Anonymous wanted to know what it means when one says that a foundation “oxidizes”.
Now, I’m no chemist (skipped Chemistry all together in school), but a simple answer is that the foundation changes color (usually turning darker or orange) when it interacts with the oils in your skin.
For a more in-depth answer, this is what I understand of the process. Oxidation occurs when an oxidative agent interacts with another substance, removing electrons from that substance. Somehow, this reaction causes a change of color. Oxidation is also responsible for the rusting of metals, which is the closest I can liken the process that foundation undergoes.
So, in this case, the oils (and possibly the oxygen present in the air) are the oxidative substances. Therefore, people with oily skin experience this product more more often than those with dry (trust me, I’ve been the victim of this many a time… it’s not fun). Quite a few seemingly-good foundations have had to be returned because of this issue. I’m not at all sure what makes some foundations oxidize and others not, though I’m certain it must be the presence of some ingredient or the ratio of one ingredient to another.