Flux

Flux is a chemical compound—usually zinc chloride, rosin, or an acid-based paste—that removes oxidation from copper tubing and fittings before you solder them together. Without it, solder won't wet the joint properly and you'll get a weak connection that leaks.

In the field, you apply flux to both the inside of the fitting and the outside of the tube before heating. The flux liquefies as the joint warms, scrubbing away oxide layer so the molten solder can form a solid bond with the base metal. Different fluxes work at different temperatures: rosin flux for general copper work, acid-core for tougher jobs, and water-soluble flux when you need easy cleanup.

The type of flux matters for code and durability. Some jurisdictions restrict acid-based flux on potable water lines because corrosive residue can attack the copper if not thoroughly cleaned. Always read the label and rinse with water when the manufacturer requires it. Using the right flux is the difference between a joint that holds for decades and one that fails in months.