Oxidation of Silver
Recently I made a sterling silver pendant (a replica based on classic Rick Owens pentagram) and I noticed the appearance of iridescent colors when oxidizing it, from light yellow to golden to a mixture tones of red violet and teal to black eventually.
It turns out to be the result of Thin Film Interference, which also accounts for the phenomenon of iridescent soap bubbles. Wikipedia has explained this pretty well. To keep it short here, a thin oxide layer is gradually thickening when the silver is heated, forming two reflection surfaces (air-oxide interface and oxide-silver interface). The thickness of the oxide layer further determines optical path difference (as straightforwardly calculated on Wikipedia) for either constructive interference (enhance a certain color) or destructive interference (diminish a certain color). Therefore, a changing of colors is observed when oxidizing the silver.
Intuitively, shorter wavelengths should be observed before longer ones since they require smaller OPD for constructive interference. But wait! Shouldn’t that mean violet and blue tones are supposed to appear earlier than other colors? I’m pretty sure I saw the light yellow color first. Unfortunately, I cannot find a definitive explanation to it but I have some theories.
- According to the condition of constructive interference, OPD needs to be equal or very close to the multiple of the wavelength. So it might be violet and blue cover less wavelength range so there might be cooler hues but they diminish fast. However, this is apparently wrong because the wavelength of violet and blue ranges roughly from 400-500 nm while yellow only spans 560-600 nm.
- Human eyes are much more sensitive to yellow-green or similar hues so that the violet and blue light actually appears but they are less noticeable.
- Sterling silver contains 7.5% copper which is positioned before silver in view of metal reactivity series and might account for the light yellow color at first. I personally would go with this hypothesis at the moment.
Hopefully I’ll find a concrete explanation to complete this section in future.
Anyway, I had fun cutting fingers :( silver when making this pentagram and will make more accessories when I have time.