Tempering
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