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256 MFXHANICAL DENTISTRY AND METALLURGY.
carefully considered. It is in this that many fail and hence
consider soldering a most difficult operation.
(i) A smoky or oxidising flame should be avoided.
(2) The heat should be applied at first very gradually
and uniformly.
(3) After the case is thoroughly heated, direct the flow
of the solder with a small, soft, and pencil-like flame, re-
membering that solder flows to the hottest point. The
heating process, whether for bridge and crown work, or
for plate work, should be conducted very gradually. In
any case the too sudden application of heat may cause dis-
placement of the pieces of solder by the puffing up or swell-
ing of the borax. But where porcelain teeth are involved
the too sudden elevation of heat may result in fracture of
the teeth or the cracking and displacement of the investment,
owing to the too rapid expulsion of moisture.
We repeat, solder flows to the hottest point. If, there-
fore, we are endeavoring to solder teeth to a plate, and the
teeth and surrounding investment have not been thoroughly
heated up, the solder, when it melts, will flow back upon the
plate. In soldering a wire or other small attachments to a
plate or band, the small piece will become heated more
quickly and attract the flowing solder to itself. The flame,
therefore, should be directed through or upon the larger
piece.
Soldering is not a difficult operation. It simply needs to
be conducted with care and intelligence.
Fluxes. —Substances coming under this head are used in
soldering, to remove oxidation and to assist in the flow of
the solder. They are employed in melting metals to com-
bine with the oxids and to protect the molten mass from
further oxidation ; in smelting ores they form a combina-
tion with the infusible substances, forming what is termed
a slag. The flux principally employed in the dental labo-