Red heat

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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[edit]

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] 426 593 709 1010
Very dark red 593 704 1010 1210
Dark red 704 814 1210 1408
Cherry red 815 870 1409 1508
Light cherry red 871 981 1510 1708
Orange 981 1092 1708 1908
Yellow 1093 1258 1910 2207
Yellow white 1259 1314 2209 2308
White 1315+ 2309+

Stirling[edit]

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[edit]

References[edit]

  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.