Tonight after reading on foundation oxidation and seeing so many contradictions, I coerced (lovingly) my husband to give me his hypothesis and had him help me round up some at home chemicals with different properties and pH's to do a casual experiment.
First of all, Brad's hypothesis was that the pigment was intensifying as it dried, so there was no chemical change, just that the fluid was evaporating, leaving a more concentrated product and therefore becoming darker or more orange.
Now, I didn't believe this entirely because it seemed to darken and to me that's a chemical change. Usually oils and/or adding liquid "intensifies" a pigment, which MUA's do all the time with eyeshadow.
This first lazy experiment I did on my own as I knew quite a few people would mention how oily skin may change the color. While some probably do, I have quite dry skin, so that wouldn't make sense with my personal situation.
What I did was add green tea seed oil to a petri dish and mixed it with the foundation (green tea "seed" oil is clear and won't add color).
Now, in this photo, the swab on the left looks slightly darker. At this point I was starting to think oil would change the overall color.
I swatched the two on white paper and I couldn't detect a difference in color.
In this next casual experiment, we took bleach, vinegar, a facial toner with alcohol, and Drano.
First of all, we took bleach and dropped it on the foundation as bleach is an intense oxidant. Second, toner or Sea Breeze was used, which contains alcohol-40. Thirdly, we used vinegar to see if it'd change the pH to a more acidic level. Lastly, we took Drano, which is very basic or high in pH (sodium hydroxide, a caustic substance).
Nadda, ZILCH, Zero, None of the foundation droplets changed in any dramatic manner, whether it be more orange or darker.
The only thing I noticed (slightly) was the Sea Breeze toner and as you know Alcohol can cause liquid to evaporate more easily.
Conclusion. The only test which results in a darker or more orange foundation was the fact it DRIED. It had nothing to do with pH.
Top swatch is after ten minutes and bottom is around one. This is the only REAL swatch difference I see, and that is after it set/dried for awhile.
May I add that the ingredients in this foundation are extremely incredible for the fact it didn't react to ANY of these chemicals, harsh and all (completely inert). Interesting stuff and completely unexpected!
I am now coining this phenomenon, "the oompa loompa dry down"