{{Short description|Practice of using colours to determine the temperature of metal}} {{Other uses}} {{Multiple issues| {{More citations needed|date=April 2025}} {{Cleanup rewrite|date=April 2025}} {{Context|date=April 2025}} }} thumb|right|Thermal radiation in visible light can be seen on this hot metalwork. 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. [[File:Wiens law.svg|300px|thumb|right|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:<ref>{{cite book|last1=Chapman|first1=W. A. J.|title=Workshop Technology, Part 1|edition=5th|year=1972|publisher=Elsevier Butterworth-Heinemann|location=Burlington, MA|isbn= 978-0713132694 }}</ref>
{| class="wikitable" |- ! rowspan="2" | Colour ! colspan="2" | Temperature [°C] ! colspan="2" | Temperature [°F] |- ! From ! To ! From ! To |- | Black red<ref group=note>When viewed in dull light.</ref> | 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 | colspan="2" | 1,315+ | colspan="2" | 2,397+ |}
==Stirling== In 1905, Stirling Consolidated Boiler Company published a slightly different set of values:<ref>{{cite book |title=A Book of Steam for Engineers |publisher=Stirling Consolidated Boiler Company |year=1905 |url=https://archive.org/stream/stirlingbookonst00stirrich |asin=B006RXDG3W|page=50}}</ref> <!-- Note, these values are COPYRIGHT SAFE coming from a text published in 1905. --> {| class="wikitable" |- ! Colour ! Temperature [°C] ! Temperature [°F] |- | Red: Just visible | {{convert|977|°F|°C|disp=table|order=flip}} |- | Dull red | {{convert|1290|°F|°C|disp=table|order=flip}} |- | Dull cherry red | {{convert|1470|°F|°C|disp=table|order=flip}} |- | Full cherry red | {{convert|1650|°F|°C|disp=table|order=flip}} |- | Clear cherry red | {{convert|1830|°F|°C|disp=table|order=flip}} |- | Deep orange | {{convert|2010|°F|°C|disp=table|order=flip}} |- | Clear orange | {{convert|2190|°F|°C|disp=table|order=flip}} |- | White heat | {{convert|2370|°F|°C|disp=table|order=flip}} |- | White bright | {{convert|2550|°F|°C|disp=table|order=flip}} |- | White dazzling | {{convert|2730|°F|°C|disp=table|order=flip}} |}
==See also== *Black-body radiation *Color temperature *Incandescence
==Notes== {{Reflist|group=note}}
==References== {{Reflist}}
Category:Metallurgy Category:Temperature
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