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)
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Last revised: 28 December 2007.