Most of us tend to think of color as residing in pigment or dye. Iron oxide in common rust is a reddish pigment; white lead is a white pigment and so on. The very thin surface layers that cause case colors are indeed very thin as noted above by Rocketman. He is right when he says that the colors are caused by the thickness of the surface layers, but it isn't a build-up of pigment that does it, it's wavelength interference.

Light passes through the surface layers, hits the substrate and is reflected back. Specific wavelengths are interfered with; others pass through and which are interfered and which pass depends upon:

1. the thicknes of the surface layer
2. the refractive index of the surface layer

I have no idea if UV or other energy will change the refractive index of the surface layers but it indeed possible as Clapper noted. The light doesn't have to change the thickness (or amount) of surface layer, it would just has to change its refractive index.

When we think of terms of pigment it is obvious that a heavier layer will be more opaque or darker, but the colors resulting from interference are generated by a different process entirely.

Think blue jays (the bird). There is no blue pigment in their feathers.

Then think the colors (blue to yellow) that you see on the surface of camera lenses. These too are interference colors generated by the anti-reflection coating on the surface (magnesium fluoride etc). A magnesium fluoride coating 1/4 wavelength thick will look blue in reflected light.

The rainbow colors generated by a spreading oil droplet on a puddle of water is the same phenomenon - the colors are not intrinsic to the oil; they are generated by interference of the light reflected through the oil from the oil/water interface.

The holy grail in case coloring is finding a non-thermal chemical way to create the surface coatings of proper refractive index. Then one could renew case colors without risking the integrity of the metal.

Note added in edit: Worn case color seems to become grayish - that is consistent with physical thinning. This doesn't represent "fading" in the common sense, it is the result of a shift in wavelength interference. In physical thinning the refractive index of the layer stays the same but its thickness decreases. Thickest layers will produce blue interference; then as the thickness decreases we see purple, red, gold, silver and finally grey. For a layer with an index of refraction of 1.5 we see blue at about 220 millimicrons, grey at <60. Note that the index of refraction hasn't changed. If photreactivity changes the refractive index then that would be a separate mechanism whereby perceived color would change.

Last edited by Gnomon; 06/04/13 06:32 AM.