The True Acid Perm

Glyceryl monothioglycolate, in a tube of its own

The reducing agent here is glyceryl monothioglycolate — GMTG. Glyceryl, with the -yl ending, however often the trade press writes it as the sugar alcohol instead; only one of those spellings names the ingredient. It arrived in the early 1970s to solve a specific problem. Permanent colour had gone mainstream, the alkaline cold wave at pH 9.5 was hard on tinted hair, and the trade wanted a perm that would not strip and muddy the colour a client had just paid for.

GMTG is unstable in solution, so it cannot sit pre-mixed on a shelf. It is supplied as a separate small tube or vial that is poured into the base lotion at the chair, immediately before it goes on your head. That is the identification tell, and it is a useful one: if your stylist mixes something from a second container in front of you and then puts you under a dryer, you are having a true acid perm. If the bottle came ready to pour, you are not.

Modern true acid lotions run at about pH 6.9 to 7.2 — which is to say, essentially neutral. “Acid perm” is now a historical name rather than a description.

Historically the category did run acid, anywhere from 4.5 up to 7.0, and the name comes from there. Today it sits just either side of neutral, close enough to the hair's own resting 4.5 to 5.5 that the cuticle hardly swells. That is the whole appeal of the chemistry, and also the whole problem with it. There is more on why pH governs everything in a perm at pH and perms.

Level of change

The teCsture Scale runs from 1, the tightest pattern, to 10, poker straight — one ladder, ten rungs. What a treatment does is move you a number of rungs. That distance is the job, and it is what the work should be priced on: the level of change, not the bottle.

A true acid perm is a small, forgiving move — and it gives back a proportion of it over three or four months.

Endothermic: the perm that has to be given heat

An alkaline perm uses pH to force the cuticle open so the reducing agent can get into the cortex. A true acid perm has no pH to spare. Something else has to lift those cuticle scales, and that something is heat, applied from outside: a plastic cap and a pre-heated hood dryer, twenty to thirty minutes, sitting still.

This is what endothermic means — the reaction absorbs heat rather than releasing it. It is the exact opposite of an exothermic perm, which brings its own warmth in a second bottle, and the two words are worth keeping straight because they describe opposite arrangements of the same idea.

Inside the fibre, less happens than with an alkaline lotion, and that is deliberate. Fewer disulphide bonds are opened, the cuticle stays comparatively flat, and more of the hair's native elasticity survives the service. The result is softer, looser, shinier and structurally weaker than a cold wave. You are buying condition and paying in curl.

The hair this suits, and the hair it will fail on

It suits fine hair, single-process tinted hair, and porous or fragile hair that still passes an elasticity test. On tinted hair in particular it remains one of the kindest conventional perms ever formulated: with almost no cuticle swelling there is very little for the colour to leach out through. It was designed for exactly that client and it still does that job.

It fails on coarse, resistant, virgin hair. A tight flat cuticle with no alkali pushing it open will not admit enough GMTG, even under a dryer. The hair comes out of the salon looking like a blow-dry and is straight again by the weekend. Thick straight Asian hair is the classic case; it wants alkaline chemistry or a digital hot perm, not this.

The failure that actually happens

Under-processing, almost every time. The dryer is not hot enough, or the client's hair is more resistant than it looked, or the cap sat badly on one side. The GMTG never gets far enough in, and the hair dries perfectly, disappointingly straight. It is not dangerous. It is just a whole afternoon and a full price for nothing, and it is why an honest stylist will talk you out of this chemistry the moment your hair looks anything like resistant.

What the service involves

1. Elasticity test. Fragile hair is exactly the market for this perm, so the test matters more here, not less — see can your hair take a perm.
2. Gentle cleanse. No hard scalp work.
3. The wind, thirty to forty-five minutes. Firm, even tension throughout. With chemistry this mild the wrap is doing a larger share of the work than usual, and a loose rod simply will not produce a curl.
4. Mixing at the chair. The GMTG tube goes into the base lotion and is applied straight away, in gloves. This is the step you can watch for.
5. Cap on, hood dryer, twenty to thirty minutes. The dryer is pre-heated. Test curls through the cycle, at the interval the manufacturer specifies for that product — current professional systems disagree with each other about when the first check should be, so there is no universal number to quote.
6. Rinse, blot, neutralise. A thorough rinse, careful blotting of every rod, then the oxidiser to close the bonds in their new positions. See neutralising a perm.

Expect two to three hours in the chair. The dryer phase is the part clients forget to allow for when they book.

The occupational allergy, told straight

This is the part of the story that consumer guides leave out, and it is the reason the category faded. GMTG is a potent delayed-hypersensitivity allergen, and the people it sensitises are not clients. They are hairdressers, who meet it repeatedly, week after week, for years.

The first paper identifying it as an occupational sensitiser in hairdressers was published in Contact Dermatitis in November 1981, and a run of studies through the 1980s filled in the picture. Three findings from that literature are worth having in plain words:

The contrast that makes the point: ammonium thioglycolate, the alkaline cold wave active, had been in use since 1943 in homes and in salons with no clearly documented cases of contact allergy. GMTG entered American salons only in 1973 and had a literature within eight years.

GMTG was withdrawn from the German market in about 1992 — a voluntary market withdrawal by the industry, not a ban.

The distinction matters and is routinely got wrong, including by sources that date it to the mid-1990s. It was a commercial decision, taken in the face of the dermatology data, and German dermatology clinics tracked hairdressers' positive patch-test rates falling steadily from that point. In Britain and the United States GMTG remains perfectly legal, sold professionally, and used under strict handling protocol. Your risk as a client is very low; the risk was always occupational.

What you get, and how long you keep it

A soft, loose, natural-looking wave, silkier and shinier than an alkaline perm, sitting closer to good natural texture than to a set. It is the most flattering perm result on fine hair there is, and it is genuinely kind to a tint.

It drops noticeably after three or four months. Not gradually and not gracefully — clients tend to notice a particular fortnight in which it stopped being a wave. Fewer bonds were rearranged, so there is less structural memory holding the shape, and once the hair reasserts itself it does so fairly decisively. There is no growing-out line to speak of, which is the compensation.

Because the wave is weak, keep the weight off it. Mousses and thin curl fluids, not creams and oils; a heavy product will drag a soft acid wave straight in one application. And the first seventy-two hours are the same as for any perm: nothing tied, clipped or pinned and no hard brushing, because oxidation carries on in air and the fibre keeps hardening for about three days. After that, follow the washing instruction for the product actually used on you.

Questions