A Plumber’s Soldering Equipment
Q: So what’s in the bag?
A: Three things: Flux, solder and a blow torch.
Plumber’s Flux
In joining copper plumbing pipes, flux serves two purposes:
First, flux is an acid that is inert at room temperature and becomes reactive at high temperatures. So why is an acid needed when soldering copper plumbing pipes? The tin-silver solder used by plumbers bonds really well to copper, but not to copper oxide. Oxides are the byproduct of oxidation, the reaction between metal and oxygen. Since oxygen is a big part of the air we breathe, copper plumbing pipes are always covered by a thin layer of oxide which has to be cleaned off before they can be joined with solder. However, at high temperatures, oxidation happens almost instantaneously. This means all the oxidation that was cleaned off returns as soon as a copper pipe is heated. The acid flux burns the oxides away as they form and gives the solder a pristine copper surface with which to bond.
Secondly, flux is a wetting agent. Once in liquid form, solder behaves just like any other liquid and forms into drops as a result of surface tension. This behavior prevents the solder from naturally flowing into and filling the joint between the two pipes. Combining the solder with the flux once both are heated and in liquid form reduces the surface tension and draws the liquid solder into the joint, filling it. The force with which the flux draws the solder is strong enough that the solder can be applied to the bottom of a plumbing pipe joint and the flux will pull it, against the force of gravity, to the top.
Plumbing Solder
Solder is a metal with a significantly lower melting point than the metals being joined. Typically solder is a an alloy, or a combination of two metals. Today, plumbers use a silver-tin alloy when installing copper pipe meant for potable water. However, up until the 1990’s lead-tin alloys were most commonly used in plumbing because lead was cheaper than silver and was more easily worked once the joint was formed. Use of lead based solder continued even after the harmful effects of lead were known because it was thought that the amount of lead entering the water supply from the soldered pipe joints was too low to cause harm. Since it is now known that even small doses of lead can be harmful, all copper plumbing for potable water is done with lead-free silver solder.
Plumber’s Blow Torch
People familiar with soldering electronic components will be familiar with soldering irons. However, when working with copper pipe a blowtorch is required. This is due to the conductive nature of copper and the sizes of the pieces that plumbers typically work with. Copper is one of nature’s best conductors, both of electricity and of heat. That’s why even a small copper plumbing pipe has sufficient surface area to dissipate heat faster than any iron could supply it. That’s why a blowtorch is typically used. Even with a blowtorch, however, it is important that the copper pipes being soldered are dry as even a small amount of water can cause the heat to dissipate too quickly, cooling the solder before it has time to make a proper joint.