Re: [Phys-L] Blue color heated steel

[John Denker <jsd@av8n.com>]

On 10/4/21 7:25 AM, M R Carabajal via Phys-l wrote:

> I would greatly appreciate a bibliographic indication on the subject of
> heating steel. I have not been able to find a clear explanation of the
> reason for the blue color, that steel presents when heated to about 600
> Kelvin. That is, to substantiate the deviations of the blackbody heating
> spectrum.

The question appears to be based on multiple false assumptions.

First of all, a black body at 600 K is not blue. It is not even
red. Insofar as it has a black body color at all, it is infrared.
Far infrared. And it would be imperceptibly dim, even if the
color were perceptible at all. Bibliographic reference: Anything
ever written about the black body spectrum. For example, search
for the term "Draper point" here:
  https://en.wikipedia.org/wiki/Black-body_radiation

One occasionally hears that steel presents a blue appearance at
600 degrees. However, that has nothing to do with black bodies.
It is blue in the same way that a blue butterfly is blue. It
reflects the blue component of ambient light. Unlike black body
radiation, it does not glow in the dark.

*AND* it is 600 degrees F, not 600 K.

The color is attributed to the formation of a film of oxide.
Reference:
  https://www.sizes.com/materials/colors_of_heated_metals.htm
and references therein.

I imagine the physical basis of the blue color is interference,
i.e. a half-wave interference layer. Reference: Anything on
Newton's rings or AR coatings.

As to why the thickness of the oxide film should have this
particular dependence on temperature, I don't know. I imagine
it depends on the chemical kinetics of oxygen and iron and
maybe carbon. The details would require considerable depth
of specialized knowledge.