Flux

What is flux?  For the metal smith, flux is an oxygen inhibitor.  That's really it.  When metal containing copper (which is sterling silver, alloyed gold and brass) is exposed to heat and air at the same time, it oxidizes and creates a scale layer that solder cannot attach to.  Solder is also made up of silver, copper and zinc and so it also needs to be protected from oxygen while it is being heated.  Flux goes onto the piece while it is clean and cool and forms a glassy barrier when melted which air cannot penetrate until it is finally burned away through extensive heat exposure (i.e. you've been heating the piece at high heat for a long time).

If you've done it right, the solder will melt and join the pieces of metal you want to join while the flux is still protecting the seam.

For years, metal smiths used fluxes containing potassium biflouride, a chemical that is fairly nasty to inhale but works great as an oxygen inhibitor.  Many of us know this kind of flux by it's brand name "Handy Flux".

The original paste flux I always used- still a great flux and still in use today

Years ago an alternative came out called "Dandix Flux", which didn't contain the potassium biflouride.  Yeah. It sucked. Big time.  It didn't hold for very long under heat, meaning the time one had to get something soldered before everything turned black and dirty was  greatly reduce.

Booooo!!!!!

I tried it and ended up going back to the more chemically Handy Flux. Until, that is, Bob Coogan at the Appalachian Center for Craft turned me on to Superior #6 brazing flux.  This stuff is just as good as the Handy Flux and it doesn't contain the potassium biflouride.  I've used it for years and highly recommend it. You can order it by the case from the company directly or by the jar from H&N Electronics in California.  That's not to say flux is completely safe, one still has to take sensible steps to minimize exposure, but at least we can cross one chemical off the list!

This is my somewhat dilapidated jar.  I've had it for years and it's still half full.

By the way, I was trained on paste flux so that is what I'm comfortable using.  I know some jewelers use Battern's, which is a yellow liquid flux and some use a combination of powdered boric acid and denatured alcohol.  I don't use those.

Battern's is really more for gold soldering in my experience- and I have no idea what is actually in it. And boric acid- well, if you accidentally ingest boric acid- there's no antidote and you can be poisoned as well from long term absorption through the skin (I've read the paste fluxes contain boric acid as well but it's already mixed in and not a powder to be mixed, which I think is somewhat safer). 

There are more products on the market of course and some new ones say they are better and safer than the old paste flux. One new one is called Firescoff.

It's supposed to be very safe.  Metal smith Polly Smith uses it in conjunction with paste flux and really likes it, especially with Argentium silver.  I have some but haven't tried it.  It's expensive so Polly suggests painting it on rather than spraying it as directed.  I'll let you know if I like it when I finally getting around to trying it.

There's also something called Cupronil, which I also have a bottle of someone probably gave me and I've never tried it.  I'll report on it as well if I ever try it!

Cupronil Flux

It's important to note there are many fluxes out there.  If you know of any other fluxes not mentioned here you really like or you have tried Firescoff or Cupronil and have some input- please leave a comment!