What it is
Magnesium sulfate is a salt with the formula MgSO4, made of magnesium cations and sulfate anions; its familiar heptahydrate form (MgSO4·7H2O) is known as Epsom salt. In cosmetics it works as a viscosity controller and bulking agent and, importantly, as an emulsion stabilizer in water-in-oil emulsions, where water droplets are dispersed in a continuous oil phase. In water-free formulas it can even generate heat through an instant reaction with water. Within sunscreens it serves these formulation roles and is not a UV filter.
Safety & warnings
Magnesium sulfate is the same salt long used in bath soaks and is generally regarded as a gentle, well-tolerated cosmetic ingredient at the small levels used to control texture or stabilize emulsions. Public references do not flag notable irritation or allergen concerns for typical cosmetic use; sensitive skin types can patch test new products as a precaution.
Origin & sourcing
Magnesium sulfate occurs naturally as the mineral epsomite and in dry lake beds and other natural deposits. It can be obtained from these natural sources or made by reacting magnesium carbonate or magnesium oxide with sulfuric acid, and it can also be derived from seawater-based magnesium.
History
The heptahydrate takes its common name Epsom salt from a bitter saline spring in Epsom, Surrey, England, where the salt was produced from springs arising where the porous chalk of the North Downs meets the impervious London clay.
Questions everyone asks
Is this the same as Epsom salt?
Yes. Magnesium sulfate's common heptahydrate form is Epsom salt, named after the springs at Epsom in England.
What does it do in a sunscreen?
It is a formulation aid used to adjust thickness and to help stabilize water-in-oil emulsions. It is not a UV filter and provides no sun protection.
Is it safe for the skin?
It is a familiar mineral salt used in bath products and is generally well tolerated at cosmetic levels. Patch test if your skin is very sensitive.