How Does Hair Dye Work?

Some hair dyes are basically fine-grained paint. But dyes that stick with you for more than a couple weeks physically and chemically change each hair. Learn how with HowStuffWorks.

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This is not my natural hair color. There are three basic chemical formulations of hair dye: temporary, semi-permanent, and permanent. Before we look at what each of them do, let’s look at the physical structure of hair.

Hair is dead stuff. Three layers of slightly different dead stuff. The core, called the medulla, is not pertinent to our interests today.

But it’s surrounded by a thick layer of cells called the cortex – no relation to your brain’s cortex. It’s where you find the pigmented melanin proteins that give hair its color.

Protecting the cortex is hair’s outermost layer: the cuticle. As hair’s armor, it’s made up of overlapping scales. Temporary dye just sticks to the cuticle – it’s more like paint, really, and it’ll usually all circle the drain with your next shampoo.

Semi-permanent dye contains molecules of pigment so tiny that they can slip between the scales of the cuticle and stick to the cortex. But it’s still more paintlike – it doesn’t chemically react with anything in the hair.

The wee pigment particles will wash back out through the cuticle’s scales with soapy water, so a semi-permanent dye lasts about 12 shampoos max.

Permanent dye, as the name suggests, is designed to stay with your hair until the hair grows or falls out. In general, permanent dye consists of two solutions. First, an alkaline chemical plus two types of particles that will come together to form the new color: dye precursors and dye couplers.

Second, the developer: an oxidizer, usually a weak solution of hydrogen peroxide. You (or your salon professional) mix ‘em together right before applying them to your hair.

The alkaline chemical (either ammonia or a gentler substitute) goes to work opening up the cuticle. For dye to be most effective, it needs access to hair’s cortex. The alkaline stuff temporarily softens and relaxes the cuticle’s scales.

The next hurdle in achieving a new hair color is getting rid of some of the existing melanin in the hair’s cortex. Enter the developer. It oxidizes the melanin molecules, breaking melanin’s double carbon-carbon electron bonds and giving up one of its own oxygen atoms to fill in the space.

The result: the melanin turns colorless and releases sulfur atoms. That’s right: part of permanent dye’s characteristic stink isn’t actually the dye at all, but an element of your hair passing into the air.

But that’s not the developer’s only job: It also kicks off the reaction that brings together the new color molecules by oxidizing the dye precursors. These are usually colorless chemicals that develop color when oxidized.

The resulting pigmented particles (called intermediates in industry lingo) are monomers that, left to their own devices, would slip through the cuticle’s scales like semi-permanent dye.

But the dye couplers react with the intermediates to form polymers of pigment that’re too big to just slip back out. That’s how permanent color resists fading through multiple washes: It’s trapped beneath the cuticle.

SOURCES:

Tro, Nivaldo. “Chemistry in Focus: A Molecular View of Our World”. Cengage Learning. Jan 1, 2015. p. 369.

[abomeh]

trying to break down the last part

hair structure from outermost layer:
1. cuticle
2. cortex (contain melanin which gives your hair color)
3. medulla

stuff in the hair dye:
1. alkaline (ammonia)
2. dye precursors
3. dye coupler
4. developer/oxidizer (hydrogen peroxide)

process:
1. alkaline open up and soften cuticles
2. developer removes existing color in melanin (by oxidizing melanin molecule)
3. developer brings in new color (by oxidizing dye precursors)
4. dye coupler helps trap the new color underneath the cuticles

[treestump]

Thank you so much! I'm currently doing a science project on the chemistry of hair dye and this helped so much!

[BrainStuffShow]

Lauren here! I fell down a research rabbit hole while writing this script -- lots of complex chemistry and microscopic materials science go into making everyday hair dyes. Now I want to research exactly how much physical and chemical damage hair dyes actually do.

[TopHatKitty]

Btw, henna is permanent. It binds to the cortex molecules. I've been doing it for a decade. Indigo is another natural dye, and over henna, it creates a bluish-black. Indigo is not permanent. If you look good with red hair and wanna commit, I recommend it.

[marriejones]

anyone else watching this while dyeing their hair?

[DomyTheMad]

*sits stunned with all this info i never had*

[pn]

Hi! I like understanding what actually goes on chemically with all these processes. Thank you. I also have a question regarding thermally rebonded hair. Would it be okay if I dyed it? Atleast semi-permanent?

[chesterrw]

i accidentally bought permanent instead of semi-permanent hair dye. is there anyway to remove the color without bleach/other harsh chemicals. (it's been one day)

[MiotaLee]

hair addiction? not really. but I've been watching lot of Guy Tang lately for inspiration for my next hair color endeavor. To answer your question: I don't know- I've been blonde(natural), brunette, black, red(temporary) pink and blue. Blue is my favorite so far though.

[VincentBalducci]

Pretty much the cortex is what holds the hair color. You cant permanently color it with out using permeant hair color that uses ammonia or MEA to open the cuticle to color the cortex. Then the small dye molecules get blow up by the peroxide so they can leave the hair. Peroxide can be super strong like 40vol some lines carry 60vol.

[nafichachowdhury]

A question, when dying your hair, should u use more developer or color

[lavenderamberli]

Thank you so much for this video. I've been stuck in the DIY haircolor echo chamber of people repeating "I don't know the science behind it."

[vigamortezadventures]

Awesome information some examples and techniques would strengthen this information but great general knowledge over all.

[EqualsThreeable]

As a man, masculine more than feminine, I am not really into my looks, other than not smelling bad and exercising. However I've always wondered what I would look like with blond hair, I naturally have dark brown hair, I think it would be fun to try for a vacation one time.

[msreinevilla]

Im distracted with her hand gestures

[Miush]

Hi, hope you'll reply!!!
I dyed my hair with loreal casting creme gloss no ammonia in black colour (my natural hair is dark brown) and it says on the package that it stays on up to 28 washes. I have already had 28 washes and it has only faded. How long do you think it'll take to the colour to be completely out of my hair?? I hate it!!! :(

Btw- does flat iron make the hair dye last longer or not?

[neilswiger]

...purple will go perfect with your black hair extensions...I'ma proff hair stylist & now there are demi permanent colors that we work with everyday...and also new permanent colors with NO ammonia...

[ronaldoist]

Yo mama joke of the day: Yo mama's hair so nappy, even moses couldnt part it

[sherri]

can the color of a home hair color kit be use without the developer ? and will it hurt my hair . just for temp color.

[Skynet_]

Me as a man honestly would like to try all the hair dyes on my hair I will honestly find it cool also is there eye pigmentation or should I go back to the 24th century?