Flux Redux by Falk Burger Named for its usefulness in making metals flow, flux in the jewelry studio is a basic soldering supply.

Falk Burger is a self-taught goldsmith of 30 years' standing based in New Mexico. The discussion of flux that appeared in this column in April prompted this expansion on the subject.

SOLDERING BASICS

Solder Flux Flux Redux Fit and Cleanliness (a.k.a. solder will not fill gaps) Solder Joints and Placement Steady Soldering

“Flux” means “flow”: the choice of this word was probably deliberate on the part of early smelters, who noticed that molten metal tended to form a mass of tiny, molten balls in a matrix of impurities that kept the globules from coalescing into a flowable mass at the bottom of the crucible. What was needed was a kind of detergent or wetting agent that overcame the extremely high surface tension of the liquid metal, dissolve interfering oxides, and wash away insoluble impurities to be skimmed off as slag, very much the same way as detergents in water dissolve grease and lift off dirt particles. Metal halide salts were found to answer this purpose: even table salt can be used as a flux.

Bench jewelers encounter fluxes and metal flow most commonly in the process of soldering, which is understood to mean “a type of welding in which the metals to be joined remain in the solid state, while the joining metal is liquid.” In practice, the properties of commercial fluxes are as follows.

FLUXING ACTION
The best fluxes for oxide removal, impurity flushing, bead running, and working visibility are paste fluxes. There is no sight more satisfying than that little bright line of solder running down a seam farther than you ever thought possible. Also, in prolonged or complex soldering, molten paste fluxes remain transparent, while liquid fluxes become so dark that observation of flowing solder is no longer possible. (I have heard some jewelers object that paste fluxes obscure the seam until just before soldering temperature is reached, but since you don't need to see the seam until then, I don't see the problem.) For those who are concerned about limiting their use of acids (pickles), it's good to remember that paste fluxes are readily soluble just in hot water.

Remember that paste flux is concentrated. When soldering a bezel to a plate, for instance, try applying just a very thin line of flux to one side of the seam using a solder pick; then place your solder pallions on the other side of the seam, using the solder pick coated with flux to pick up the pallions. As you heat, you can just observe what happens, which is why paste fluxes are so popular.

As useful as paste flux is and as much as I use it (I almost never use liquid flux), there is one downside to it: toxicity. The warning label is quite straightforward: “Fumes and gases can be dangerous to your health. Burns eyes and skin on contact. Can be fatal if swallowed.” The fluorides in paste fluxes are the reason I use them sparingly and with ventilation. My use of paste flux is limited to small parts fabrication and assembly - bezels, joints, catches - and cracking a nearby window is adequate for my comfort. When soldering large pieces or large numbers of pieces, I recommend increased ventilation.

However, I don't use paste flux to coat large pieces for the prevention of fire scale. The volume of fumes increases proportionately with the volume of flux, and in annealing hollowware, the worker stands over the piece, more directly in the rising column of increased fumes. Only once have I breathed in the fumes, trying to get a better look at a large brooch I was soldering, but the intense, burning sting in my nose and eyes made me jump back involuntarily (luckily, I didn't knock anything over).

Besides, just as Faberge produced cigarette cases that were deliberately fire-scaled, I consider fire scale a legitimate surface. As attractive as virgin metal, it poses no problem unless you polish through it. The solution? Finish your work completely, short of stone-setting. Heat the piece without flux once or twice more to achieve an even oxide coat. Pickle, set, and burnish or buff lightly.

If you must banish fire scale completely, there is another way. Dip your piece in a saturated solution of boric acid in alcohol and, holding the piece with tongs and away from the open dish of alcohol, light the still-wet piece and enjoy watching the blue-orange flame die off, leaving behind a perfect, unbroken frost of boric acid.

Some final tips: Dendix brand paste flux is a high-performance flux that contains no fluorides. I always use distilled water to thin fluxes because tap water generally contains carbonates that act as an anti-flux after repeated thinning. I also reconstitute dried paste flux cold with a ceramic mortar and pestle; I used to heat fluxes, but on cooling, there was crystal growth and the mixture became grainy. Try using a fine solder pick as a paste flux applicator. You can control delivery by adjusting the viscosity of the paste.

For liquid fluxes, try digging up an old dropper bottle. Wearing safety glasses, heat the well-dried glass barrel of the dropper right where it begins to thin toward the tip, rotating the barrel in the stationary torch flame until the glass softens. Then pull on the tip with tweezers or pliers to produce a long, thin nozzle for precise delivery (or removal) of liquid flux. After the glass has cooled, break the nozzle off at the length you want and flame-polish the aperture. This avoids the messiness of flux brushes and that occasional stray hair that snags the work, usually into your lap!

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