Yesterday we talked about how soldering works on a molecular level to help us get ready for the soldered and wirework earring tutorial I'll be posting soon. Today we'll be talking about flux. As I mentioned yesterday, it's the compound that prevents the oxidation of copper at high temperatures so that the solder can flow into the molecular lattice of the silver you'll be soldering (remembering, of course, that sterling silver is 7.5% copper).
Flux absorbs oxygen through a chemical reaction. It begins to melt as the metal absorbs the heat from the flame; at soldering temperatures it then turns into a kind of glass. That's one reason that flux works well as a flow agent in enameling and ceramic glazes, as well as in soldering. There are, of course, different kinds of flux: different fluxes for ceramics, enameling, and soldering. Among soldering fluxes alone there are different brands and each brand or chemical formulation serves one of two different purposes: flow flux or barrier flux.
Part of what can make soldering seem so complicated is that there are so many steps: filing and cleaning, fluxing, choosing solder and applying it, the heating of the project to soldering temperature, quenching, pickling, and finally cleaning and polishing. So, when any individual step has a lot of different options, it can feel overwhelming. I hear you! But what I've learned is that having those options means that you have more opportunities to make your piece perfect by tailoring the process to suit your specific needs. I think it's important to remember that and use it as a goal for continuing to learn and perfect every component of your craft. It's also important to know that it is absolutely possible to approach soldering in a streamlined way, using the same products and process on a wide variety of projects resulting in good bonds and aesthetic results right off the bat.
For a long time I only used a flow flux in my soldering and was content to pickle, sand, and polish the firescale off of my work. That works. But, to cut down cleaning time, and to maximize the strength of your work (and not filing layers away from your piece), it's good to know that barrier fluxes are available.
To expand upon what I touched on yesterday, flow fluxes are the fluxes that prevent oxidation at the seam or joint that you want to solder. Barrier fluxes prevent oxidation on the rest of the piece. While there will be those that argue with me, it's really only essential to have a flow flux. That being said, you also need to know that every second the metal is exposed to heat the copper is oxidizing, so you want to use the largest flame appropriate for the job to complete the soldering as quickly as possible in order to minimize the firescale. That's the part that takes practice - knowing how much heat the piece needs and your margin of error between developing unrecoverable firescale and melting your work to kingdom come. Know that there is a workable margin there, but also know what you're trying to avoid. Fear not. I'll talk more about the application of heat in another post.
So, which fluxes are which?
|Flow Flux||Barrier Flux|
|My-T-Flux||Pripp's Flux/boric acid and denatured alcohol|
|Handy Flux||Stop Ox|
I think that understanding that there are two different kinds of flux is the most important think to know about flux - that you can't just use any flux for any job. Personally, I've used Pripp's Flux, boric acid and denatured alcohol, and cupronil for barrier fluxes. I like the Pripp's/boric acid fluxes well. For me, the cupronil has tended to leave a copper colored coating where I didn't want it. For flow fluxes I'll be honest off the bat and say that I worship at the altar of paste solder, which has flux in the paste. Hooray! I've also used and liked My-T-Flux and Handy Flux, and liked both well. As always, though, read reviews for yourself and check it out on your own. But, for a buying recommendation, I'd say start with either My-T-Flux or Handy Flux and Pripp's for a barrier flux, depending on what's more easily available to you.
Here is a video from YouTube that I love showing the soldering process using barrier flux and flow flux on a gold ring:
Just in case I didn't nerd-it-up enough for you here, a great article to read for a more thorough understanding of flux is at Ganoksin.
Thanks for stopping by!