Most everyone has gone into a salon for a cut and color. Usually this consists of showing your stylist your end goal, them working some magic, and you leaving with different colored hair. But – have you ever wondered how and why this is possible?
What is hair?
First things first. To understand why we can color, perm, and all that, we have to think about what hair is made up of.
There are two main structures that make up your hair. The first is the hair follicle which is found in the skin. The second is the hair shaft which is what you see outside of the skin. Both your hair and nails are primarily made up of a strong protein called keratin (you may have heard of this since it is often advertised in hair care products). As your hair lengthens, more cells are added to the bottom of the hair shaft within the hair follicle. These new cells push out the older cells making your hair longer. Once the hair is outside of your scalp, the cells making up the hair shaft are no longer living. When we get our hair done, we are always changing the hair shaft, not the follicle.
The structure of the hair shaft is made up of three layers. The medulla in the center is a layer only found in thicker hair types and does not have much known function. The middle layer is called the cortex and contains pigments called melanin that are responsible for the color of you hair. Eumelanin is a form of melanin responsible for black and brown whereas phaemelanin adds the blonde and red shades (1). Finally, the outer layer is called the hair cuticle. This layer is composed of overlapping cells that act as protection for the inner layers.
Now, whether your hair is curly or straight is more dependent on the hair follicle than the hair shaft. First, the shape of the follicle can influence your hair type. Straight hair follicles are usually round and curly hair follicles are more oval (2). Also, the angle of the follicle matters too. If the follicle is straight up and down you probably have straight hair. If this follicle is angled, you probably have curly hair. One final influencing factor is the bonds present between the keratin proteins within the shaft cortex. Keratin contains an amino acid called cysteine which contains sulfur. Cysteine has a special property that allows it to bind to other cysteines at the sulfur group. These bonds are known as ‘disulfide bonds.’ When a lot of these bonds form with cysteines farther down the shaft, this contributes to curling of the hair.
Color
Like I mentioned above, there are two types of melanin that contribute to the color of your hair. The absence of these pigments results in white or gray hair as you age. When you dye or bleach your hair, you need to be able to access the cortex so that you may deposit or remove pigments into this layer. Below I explain the differences between the types of dyes:
- Permanent – Permanent hair dyes are considered permanent because they are able to permeate the hair shaft into the cortex. To get to the cortex, the cuticle must first be opened to deposit the dye. Ammonia is normally used to open the cuticle. After the cuticle has been opened, oxidizers like peroxides work to remove the preexisting color pigments followed by bonding the new color to the cortex. Remember me talking about cysteine containing sulfur? This is why you get a smell with permanent dyes because the peroxide breaks these disulfide bonds (3).
- Demi- and Semi-permanent – Demi-permanent dyes contain a lower concentration of ammonia and peroxides. Therefore, they only lift the color of your hair by about one ‘shade’. Semi-permanent dyes contain even lesser amounts of ammonia and peroxides. This makes them last just a little longer than temporary dyes (4).
- Temporary – Temporary dyes are considered direct dyes because they are already developed before applied to your hair. They don’t contain peroxides or ammonias like the other dyes do, so there is no color-lifting involved in this process. Because of this, the dye never penetrates through the cuticle which is why temporary dyes wash out after a few washes.
- Lightening – The process of lightening is similar to that of permanent dyes but without the adding back of pigments. Bleach contains high levels of peroxide which is why bleaching is often damaging to hair.
Perms and Straightening
While not nearly as common anymore, perms are also possible due to chemistry! (I will say I had a perm for a few years in college…) The reason we can curl (and straighten) our hair goes back to the structure of the bonds within the hair shaft.
When you perm hair, what you’re actually doing is breaking and reforming the disulfide bonds between the cysteine amino acids in the keratin proteins. These bonds are fairly strong and require reducing agents to break them apart (5). Once these bonds are broken, the molecules can move around and adjust to the shape of the curler. A neutralizer, like hydrogen peroxide, is then added to remake disulfide bonds. Once complete, your hair is locked into its new shape.
Summary
So that about sums it up! We owe our perfect hair to the hair biologists and chemists who figured all this out for us. I’m someone who loves to change up my hair so I am thankful that they did 🙂
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