Soldering paste is made from either an alloy or pure metal, and can be made from nickel, jeweler's gold, copper, aluminum, silver or lead, though soldering paste for copper installation is generally made from atomized filler metal, flux and a paste binder to help join the parts together.
Once applied to the copper surface, the flux helps clean any residue or dirt from the coupling/pipe surfaces not previously sanded away with emery cloth. Upon application of heat, flux is released from the paste first, which prevents any oxidation on the copper while the heating process is carried out.
Capillary action (also caller capillarity) is the process in which liquid tends to be drawn into narrow gaps, even against gravity. The space between the copper pipe ends and the coupling is 0.004 inch, which draws the melted solder into the gap making a water-tight seal. In scientific terms, capillary action occurs when intermolecular action between the liquid (melted solder) and solid (copper) is greater than the cohesive molecules in the liquid.
After pushing the pipe ends into the coupling's inlets, heat the coupling with a propane torch and touch the solder to the seams where the pipes enter the coupling. As the solder melts to a liquid on contact with the seams, it is drawn between the pipes and inner coupling to form a watertight seal.