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The practice of using colours to determine the temperature of a piece of (usually) ferrous metal comes from 
The practice of using colours to determine the temperature of a piece of (usually) ferrous metal comes from blacksmith
A blacksmith is a metalsmith who creates objects primarily from wrought iron or steel, but sometimes from #Other metals, other metals, by forging the metal, using tools to hammer, bend, and cut (cf. tinsmith). Blacksmiths produce objects such ...
ing. Long before thermometer
A thermometer is a device that measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermomete ...
s 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:Stirling
In 1905, Stirling Consolidated Boiler Company published a slightly different set of values:See also
* Black-body radiation *Color temperature
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
* Incandescence
References
{{Reflist Metallurgy Temperature