Tempering
3 of the 7 encyclopedias on this shelf carry an entry for Tempering. Both are reproduced below, so you can see where they agree and where they differ.
American Watchmaker and Jeweler (1892)
Before tempering, the surface of the object must be thoroughly cleaned and freed from grease by the application of oilstone dust, emery, or some like scouring agent. The object should not be handled with the fingers after cleaning, or it will be difficult to obtain the requisite tint. The following table by Stodart will be valuable to the student: 1 430 deg. F Very Pale Straw Yellow..... 220 deg. C 2 450 deg. F A Shade Darker Yellow..... 235 deg. C 3 470 deg. F Darker Straw Yellow..... 245 deg. C 4 490 deg. F Still Darker Straw Yellow..... 255 deg. C 5 500 deg. F Brown Yellow..... 260 deg. C 6 520 deg. F Yellow tinged with Purple..... 270 deg. C 7 530 deg. F Light Purple..... 275 deg. C 8 550 deg. F Dark Purple..... 290 deg. C 9 570 deg. F Dark Blue..... 300 deg. C 10 590 deg. F Paler Blue..... 310 deg. C 11 610 deg. F Still Paler Blue..... 320 deg. C 12 630 deg. F Light Bluish Green..... 335 deg. C After letting an object down to the required color it should be allowed to cool gradually, and no artificial means used to hasten the cooling. A piece of steel may be let down to the same color several times without in any way injuring it or altering its properties. Tempering of small articles is performed satisfactorily by means of the bluing pan. (See Fig. 38). Small articles are also tempered by placing them in a vessel, say a large spoon, covering them with oil and heating them to the requisite degree. This is a favored method of tempering balance staffs and similar articles. The temper is usually judged by the color of the smoke; Saunier gives the following rule: When smoke is first seen to rise, the temper is dark yellow (or No. 2). Smoke more abundant and darker (No. 5). Black smoke still thicker (No. 7). Oil takes fire when lighted paper is presented to it at No. 9. After this the oil takes fire of itself and continues to burn. If the whole of the oil is allowed to burn away No. 12 is reached. The Color of Steel at Various Degrees of Temperature. The following table gives the temperature corresponding to the various colors of steel when heated. 1 980 deg. F Incipient Red..... 525 deg. C 2 1290 deg. F Dull Red..... 700 deg. C 3 1470 deg. F Incipient Cherry Red..... 800 deg. C 4 1650 deg. F Cherry Red..... 900 deg. C 5 1830 deg. F Clear Cherry Red..... 1000 deg. C 6 2010 deg. F Deep Orange..... 1100 deg. C 7 2190 deg. F Clear Orange..... 1200 deg. C 8 2370 deg. F White..... 1300 deg. C 9 2550 deg. F Bright White..... 1400 deg. C
Collier's New Encyclopedia (1921)
in metal work, the process of producing in a metal, particularly steel, that peculiar degree of hardness and elasticity which adapts it for any of the purposes to which it is to be applied. The malleable metals generally increase in hardness by being hammered or rolled, and hammer-hardening that is, hammering without the application of heat, is frequently employed for hardening some kinds of steel springs. Steel is for most purposes hardened by plunging it while hot into water, oil, or other liquid to cool it slowly. Nearly every kind of steel requires a particular degree of heat to impart to it the greatest hardness of which it is susceptible. If heated , and suddenly cooled below that degree, it becomes as soft as iron; if heated beyond that degree, it becomes very hard, though brittle. Bronze is tempered by a process reverse to that adopted ted with steel. Cooling bronze slowly hardens it . The sudden cooling makes it less frangible , and is adopted with gongs. A method of tempering much practiced is by the use of electricity, by means of which the article to be tempered is heated.
The Encyclopedia of Founding (1892)
When steel is suddenly cooled from a high temperature it becomes hard and brittle, but when slowly cooled it is very tough and pliable. The process of bringing steel to the several degrees of hardness for use in the arts and manufactures is called tempering. This term is usually applied to mean a combination of the hardening and annealing processes. According to the temperature to which the hardened steel has been heated before annealing, so is the diminution of the hardness affected by the process. See STEEL; RESTORING BURNT STEEL; DIES, TO HARDEN; ANNEALING.