_Another way to Case-Harden._—Collect such articles of animal origin as cows’ horns, or hoofs of either cows or horses, or leather trimmings from about the shoe-shops, or old cast off boots or shoes, and burn them until sufficiently charred to admit of being easily pounded into a powder. Having finished up the article to be hardened, ready for the final polish, place it in an iron box, and surround it completely on all sides by a packing of the powder. Pour into the box, until the powder is made moist, a saturated solution of common salt in urine. Next close the box and seal it until airtight, with wet and well-worked clay, then put it into the furnace and blow up gradually until heated to a cherry red. Don’t run the heat any higher, but hold it at that about five minutes, then take out and plunge at once into the slack-tub. By this means a piece of soft malleable iron is rendered as hard as hardened steel. Some workmen contend that the salt solution is of no particular importance—that just as good results will come of packing in the animal charcoal alone. The iron box, though very convenient when a good deal of case-hardening is to be done, is not an absolute necessity. If the article, surrounded by the animal charcoal, is incased in a ball of stiff and well-worked clay, and then exposed to the proper heat and slacking, the results will be the same as if heated in an iron box. _Another Formula._—In earlier times, when guns were more in use than either agricultural or mechanical implements, and there was a gunsmith’s shop at almost every cross-road, they had a way of case-hardening that was much more simple than either of the foregoing, and yet quite effectual. Scraps of old leather, as cut from old boots or shoes, were tightly wrapped and tied around the piece of iron to be made hard, to the extent of several thicknesses. Around this was placed a layer of sand and salt in equal proportions, to the thickness of half an inch. The sand and salt was dampened with water to make it stick together. A layer of plastic clay, an inch in thickness, was worked around the whole, and the ball, so made, was exposed to heat at about the cherry-red degree, sufficiently long to consume the leather, when it was dropped suddenly into the slack-tub. _Still Another Formula._—Make a powder of pulverized prussiate of potash, sal-ammoniac and saltpetre in equal parts. Heat the iron to cherry-red and sprinkle thoroughly on all sides with the powder, then immediately plunge into the slack-tub. Some smiths contend that the pulverized prussiate of potash, used in the same way, is entirely effectual without the other ingredients. _To Chill Cast Iron._—Make a powder by pulverizing together, salt, 2 lbs.; saltpetre, ½ lb.; alum, ½ lb.; ammonia, 4 ozs., and salts of tartar, 4 ozs. Heat the iron to cherry-red, sprinkle thoroughly with the powder and then plunge into cold water.