All steels that are worked under hammers and heated to any degree of
temperature, when finished should be softened by heating, so that the
unequal strains caused by the working of the metal may be neutralized.
When the work is entirely finished it is placed in a furnace or forge
fire and heated red hot. Then the fire is either withdrawn from the
furnace, or the blast is shut off from the forge, and the metal is
allowed to cool off as the fire goes down. This softens the metal and
saves it from the danger of unequal strains.
Hard or high carbon steels are treated in a different way. The metal
is heated red hot, then it is placed in a box filled with slack lime.
The lime completely covers the piece. The cold air is prevented from
striking it and cooling it off too quickly. When no lime is at hand
wood ash can be used.
Sometimes the steel is heated red hot, then held until the red
entirely disappears. Then it is plunged into cold water. This process
is known as water annealing and is a process used for tool steel when
quick work is required. It softens the metal so that it can be filed
and worked quite easily.