Articles of malleable iron and cast iron are as easily case-hardened as wrought iron. A poor quality of steel is benefited by the operation, as the metal imbibes carbon in which it was before deficient. _Material for Case-Hardening._—For case-hardening, bone-dust is the article most readily obtained and it is clean and neat to use; but it will not produce the mottled tints that charred or burned leather will give. The leather may be prepared by cutting up old shoes or boots, putting them in an old pan and setting the mass on fire. Let it burn until it is a charcoal that will readily crumble in pieces by using a little force. Grind this charcoal to a fine powder by pounding in a mortar or by running it through an old coffee or spice mill. Pack the work with the powder, the same as bone-dust. Bone-black may be used the same as bone-dust, but it is not very satisfactory in its results. It is also dirty to use and to have around a shop. Ivory dust will also answer the same purpose as bone-dust. Gun guards, straps, and long pieces of work will become shorter by case-hardening, and it is best not to fit such pieces into the stock until after they are hardened. If it be desired to have a portion of the work left soft and the other parts hardened, securely cover the places to be left soft, with a coating of moist clay, and this will prevent the hardening material from coming in contact, and, consequently, it will have no opportunity to absorb carbon and harden when put in the cold water. It may also be observed that articles that are case-hardened will not rust so readily as those not so treated. If the articles be quite thin and there be danger of their cracking by sudden chilling, the water may be warmed a little, or a film of oil may be spread on the water which will tend to prevent a too sudden contraction of the articles while cooling. If it be desired to have the work present the colors or mottled tints as seen on some kinds of case-hardened gun work, the surface of the work before being put in the receptacles containing the burnt leather, must be nicely polished and then buffed or burnished. The higher the finish the more brilliant will be the colors. In using prussiate of potash to case-harden, the potash must be finely powdered, the work heated and dipped in, or if the work be large the potash must be spread over it. The work must be hot enough to fuse the potash, and if it become somewhat cold by removing from the fire it must be reheated, removed quickly from the fire and quenched in cold water.