Letter Re: A Useful Heated Steel Color Emission Chart

Dear Jim,
That chart that you posted on Saturday only applies to steel alloys. At a given temperature, oxidation will show those colors. The brighter colors are incandescent colors that are emission, not absorption colors. This is a good reference for heat treating metal, but only works if you know the alloy in question.

It is actually best to use that chart on overcast days or in shadow. Bright sun will generally cause one to excessively heat in the incandescent range, leading to red-shortness, cracks, decarburizing and burning. – Michael Z. Williamson