It is often necessary to join together two pieces of iron, or the ends
of the same piece, as in forming a circle, so that the joint will form
one solid mass. When this is done the pieces are said to be welded
together. Most of the problems treated so far have had to do with
single pieces of iron. Often many pieces are welded together to make
one forging, as in making the horseshoe. This is called built up work.
It is possible to weld any iron surfaces provided no oxide of iron
is formed upon the surface. If such a coating occurs welding is
impossible. Iron heated in the air absorbs oxygen, which results in
the oxide formation. There are two ways of guarding against this
difficulty: (1) Suitable fluxes, and (2) reducing fires.
The best flux is a sharp sand, usually beach sand. This is sprinkled
over the ends to be welded. The sand fuses on to the ends of the iron
and excludes the air. The reducing fire is one that consumes all the
oxygen that is forced into it by the blast. This is obtained by having
a thick bed of fire for the air to pass through before coming into
contact with the iron and by keeping the blast pressure low enough
to enable the fire to burn all that is admitted. A thick fire, so to
speak, should always be kept.