colors of heated steel

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 Holcomb Steel data that was closest to theirs. The variation demonstrates how unreliable this method is even in the hands of careful observers.

Temperature Judged by Color
Color	Halcomb Steel		Howe	White & Taylor
Color	degrees F	degrees C	degrees C	degrees C
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
Dark red	1292	700	550-625	635
Dull cherry-red	1472	800		677
Cherry-red	1652	900	700	746
Bright cherry-red	1832	1000	850	843*
Orange-red	2012	1100		899
Orange-yellow	2192	1200	950-1000	941
Yellow-white	2372	1300	1050	996
White welding heat	2552	1400	1150	1079
Brilliant white	2732	1500		1205
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.

3. Maunsel White and F. W. Taylor.
Transactions of the American Society of Mechanical Engineers, 1899.

Colors for Tempering
Color	degrees F	degrees C
Very pale yellow	430	221.1
Light yellow	440	226.7
Pale straw-yellow	450	232.2
Straw-yellow	460	237.8
Deep straw-yellow	470	243.3
Dark yellow	480	248.9
Yellow-brown	490	254.4
Brown-yellow	500	260.0
Spotted red-brown	510	265.6
Brown-purple	520	271.1
Light purple	530	276.7
Full purple	540	282.2
Dark purple	550	287.8
Full blue	560	293.3
Dark blue	570	298.9
Very dark blue	600	315.6

Source: Halcomb Steel Co. (1908)