Glycerin

Glycerin is the backbone of every fat. Triglycerides, the fats and oils of animals and plants, are three fatty acids attached to one glycerol molecule, and splitting them apart releases it. Soap works were producing it in quantity long before anyone noticed it was there.^[1]

In a cosmetic it is a humectant: it holds water. It is cheap, it has been in continuous use for two centuries, and it appears on more labels than almost anything else.

Glycerol is propane-1,2,3-triol: three carbons in a row, each carrying a hydroxyl group, and nothing else. Those three hydroxyls account for everything it does. They hydrogen-bond to water, which makes glycerol hygroscopic, drawing water vapour out of the air and holding it, and miscible with water in all proportions. They also hydrogen-bond to each other, which is why the liquid is syrupy.^[2]

Commercially it comes from splitting fats and oils, historically tallow and now mostly vegetable oils, and very large quantities are recovered as a by-product of biodiesel manufacture. A synthetic route from propylene exists but supplies little of the market. The plant origin recorded for this entry follows that dominant present-day feedstock; the tallow of the soap works, which is where the history below begins, was an animal one.

Carl Wilhelm Scheele obtained glycerol in 1779 by heating olive oil with litharge, a lead oxide. A sweet syrup was left behind. He recognised it as a common constituent of fats and oils generally and called it the sweet principle of fats. The name it ended up with, glycerine, was built from the Greek glykys, sweet.^[3]

Two subsequent developments made it abundant. Nitroglycerin, and then dynamite, gave glycerol a large industrial market and made it scarce for everything else in wartime. Industrial soap-making supplied it steadily as a by-product. Glycerine preparations for chapped and roughened skin were a staple of nineteenth-century commercial beauty culture, and glycerine and rose water survived as a recognisable product well into the twentieth.^[4]

Glycerin holds water in the stratum corneum, the dead outer layer of skin whose flexibility depends on its water content. Dry stratum corneum cracks; hydrated stratum corneum bends.

The mechanism is not purely passive. Glycerol is one of the skin's own natural moisturising factors, and it is carried into the epidermis by aquaporin-3, a membrane channel that transports glycerol as well as water. Mice lacking aquaporin-3 have about a third of the normal glycerol in the stratum corneum, along with poor hydration, reduced elasticity and slow barrier repair, and supplying glycerol corrects all three.^[5] That is an animal model rather than a claim about a face cream, but it establishes that glycerol has a physiological role in skin and is not merely sitting on it.

The second function on this page is solvency, and it is a real one rather than a label courtesy. Because glycerol hydrogen-bonds so freely it dissolves a great deal that water alone will not hold in solution: many botanical extracts are prepared in a glycerin and water mixture rather than in alcohol, and the glycerite that results is what an ingredient list calls an extract. The same property lets it carry actives into a water phase that would otherwise reject them.

A frequent assertion holds that in very dry air glycerin reverses and pulls water out of the skin. It is repeated widely and supported thinly. In practice glycerin is used alongside an occlusive, which holds whatever water is present in place.

Glycerin is not listed among the substances prohibited or restricted by the annexes to Regulation (EC) No 1223/2009, and is in ordinary cosmetic use in the European Union.^[6] Its one consequential appearance in regulatory history is as the substance that was not used.

In 1937 the S. E. Massengill Company of Bristol, Tennessee dissolved sulfanilamide in diethylene glycol to make a raspberry-flavoured liquid medicine. Diethylene glycol is sweet, syrupy and cheap, and resembles glycerol in the respects that mattered to the formulator. It is also a potent kidney poison. More than a hundred people died, many of them children, and accounts differ on the exact number.^[7]

No law then required a manufacturer to establish that a product was safe before selling it. The Federal Food, Drug, and Cosmetic Act of 1938 followed directly.^[7] Among the provisions that Act introduced was one extending federal control to cosmetics and therapeutic devices, which had stood outside it until then.^[8]