When handling brass pieces having thin sections there is danger of melting
the brass and causing it to flow away from under the flame, with the result
that the work is ruined. If, in the judgment of the workman, this may
happen with the particular job in hand, it is well to build up a mould of
fire clay back of the thin parts or preferably back of the whole piece, so
that the metal will have the necessary support. This mould may be made by
mixing the fire clay into a stiff paste with water and then packing it
against the piece to be supported tightly enough so that the form will be
retained even if the metal softens.
_Brazing._--With the work in place, it should be well covered with the
paste of flux and water, then heated until this flux boils up and runs over
the surfaces. Spelter is then placed in such a position that it will run
into the joint and the heat is continued or increased until the spelter
melts and flows in between the two surfaces. The flame should surround the
work during the heating so that outside air is excluded as far as is
possible to prevent excessive oxidization.
When handling brass or copper, the flame should not be directed so that its
center strikes the metal squarely, but so that it glances from one side or
the other. Directing the flame straight against the work is often the cause
of melting the pieces before the operation is completed. When brazing two
different metals, the flame should play only on the one that melts at the
higher temperature, the lower melting part receiving its heat from the
other.
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