Heat the capping steel, which
should be thoroughly clean, until it is almost red hot, dip it quickly
into a little of the flux, and then put it into a mixture consisting of
equal parts of sal ammoniac and powdered solder until it is covered with
bright solder. Put the cap on the can and apply the hot capping steel
covered with the solder. Hold this device firmly, press it downwards,
and turn it slowly as the solder melts and thus joins the cap to
the can.
48. After the caps are soldered in place, the air inside the cans must
be driven out through the small vent, or opening, usually in the center
of the cap, and the cans made air-tight. Therefore, place the cans into
boiling water to within 1/2 inch of the top and let them remain there
for a few minutes. Usually, 3 minutes in boiling water is sufficient.
Immediately after _exhausting_, as this process is called, apply a
little of the flux as in capping, and, with the tipping iron well heated
and a strip of solder, seal the hole in the caps. After this is done,
test each can for leaks by submerging it in water. If bubbles arise, it
is an indication that the cover is not tight and must be resoldered.
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