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Sara M. Sanford is a professional jeweler and founding member and past president of the Creative Metal Arts Guild in Portland, Oregon. Her detailed examination of soldering continues with this discussion of flux.
You must have flux in order to join metal together in the basic metalsmithing process of soldering. Flux is a liquid or paste compound that is applied to the metals being soldered. By keeping the metals clean of oxides, flux helps solder flow (see “Solder,” Technically Speaking, December, 1998). Flux also acts as a temperature indicator: by observing the changes in the flux as heat is applied, you can gauge when the solder is about to flow. Liquid flux, sold by trade names such as Battern's© Self-Pickling Flux or Solder Fast™ High Temperature Flux, can be used with gold, silver, copper, brass, or bronze. Liquid flux is usually used on gold and on other metals for delicate soldering operations such as joining a bezel. The main advantages of liquid flux are one, that it is reasonably transparent and therefore will not obscure the solder seam during heating, letting you watch the soldering process closely; and two, it will flow more easily into solder joints than will paste flux. Liquid flux also has a wider temperature range (1100°F to 1700°F). Any flux forms a glaze on metal that acts as a barrier to air, and this prevents the formation of oxides that produce the darkening called fire scale. The wider the coverage of the flux, the greater the protection from fire scale. With relatively little coverage, however, liquid flux offers only minimal protection from fire scale. Paste flux, sold by trade names such as Handy Flux© or Grifflux #1©, is a white, water-based compound containing borax and boric acid. Like liquid flux, paste flux can be used on gold, silver, copper, brass, and bronze. It can also be used on steel, although there are fluxes manufactured specifically for use when soldering steel with silver solder. Paste flux is commonly used on silver and base metals, especially when working on larger pieces such as hollowware, as it has much better covering capability than liquid flux. As a result, paste flux can also help a good deal in limiting fire scale. Paste flux has its disadvantages as well. It is an opaque white that obscures the joint from view until just before the soldering temperature is reached. It also has a narrower temperature range (1100°F to 1500°F) than liquid flux. The old traditional flux was a borax cone ground with water on a piece of slate to make a paste. Paste flux can also be made from borax (but do not use Boraxo!) and boric acid in varying proportions, sometimes with other ingredients added. Most jewelry and metalsmithing how-to books contain formulas that you can consult if you want to experiment with making your own flux. Which flux to use when? My suggestions:
After selecting the type of flux and applying it, you may discover that you have used too much. As they become hot, both liquid and paste flux will bubble up and may displace solder pallions, and any excess flux will only make this more likely. How much is too much? Only experience can really teach you that, but if, for instance, your liquid flux forms huge puddles, you've probably used too much. Excess liquid flux can be wicked away with a dry brush or a piece of tissue before heating to help prevent the solder from being displaced. Another useful product for this is the “tortillian,” a tightly rolled, pointed paper stick used to smooth and blend pastel and charcoal drawings, which is sold in artist's supply stores. A fairly dry brush is necessary to remove excess paste flux. To minimize solder displacement during heating, heat the flux gently until the water in it evaporates and the solder pallions settle down. Replace any solder pieces that have moved. When you begin to apply heat, paste flux will remain an opaque white, while liquid flux will go from transparent to semi-transparent and grainy. As the melting temperature of the solder is approached, both fluxes will turn transparent. This is the signal to zero in on the solder seam, concentrating the heat on the area to which you want the solder to flow. Regardless of which flux you use, one of the most important factors is to keep the flux clean. Pour small quantities of either flux into a small glass container with a wide mouth (plastic should not be used on the soldering bench), such as a sample jar used for jams or mustards. It should have a tightly fitting lid, as both fluxes are water based and will evaporate if left uncovered. Paste flux will dry out and harden if not used regularly. Add water periodically to prevent this. If the paste flux does dry out, it can be reconstituted by adding a small amount of water and heating the uncovered jar in a microwave oven, stirring frequently; or by placing the uncovered jar in a pan of boiling water (at a level that will not flow into the jar) on top of the stove, heating and stirring until it reliquifies, adding small amounts of water as necessary. A last word on flux brushes. Since the bristles do tend to burn down quickly, many jewelers use very cheap brushes. I think this is a mistake. You need not use a sable hair brush, but I recommend using at least a camel hair brush for fluxing and for placing small pallions of solder. The coarser hairs of a cheap brush will not pick up solder pallions as easily, nor will it dispense the flux as smoothly along the seam. Using quality tools always makes the job easier.
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