_Aluminum._--This is the most difficult of the commonly found metals
to weld. This is caused by its high rate of expansion and contraction and
its liability to melt and fall away from under the flame. The aluminum
seems to melt on the inside first, and, without previous warning, a portion
of the work will simply vanish from in front of the operator's eyes. The
metal tends to run from the flame and separate at the same time. To keep
the metal in shape and free from oxide, it is worked or puddled while in a
plastic condition by an iron rod which has been flattened at one end.
Several of these rods should be at hand and may be kept in a jar of salt
water while not being used. These rods must not become coated with aluminum
and they must not get red hot while in the weld.

The surfaces to be joined, together with the adjacent parts, should be
cleaned thoroughly and then washed with a 25 per cent solution of nitric
acid in hot water, used on a swab. The parts should then be rinsed in clean
water and dried with sawdust. It is also well to make temporary fire clay
moulds back of the parts to be heated, so that the metal may be flowed into
place and allowed to cool without danger of breakage.

Aluminum must invariably be preheated to about 600 degrees, and the whole
piece being handled should be well covered with sheet asbestos to prevent
excessive heat radiation.

The flame is formed with an excess of acetylene such that the second cone
extends about an inch, or slightly more, beyond the small blue-white point.
The torch should be held so that the end of this second cone is in contact
with the work, the small cone ordinarily used being kept an inch or an inch
and a half from the surface of the work.

Welding rods of special aluminum are used and must be handled with their