Encyclopedia · chemical treatment
Ammonium thioglycolate
The classic reducing agent in perms and thio relaxers; it breaks hair's disulfide bonds so the shape can change, with some loss each time.
What it is
The salt of thioglycolic acid, used in permanent waves, thio straighteners and depilatories. It is a regulated cosmetic ingredient in the US, EU and UK.
How it works
Thioglycolate reduces disulfide bonds to thiols, and a neutraliser (usually peroxide or bromate) oxidises them back. Raman work shows reduction lowers the disulfide signal, high-temperature stress relaxation after reduction speeds up disconnection, and the oxidation step raises cysteic acid in the cuticle region. Kuzuhara 2016 About 20% of cystine is reduced and largely recovered in a normal perm, according to a review of earlier work (see bond builders).
What the studies show
- Raman spectroscopy of waved hair: reduction lowered disulfide content, high-temperature relaxation worsened it and oxidation raised cysteic acid. Kuzuhara 2016
- On hair swatches, sodium hydroxide, guanidine hydroxide and ammonium thioglycolate each gave about 2.5 ug/g protein loss versus 1.12 in untreated hair, and glyoxylic acid gave 3.5. Break force was similar across products. Velasco 2022
How it's used in studies
A salon perm typically uses reduction for 10-20 minutes followed by a neutraliser. This is practice norm, not taken from a cited study.
Side effects and safety
Strong smell, scalp irritation and burns if left on too long, and breakage on bleached hair. Allergic contact dermatitis has been reported in hairdressers. Do not perm bleach-damaged hair.
Bottom line
It works as designed but is a damaging process, especially stacked on bleach. Skip it on already compromised hair and do a strand test first. See relaxers, perms and smoothing.
Studies 2
Kuzuhara A, Hori T. Internal structural changes in keratin fibres resulting from combined treatments of waving and stress relaxation: Raman spectroscopy. Int J Cosmet Sci. 2016
Result: High-temperature stress relaxation after reduction sped up disconnection of disulfide bonds, and the oxidation step raised cysteic acid in the cuticle and outer cortex.
Read the study ↗Velasco MVR et al. Impact of acid (Progressive Brush) and alkaline straightening on the hair fiber. Int J Trichology. 2022
Tested: sodium hydroxide, guanidine hydroxide, thio and glyoxylic acid straighteners
Result: Alkaline relaxers and thio raised protein loss to about 2.5 ug/g versus 1.12 untreated; glyoxylic acid reached 3.5; break force was similar across products.
Read the study ↗Related
- Hydrogen peroxideThe oxidiser in every bleach and permanent dye; it lifts colour by destroying melanin but also oxidises the hair's own proteins, so damage rises with strength and repeat use.
- Sodium hydroxide and guanidine relaxers (lye and no-lye)Chemical relaxers permanently straighten tightly curled hair; they work, but are linked to scalp burns, breakage and cancer associations still being investigated.
- Relaxers, perms and smoothing treatmentsChemical straighteners and perms all damage hair, and some carry scalp, respiratory, kidney or cancer-association concerns worth knowing before you book.