In order to harden tools without making them warp or having their interior become too hard and brittle, the following compound will be found desirable:
500 parts Peruvian Bark.
25 parts Common Salt,
15 parts Yellow Prussiate of Potash,
15 parts Saltpetre,
50 parts Hartshorn Shavings, 100 parts Black Soap.
Spread the soap in a layer 39-100 of an inch deep, then scatter on it the pulverized mixture of the other substances and knead the paste thus formed. Continue this kneading until the mass is formed into a stick about 2 inches in diameter. Allow this compound to dry for twenty-four hours before using, then care must be taken to heat the cast-steel article only to dark red.
Then the portions to be hardened are coated with the compound and cooled off. When this is done the interior as well as the portions not coated remain soft and tough, while the coated portions become hard as glass. To find the degree cast-steel should be heated for this, it is perhaps well to experiment with a round steel wire, about 1/2 inch thick.