How do hair perms work?

The secret to perfect curls is some simple hair chemistry

A perm, or permanent wave, is a style created by exploiting your hair’s chemistry. Hair is made from flat, overlapping cells covering a core of fibres. These fibres are made from a protein called keratin.

Keratin is made from small units, woven together into two-stranded coils. Four of these coils are in turn coiled together, and these are joined end-to-end to make structures called protofilaments. Eight protofilaments wind up to make up intermediate filaments, which clump together to make macrofilaments, which are glued into strands by the fatty membranes of spindle-shaped cells. It’s these structures that are key to curls.

The keratin molecules contain a component called cysteine. It’s an amino acid – one of the building blocks of protein – and it contains sulphur. A sulphur from one cysteine can form a strong bond to a sulphur from another cysteine, which means that neighbouring keratin molecules can become linked together. Depending on where these tiny links are made, they could hold the hair poker straight or twist it into ringlets.

Cold perms use a chemical called ammonium thioglycolate to break the bonds so that the hairs can be twisted into a new shape. Then hydrogen peroxide is added to allow new bonds to form between the strands, setting the curl.

 This article was originally published in How It Works issue 099

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