A chemical depilatory is a cosmetic preparation used to remove hair from the skin. Common active ingredients are salts of thioglycolic acid and thiolactic acids. These compounds break the disulfide bonds in keratin and also hydrolyze the hair so that it is easily removed. Formerly, sulfides such as strontium sulfide were used, but due to their unpleasant odor, they have been replaced by thiols.
The main chemical reaction effected by the thioglycolate is:
- 2 HSCH2CO2H (thioglycolic acid) + R-S-S-R (cystine) → HO2CCH2-S-S-CH2CO2H (dithiodiglycolic acid) + 2 RSH (cysteine)
Chemical depilatories contain 5–6% calcium thioglycolate in a cream base (to avoid runoff). Calcium hydroxide or strontium hydroxide maintain a pH of about 12. Hair destruction requires about 10 minutes. Depilation is followed by careful rinsing with water, and various conditioners are applied to restore the skin's pH to normal. Depilation does not destroy the dermal papilla, and the hair grows back.
Depilatory ointments, or plasters, were known to Greek and Roman authors as psilothrum. In Jewish lore, King Solomon is said to have discovered a chemical depilatory made from a mixture of lime and water and orpiment (arsenic trisulfide).
- "Hair Preparations", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), 2007, doi:10.1002/14356007.a12_571.pub2
- Arza Seidel; et al., eds. (2013), Kirk-Othmer Chemical Technology of Cosmetics, p. 44
- "PSILOTHRUM", Dictionary of Greek and Roman Antiquities (3rd ed.), 1890
- Colloquy of the Queen of Sheba, taken from the Aramaic Targum Sheni of Megillat Esther