FLUX

There are many different kinds of flux used, but they all provide the same basic function. This function is to provide an oxygen barrier to the area being joined, so that it stays clean and does not develop oxides. Fluxes are usually water or alcohol based, either a thin paste or a liquid. They frequently use borax as a key ingredient.

Different fluxes have different characteristics, and are chosen for the type of work one expects to be doing. For example, a particular type of flux might be great if you're expecting to do a lot of high-temperature lengthy soldering jobs. Another might be more appropriate if you're doing general fabrication (less intensity of heat). Still different kinds of flux is used in casting, for repair work or with ferrous metals.

My personal favorite... the one I "grew up" with, is Handy Flux. It's a borax and water based white paste that's fairly common. One of the advantages of this paste-type flux is that it can be used as a temperature indicator. When the flux reaches about 1100ÂºF, the borax melts and the flux turns clear and watery. At this point, you know you're approaching the soldering temperature.

What the flux is actually doing at this point, is absorbing oxygen and preventing oxides, or tarnish, from developing. With sterling silver (92.5% pure) this is especially important, since the 8.5% copper content oxidizes quite readily when heated. Soldering copper has much the same problem, since it's all copper. Fine (99% pure) silver doesn't oxidize as readily, since it has no copper content.

Preventing the oxide from forming is important; and not just because it saves you from having to remove it after soldering. Preventing oxides from forming is important because they will inhibit the solder from flowing, or cause it to flow where you don't want it.

Flux can also become "spent" during the soldering process. After it has absorbed all the oxygen it can, a flux can turn greenish-blue, especially around the edges. This can occur if you are doing several solder joints during a single operation, or if you are having difficulty with a join and it's taking longer than expected. This is why different fluxes are made. Some fluxes are more "durable" or can handle higher temperatures.

If you reach the point that your flux is spent, it's time to stop and back up. Turn off the torch and drop the work in the pickle solution. One may attempt to add more flux if you're a gambler, but at the risk of overheating the metal. The best bet is to step back and troubleshoot. Repickle, reclean, check the fit and set it up again. If necessary, get a cup of coffee, step out for some fresh air or have some lunch. Getting some distance from the problem usually helps on the second attempt.