What is Flux?
Flux is part of a dynamic duo of products that allows you to connect copper pipe and fittings by “sweating” or “soldering.” Flux is an acidic paste that comes in a little round tin. The soldering process begins by cleaning the inside (or female portion) of the fitting and the outside (or male portion) of the pipe with a stiff brush and fine grit emery paper. The paste is then applied to the fitting and pipe with a small brush and then the two parts are fitted together. (Use a rag to wipe off any excess material.) Heat is applied to the joint, then wire solder is applied to the perimeter of the joint; the paste helps draw the solder in to create a water-tight joint.
Since flux creates a pathway for drawing in the solder, keep the flux confined to the joint area so it doesn’t welcome solder into places it shouldn’t go.