Some hardeners prefer cyanide of potassium to lead for heating cutting tools, dies, etc. It is a white transparent salt which melts at a fairly low temperature, and should be carefully used as it is a virulent poison. The fumes are very injurious and the furnace should be enclosed with a hood connecting with a chimney or ventilating shaft. This bath is used a great deal in gun shops for hardening certain pieces on which it is wished to secure ornamental color effects. To get the color effect an air jet is forced up from the bottom of the quenching tank, as shown in Fig. 182, and the air coming in contact with the piece to be hardened gives it a variety of colors.


Casehardening also can be done with cyanide, although no great depth of case is secured. The cyanide is melted in a cast-iron pot in a furnace and then the work to be casehard-ened is entirely immersed in the cyanide which is heated to a dark cherry red. The work should be suspended by fine iron wires. When the work has been thoroughly heated through, it can be removed and quenched 15 or 20 minutes afterward, and a casing of suitable depth for ordinary purposes is insured. Increasing the length of time of immersion will simply add to the depth of the casing, but 30 minutes of heating will give a very deep casing. The work can be dipped in clear cold water immediately after having been removed from the cyanide bath, or it may be permitted to cool, and may be reheated and hardened in the same manner as in connection with pack hardening. When small pieces are to be heated in cyanide, it is best to use wire baskets, which must be made so that the liquid has free access to all the surfaces of the finished pieces.