Sidebar · The chemistry

The chemistry

The chapters run along a chronological spine; the chemistry does not. Beneath every permanent wave from Nessler's 1906 brass rods to the modern salon treatment lies a single molecular transaction — the breaking and re-forming of the disulfide bond in keratin. This sidebar gathers that science in one place. It is the reference the chemical-era chapters draw on: Chapter 7, The cold wave, where it is first named; the acid perm and the texture renaissance that follow; and the earlier thermal machines, which exploited the same chemistry unknowingly, through heat.

A note on certainty. The disulfide-bond chemistry itself is textbook and is stated plainly here. The dates attached to specific reagents — the sulfite cold wave of around 1932, the thioglycolate cold wave of around 1940–41, the acid perm of the 1970s, the bond-builders of the 2010s — are carried with the same caveats as in the host chapters: established as chemistry, approximate as history.

1 · Intact native hair S–S disulfide bond holds the shape 2 · Reduced waving lotion S–H H–S reducing agent donates electrons bond splits → two thiols; hair pliable 3 · Re-oxidized neutralizer S–S bonds re-form in the new shape shape held reduce re-oxidize the same S–S bond, broken on the rod and re-formed in the rod's curve
Fig. 1. The single transaction that every permanent wave performs. (1) In native hair, two sulphur atoms on neighbouring keratin chains are joined by a disulfide bond (S–S), locking the chains into the shaft's inherited shape. (2) A waving lotion — a reducing agent — donates electrons to that bond, splitting it into two free thiol (S–H) ends; the chains slip free and the hair becomes pliable. (3) Once the hair is wound and held in the new curve, a neutralizer — an oxidizer — withdraws electrons again, and the sulphur atoms re-link, now in whatever position the rod imposed. The re-formed bond is chemically identical to the original, which is why the wave survives washing. (Author's diagram, echoing the cold-wave figure of Chapter 7.)

The bond that holds the shape

A hair is not a fibre in the simple sense; it is a bundle of long protein chains, made largely of keratin, twisted and bundled along the shaft. What gives those chains a stable shape — what makes one head of hair hang straight and another curl — is not the chains themselves but the cross-links between them. The most important of these, and the one the entire permanent-wave industry is built on, is the disulfide bond: a direct chemical link between two sulphur atoms, one on each of two neighbouring keratin molecules.

The disulfide bond is strong, covalent, and — crucially — water-resistant. Hydrogen bonds, the weaker cross-links keratin also forms, dissolve the moment the hair is wet and re-form as it dries, which is why a setting paste or a curling iron could only borrow a shape for a day. The disulfide bond does not dissolve in water. It holds its position through a washing, through humidity, through years. Straight hair and curly hair differ, at the molecular level, largely in the pattern of these disulfide cross-links along and between the keratin chains. To change the shape of hair permanently is therefore to change the pattern of its disulfide bonds — and to do that, the bonds must first be broken.

Every lasting curl in history is a rearrangement of the same bond. The iron could not touch it; the wig could only hide it. The permanent wave begins the moment chemistry learned to break it open and re-form it at will.

Reduce, shape, re-oxidize

The mechanism is a sequence of three steps, and it is the same whether the wave is set by Nessler's 1906 heat-and-alkali machine or by a modern cold-wave bottle. Reduce. A reducing agent — the waving lotion, or in the thermal era, heat combined with an alkali — donates electrons to the disulfide bonds, breaking each S–S link into two S–H groups called thiols. With the cross-links open, the keratin chains are free to move past one another, and the hair becomes pliant. Shape. The loosened hair is wound under tension on a rod — spiral in the early machines, flat (croquignole) in Mayer's 1924 system, narrow in the modern salon — and as the open bonds settle, the shaft takes the rod's curve. Re-oxidize. The rods stay in; a neutralizer, an oxidizing agent, is applied; it withdraws electrons from the thiols, and the sulphur atoms re-link — now in whatever geometry the rod has imposed. The re-formed disulfide bonds are chemically identical to the originals. The shape they now lock in survives water exactly as well as the native shape did.

That is the whole mechanism, and the diagram above traces it. The thermal machines of 1906 to the 1940s accomplished the reduction with sustained heat and alkali — the borax pastes of Nessler's London, the steam sachets of Mayer's REALISTIC system, the croquignole heater sleeve. The cold wave that displaced them accomplished the same reduction with a reagent at room temperature. The chemistry is identical; only the means of breaking the bond changed. This is why Chapter 4 and Chapter 3, though they predate the named chemistry by decades, are properly part of the same story: they were operating on the disulfide bond before anyone had named it.

