Red heat

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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 visible color of an object heated to incandescence (from 550°C to 1300°C)
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[2] 426 593 799 1100
Very dark red 594 704 1100 1299
Dark red 705 814 1300 1497
Cherry red 815 870 1498 1598
Light cherry red 871 981 1599 1798
Orange 982 1092 1799 1998
Yellow 1093 1258 1999 2296
Yellow white 1259 1314 2297 2397
White 1315+ 2397+

Stirling[]

In 1905, Stirling Consolidated Boiler Company published a slightly different set of values:[3]

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[]

References[]

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