Oxidation during sintering process

[wwdragonrider]

I am drawing a blank, what are the colors associated with Oxidation during the different phases of iron sintering. I remember the colors are Blue, Matte, Gray, Straw, and frosted but I cant remember the phases or causes for each color.

 

[mcguire]

The different colors are from different thicknesses of oxide film. They are interference colors, like oil on water. The difference in color doesn't come from different phases. The darker the color, the thicker the oxide layer.