The color of metals
Automated disclaimer: This post was written more than 15 years ago and I may not have looked at it since.
Older posts may not align with who I am today and how I would think or write, and may have been written in reaction to a cultural context that no longer applies. Some of my high school or college posts are just embarrassing. However, I have left them public because I believe in keeping old web pages alive—and it's interesting to see how I've changed.
Why are metals always gray or yellow? Why not other colors, like red, or green, or blue?
Markus Wolff says:
Because then all those car paintshops would be out of business.
Tim McCormack says:
:-P Nice one.
Then again, would they? I mean, if you want a nice golden sheen to your car, you don't have the body made of gold or brass.
Neosamurai85 says:
I want a car with a body of brass... so it can turn green and become the "Moss Mobile." It would be the greatest dirty pimp wagon in the world!
I'd pimp elves in it.
Tim McCormack says:
I bet a copper-plated car would be cool. The metal would slowly turn from orange to green, with streaking. It would turn out looking like an amazing custom paint job!
anon says:
(i) When light falls on a metal, electrons below the fermi level absorb the energy and are excited to higher levels.
(ii) So, there's a negative electron at a higher energy level and a positive "hole" at the lower energy level.
(iii) Since metals are good conductors of electricity (ie, the valance band and conduction band overlap), alternating currents are induced at the surface due to the electron-hole pairs.
(iv) These currents cause the metallic color, when the electrons fall back to the original energy level and remit the light.
(v) If all colors are absorbed and emitted equally well, you get the metallic, shiny color. For other metals, the ability to absorb and remit varies with color (ie, energy). The yellowish and reddish colors occur because there is a poorer efficiency in this absorption and emission at the blue end of the spectrum in those metals.
Tim McCormack says:
Very interesting! This is the first explanation I've seen regarding this phenomenon. (It's easy to find explanations, though longer, of parts i-iv, but I've never seen part v before.)
I suppose the question now is why some metals have poorer energy conversion efficiency with higher-frequency photons. :-) Seriously, though, I see how differences in absorption/emission efficiency would occur with higher and lower frequency photons, and how that would result in a bias in the observed colors of metals. (I suppose this would have something to do with the black-body radiation graph that I vaguely remember from physics class.)
Wim says:
Hi gentlemen, I came across some steel alloy which I would like to identify. It was used in the center hub of a small steering wheel. It had a light yellow color, almost like brass, but whiter, and it was incredible hard. I mean a file slipped right over it without bite. I finally could lathe it with a cutting bit slightly harder than that and the chips sparked off and turned dark blue hot. It was magnetic. What steel could that be?