Before infrared pyrometers became available, blacksmiths and other metalworkers judged the temperature of heated steel and iron by its color. Unfortunately, it isn’t possible to do so very accurately1, especially at temperatures above 1200 °C.
Tables comparing temperature and color appeared at least as early as 1836 (Pouillet). The one below shows three attempts at correlating temperature and color. The verbal descriptions given by Howe2 and White and Taylor3 have been omitted and their temperatures placed with the verbal description in the Halcomb Steel data that was closest to theirs. The variation demonstrates how unreliable this method is even in the hands of careful observers.
|Color||Halcomb Steel||Howe||White &
|Red heat, visible in the dark||752||400||470|
|Red heat, visible in the twilight||885||474|
|Red heat, visible in the daylight||975||525||475||532|
|Red heat, visible in the sunlight||1077||581||556|
|White welding heat||2552||1400||1150||1079|
|Dazzling white (Bluish-white)||2912||1600|
*Heat at which scale forms and adheres (scale on iron heated to higher temperatures falls off when the iron is cooled in air).
1. Bureau of Standards, Bulletin Number 2. (1905)
2. H. M. Howe.
Maunsel White and F. W. Taylor.
Transactions of the American Society of Mechanical Engineers, 1899.
|Very pale yellow||430||221.1|
|Very dark blue||600||315.6|
Source: Halcomb Steel Co. (1908)
Hardening and Tempering Tools and Metals.—The following is the colour and temperature required:— Pale straw, 430° Fah., for lancets, &c.; dark yellow, 470° Fah., for razors, &c. ; dark straw, 470° Fah., for penknives; clay yellow, 490° Fah, for chisels and shears; brown yellow, 500° Fah., for adzes and plane irons ; very pale purple, 520° Fah., for table-knives; light purple, 530° Fah., for swords and watch-springs ; dark purple, 550° Fah., for softer swords and watch-springs ; dark blue, 570° Fah., for small fine saws; blue, 590° Fah., for large saws; pale blue, 610 Fah., for saws, the teeth of which are set with pliers; greenish blue, 630 Fah., for very soft temper. To obtain the proper temper lay the metal on a lump of iron heated to a sufficiently strong heat in the forge or other fire. The desired temper may be thus secured with the greatest facility and exactitude, as the clean bright metal shows the degrees of oxidation from the blue upwards most distinctly, which oxidation can be arrested at will. Cleanliness, or rather brightness of surface, is essential.
Workshop Receipts, for the Use of Manufacturers, Mechanics and Scientific Amateurs.
London: E. and F. N. Spon, [no date, but 1873]
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Last revised: 12 November 2013.