Gnomon, I believe the ability of an iron oxide layer to interfere with light wavelength, refract, etc. to cause different colors to appear is well known.
But, as Doctor Gaddy pointed out, the example of an optic lense or drop of oil might not be good analogies to gun finish colors. Those examples are true optic occurrences, the color changes as the viewing angle changes because the light coming into the eye has passed through a slightly different thickness of oil or lense coating.
Gun cases colors don't change to any appreciable amount when it's viewed at different angles.
ClapperZ, if someone had a kiln what easy experiment would you be considering to show the variety, complexity, reactivity and instability of gun type case colors. Does oxygen have an energy favorable drive to react with iron oxide that's already formed on the surface of a gun.
