To harden the iron arms of cart axles, place them in an iron box about three times the size of the proper box, sealing up the front end quite close; pack up the space between the axle and box with crushed bones and shreds of leather, close up the back end with clay or other substance so that it is air- sight, and place in a furnace with a good heat for about eight hours, when the bone and leather should be consumed. Allow to cool, fill up the space with powdered potash, replace in the furnace again until it is consumed, then take it out, and allow to cool until black hot, when it should be cooled out in a tub of strong salt and water. To harden the insides of boxes, make them fairly hot, charge the insides with potash,and revolve them until the potash is consumed; repeat this, then cool out as before.

In Moxon's " Mechanical Exercises" the method of i hardening is thus described: Take cow-horn or hoof, dry it thoroughly in an oven, then beat it to powder; put to it an equal quantity of bay salt, and mingle them with white wine vinegar. Lay some of this mixture upon loam, and cover the iron all over with it; then wrap the loam all about it, and lay it on the hearth of the forge to dry and harden. Put it into the fire when dry; when it attains a blood-red heat, withdraw, and allow to cool out.