While goldsmiths normally strive to have their solder flow easily, there are times you don’t want it to flow, such as spilling over a decorated surface, if it ‘freezes’ a mechanism, or if it threatens to open a gap in a seam soldered earlier in a construction.
The best way of not melting earlier seams is to learn heat control and use heat sinks creatively to suck heat from the immediate area where the threatened join is. I use small blocks of steel for this purpose. Repair jewelers sometimes use stacks of copper pennies as heat sinks – but make sure they are pre-1974 or so ( as later ones have zinc cores which can melt out suddenly with unfortunate consequences. A solder’s melting temperature rises every time it is heated so there is the (admittedly mostly mental) assistance of earlier seams melting at a higher temperature than the same solder you are using again for a join.
Heat sinks won’t work well with silver objects, as silver conducts heat so rapidly. Instead, for silver constructions and for especially delicate jobs a solder flow retardant can be really useful. A paint-on solder flow retardant is easiest to use.
There are commercial solder flow retardants, but I’ve never needed to use them. Here are some of the ways that goldsmiths use to prevent solder flowing.
Polishing Machine Rouge: Some people use a little rouge powder from the polishing machine dust mixed with a small amount of oil which works well. Jeff Demand of Toronto likes to make a heat the rouge until it melts and then paint it on.
Pencil Lead: a graphite stick or soft pencil will leave a barrier that solder won’t flow across. Pencil leads also work to line up tube sections when constructing a hinge. Liquid graphite lubricant that can be found in for automotive supply stores can be used.
Scrap of Rubber: One can take a small piece of rubber (vulcanized mold chunk, a slice off a piece of an old torch hose etc) and, holding it in tweezers, rub it on the metal part to be protected while it is hot. The rubber melts on with a truly nasty smell (use ventilation) and leaves a brown slimy coat on the metal. This too works well.
Yellow Ochre: Old timers used yellow ochre with water. I find that it has a tendency to “bleed” into the flux and make the soldering problematic. It is somewhat better mixed with a little oil instead of water but I find it difficult to remove from the piece and do not use it.
Maalox® and “White-Out”: My favorite is Papermate© correction fluid. There are solvent based and solvent-free water based versions. In my experience the water based version does not come off the metal after soldering as easily as the solvent based one. I like the solvent based one for this reason.
I wrote to the Papermate company listing all the metals, chemicals and temperatures that their product would come in contact with in standard use by metalsmiths. In their return letter no mention of chemical interactions with the white pigment was made and they felt the main danger lay in the solvent used: 1.1.1. trichloroethylene which is a mutagen and carcinogen. This leads me to think that the white material is something inert (chalk). 1.1.1. trichloroethylene breaks down when heated to form chlorine gas, hydrogen chloride and phosgene gas, all highly toxic. Their lab ran evaporation rate tests and found that because it skins over the solvent does not evaporate as quickly as they had thought. About twenty minutes after application the solvent will for all intents and purposes have evaporated. There is a safety problem here: it would be wise to use water rather than solvent based materials. Ceramist’s ‘kiln wash’ might work in the same way.
I have a suspicion that Papermate® and similar products are mostly calcium carbonate, what print makers call ‘whiting;’ and what the rest of us call ‘chalk’. The cheapest source by volume for chalk in solution is probably Maalox® though I have not yet tried it as a solder flow retardant.
Robert Kaylor of Boise, Idaho uses China White, a standard graphic artists material made mostly of chalk as an effective solder flow retardant. It comes as a liquid in a tube or in a dry cake which one uses like a water color cake with a brush and a little water.