is a result of your unique genetics.
Structure of a Hair Strand
Each strand of hair is made up of three components: the cuticle, cortex, and medulla.
The cuticle is the outer protective layer of the hair, made up of tiny overlapping scales. When the scales of the cuticle lay flat, moisture stays in the hair. Chemical processes like coloring or bleaching can open the cuticle, causing the scales to lift open, allowing access to the cortex. When the cuticle stays open, due to hair damage, the cortex can be underhydrated.
The cortex is the middle layer of the hair, making up most of the mass. The pigment in your hair is stored in the cortex, giving your hair its natural color. When you color your hair, the cortex is the part of the hair where those color pigments will be deposited.
The medulla is the innermost part of the hair strand, usually only found in thicker hair types. It is made up of thin, soft transparent cells and air spaces.
Types of Hair Coloring
There are three types of hair coloring: permanent, demi-permanent, and temporary. Permanent hair coloring binds to the proteins in the cortex, while demi-permanent and temporary hair dyes do not bind to the cortex or alter the pigment in the inner layer.
Temporary Hair Coloring
Also known as 24-hour hair coloring, this type of hair dye deposits pigments on the outer cuticle. Generally, no oxidizer is used to open the cuticle. Some molecules may slip in through the cuticle and attach to the inside of the cuticle, but most of the color is removed after a few times of washing your hair.
Demi Permanent Coloring
In demi-permanent coloring, an alkalizer like monoethanolamine (MEA) is used to open the cuticle scales. A weak oxidant like hydrogen peroxide is mixed the dye precursor to create medium-sized molecules which enter the cuticle and stick to the inner cuticle and the outer cortex. The weak peroxide washes a small amount of pigment out of the hair, creating a mild color enhancement. The medium sized coloring molecules eventually escape through the cuticle. A conditioner is usually applied to the hair to seal the cuticle.
Permanent Hair Coloring
Permanent hair coloring uses a stronger alkalizer, usually ammonia, to open the cuticle so color pigments can be deposited in the cortex. The precursor is mixed with hydrogen peroxide to create large pigment molecules which bind to the cortex proteins. These coloring molecules are also larger so they are less likely to escape the hair strand. Permanent hair coloring is extremely resistant to washing, so the color lasts a long time.
| Type of Hair Coloring | Color Duration | Type of Pigment Binding | Alkalizer Used |
|---|---|---|---|
| Permanent | Until hair grows out | Hair cortex, strong bind | Strong alkalizer, such as ammonia or monoethanolamine |
| Demi-Permanent | Up to 24 washes | Inner cuticle, weak bind | Weak alkalizer, such as monoethanolamine |
| Temporary | Up to 8 washes | Outer Cuticle, weak bind | None |
How Does Permanent Hair Coloring Work?
There are two components of permanent hair dye that are mixed immediately before the coloring application. An alkaline agent, like ammonia, is used to open the cuticle scales so that the coloring pigments can penetrate the hair shaft. This is usually combined with the pigment molecules, eumelanin and pheomelanin in the dye. The other component is an oxidizer, like a hydrogen peroxide mixture, which unbinds the existing melanin from the hair shaft, so the new pigments will attach to the cortex.
In the oxidation process, the double carbon electron bonds are disintegrated, and an oxygen molecule fills the void. As the natural melanin in your hair loses its color, and the chemical bonds are broken, sulfur atoms are released during the chemical process. The hair dye “smell” comes from the oxidation of the existing melanin molecules, not the hair dye.
The hair coloring molecules enter the hair strand through the opened cuticle and bind to the cortex. The pigment molecules, which before were unseen to the naked eye, now combine and show up as color in the dye and in the hair. The molecules react with chemical couplers which create coloring molecules which are too large to easily escape the cortex. The end result is a permanent color that will remain in the hair strand indefinitely.
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