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.
3. 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]
We have reformatted his list for ease of reference.
The Thirteenth Book of Natural Magick: Of Tempering Steel
Then, as Pliny saith, it is commonly made soft with Oyl, and hardened by Water. It is a custome to quench thin Bars of Iron in Oyl, that they may not grow brittle by being quenched in Water. Nothing hath put me forward more to seek higher matters, then this certain Experiment, That Iron may be made so weak and soft by Oyl, that it may be wrested and broken with one's hands : and by Water it may be made so hard and stubborn, that it will cut Iron like Lead.
I have said how Iron may be made softer, now I will shew the tempering of it, how it may be made to cut sharper. For the temper of it is divers for divers uses. For Iron requires several tempers, if it be to cut Bread, or Wood, or Stone, or Iron, that is of divers liquors ; and divers ways of firing it, and the time of quenching it in these Liquors: for on these doth the business depend. When the Iron is sparkling red hot, that it can be no hotter, that it twinkles, they call it Silver; and then it must not be quenched, for it would be consumed. But if it be of a yellow or red colour, they call it Gold or Rose-colour : and then quenched in Liquors , it grows the harder: this colour requires them to quench it. But observe, that if all the Iron be tempered, the colour must be blew or Violet colour, as the edge of a Sword, Rasor or Lancet : for in these the temper will be lost if they are made hot again. Then you must observe the second colours; namely, when the Iron is quenched, and so plunged in, grows hard. The last is Ash colour : and after this if it be quenched, it will be the least of all made hard. For example:
The temper of a Knife to cut Bread.
I have seen many ingenious men that labored for this temper, who, having Knives fit to cut all hard substances, yet they could scarce fall upon a temper to cut Bread for the Table. I fulfilled their desire with such a temper. Wherefore to cut Bread, let the Steel be softly tempered thus: Heat gently Steel, that when it's broken seems to be made of very small grains; and let it be excellent well purged from Iron: then strike it with a Hammer to make a Knife of it : then work it with the File, and frame it like a Knife, and polish it with the Wheel: then put it into the Fire, till it appear Violet-colour. Rub it over with Sope, that it may have a better colour from the Fire: then take it from the Fire, and anoynt the edge of it with a Linen cloth dipt in Oyl of Olives, until it grow cold; so you shall soften the hardness of the Steel by the gentleness of the Oyl, and a moderate heat. Not much differs from this,
The temper of Iron for Wood.
Something harder temper is fit to cut wood; but it must be gentle also : therefore let your Iron come to the same Violet-colour, and then plunge it into water: take it out ; and when it appears Ash-colour, cast it into cold water. Nor is there much difference in
The temper for Instruments to let blood.
It is quenched in Oyl, and grows hard; because it is tender and subtile: for should it be quenched in water, it would be wrested and broken.
The temper of Iron for a Sythe.
After that the Iron is made into a Sythe, let it grow hot to the colour of Gold, and then quench it in Oyl, or smeer it with Tallow, because it is subtile Iron; and should it be quenched in waters, it would either crumble or be wrested.
John Baptista Porta. [Giambattista della Porta]
Natural Magick. Book 13.
London: Thomas Young and Samuel Speed, 1658.
The first known edition of della Porta's book, in Latin, was published in Naples in 1558, and became an international bestseller. A much enlarged edition appeared in 1589, of which this is a translation. The translator's identity is not known. Basic Books published a facsimile reprint in 1957.
Della Porta was an extremely prominent figure in those years in which experimental evidence gradually replaced the authority of ancient writers. He founded what is perhaps the first scientific society, the Otiosi, a group who met in della Porta's home. He published prolifically, and later was Vice-President of the Academy of the Lynxes. Galileo was a fellow member.
Tempering.— Steel in its hardest state being too brittle for most purposes, the requisite strength and elasticity are obtained by tempering—or, letting down the temper, as it is termed—which is performed by heating the hardened steel to a certain degree and cooling it quickly. The requisite heat is usually ascertained by the color which the surface of the steel assumes from the film of oxide thus formed. The degree of heat to which these several colors correspond are as follows:
|At 430°, a very faint yellow.
At 450°, a pale straw color.
|Suitable for hard instruments, as hammer faces, drills, etc.|
|At 470°, a full yellow.
At 490°, a brown color.
|For instruments requiring hard edges without elasticity; as shears, scissors, turning tool, etc.|
|At 510°, brown with purple spots.
At 538°, purple.
|For tools for cutting wood and soft metals; such as plane-irons, knives, etc.|
|At 550°, dark blue.
At 560°, full blue.
|For tools requiring strong edges without extreme hardness; as cold-chisels, axes, cutlery, etc.|
|At 600°, grayish-blue, verging on black.||For spring-temper, which will bend before breaking; as saws, sword-blades, etc.|
If the steel is heated higher than this, the effect of the hardening process is destroyed.
Wm. T. Egan, compiler.
Hall, Kimback & Co's Illustrated Catalog.
Chicago: by the company, 1890. Page 295.
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Last revised: 16 August 2019.