What Is Actually Happening to the Pigment
Foundation oxidation is a chemical reaction between the iron oxides in your foundation's pigment and the sebum, sweat, and oxygen on your skin's surface. The oils your skin produces break down the pigment molecules over time, shifting their colour toward orange and brown. This is not the foundation drying down to its true shade, which happens in the first five minutes. Oxidation is a slower drift that continues for an hour or more after application.
