(1) Softening. (a) Heat your steel to dull redness, hold it in some dark or shady nook or 3 corner until you can just see the least possible tinge of redness, then cool immediately in water at the ordinary temperature, and you will be able to file or turn it with very little difficulty. There are times when the delay of the ordinary process is extremely inconvenient-as in the filing of turning tools of a particular shape, re - annealing steel when the skin is taken off, etc, and then - this mode answers admirably. (b) Make the steel red - hot, then put it in a heap of dry sawdust till cold, when it will be found to be quite soft, (c) Place a quantity of newly - burnt lime in a damp place, where it will fall in the form of flour; put it in an iron box. Heat the articles to dull red, clean off all scale, put in lime, and completely cover with lime; cover box over with iron lid, and leave until cold. The more lime and larger the box the better. Keep airtight if possible.
(2) After being tempered, the volume of the tool is slightly increased, and consequently its specific gravity is decreased. As the expansion or increase of volume is so very slight, it is quite immaterial which is plunged in the liquid first; however, every moment the edge is kept out it is cooling, and the tempering may be rendered defective thereby. Mercury tempers the hardest, then water, then salt water, then oil of various kinds-as whale - oil. As oil cools the metal more slowly, it is not tempered so hard, but the tenacity is increased.
(3) It is said that the engravers and watchmakers of Germany harden their tools in sealing - wax. The tool is heated to whiteness and plunged into the wax, withdrawn after an instant and plunged in again, the process being repeated until the steel is too cold to enter the wax. The steel is said to become, after this process, almost as hard as the diamond, and when touched with a little oil or turpentine, the tools are excellent for engraving, and for piercing the hardest metals. .
(4) Tools deeply dipped, and with the temperature let down slowly, are the best tempered. For instance, a carpenter's chisel may be heated in a tube (covered with coke until the whole is at a red heat) until it is at a heat slightly more than blood - red; then lower vertically down to the bottom of a pailful of tepid water; when cold, take it out and polish it. Get a bolster, or large nut,' heat it to whiteness, and traverse the chisel through and back until the surface has an orange or gold colour; cool out. Taps, dies, half - circular cutters, etc, so tempered, stand wear and tear much the best.
(5) The forging and tempering of iron or steel can be greatly enhanced, according to Blass, by dipping the metal, in whatever form, in fused salt. This dipping in salt is also well adapted for annealing steel without the oxidation of the surface. If the metal be rusted, it must be allowed to remain some time in the bath. Borax can with good effect be mixed with the salt. Metal " purified "by such an immersion is very susceptible to galvanic depositions, and can easily be coated with copper, zinc, tin, nickel, silver, etc. For iron in the spongy or powdered state, as obtained from the reduction of the ores, the salt bath is especially adapted, for it augments the combination of the particles by making their surfaces free from impurities. To prepare the bath for an application as here proposed, the salt must be fused in a puddling furnace, and the iron sponge, with the addition of a flux, be added in small quantities, so as not to vitrefy the salt. The iron is left inthe furnace till the flux has combined with all the impurities, and formed a slag, whereupon the iron is taken out and forged together. While the iron is in the furnace, it should be constantly covered with the 6alt, so that oxidation be prevented.
For the hardening of iron, the salt is fused in a convenient vessel and the object immersed, and from time to time a small quantity of potash ferrocyanide is added - 1 lb. or 2 lb. per 100 lb. of iron. The articles, according to their thickness, are permitted to remain 5 to 30 min. in this bath, and are then plunged in water containing, in 100 parts, 1 of hydrochloric acid, 5 of wine vinegar, and 1 of salt. If the objects are to have a silver lustre, they should be immersed for a few minutes in a mixture of 3 parts wine vinegar and 1 of hydrochloric acid.
(6) Steel punches,' or other small implements, particularly engraved dies, when, by accidental exposure to' too great heat, they have become spoiled, or, as the blacksmith says, burned, may be restored by the following mixture: 2 oz. bichromate of potash, 1 oz. pure nitre, 1 oz. gum - aloes, 1 oz. gum - arabic, and 2 oz. rosin. The whole having been well powdered and mixed, the piece of steel is heated to low redness, and the powder sprinkled over it. It is then heated again to low redness, and cooled. This makes the piece very hard again. The amount of nitre may be doubled, and that of the rosin taken 10 times greater, to produce a higher temper.
(7) In tempering a tool for boring cylinders, turning rolls, or any large pieces of cast - iron, let it be as hard as water will make it, taking care not to heat it more than to cherry - red. In the second heating, after the tool is hardened, the first perceptible tint is a light straw - colour, which makes its appearance when the heat reaches 430° F. (221° C). This gives the metal a maximum of hardness, with a certain amount of elasticity, fitting it for lancets, razors, and surgical instruments. At 470° F. (243° C.) a full yellow is produced, which is the temper employed for penknives, scalpels, and fine cutlery. The temperature of 490° F. (254° C.) gives a brownish - orange, suitable for shears and chisels used for cutting iron. At 510° F. (266° C.) the brownish - yellow becomes flecked with purple, the tint for pocket - knives. 520° F. (271° C.) gives a bluish purple, fit for table cutlery; while the different shades of blue from 530° to 570° F. (277° - 299° C.) indicate a temper proper for 'watch - springs, sword - blades, saws, and instruments requiring great elasticity. Beyond this temperature the metal becomes too soft to be used for cutting instruments.