The word ‘flux’ is originates from the Latin ‘Fluxus’ meaning ‘flow’. Flux is a cleaning agent applied to copper pipe joints before soldering.
Flux is usually made from either zinc chloride or zinc ammonium chloride.
When flux is applied to the piping, it chemically cleans the surface of any oxides that are present on the pipe’s surface, by dissolving them.
When flux is at room temperature, its chemical state is inert (chemically inactive).
When flux is used during soldering, it allows the solder to move (flow) easily over the surface, helping to tightly seal around the pipe joint.
Flux should be applied using a special flux/acid brush (flux may damage the bristles or cause them to fall out of a normal brush). An acid flux brush is a brush made with stiff durable bristles, usually made from black horse hair.
Once the joint has been soldered, all remaining flux should be removed. Flux will need to be cleaned off the pipe work, because when it is heated and cooled, it becomes alkaline and leaves residues, which are corrosive to the pipe work.