A: Pripp’s flux will do it. Pripp’s flux is a mix you make up yourself, and it works pretty much the same as a borax coat, which is the older and more traditional method. Classical silversmiths would often go through several sequences of “burning on” a borax coat before annealing or soldering, but it doesn’t work quite as easily or as well as Pripp’s.

I’ve been a gold and silversmith since college, and learned Pripp’s from Prof. Fred Fenster at the University of Wisconsin, who proclaimed in that 1972 sophomore class that at other schools people sometimes complained about firescale, but “here at U.W. it never gives us a problem”. This, by the way, was taught from the first moment we were shown how to light a torch, just to give you an idea of how important and basic a technique Fred felt it was. It’s named after Jack Pripp, who taught at Rochester for many years, and is considered one of the fathers of the American metalsmithing community.

To make it, you will need: a quart of water, 120 grams boric acid, 80 grams each TSP (trisodium phosphate) and borax. Boil to dissolve it (you might have to add a little more water. It’s the 3:2:2 ratio that’s important, not the concentration.). The Borax you can get at the supermarket, in the laundry area. (Borateem is just borax—the little green flecks they put in there too don’t seem to matter). TSP (trisodium phosphate) is a strong alkaline cleaner often used in cleaning walls and the like before painting. You can usually get it in paint or hardware stores, but be sure it’s actually trisodium phosphate. Because it’s rather a caustic (though reasonably safe) material, some stores carry a substitute, which may be confusingly labeled. (e.g. TSP brand wall-cleaner no longer contains TSP.) Read the box carefully. The substitute doesn’t work for this purpose. If you happen to have a chemical supply house around, you can also use disodium or monosodium phosphates. But the trisodium formula seems to be the most common.

You apply it (and this is an important detail) by spraying it on the silver while gently heating the silver up enough so the spray dries on contact, as opposed to hitting as a liquid and bubbling/boiling off. The best sprayers by far are the cheap little two-tube-with-a-hinge mouth atomizers that ceramics folks sometimes use for applying glazes. It gives a much finer and more uniform spray than any sprayer bottle I’ve seen, and cannot clog.

To use it, you’ gently brush the metal with the flame, then with quick short puffs on the sprayer, put the Pripp’s flux on a little at a time. The idea is to coat the entire piece with a thin white crusty coating, thick enough so that reflections from the metal are no longer visible, but no more. Be careful, as you do this, neither to let the metal cool so much that the flux stays liquid (it doesn’t coat evenly then), or that the metal gets so hot that it starts to discolor. It’s easy enough, but takes a little practice at first. Coat all the parts of your assembly, then let them cool, set up the joint, and with the addition of the smallest amount of additional soldering flux only in the joint (see below) and solder, do the soldering job.

Pripp’s is a much less active flux than the paste fluxes, and doesn’t burn off easily (though with enough overheating you can do it), so it gives continuous protection, and thereby completely prevents fire scale. It will work as a soldering flux all by itself if your metal and solder are both completely clean before you start, and if your heat control is good. Paste fluxes such as the “Handy” or Griffin brands, oddly enough, seem to provide little or no firescale protection. In fact, with some metals (like white golds) you’ll find the firescale is worse where the flux was. This is why you don’t want to use much, and should keep it only in the join area. But they are so very active while still fluid that they greatly promote solder flow, so many of us use them anyway. Battern’s self-pickling flux is somewhere in between—it lasts longer and doesn’t give quite the fire scale problem, but also doesn’t protect quite as well.

In my work, for simple repairs to already-made silver jewelry, I usually just use a boric acid/alcohol coat, solder with paste flux, and clean up later, as most of these pieces already have fire scale, and for a single quick ring shank solder job or what have you, it’s not worth the trouble to bring out the Pripp’s. But if I’m making something from scratch, then (with a few exceptions), every last annealing or soldering step is done with Pripp’s coating everything.

The added time and bother is more than paid back when it’s time to finish the piece—when there’s no surface oxide and no fire scale, then the piece can be polished out as easily as gold work. This coating, if you are careful and don’t pickle it off after soldering, can usually last through several soldering cycles; so for some complex assemblies; if you’ve got everything fitted before hand, you may only need to coat the parts once for a number of sequential soldering steps. Also, since the sprayers tend to cover rather more area than just your silver (like the tools and bench areas behind your soldering area), you will want to set up some sort of simple shield behind the area you’re using for spraying on the flux to catch that over spray. This saves a lot of mess.