(1) After toothing comes hardening, the toothed plates being heated to a light cherry - red, and then plunged into a bath composed of whale - oil, tallow, rosin, and beeswax. The plates, after hardening, should be as brittle as glass. They are covered with scale, grease, and dirt, which is removed by scraping and scouring with sawdust. They come out buckled, and require to be flattened. This is done between heated dies brought together by hydraulic pressure. The dies are circular in form, horizontal in position, and about 5 to 6 ft. in diameter. They are enclosed in a furnace with an adjustable blast, and are revolved, to keep the temperature even. The proper colour for hand - saws is a blue, corresponding to spring temper.
(2) Dr. Hartmann recommends the addition of about 1 lb. rosin to the usual hardening mixture, consisting of 4 3/4 qt. train - oil, 2 lb. beef tallow, and 1/4 lb. beeswax, as rendering it suitable for hardening some articles that do not harden sufficiently without it, although the proper amount of rosin can only be determined by experience, since excess of it may render the articles too hard and brittle. After use for some months, the mass loses its hardening power, and the trough must be thoroughly cleaned of it before fresh mixture is placed in it. He also recommends another mass, consisting of 95 qt. spermaceti oil, 20 lb. melted tallow, 4 1/2 qt. neat's - foot oil, 1 lb. pitch, and 3 lb. rosin. The pitch and rosin are melted together, the other ingredients added, and the mass is heated in an iron vessel until all the moisture is expelled, and the heated mass ignites from a burning splinter held upon it; the flame is thereupon immediately extinguished by means of a tightly - fitting cover.
In employing either mixture for saw - blades, these, after being heated in a suitable furnace, are placed vertically in long troughs filled with the mixture, with the toothed edge, or that intended for the teeth, down, and as soon as they are sufficiently cooled, they are removed and wiped with leather until but a film of grease remains upon them, and are then placed flat over a coke fire until the coating ignites, by the burning of which their brittleness is diminished, and the requisite elasticity imparted. - If they are to be very hard, but a small portion of the coating is allowed to burn off; but if softer, it may be allowed to burn until it goes out.