The two chemistries

Once the reduce-reshape-reoxidize principle was grasped, the search became one for a reagent that could perform the reduction at room temperature — safely, quickly, and pleasantly enough for the salon. Two were found, roughly a decade apart, and the distinction between them is the single most-confused point in the popular record.

The first was sulfite. A sulfite or bisulfite solution, applied to wound hair, reduces a portion of the disulfide bonds at room temperature over the course of an hour or more. The sulfite cold wave is generally dated to around 1932; the names most often attached to it are Clark and Speakman, though the precise priority is not uniformly recorded. It worked — a wave without electricity and without the chandelier — but it was slow, smelly, uneven, and prone to scalp irritation. It was a proof of concept more than a finished product.

The second was thioglycolate. An alkaline solution of a thioglycolate salt — most commonly ammonium thioglycolate — reduces the disulfide bonds far more efficiently, at a workable speed and with a controllable result. The thioglycolate cold wave is credited to work around 1940–41, the names McDonough and Evans most frequently attached, with Arnold F. Willatt associated with its commercialisation; the apportioning of credit is not settled in the open record. What is not contested is the consequence: the thioglycolate reagent was the one the salon trade actually adopted, and it is the chemistry the modern cold-wave perm descends from. The full history of both reagents — the wartime rationing that compressed their adoption — is told in Chapter 7.

Alkaline vs acid

The thioglycolate cold wave had a signature failing. Its waving lotion works at a high pH — roughly 9 to 10, distinctly alkaline — because the thioglycolate ion is most active as a reducing agent in that range. The alkalinity is functional, but it is also rough on the hair: it swells the shaft, roughens the cuticle, and — if the lotion is left too long or rinsed too late — over-reduces, leaving the hair brittle, frizzy, or both. The familiar "frizz" of a bad perm is the visible signature of an alkaline process pushed past its tolerance. Done well, a thioglycolate wave is durable and even; done badly, it is the cold wave's lasting reputation for damage.

The answer arrived in the 1970s in the form of the acid perm. The active agent here is glyceryl monothioglycolate, a milder reducing species that works nearer to the hair's own acidity — at a pH closer to neutral than the alkaline cold-wave lotion, and so gentler on the cuticle. The acid perm processes more slowly and at a lower temperature than the cold wave, often under gentle heat, and it is the chemistry that carried the permanent wave through the boom years of the 1970s and 1980s. It is the subject of the chapter that follows the machineless era. The trade-off is structural: mildness against time, gentleness against the brisk economics of the alkaline bottle.

Damage, and the bond-builder

Why does a perm damage hair at all, when the disulfide bond is, in principle, re-formed by the neutralizer? Because in practice it is not all re-formed. The waving lotion breaks more bonds than the neutralizer can re-link in the time available; some sulphur atoms pair off incorrectly, forming stray cross-links (the chemistry of the characteristic perm smell), and some remain as free thiols, leaving the shaft weakened. Over-processing breaks bonds the neutralizer never restores; under-processing leaves the wave uneven. The fraction of disulfide bonds permanently lost is the molecular measure of perm damage, and it accumulates across successive treatments.

The most recent chapter of the chemistry is the attempt to repair that loss directly. From the early 2010s, a class of treatments called bond-multiplier or bond-builder products — widely associated with the Olaplex brand from around 2014, though the underlying chemistry has since proliferated across many manufacturers — introduced a third step into the reduce-reshape-reoxidize sequence: a molecule that seeks out the broken disulfide bonds and re-links them during and after the treatment, before the neutralizer's incomplete work becomes permanent. By re-linking a portion of the bonds the conventional process would have lost, these treatments extend what a perm can do to already-processed hair, and they underpin the modern "texture" services that have carried the perm back into fashion. They do not abolish the chemistry of damage; they manage it. The same disulfide bond Nessler broke with brass and borax in 1906 is, a century and a quarter later, the bond the modern bottle is sold on the promise of rebuilding.

The permanent wave's whole history is the history of one bond — broken open, re-formed, broken open again. Every generation of the chemistry has been a new answer to the same question: how to break it cleanly, and how much of it can be put back.

Sources & further reading

Scope. The cross-cutting science reference for the permanent-wave history — what a permanent wave actually is, at the molecular level, gathered rather than threaded through the chapters. It builds on Chapter 7, The cold wave and the war, where the disulfide-bond chemistry first enters the narrative, and looks back to Chapter 4 and Chapter 3, whose thermal machines performed the same chemistry before it was named. The acid perm of the 1970s and the bond-builders of the 2010s are mentioned here in prose; their full chapters are forthcoming. See also the machines sidebar for the apparatus side of the thermal era. The chapter index and the timeline place this material in sequence.