Sweating is a method of joining metal tubing and fittings, usually of copper pipe and brass. Solder fittings are also called capillary fittings, because of the capillary action that draws solder into the joint.
The heat source used for sweating is normally a propane blowtorch, but a high wattage soldering iron can also be used. The solder should be flux-free, alloys such as tin-silver or tin-antimony made specifically for pipe can be used, they are environmentally friendly and generally stronger than tin/lead solder.
To prepare the pipe for soldering, the surfaces to be soldered must first be cleaned. This can be done with fine sandpaper, abrasive pads and/or a wire brush. For optimal strength, the polishing direction should be perpendicular to the forces on the resulting joint, so the small grooves created by the polishing increase the friction between the parts.
Next, a layer of flux paste is applied to the surfaces that are to be soldered. this dissolves oxides and similar impurities, and helps the solder flow and stick to the pipe. Excess flux can be wiped off before soldering, and any remains should be cleaned off afterwards, since flux remaining in the pipe can lead to corrosion. Alcohol works well for this purpose.
The parts that are to be soldered are then clamped down and heated with the torch. Ideally the joint should be mounted so the seam is vertical, this will let solder flow down both sides and any excess will drip out at the bottom.
The flame should be applied to both parts of the joint, to ensure that they both receive enough heat to melt the solder. When the solder melts when touching it, the metal is hot enough. Solder is then applied to the gap near the top, and will flow into and fill the joint. The joint is full when solder is visible around the rim, or when a drop starts to form at the bottom of the joint. Excess solder can be wiped off using a wet cotton cloth (synthetic cloth may melt).
The parts will become very hot, even well away from the soldering area, so they should be kept away from materials that could be damaged from the heat. The surface beneath may be subjected to dripping soldering paste and solder, and need to be able to withstand this. Wood will work but may get scorch marks and grease spots, metal or concrete is recommended.
When soldering several joints close to each other, it is recommended to solder them all in the same operation, to avoid re-melting of previously made joints.
It is advisable to have a bowl of cold water nearby, in case of burns or fires, or to cool down parts if needed. The finished joints should be left to cool naturally if possible.