The
different grades are of varying hardness, the harder kinds melting at
higher temperatures than the soft ones and producing a stronger joint when
used. The reason for not using hard spelter in all cases is the increased
difficulty of working it and the fact that its melting point is so near to
some of the metals brazed that there is great danger of melting the work as
well as the spelter.
The hardest grade of spelter is made from three-fourths copper with
one-fourth zinc and is used for working on malleable and cast iron and for
steel.
This hard spelter melts at about 1650 and is correspondingly difficult to
handle.
A spelter suitable for working with copper is made from equal parts of
copper and zinc, melting at about 1400 Fahrenheit, 500 below the melting
point of the copper itself. A still softer brazing metal is composed of
half copper, three-eighths zinc and one-eighth tin. This grade is used for
fastening brass to iron and copper and for working with large pieces of
brass to brass. For brazing thin sheet brass and light brass castings, a
metal is used which contains two-thirds tin and one-third antimony. The
low melting point of this last composition makes it very easy to work with
and the danger of melting the work is very slight. However, as might be
expected, a comparatively weak joint is secured, which will not stand any
great strain.
All of the above brazing metals are used in powder form so that they may be
applied with the spatula where the joint is exposed on the outside of the
work.
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