Flux is a compound commonly used in plumbing applications where two joints are connected using solder. This is a process known as "sweating" pipes together. Combined with solder, flux is a key part of the sweating process. Understanding more about flux and how to apply it will help you properly connect your copper fittings.
Flux is an acidic paste that is water-soluble, which makes cleaning up simple by using water and a cotton rag. Flux is applied to all of the pieces of copper that you want to connect together. The flux paste will draw in the solder after the pipe is heated using a blow torch. The acid in the flux attracts the solder into the seam between the two copper pieces and fills the gap and creates a semi-permanent bond between the pieces.
Preparing for Flux
It's critical that two copper pieces that you want to connect together are very clean prior to sweating with flux and solder. Clean the surface of the end of the copper pipe with a fine-grit sandpaper until the surface of the copper is bright and shiny. Use a copper wire brush to swab out the inside of the copper fitting that the pipe will go into. It's also important to remove burs from the end of the copper pipe by rubbing the sandpaper against the edges of the end of the pipe. This will give the flux a strong surface to bond to.
Flux is applied using a special flux brush that is available at most hardware stores. Dip the brush in the tin of flux and coat both sides well with the flux. Apply the flux on the shiny part of the copper pipe and the inside of the copper fitting. Flux must be applied to all of the surfaces where you want solder to go. But be careful not to spill flux on the pipe as solder will travel anywhere on a pipe that you apply flux.
Finishing the Connection
After you apply flux to the inside of the fitting and the outside of the copper pipe, press the pieces together and heat the connection using a gas torch. When the fitting has been heated for about 30 seconds, press the end of a soldering reel to the seam of the connection. The flux will draw the solder into the fitting immediately. The connection is set when the solder bubbles out of the end of the seam.