Sweating a brass fitting onto a pipe is simply another way of soldering except that, instead of heating the solder directly with the soldering iron, you heat the parts of the joint with a propane torch. You then introduce the solder into the hot joint, where the solder melts and fills between the two parts of the joint. Heating the fitting causes the fitting to expand slightly. With the solder filling the space between fitting and pipe, a firm seal is made because when the fitting cools, it contracts.
Instructions
Clean the fitting you plan to sweat onto the pipe with steel wool or a wire brush. Clean the end of the pipe you'll sweat the fitting to, also with steel wool or a steel brush.
Apply a thin coat of flux to the surfaces, on both the pipe and the brass fitting, that you just cleaned. Set the fitting onto the pipe and give the fitting a twist to be sure that the flux spreads evenly between the two.
Heat the base of the brass fitting with a propane torch. Move the torch in a circular motion to ensure that heat is applied evenly.
Stick the soldering wire into the joint when the flux begins to bubble. If the brass fitting is hot enough, the solder will melt and flow into the joint.
Remove the torch from the brass fitting when the solder flows into the joint. Let the fitting cool before continuing. The solder should return to a solid state when the joint is properly cooled.