Mark Nelson's Tips: Mixing Boric Acid Barrier Flux

This illustrated recipe explains how to make your own barrier flux with alcohol and boric acid.

Last edited: 10/16/2019
Boric Acid, denatured alcohol and a mason jar

Mark Nelson's Tips: Mixing Boric Acid Barrier Flux

When soldering standard sterling silver, brass, copper, nickel, bronze, gold or silver-filled pieces you will want to use two types of flux. One flux to protect the metal from firescale and another to help the solder flow. Our Jewelry Tech Team calls these a "barrier flux" and a "flow flux."

There are several barrier fluxes on the market that you have to spray onto a piece, like Cupronil, Stop Ox, or Firescoff. Another alternative is to use one that you dip into or paint on. This second type is a boric acid powder/denatured (or methyl) alcohol mix. It's an old recipe that has been around for a very long time and it works great to protect the metal from oxidizing during soldering. It's homemade and easy to mix.

Here's what you need:

  • Boric acid
  • Denatured alcohol or methyl alcohol (denatured alcohol can be found at your local hardware store, in the paint department). Both will work but we have found that Isopropyl (rubbing) alcohol does NOT work very well.
  • An air-tight container—I prefer a glass container with a well-sealing lid like a mason or candle jar. But anything that seals tight will work. If the seal is not tight the alcohol will evaporate.

Here is how we do it:

Pour the alcohol into the jar. An inch or two is really all you need in most cases.

Pouring denatured alcohol into a mason jar

Next, add the boric acid until you can see that the ratio of powder to alcohol is about 50/50 (this is why using glass is handy). A 50/50 ratio is just a standard recommendation—some like to use less boric acid, some like more. I try to keep the acid at no more than 60%.

Adding boric acid to denatured alcohol in a mason jar

To use the mix, you can either dip the piece into the solution or paint the solution on. I highly recommend painting small pieces like jump rings, since they're easy to drop into the solution but nearly impossible to retrieve! If you dip, then be sure to swirl and mix the solution; otherwise you won't get a good coating.

Mixing and applying the boric acid solution to a piece of jewelry

Once the mix is applied, you can either wait for the alcohol to burn off or light it and let it burn off. I like burning it off because it's quicker—and it makes a pretty blue/green flame! You should be left with a nice, even, powdery coating.

Burning off the denatured alcohol from a jewelry piece to leave behind a coating of boric acid

Now you are ready to apply the flow flux and the solder and do the soldering.

Flow fluxes include Handy Flux, My-T-Flux, Rio Ready-Flux, and Grifflux among others. These make horrible barrier fluxes and don't protect the surface very well. They typically bead up and give you a polkadot firescale pattern. Which could be cool if that's the kind of thing you are looking for—but normally it's not!

Some more things to consider:

  • The alcohol is flammable so BE SURE TO BE CAREFUL! In the past, I have accidentally lit the jar on fire. If that happens, just put the lid back on and the flame will go out.
  • Over time the container will get crusty from the boric acid, but it's very easy to clean. Just scrape off the coating with your fingers and then soak the jar in water. After a short soak the container will be good as new.
  • The boric acid isn't particularly harmful; it is sometimes used as a medicinal scrub. That said, you should avoid getting it in your eyes or ingesting it. As always, it's a good idea to consult the SDS (Safety Data Sheet).