k, arid has the advantage of high fusing points. The hardest
contains from 4 parts of silver to 1 of copper; the softest 2 of silver
to 1 of brass wire. Borax is the flux used, with silver solder as with
spelter.

The soft solders are composed mainly of tin and lead. They occur in a
large range. Common tinner's solder is composed of equal parts of tin
and lead, and melts at 370 deg. Fah. Plumber's solder has 2 of lead to 1
of tin. Excess of lead in plumber's solder renders the solder difficult
to work, excess of tin allows it to melt too easily. Pewterers add
bismuth to render the solder more fusible, e.g. lead 4, tin 3, bismuth
2; or lead 1, tin 2, bismuth 1. Unless these are cooled quickly the
bismuth separates out.

The essentials of a soldered joint are the contact of absolutely clean
surfaces, free from oxide and dirt. The surfaces are therefore scraped,
filed and otherwise treated, and then, in order to cleanse and preserve
them from any trace of oxide which might form during subsequent
manipulation, a fluxing material is used. The soldering material is
compelled to follow the areas prepared for it by the flux, and it will
not adhere anywhere else. There is much similarity between soldering and
welding in this respect. A weld joint must as a rule be fluxed, or metal
will not adhere to metal. There is not, however, the absolute need for
fluxing that there is in soldered joints, and many welds in good fibrous
iron are made without a flux. But the explanation here is that the metal
is brought to a temperature of semifusion, and the shapes of joints are
generally such that particles of scale are squeezed out from between the
joint in the act of closing the weld. But in brazing and soldering the
parts to be united are generally nearly cold, and only the soldering
material is fused, so that the conditions are less favourable to the
removal of oxide than in welding processes.

Fluxes are either liquid or solid, but the latter are not efficient
until they fuse and cover the surfaces to be united. Hydrochloric acid
(spirits of salts) is the one used chiefly for soft soldering. It is
"killed" by the addition of a little zinc, the resulting chloride of
zinc rendering its action quiet. Common fluxes are powdered resin, and
tallow (used chiefly by plumbers for wiped joints). These, with others,
are employed for soft solder joints, the temperature of which rarely
exceeds about 600 deg. Fah. The best flux for zinc is chloride