Technical Briefs deals with technical concerns of interest to a wide audience, addressing topics as diverse as vanguard technology and ancient arts. It will also answer questions from readers and respond to suggestions, always trying to explain in simple terms the magic and wonder of working with metal. For this article, Tim talks about fluxes.
I am a self-taught metalsmith with a question about fluxes. I’ve read most of the standard textbooks and, while I find the same fluxes mentioned in most I can’t find a clear first choice. Which is the best?
– Desperately Seeking Fluxes
I sympathize with your confusion, but l can’t recommend a best flux either. There isn’t one. The choice of a flux depends on several factors, all related to the type of work, its scale and the maker’s personality.
The purpose of flux is to inhibit the formation of oxides while providing a surface along which solder can flow This is accomplished by absorbing oxygen before it can reach the metal to form oxides and by remaining liquid at high temperatures. These two qualities, the absorbing power and its gooiness, are what to look for in fluxes.
In situations where a small amount of heat is applied for a short period of time, and where the metal being joined is one that has only a slight tendency to combine with oxygen (e.g., silver or gold), a light-duty flux will do. This category includes the fluoride-based yellow or greenish liquids you have probably read about. These are especially appropriate to straightforward jobs like ring sizing, simple wire construction and so on.
An advantage of these fluxes is that they do not build up flux glass, the tough enamel-like residue of borax fluxes. They are called self-pickling, that is, they do not need to be cleaned in pickle after soldering. In situations that call for speed or when a pickle pot is unavailable, these would be the fluxes to use. Another, less important advantage of these fluxes is that they are a watery liquid. This makes them easy to apply with a brush and allows for spraying of the workpiece with an atomizer. If you do this, avoid breathing the mist and vapor created.
A popular and familiar product is a white paste called Handy Flux. The biggest advantage of this flux is its much greater ability to absorb oxygen. It is preferred for operations that involve oxide-forming base metals like copper and brass and for situations that call for a lot of heat. Even then, the absorbing power of the flux may be exhausted. To see this for yourself, try this experiment: Cut a piece of copper a couple inches square, clean it and coat it with flux. Set it onto a soldering block and apply heat to a spot in the center. First, the water from the flux will boil away, leaving a white crust. The flux will melt, take on a reddish-brown color and sag outward. Next, the color will burn away, leaving the flux clear and the copper a bright orange. Depending on the thickness of the flux layer, this state will last for a minute or two. Then, around the edge of the plate, you’ll see the copper start to turn dark. Keeping the flame fixed in the center of the sheet, you’ll see the ring of dark green, oxidized copper creep in toward the flame. These are the copper oxides forming. Within a couple minutes the circle of clean copper in the center will have shrunk to a dot and then disappear. This illustrates the absorbing power of the flux and the result of the flux becoming saturated.
If you allow the copper to air cool, examine the flux glass and you’ll see how tenacious it can be. Removing it with abrasion is difficult and risky, since the file or abrasive paper tends to slide off the glass and wear down the metal around it. The solution is to dissolve the flux in pickle. In warm Sparex, most flux glass deposits will come off in about five minutes. Especially thick coatings may take longer. The residue can be rinsed off in hot water, but it takes longer.
Besides its oxygen-absorbing power, borax-based flux has other features. One is the fact that it always turns clear at 1100°F, making it a reliable temperature indicator. Another quality, both loved and hated, is the way it acts like a glue. Handy Flux can help hold a chip of solder in place when used respectfully, but will stick work relentlessly to the soldering block in unfamiliar hands. Experience is the only guide to controlling this quality.
A third choice in fluxes is the one called Prip’s Flux. This is a mixture you can make yourself (I’m sure you’ve seen the recipe in your reading). It is a liquid like the light-duty flux, but shows a great strength to protect metal against oxides. Prip’s does not have the wetting power of the other two, so it is more difficult to develop a complete coating. A recommended method is to warm the metal, brush or spray on the flux, then warm the metal again and repeat the process. This must be done several times to establish an air-tight shield against oxidation. On the positive side, Prip’s is not water soluble, so if you cool the piece in water rather than pickle, the flux layer is left relatively undisturbed.
Another solution you might have read about is a mixture of alcohol and boric acid. Before soldering, a jewelry piece is dipped in a concentrated solution of these two ingredients, set on the soldering block and ignited. The alcohol burns off, leaving a thin white layer of borax. This is a good light-duty coating to prevent scale formation, but does not contribute much to the actual flowing of the solder. Many people use this method then apply a small amount of diulte Handy Flux to the joint area.