pH and Perms

One journey, in one direction and back

pH stands for potential hydrogen. It runs from 0 to 14, seven is neutral, and the one property of it that matters at the basin is that the scale is logarithmic. Each whole number is a tenfold change, not a single step.

Hair sits at about 5. An alkaline perm lotion sits at about 9. That is not twice as alkaline. It is somewhere around thirty thousand times fewer free hydrogen ions in the water surrounding the fibre. Which is why perm timing is unforgiving, why ten extra minutes is not a small variation on the service you agreed to, and why every stage of a perm is defined by a number rather than by how it looks.

A perm is one controlled journey: from about pH 5, up towards 9, held there while the shape is decided, then down to about 3 and left to settle back at 5.

I would rather a client understood that arc than memorised the name of a product. Everything below is detail hung on it.

Where hair sits when nobody is doing anything to it

Healthy scalp and healthy hair carry a thin acidic film, typically pH 4.5 to 5.5, made of sebum, sweat and the residue of the skin’s own barrier. At that range the cuticle scales lie flat and tightly overlapped, the salt bridges and disulphide bonds inside the cortex sit in stable equilibrium, and the fibre is at its strongest, shiniest and least reactive.

Every single product used in a perm is formulated away from that range on purpose. None of them is designed to be comfortable for the fibre. They are designed to be reversible, which is a different thing, and the whole craft of the service is in getting back.

Why this decides which lotion you are given

Hair with a tight, intact cuticle and a full lipid layer resists an alkaline solution for a long time. Hair that has been coloured, heat-damaged or weathered has lost some of that layer, so an alkaline lotion gets in far faster than the stylist may expect. This is the point clients get backwards most often: damage makes hair dangerously easy to process, not hard. A strong lotion on porous hair is not a stronger result, it is a faster failure. That is why porosity is measured before a strength is chosen, and why the honest answer to “can my hair take this” is sometimes no.

Phase one: up the scale, and the ceiling at the top

The lotion does two jobs, and only one of them is the famous one. The reducing agent — ammonium thioglycolate in most alkaline waves — breaks disulphide bonds. But it cannot break a bond it cannot reach, and the cuticle is a waterproofed roof of overlapping scales built specifically to keep things out. The alkalinity is what opens the roof.

Raise the pH towards 9 and the scales swell, lift and separate. The reducing agent diffuses through the gaps into the cortex and gets to work. Take the alkalinity away and you have a bottle of chemistry sitting on the outside of the hair, doing very little. That is the entire relationship between the pH of a lotion and its strength.

Note what the word “acid” is doing there. A true acid wave is acidic relative to an alkaline wave, not acidic in the way vinegar is. At 6.9 to 7.2 it is essentially neutral water with a milder reducing agent in it. Older textbooks quote acid waves as low as 4.5; those figures belong to an earlier generation of product and are not what is on the shelf now.

Phase two: five minutes of nothing, which decide everything

When the test curl says the reduction has gone far enough, the hair is rinsed with warm water — still wound, still on the rods, for at least five minutes. Tap water sits at roughly 6.5 to 7.5, near enough neutral.

The rinse is a physical step, not a chemical one. It flushes residual thioglycolate out of the fibre and stops the reduction going further. It does not restore the acid mantle. It does not close the cuticle. It does not rebuild anything. The hair comes out of that rinse exactly as alkaline, exactly as swollen and exactly as plastic as it went in, minus the lotion.

Between the rinse and the neutraliser, the hair has no structure of its own. Its shape is being held entirely by a piece of plastic.

That sentence is why this page exists. Every decision made about technique — rod diameter and profile, base control, direction, how much tension went into the wind — is banked in those minutes and cannot be revised afterwards. The chemistry has opened the bonds; the rod is the only thing deciding where they will close.

It is also the most mechanically fragile the hair will be all day. Each rod is blotted with a towel, thoroughly, to get the water out before the neutraliser goes on — and blotted, never dragged or pulled, because the bonds holding the fibre together are open. A stylist who is rough with a towel at this exact moment does more damage in thirty seconds than the lotion did in twenty minutes.

Phase three: back down, hard

A peroxide neutraliser is formulated distinctly acidic, generally pH 2.5 to 4.0, and it is doing two entirely separate jobs in the same five minutes.

1. Chemical. The oxidising agent takes hydrogen off the free thiol groups left by the reduction, and the sulphur atoms cross-link again in whatever positions the rod has put them in. This is the part that makes the curl permanent.
2. Mechanical. The sharp drop from about 9 to about 3 makes the swollen cuticle scales contract, flatten and clamp down over the cortex. The new bonds are shut inside a closed fibre, and the hair stops being plastic and starts being hair again.

A sodium bromate neutraliser works at pH 6.0 to 7.0 instead — near neutral. It does the first job perfectly well and the second job barely at all, which is why bromate services run to ten or fifteen minutes rather than five and usually finish with a separate acidic conditioner to do the cuticle work the neutraliser did not. The trade-off is worth it on tinted hair, where a low-pH oxidiser will take colour with it. The full comparison is on neutralising a perm.

Phase four: the seventy-two hours you take home

The service does not finish when you leave. Seventy-two hours. Neutralising reforms the bonds but it does not finish the job on the spot — oxidation carries on in air and the fibre keeps hardening for about three days. In that window what you press into the hair tends to stay: no tight ties, no clips, no hard brushing, and go gently with heat. The reason is chemical rather than superstitious.

Two things matter in it. Aftercare products should be acidic, in the 4.5 to 5.5 range, because the faster the acid mantle re-establishes the sooner the cuticle is properly closed. And handling should be gentle: a wide-tooth comb, no dragging a heavy multi-row styling brush through a curl whose bonds are still stiffening. Mechanical stress applied in those three days does not wash out.

pH 12 to 14, and why it is not simply a stronger perm

Hydroxide relaxers — sodium, calcium, guanidine — sit at pH 12 to 14. On a logarithmic scale that is not one notch beyond an alkaline perm. It is several orders of magnitude beyond it, and it is not doing the same chemistry harder.

A thio perm breaks a disulphide bond and puts it back together in a new position. A hydroxide relaxer performs lanthionisation: it removes one of the two sulphur atoms from the pair, permanently, leaving a single-sulphur lanthionine link that cannot be reduced and cannot be rebuilt. There is nothing there for a perm lotion to open and nothing for a neutraliser to close.

Which is why the rule is absolute and runs both ways: thio and hydroxide never meet, in either direction. Not relaxed hair permed, not permed hair relaxed. That comparison has its own page at perm versus relaxer.

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