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Speaker 1: Welcome to brain stuff from how stuff works. Hey, brain stuff,

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Speaker 1: it's Christian Sager here. There are three basic chemical formulations

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Speaker 1: of hair dye, temporary, semi permanent, and permanent. Before we

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Speaker 1: look at what each of them do, let's look at

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Speaker 1: the physical structure of hair. Hair is dead stuff, three

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Speaker 1: layers of slightly different dead stuff. The core, called the medulla,

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Speaker 1: is not pertinent to our interests today, but it's surrounded

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Speaker 1: by a thick layer of cells called the cortex. And

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Speaker 1: that's no relation to your brain's cortex. It's just where

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Speaker 1: you find the pigmented melanin proteins that give hair its color.

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Speaker 1: Protecting the cortex is hair's outermost layer, the cuticle, and

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Speaker 1: this is hair's armor, made up of overlapping scales. Temporary

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Speaker 1: dye just sticks to the cuticle. It's more like paint really.

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Speaker 1: It will usually circle the drain with your next shampoo.

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Speaker 1: Semi permanent dye contains molecules of pigments so tiny that

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Speaker 1: they can slip between the scales of the cuticle and

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Speaker 1: stick to the cortex. But it's still more paint like.

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Speaker 1: It doesn't chemically react with anything in the hair. The

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Speaker 1: little pigment particles will wash back out through the cuticle

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Speaker 1: scales with soapy water, so a semi permanent die lasts

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Speaker 1: about twelve shampoos max. Now, both temporary and semi permanent

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Speaker 1: dyes can sometimes stain very light colored hair, but permanent dye,

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Speaker 1: as the name suggests, is designed to stay with your

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Speaker 1: hair until the hair grows or falls out. Now there

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Speaker 1: are as many formulas for permanent dye as there are

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Speaker 1: proprietary brands, and in general, permanent dye consists of two solutions. First,

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Speaker 1: an alkaline chemical plus two types of particles that will

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Speaker 1: come together to form the new color, die precursors and

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Speaker 1: die couplers. Second, the developer. This is an oxidizer, usually

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Speaker 1: a weak solution of hydrogen peroxide. You or you know

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Speaker 1: your salon professional mix the two together right before applying

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Speaker 1: them to your hair. The alkaline chemical, either ammonia or

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Speaker 1: more gentle substitute, goes to work opening up the cuticle.

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Speaker 1: For dye to be most effective, it needs to access

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Speaker 1: the hair's cortex. Alkaline stuff temporarily softens and relaxes the

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Speaker 1: cuticle scales. The next hurdle in achieving a new hair

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Speaker 1: color is getting rid of some of the existing color.

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Speaker 1: That's the melanin that's in the hair's cortex. Enter the developer.

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Speaker 1: It oxidizes the melanin molecules, breaking melanin's double carbon carbon

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Speaker 1: electron bonds and giving up one of its own oxygen

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Speaker 1: atoms to fill in the space. The result, the melanin

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Speaker 1: turns colorless and releases sulfur atoms. That's right, part of

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Speaker 1: permanent dyes characteristic stink isn't actually the die at all,

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Speaker 1: but an element of your hair passing into the air.

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Speaker 1: But that's not the developer's only job. It also kicks

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Speaker 1: off the reaction that brings together the new color molecules

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Speaker 1: by oxidizing the die precursors. These are usually colorless chemicals

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Speaker 1: that develop color when oxidized. The resulting pigmented particles, called intermediates,

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Speaker 1: are monomers that, left to their own devices, would slip

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Speaker 1: through the cuticle scales like semi permanent dye, But the

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Speaker 1: die couplers react with the intermediates to form polymers of

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Speaker 1: pigment that are too big to just slip back out,

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Speaker 1: and that's how permanent color resists fading through multiple washes.

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Speaker 1: It's trapped beneath the cuticle check out the brain stuff

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Speaker 1: channel on YouTube, and for more on this and thousands

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Speaker 1: of other topics, visit how stuff works dot com.