red heat

{{Short description|Practice of using colours to determine the temperature of metal}}

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File:Hot metalwork.jpg

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. 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.]]