Straw yellow (about '460° F.) : Dies, taps, drills, punches, reamers.
Brown yellow (about F.) : Gouges, plane irons, twist drills, cooper tools, wood bbring cutters.
Light purple (about F.) : Augurs, sur gical instruments, cold chisels, edging cutters.
Dark purple (about 550° F.) : Axes, gim lets, needles, hacic-saws, screwdrivers, springs,. wood saws.
Some tools are of such a shape that they cannot be tempered in the manner here de scribed, but must have their temper °drawn° to the desired color by reheating the piece be tween hot iron plates, or in a hot iron ring. Springs are often tempered by a different method, known as °oil tempering.° In carrying out this method, the piece is first hardened by heating to a bright red heat and then quenching by plunging the whole piece in water or in oil. The article to be tempered is then wetted with oil and gradually and uniformly heated until the oil upon it blazes up, when the piece is again quenched in the oil. This process of heating to the ignition point of the oil and then quenching is repeated until it has been per formed three times, after which the piece is said to be “oil tempered," and is ready for use.
In the early days of steel-worlcing in the United States, it was common to import water in casks from Sheffield, England, for hardening and tempering purposes, as it was believed that there is some special virtue in the water that had been used for so long, and with such eminent success, in that city. There was prob
ably little or no foundation for this belief, and yet it is known that substances that may he in solution in the water that is used for quenching often have an important influence upon the product. Many artisans dissolve salt or cyanide of potassium in the water that they use for this purpose, and there is considerable ground for the belief that such dissolved substances do exert an influence upon the character of the product, which is out of all apparent proportion to the strength of the solutions containing them. In particular, it may be noted that there is a deeply-rooted belief among blacksmiths and other artisans who work with metals that a piece of steel cannot be hardened by heating it and then quenching it in water that contains soap, even in small amounts.
The art of tempering cannot be adequately presented in a short article, and those who are skilled at it maintain (probably quite justifiably) that the only way to learn it is by actual ex perie.nce in the shop. Different steels may require radically different treatment, and special implements (razors, for example) may call for years of study before they can be tem pered satisfactorily.