When the two ends have been given this shape they should not fit as closely
together as might be expected, but should touch only at the center of the
area to be joined (Figure 51). That is to say, the surface of the beveled
portion should bulge in the middle or should be convex in shape so that the
edges are separated by a little distance when the pieces are laid together
with the bevels toward each other. This is done so that the scale which is
formed on the metal by the heat of the fire can have a chance to escape
from the interior of the weld as the two parts are forced together.

[Illustration: Figure 51.--Proper Shape of Scarfed Ends]

If the scarf were to be formed with one or more of the edges touching each
other at the same time or before the centers did so, the scale would be
imprisoned within the body of the weld and would cause the finished work to
be weak, while possibly giving a satisfactory appearance from the outside.

_Fluxes._--In order to assist in removing the scale and other
impurities and to make the welding surfaces as clean as possible while
being joined, various fluxing materials are used as in other methods of
welding.

For welding iron, a flux of white sand is usually used, this material being
placed on the metal after it has been brought to a red heat in the fire.
Steel is welded with dry borax powder, this flux being applied at the same
time as the iron flux just mentioned. Borax may also be used for iron
welding and a mixture of borax with steel borings may also be used for
either class of work. Mixtures of sal ammoniac with borax have been
successfully used, the proportions being about four parts of borax to one
of sal ammoniac. Various prepared fluxing powders are on the market for
this work, practically all of them producing satisfactory results.