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 Christian Sager.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain stuff from how stuff works. Hey, brain stuff,
it's Christian Sager here. 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

(00:24):
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. And
that's no relation to your brain's cortex. It's just where
you find the pigmented melanin proteins that give hair its color.
Protecting the cortex is hair's outermost layer, the cuticle, and

(00:45):
this is hair's armor, made up of overlapping scales. Temporary
dye just sticks to the cuticle. It's more like paint really.
It will usually circle the drain with your next shampoo.
Semi permanent dye contains molecules of pigments so tiny that
they can slip between the scales of the cuticle and
stick to the cortex. But it's still more paint like.

(01:08):
It doesn't chemically react with anything in the hair. The
little pigment particles will wash back out through the cuticle
scales with soapy water, so a semi permanent die lasts
about twelve shampoos max. Now, both temporary and semi permanent
dyes can sometimes stain very light colored hair, but permanent dye,

(01:29):
as the name suggests, is designed to stay with your
hair until the hair grows or falls out. Now there
are as many formulas for permanent dye as there are
proprietary brands, and in general, permanent dye consists of two solutions. First,
an alkaline chemical plus two types of particles that will

(01:49):
come together to form the new color, die precursors and
die couplers. Second, the developer. This is an oxidizer, usually
a weak solution of hydrogen peroxide. You or you know
your salon professional mix the two together right before applying
them to your hair. The alkaline chemical, either ammonia or
more gentle substitute, goes to work opening up the cuticle.

(02:12):
For dye to be most effective, it needs to access
the hair's cortex. Alkaline stuff temporarily softens and relaxes the
cuticle scales. The next hurdle in achieving a new hair
color is getting rid of some of the existing color.
That's the melanin that's in the hair's cortex. Enter the developer.
It oxidizes the melanin molecules, breaking melanin's double carbon carbon

(02:36):
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 dyes characteristic stink isn't actually the die at all,
but an element of your hair passing into the air.

(02:56):
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 die precursors. These are usually colorless chemicals
that develop color when oxidized. The resulting pigmented particles, called intermediates,
are monomers that, left to their own devices, would slip

(03:19):
through the cuticle scales like semi permanent dye, But the
die couplers react with the intermediates to form polymers of
pigment that are too big to just slip back out,
and that's how permanent color resists fading through multiple washes.
It's trapped beneath the cuticle check out the brain stuff

(03:45):
channel on YouTube, and for more on this and thousands
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