The Mouth Blowpipe.
—Although soft-soldering is usually associated with the use of a copper bit, quite a number of jobs can be done without one, using instead a bunsen burner or, more generally, a mouth blowpipe, which is an inexpensive appliance, useful for both hard and soft soldering, and with either gas, candle, or a methylated-spirit flame. Three shapes of mouth blowpipe are shown in [Figs. 23] to [25]. In a blowpipe flame there are three cones, X, Y, Z ([Fig. 26]). X is a non-luminous cone, consisting of a mixture of atmospheric air and unburnt combustible gases (each with a low temperature); Y is a luminous cone, composed of burning gases (carbon and carbonic acid being in excess); and Z is a cone the oxygen in which renders it less luminous and free from combustible materials, its temperature being exceptionally high, especially where the cone comes in contact with the point of the cone Y. Because of its properties, Z is termed the oxidising or outer flame, whilst Y is known as the inner or reducing flame, because when it is applied to some easily reducible substance—say, lead oxide—the oxygen in the substance heated mingles with the unburnt carbon in the cone of the flame and produces carbonic oxide, the lead being thus separated or reduced. The blowpipe flame is one of intense heat, even that produced by blowing a common candle being capable of melting metallic fragments when they are supported on a bed of charcoal. The pointed flame gives the greatest heat, and this can be produced simply by increasing or decreasing the space between the flame and the article to be soldered or the metal to be melted.
The particular advantage of a blowpipe is that it gives a fierce heat at a very localised area, beyond which the solder does not run, and it enables spots to be soldered, or parts to be unsoldered, adjusted and re-soldered without allowing heat to stray and cause trouble at other places. A useful little addition to the ordinary blowpipe is a small washer soldered on near the mouth end (see [Fig. 23]), the object of this being to raise this part off the bench and so keep it from contamination with dirt, filings, etc., which are unpleasant to the lips. Sometimes the washer is made elliptical and slightly concave to fit the lips, so that it forms a convenient stop or steady when the blowpipe is held between the teeth without help from either hand.