Notes on Soldering Silver

Silver is joined by a process that silversmiths call soldering by convention, but that metalurgists call brazing. True soldering involves only wetting of the bulk metal by the solder, essentially a surface process. Brazing is done at higher temperature, and involves diffusion exchange between the bulk metal and the braze. Sterling silver is .925 Ag, .075 Cu. A very hard (very high-melting) jeweler's silver "solder" is .75 Ag, .225 Cu, and .025 Zn. That melts at 1841F, and is suitable for fine silver, but it's too close to sterling's melting point for most uses. In use, as the liquidus temperature is maintained, silver diffuses into the solder, and zinc and copper diffuse out, raising the melting point and delocalizing the joint. The color is close enough so that the joint is invisible when polished. The tarnish properties are different, and a slightly tarnished piece with joints that have cooled soon after melting and wetting will show a fine dark line at the joint. When the heat is held for a minute or so, this "line" becomes too diffuse to be noticed by eye (but a photometer can show that it's still there).

The common silver solders that jewelers use go by the names "easy", "medium", and "hard". These names refer to the melting points; Easy has the lowest. "Medium" is the easiest to use because it flows - wets the sterling - most readily. "Easy" is the most difficult. The right flux is important, too. There are liquid fluxes that clean up easily and work well for experienced users. Paste fluxes don't burn out with extended heating, and are often better for beginners and for large jobs where it can be a long time from the first joint to the last. The glassy residue can be removed by boiling in water. Painting the entire piece with paste flux can minimize the risk of forming "fire scale".

To help clean the piece after soldering, it is quenched in "pickle", a moderately strong sulfuric acid bath, or a pretty concentrated solution of sodium bisulfate. Hydrochloric (muriatic) acid is better for copper and brass. (Platers call it "bright dip".) Use two containers, one for the pickle, the other for rinse water. Pickle will eat holes in cotton and other vegetable fibers, and it can splash when a hot piece is dropped into it. Wear an apron. Don't pick up a piece in the pickle with iron tongs. Some dissolved copper will plate onto the silver because of the galvanic action. Copper tongs are standard; stainless steel might work. A hook of copper wire might suffice, and I don't hesitate to use my fingers if I have no broken skin.

A piece of firebrick makes a good work surface. For small pieces, and especially when soldering with an alcohol lamp and blow pipe, a charcoal block is dandy.

Silver solder will not fill visible gaps. Its strong wetting action causes it to flow between surfaces that are in contact. It flows toward hotter parts, and will flow on the surface away from the intended joint if the joint is somewhat cooler than the bulk in one side. Ocher or jewelers rouge can be made into a thin paste with water and applied as an anti-flux to keep the solder away from where it isn't wanted. India ink works too, and seems to be the preferred anti-flux for making hinges.

I use cut pieces of solder; these are approximately one-millimeter squares (or whatever shape results) cut with shears from sheet. Silver solder also comes as wire, ribbon, and occasionally, pre-cut pieces. (I find wire or ribbon occasionally, but rarely, useful.) The pieces are placed along the joint at intervals between 1/4 and 1/2 inch, depending on the thickness of the material for butt joints, the size of the desired fillet for tee joints, and so on. A single piece suffices to fasten a post to an ear ring or butt-join 16-gauge wire. The pieces can be placed by touching one -- but not the cup in which it lies -- with a fine brush moistened with flux. When the brush is touched to the fluxed joint, it will come off the brush and stay on the joint.

An iron poker is handy for leading the molten solder when it is reluctant to flow and to push an unmelted piece back into place if the action of the flux has moved it. Keep the tip of the poker hot, so that something touched by it won't be chilled. Some sacrifice a scriber for this. I made mine from 1/8" square rod, twisted where I hold it, and ground to a point. The wire from a hanger (a thick one is nicer) folded so that about 6" is double and 3 inches is single makes a nice poker too. Twist the doubled part to make a comfortable grip and point the free end. Don't leave a hot poker lying around; quench it in the rinse water before putting it down.

Silver-soldering fluxes are either pastes or liquid, but in either case, they contain salts that have water of crystalization which is expelled by the heat of the soldering torch. Care is needed to expel the water slowly enough so that the solder pieces are not displaced (or popped across the room!).

Polishing takes more work than the rest. There are polishing wheels that can replace grinding wheels, and Dremel tools are good for small work. The abrasives you will need are Tripoli (rottenstone) for the first go (cut) and rouge for finishing (color), both in stick form. I have seen these in Sears. DON'T get Tripoli on your rouge wheels. Keep them separate and wash the work with soapy water and a toothbrush to remove all traces of a coarse abrasive before going to a finer. (Just like grinding a mirror.)

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