Apparently, this is totally a common issue with certain vitamin C serums, and I made it my day's work to get to the bottom of why it happens. There are scientific reasons the serums go from being clear to having an orange-brown tint: Over time, ascorbic acid oxidizes into erythrulose, according to cosmetic chemist Michelle Wong of LabMuffin—which is a key ingredient in self-tanners.
"Erythrulose is an ingredient in fake tans, and is often included alongside the more commonly found dihydroxyacetone or DHA," notes Wong in a post on her site. "These ingredients react with proteins in the dead stratum corneum layer of your skin to produce brown compounds called melanoidins that stay on your skin until the dead cells come off (after around a week)."
Don't worry, though—this is not to say you have to ditch the buzzy and effective skin-care ingredient completely. You've just got to ensure you've got a fresh vitamin C serum, and are using it correctly. First up? According to Wong, look for a vitamin C that's more oxidation-resistant (AKA not an ascorbic acid, which she says is the most unstable). Instead, Wong says to look for vitamin C listed as the following variations: magnesium ascorbyl phosphate, sodium ascorbyl phosphate, ascorbyl tetraisopalmitate, or tetrahexyldecyl ascorbate (*opens Notes app immediately*).
It's also helpful to prevent the oxidation in the first place, which Wong says can be done by purchasing a vitamin C serum that also contains vitamin E and ferulic acid, which ups the antioxidant amount, and to use the serum at night and away from direct sunlight. We've even heard rumors of people storing their vitamin C serums in the refrigerator to preserve it as much as possible.
Your application method can help with the staining dilemma, too—Wong recommends applying your serum evenly over your entire face. "Apply the serum as evenly as possible and regularly exfoliate your face so that the tan fades evenly," she says. "If you notice streaking, increase the waiting time between applying your vitamin C serum and your next product." Then, after application, she advises to wash your hands (since they tend to have more dead skin cells than your face). "Just wash with soap after applying the vitamin C serum." Now nothing can stand in the way of that glow.
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