Red heat

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Thermal radiation in visible light can be seen on this hot metalwork. Hot metalwork.jpg
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.

Contents

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. Wiens law.svg
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]

ColourTemperature [°C]Temperature [°F]
FromToFromTo
Black red [note 1] 4265937991,100
Very dark red5947041,1001,299
Dark red7058141,3001,497
Cherry red8158701,4981,598
Light cherry red8719811,5991,798
Orange9821,0921,7991,998
Yellow1,0931,2581,9992,296
Yellow white1,2591,3142,2972,397
White1,315+2,397+

Stirling

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

ColourTemperature [°C]Temperature [°F]
Red: Just visible525977
Dull red6991,290
Dull cherry red8001,470
Full cherry red9001,650
Clear cherry red1,0001,830
Deep orange1,1002,010
Clear orange1,2002,190
White heat1,3002,370
White bright1,4002,550
White dazzling1,5002,730

See also

Notes

  1. When viewed in dull light.

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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.