In the wetshaving community "butterscotch" shaving brushes became sort of a sought-after commodity. They're basically plastic brushes originally meant to mimic

In the wetshaving community "butterscotch" shaving brushes became sort of a sought-after commodity. They're basically plastic brushes originally meant to mimic ivory in color that have oxidized to a sort of butterscotch color. Originally there was a lot of interest in restoring them to their original ivory-like color. If you search there's lots of advice on this, like exposing them to sunlight, or using UV lights and/or peroxides. Eventually though I think there got to be an interest in the aged color, and interest in restoring the color declined. I'm not a dentist, but I have family who are, and they explained similar processes are involved in teeth whitening. Basically a peroxide gel that gets put on the teeth with a custom fitting, and then exposed to UV light.