Red heat
The practice of using colours to determine the temperature of a piece of 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.
. 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:Stirling
In 1905, Stirling Consolidated Boiler Company published a slightly different set of values:
| Colour | Temperature | Temperature |
| Red: Just visible | | - |
| Dull red | | - |
| Dull cherry red | | - |
| Full cherry red | | - |
| Clear cherry red | | - |
| Deep orange | | - |
| Clear orange | | - |
| White heat | | - |
| White bright | | - |
| White dazzling | | - |
