Firescoff flux occasionally for a while, but it is fairly expensive so I like to save it for soldering operations that tend to cause the most firescale, like soldering pin findings onto the back of a brooch.
I read that plain borax has been used as a flux historically, so I decided to give it a try. I started by mixing borax powder (the laundry kind) with water to make a paste, but the grains were too big, making a gritty mixture that was hard to apply to the metal. After some experimentation, I figured out that the trick is to use a mortar and pestle to grind up the borax and water to make a finer, more "paintable" paste.
You can also use denatured alcohol instead of water, which has the advantage of burning off quickly when the torch is applied, instead of boiling off like water does. Denatured alcohol is kind of nasty stuff though, so I decided to try water first.
So how does it work? Well, not as well as good old trusty handy flux, but well enough. It doesn't stick to the metal the way paste flux does, so it helps to get the metal nice and clean first so the borax and water mixture doesn't bead up on the metal surface. The borax also kind of puffs up when heated, before it melts down to a clear coating. It takes a little getting used to. I think I will keep working with it for a little while and see how it goes.