In blacksmithing, red heat is the practice of using colours to determine the temperature of a piece of metal (usually iron or steel). 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.
Chapman According to Chapman's Workshop Technology, the colours which can be observed in steel are:
Colour Temperature [°C] Temperature [°F] From To From To Black redWhen viewed in dull light. 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:
Colour Temperature [°C] Temperature [°F] Red: Just visible Dull red Dull cherry red Full cherry red Clear cherry red Deep orange Clear orange White heat White bright White dazzling
See also Black-body radiation Color temperature Incandescence
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