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Aluminum and titanium form clear oxide layers. Aluminum oxide layers
quickly become too thick to show any thin film interference. But titanium
can be controlled carefully and can produce any color (almost, thin films
fail at a few).
But if you are machining steel stock and get it too hot, it always turns
blue. Either it produces a layer of clear oxide that has a very specific
thickness, or there is another explanation. Iron oxides are usually red,
yellow, or black. None of those suggests clear, or blue.
On Mon, Oct 4, 2021 at 10:20 AM John Denker via Phys-l <
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
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:
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.
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.
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