Naked Science Forum

General Science => General Science => Topic started by: katieHaylor on 02/07/2018 09:15:42

Title: How does temperature affect the colour of metals?
Post by: katieHaylor on 02/07/2018 09:15:42

Tobey asks:

Why do metals turn red when hot, and why do some metals, like titanium, stay colored after they cool?

Can you help?

Title: Re: How does temperature affect the colour of metals?
Post by: RD on 02/07/2018 11:01:23

Quote from: katieHaylor on 02/07/2018 09:15:42

Tobey asks:
... why do some metals ... stay colored after they cool?

The colours are due to oxides which have formed during the heating process : the metal has reacted with the oxygen in the air ...

(https://upload.wikimedia.org/wikipedia/commons/thumb/8/89/Tempering_standards_used_in_blacksmithing.JPG/800px-Tempering_standards_used_in_blacksmithing.JPG)
https://en.wikipedia.org/wiki/Tempering_(metallurgy)#Tempering_colors

Title: Re: How does temperature affect the colour of metals?
Post by: evan_au on 02/07/2018 11:43:28

Part of the color change is black-body radiation - as an object gets hotter, it tends to emit more energy at higher frequencies, so a yellow glowing piece of metal is hotter than the same piece of metal glowing a dull red.
But this color change goes away after the metal cools down.
See: https://en.wikipedia.org/wiki/Red_heat

Part of the color change is chemical. At higher temperatures, chemical reactions like oxidation (or "rusting") to occur much more quickly. Heating steel wool in a flame can can quickly produce a discolored layer of rust on the surface.
This color change remains after the metal cools down.

Titanium naturally forms a thin, clear protective oxide layer that inhibits further oxidation.

However, under that protective layer, titanium is still a very reactive metal - it is even able to react with nitrogen at high temperatures; we normally consider nitrogen to be a fairly inert gas.

Maybe the color change you see is the reaction product with some other substance at high temperatures?
See: https://en.wikipedia.org/wiki/Titanium#Chemical_properties

Title: Re: How does temperature affect the colour of metals?
Post by: Bored chemist on 02/07/2018 11:48:45

The colours of the oxide film are not because the oxide is coloured.
They are more like the colours you get from thin films of oil on water.

Title: Re: How does temperature affect the colour of metals?
Post by: chris on 04/07/2018 17:45:26

Quote from: Bored chemist on 02/07/2018 11:48:45

The colours of the oxide film are not because the oxide is coloured.
They are more like the colours you get from thin films of oil on water.

Why does the temperature affect the colour?

Title: Re: How does temperature affect the colour of metals?
Post by: Bored chemist on 04/07/2018 17:49:55

The higher the temperature (and, to an extent, the longer you keep it hot) the thicker the oxide layer gets.

Title: Re: How does temperature affect the colour of metals?
Post by: chris on 04/07/2018 18:32:55

Thanks - I suspected that would be the case but thought I'd better check...