What it is
Glycerin is a colorless, odorless, sweet-tasting viscous liquid with the formula C3H5(OH)3, carrying three hydroxyl groups that make it fully miscible with water and strongly hygroscopic. Those hydroxyl groups let it bind water and pull moisture toward the skin, which is why it acts as a humectant. It is also a normal component of the skin's own moisturizing system, and research on cosmetic formulas links higher glycerin content with better measured skin hydration. In sunscreens it commonly supports the texture and feel of the base while helping offset the drying tendency of some other ingredients.
Safety & warnings
Glycerin is among the most widely tolerated cosmetic humectants and is generally suitable for sensitive and acne-prone skin, with cosmetics references noting it can even help protect against irritation from over-washing. The main practical caveat is that very high or near-pure concentrations applied in a dry environment can, in principle, draw water from deeper skin layers rather than from the air, so finished products use it at moderate levels balanced with other ingredients.
Origin & sourcing
Glycerin is found naturally in plant and animal fats, where it forms the backbone of triglycerides. It is most often recovered as a co-product when fats and oils are saponified or hydrolyzed, for example reacting a triglyceride with sodium hydroxide to yield glycerol plus soap. It can also be made synthetically from propylene, via allyl chloride and epichlorohydrin that is then hydrolyzed to glycerol, a route that grew in importance during the twentieth century.
History
The compound was first obtained by the Swedish chemist Carl Wilhelm Scheele while working with fats and olive oil. The French chemist Michel Eugene Chevreul later coined the name glycerine, around 1811, to replace the older description sweet principle of fat; the term entered English use in the following decades, and the form glycerol became standard in the twentieth century.
Questions everyone asks
Does glycerin moisturize the skin?
Yes. It is a humectant that attracts and holds water, and cosmetic studies link higher glycerin content in a formula with better measured skin hydration.
Is glycerin natural or synthetic?
It can be either. It occurs naturally in plant and animal fats and is often recovered from them, but it can also be made synthetically from propylene.
Is glycerin safe for sensitive or acne-prone skin?
It is generally well tolerated and non-comedogenic, and cosmetics references describe it as suitable for most skin types, including sensitive skin.