Red heat

For other uses, see Red heat (disambiguation).
Thermal radiation in visible light can be seen on this hot metalwork.

The practice of using colours to determine the temperature of a piece of (usually) ferrous metal comes from blacksmithing. Long before thermometers were widely available, it was necessary to know what state the metal was in for heat treating it and the only way to do this was to heat it up to a colour which was known to be best for the work.

The peak wavelength and total radiated amount vary with temperature according to Wien's displacement law. Although this shows relatively high temperatures, the same relationships hold true for any temperature down to absolute zero. Visible light is between 380 and 750 nm.

Chapman

According to Chapman's Workshop Technology, the colours which can be observed in steel are:[1]

Colour Temperature [°C] Temperature [°F]
From To From To
Black red[note 1] 426 593 799 1,100
Very dark red 594 704 1,100 1,299
Dark red 705 814 1,300 1,497
Cherry red 815 870 1,498 1,598
Light cherry red 871 981 1,599 1,798
Orange 982 1,092 1,799 1,998
Yellow 1,093 1,258 1,999 2,296
Yellow white 1,259 1,314 2,297 2,397
White 1,315+ 2,397+

Stirling

In 1905, Stirling Consolidated Boiler Company published a slightly different set of values:[2]

Colour Temperature [°C] Temperature [°F]
Red: Just visible 525 977
Dull red 699 1,290
Dull cherry red 800 1,470
Full cherry red 900 1,650
Clear cherry red 1,000 1,830
Deep orange 1,100 2,010
Clear orange 1,200 2,190
White heat 1,300 2,370
White bright 1,400 2,550
White dazzling 1,500 2,730

See also

Notes

  1. ^ When viewed in dull light.

References

  1. ^ Chapman, W. A. J. (1972). Workshop Technology, Part 1 (5th ed.). Burlington, MA: Elsevier Butterworth-Heinemann. ISBN 978-0713132694.
  2. ^ A Book of Steam for Engineers. Stirling Consolidated Boiler Company. 1905. p. 50. ASIN B006RXDG3W.