2. Pale straw-color—450°. Still very hard, suitable for the faces of hammers and anvils.
3. Full yellow—470°. Shears and scissors.
4. Brown—490°. Gravers and turning-tools for hard metals; also percussion-lock gun tubes.
5. Brown, with purple spots—510°. Wood-working tools and most of the steel parts in a gun-lock with the exception of the springs; also knives of all sorts for cutting wood.
6. Purple—538°. Butcher-knives and other flesh-cutting implements.
7. Dark blue—550°. Tools requiring strong cutting-edges without extreme hardness, as case-knives.
8. Full blue—560°. Chopping-axes.
9. Grayish blue, verging on black—600°. Springs, saws, swords, and the like.
Various other methods of tempering steel are sometimes recommended, as with oils, tallow, lead, mercury and divers solutions; but since the matter-of-fact gunsmith will find use for none of them, it is not deemed proper to encumber this book with anything further on the subject of tempering. It might be well to state, however, that the hardest degree to which steel can be brought is secured by heating the piece to a light yellow and instantly plunging it into cold mercury.
To Restore “Burnt” Steel.—Pulverize together two parts horn or hoof filings; one part sal ammoniac; one part charcoal, and one part common soda. When thoroughly ground together, work in tallow enough to make a kind of wax or paste. Bring the damaged steel to a bright cherry-red heat, and then cover with the paste, leaving it to cool gradually. The process may be repeated several times with profit if considered necessary. While a piece of badly-burnt steel may not be entirely restored by this process, it can be much improved. Entire restoration is scarcely possible.