For when you need to solder but do not have a soldering iron.
If you apply flame directly to a solder connection, the wires oxidize and the solder joint is typically poor. The trick is to carry one** piece of wire beyond the joint and apply the flame to that piece. It will act as the soldering iron.
I've used matches, candles, oil lamps, etc. It works. Just be patient and let the joint heat up.
Be careful using an open flame that you don't set other things on fire. Use standard precautions for soldering (eye protection, etc.)
Never use a lighter longer than the manufacturer's recommendations.
Some Notes I Made On This A Long Time Ago
Posted to youtube 20140603
"Ironless" Field Soldering Technique: Occasionally you may find yourself in a place where you'd like to finish a project, perhaps just soldering a couple of wires together, but don't have your electric/gas soldering iron handy. It might be working away from your home or in the field.
Not long ago I was out replacing the AC cord on a power supply and forgot my soldering iron. The options were -- a) twist, close and wait until later to tear it down and finish it up; b) stop, go out and buy a soldering iron, c) go home and get my soldering iron, d) find a new alternative. If you are like me, you really, really despise reopening a project later to finish it up. Maybe it feels too much like fixing a mistake. Stopping and going out to buy one, or driving further to go get mine was a real morale/time killer. Need is the mother of invention, so in this case she gave birth a new alternative.
I've tried soldering using matches, torches, cigarette lighters and a few other acts of desperation. None of them ever gave me a good solid joint. The problem with flame based soldering was that exposing the wires to be soldered, and the solder, directly to a flame creates a "dirty" joint, a joint that is not electrically nor mechanically sound. Here's the trick:
Bare a 1.5 - 2 inches of wire on one** wire, and a normal amount for the other (maybe ½ to 3/4 inch). Make sure the wires are clean. The longer part will be left sticking out of the joint and will be heated to make the "soldering iron". The heating element will be a cigarette lighter or match. The difference here is that the tip stays behind as part of the joint.
Add shrink wrap if you want it. Twist the wires to be soldered together to create a good mechanical connection. You can either twist the two wires to be soldered in side by side in parallel, or you can twist the two wires to be soldered together in a straight line, then wrap a (solid) copper wire around the joint. (The second way the joint will be bigger, but you can join much thinner wires).
Position the joint away from other things that the flame would damage, such as other parts of the circuit or the case. Heat the "free" end of the solid copper wire, that is the end that is not twisted with the wires you are soldering together. Get as much of it in the flame as possible, but keep the joint out of the flame. The joint will heat up quickly. Touch the solder to the "soldering copper" where it is twisted in/around the wires as you would when soldering with an iron.
I recommend a standard rosin core, fine gauge solder. Thicker solder requires more heat and patience. Allow the joint to cool and clip off the excess wire. Add twist caps, tape, shrink wrap or otherwise insulate and finish off the joint.
** Note: you can carry both out for better heat conduction, but it's usually not necessary and it wastes more wire.