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Saws and springs are generally hardened by being immersed in various compositions of oil, suet, wax, and the like materials. The usual way of proceeding is to heat the saws in long furnaces and then to immerse them horizontally and edgewise in a long trough containing the composition; two troughs are commonly used alternately. Part of the composition is wiped off with a piece of leather, when the articles are removed from the trough; they are then heated one by one over a clear coke fire until the grease inflames; this is called "blazing off." A greatly recommended composition consists of two pounds of suet and a quarter of a pound of bees wax to every gallon of whale oil; these are boiled together, and will serve for thin articles and most kinds of steel. The addition of black resin, to the extent of about one pound to the gallon, makes it serve for thicker pieces, and for those it refused to harden before; but the resin should be added with judgment, or the articles will become too hard and brittle. The composition is useless when it has been constantly employed for about a month; the period depends, however, on the extent to which it is used, and the trough should be thoroughly cleaned out before the new mixture is placed in it. The following recipe is recommended; Twenty gallons of spermacetic oil; twenty pounds of melted and strained beef suet, one gallon of neatsfoot oil; one pound of pitch; three pounds of black resin. These last two articles must be previously melted together, and then added to the other ingredients; the whole must then be heated in a proper iron vessel, with a close cover fitted to it, until the moisture is entirely evaporated, and the composition will take fire on a flaming body being presented to its surface; the flame must be instantly extinguished again by putting on the cover of the vessel.
When the work is thick, or irregularly thick and thin, as in some springs, a second and third dose is burned off, to insure equality of temper at all points alike. When the saws are wanted to be rather hard, but little of the grease is burned off; when milder a large portion; and for a spring temper, the whole is allowed to burn away. Gun-lock springs are sometimes literally fried in oil for a considerable time over a fire in an iron tray. The thick parts are then sure to be sufficiently reduced, and the thin parts do not become the more softened from the continuance of the blazing heat. Springs and saws appear to lose their elasticity after hardening and tempering, from the reduction and friction they undergo in grinding and polishing. Toward the conclusion of the manufacture, the elasticity of the saw is restored principally by hammering, and partly by heating over a clear coke fire, to a straw color; the tint is removed by very diluted muriatic acid; after which the saws are well washed in plain water and dried. WK
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