The process of hardening the surface of iron or steel is called case-hardening. The piece to be treated is first heated to a bright red and the surface rubbed with prussiate of potash. When it has cooled to dull red, it is immersed in cold water. When the piece is to be highly finished, the finish may be obtained by heating it in a cast iron pot containing red-hot lead. A good case-hardening mixture may be made with three parts prussiate of potash and one of sal ammoniac. The temperature for cherry red is given at 1832° F. Machine nuts, set screws, wrenches, and collets are samples of such work.