Without heat, the soldering process just doesn’t happen, right? Here, we take a look at the different gases used in soldering and how to achieve the flame balance needed for soldering success.

Fuel Gases

Heating jewelry pieces during soldering is done with torches, which mix either oxygen or air with a gas. Hydrogen, natural gas, acetylene, propane, and butane are the fuel gases most commonly used.

It’s All In The Chemistry

One of the most important steps to achieving the perfect flame is to control your fuel to oxygen ration. Consider the combustion of natural gas — mostly methane (CH4). When completely burned, it reacts with oxygen.

If there is excess oxygen in your oxygen/natural gas mixture, you’ll wind up with an oxidizing flame. The oxygen not captured by the flame can create oxides on the surface of the metal you’re soldering. This is a flame you definitely don’t want. You’ll know if you created an oxidizing flame by its appearance — pointed, with a conical shape forming at the head of the torch making two flame cones (an bluish, inner flame cone and a dark blue outer cone).

At the other end of the spectrum is a flame with oxygen below the level needed for complete combustion. This results in a reducing flame — another type you don’t want. Its appearance consists of a soft-edged, bushy yellow flame. Reducing flames contain carbon monoxide instead of carbon dioxide and water vapor. Without enough oxygen in the mix, the flame will be cooler, causing unburned fuel particles to contaminate the metal.

It takes practice to achieve the correct mix of fuel and oxygen. The perfect flame contains a balance where there is sufficient oxygen to burn all the fuel present and nothing more.

Flame Effects

High-karat golds and platinum are not adversely affected by heating with a slightly oxidizing flame.  Lower-karatage alloys, however, require a neutral or slightly reducing conditions to prevent oxidation of the base metal.

When using hydrocarbon fuels, it’s best to avoid using a reducing flame on platinum, because it makes the carbon within the metal brittle. Specifically, acetylene torches should be avoided. Acetylene flames are considered to be dirty for leaving flakes of carbon on the soldered pieces.

If you haven’t read our article on the Top 10 Soldering Mistakes in our online library, you may want to check it out. You can even check out our basic tips article as well.

Do you have fuel or flame balancing tips you’d like to share? Enrich this article with your comments.


[Adapted from: The Stuller Metals Catalog, volume 76]