Soldering Flux – The Stained Glass Magic Elixir?

For those of us that work with stained glass, "soldering flux" is one of those "gotta use it" items that we know we need, but we just do not like to talk about. Shhh, it's secret stuff! Whether we use the liquid, prefer the paste, or swear by the gel, we understand "what" it does, but we really do not know the "how" or the "why". Today, I'll attempt to fill-in the blanks, and answer those questions.

The way I see it, fluxes should not be categorized by their form factor: Liquids , pastes , or gels all work well, and it's very much a matter of preference which to use. The addition of glycerin or emulsifiers can change the form factor, but they really do not participate to a large extent in the chemistry that makes a flux "flux".

Which begs the question, "What does a flux actually do?" During the soldering action, a flux is responsible for three tasks:

  1. Dissolving of the metal oxide (MO) layer of the metal,
  2. Protecting the metal surface against further oxidation, and
  3. Enabling the "wetting" action of the solder, by lowering surface tension.

Some may argue that a fourth task might be, "creating a strong bond between the base metal and the solder", but I excuse that by the time the bonding takes place, the flux has done it's job, and has left the scene. And this is really the "why" answer; we use flux so that we can create a strong bond between the foil (or came) and the solder. In other words, we want good solder lines .

In order to understand "how" flux performances it's magic, we need to categorize them according to their chemical composition. I prefer to break fluxes down into three categories this way:

  • Highly-active, acid-based fluxes,
  • Rosin-based environmentally-friendly "safety" fluxes, and
  • Mild restoration fluxes.

Let's look at each of these in detail:

Acid-based fluxes
This category of fluxes includes most of the "everyday" fluxes used by glassers. They contain one or more salt compounds, typically ammonium chloride (aka, Sal Ammoniac) and / or zinc chloride (aka, Killed Spirits), which are highly reactive with the metal oxides (aka, tarnish) on the surface of foil or came. These fluxes usually also contain a strong acid, such as hydrochloric acid, as a "pickling" agent.

One note about acid-based fluxes; they must be removed from your stained glass project, by washing with soap and water and / or neutralizing with "Kwik-Clean", after soldering is complete. If the flux is left on, "white mold" will develop over the course of time on solder lines. This "white mold" is not really "mold" at all, but a continued reaction between the flux and the base metal, resulting in a moldy-looking formation of salts; namely lead nitrate (a carcinogen) or stannous chloride .

Rosin-based safety fluxes
A second type of flux, one that I prefer, is made from certain species of pine trees, specifically the pine tar from these trees. If we "distill" the pine tar, we end up with "rosin" (aka, colophony), which is a blend of mild acids and other compounds. If you've ever heard of "rosin-core solder" used in electronics, this is the same stuff wrapped in solder, albeit in a slightly different form factor. (Note: rosin-core solder should not be used for stained glass).

I like these fluxes because they are environmentally friendly; you are not washing zinc chloride down the sink when you clean your project. And because they are milder, they can be left on your foolish or came for a longer time, without fear of pitting or corrosion.

Cleanup is easier with a rosin-based flux, because it easily dissolves in water, unlike the "greasier" acid-based fluxes.

Mild restoration fluxes
These fluxes are used for restoration work, primarily lead came. They have a higher pH, and as a result, are milder. The fluxes that fall in this category are oleic acid and steric acid .

They are also a good choice for beginners learning lead came, but are not recommended for copper foil. They do not have the punch needed to pickle the copper.

Final Word
I hope I've taken some of the mystery out of stained glass flux, so that you can better understand the "why's" and "how's" of stained glass.

As always, whenever you are working with a soldering iron, you should work in a well-ventilated area, as the fumes given off (zinc oxide and acid fumes) can cause respiratory problems down the road. Also, if your skin comes in contact with flux, be sure to wash well with soap and hot water.

And always wear your safety glasses with side-shields! Molten solder can splash up in your face, so always work safely.