“Solder flows toward heat!”
By Sara M. Sanford

SOLDERING BASICS

Solder Flux Flux Redux Fit and Cleanliness (a.k.a. solder will not fill gaps) Solder Joints and Placement Steady Soldering

There are three basic types of solder joints: sweat, which is surface to surface (joining two flat sheets of metal, one on top of the other); strip, or edge to surface (joining a bezel to its base); and butt, or edge to edge (joining two sheets abutting, or side by side, as in marriage of metals or as in two wires end to end). Each type of joint calls for a different approach to solder placement and heat control, but for all three types of joints, the main factor is the same and cannot be overemphasized:

Solder flows toward heat.

This simple fact dictates where the solder is placed and how the heat is applied in any solder joint. Disregarding this principle causes more frustration and disasters than anything else except a poor fit.

Sweat-solder joint, surface-to-surface

FOR SWEAT SOLDERING, or joining two pieces of sheet material, one on top of the other, the easiest way is to flow the solder over the underside of the top (usually smaller) piece, then flip this over onto the bottom (larger) piece.

After checking the fit and cleaning both surfaces, start by fluxing the underside of the top piece and the top of the bottom piece. If the areas to be joined are large, say bigger than about a half inch square, paste flux is the better flux to use. If the areas are smaller, then liquid flux can be used. (See “Flux” in Technically Speaking, April, 1999.)

Place the solder pallions on the underside of the top piece. How much solder to use is something learned only by experience, but a good rule of thumb is to cover about 20 percent of the area with solder (when using thin gauge sheet solder). Too much solder will flow out of the joint and flood the bottom piece, while too little solder will result in a bad joint. Solder has a tendency to pull away from the edge of the metal, so place solder as close to the edge as possible. Sweat soldering will use more solder than any other type of joint, so larger pieces of solder can be used.

If the top piece of metal extends beyond the edge of the bottom piece, mark the underside of the top with a scribe where the edge of the bottom piece will be. To prevent the solder from flowing over the extension where you don't need it, you can coat the area with a thin paste of yellow ochre (yellow ochre will be included in an upcoming discussion on antifluxes). Leave at least a one-millimeter border between the yellow ochre and the fluxed area so they won't mix and contaminate the flux.

Place the smaller piece of metal, with flux and solder side up, on a mesh screen on top of a tripod. This allows you to apply the heat from beneath and take advantage of that incontrovertible fact that solder flows toward heat. Use a titanium or steel poker to spread the solder once it becomes liquid, “tinning” the surface with solder. If the flux doesn't burn off and leave black spots (oxidized bare metal), you can simply reflux the top piece after it cools slightly, flip it over, and position it on the bottom piece. If the flux does burn off and there are black spots, pickle the piece, lightly sand and thoroughly clean it, and reflow the solder, adding flux and more solder if necessary.

Flux the top piece, then position it on the bottom piece. Place both pieces on the mesh screen on the tripod, and again, heat from beneath; the bottom piece is usually the larger one and will take longer to heat. Using the flux as a temperature indicator (also described in “Flux,” April, 1999), continue heating from underneath until the flux turns glassy, then bring the torch flame up on top and heat until the solder flows. This can be difficult to see: look for a liquid silver line where the two pieces are joined.

Slight pressure can be applied with a poker to the top piece as the solder begins to flow, but if you try this, be careful not to apply too much pressure, as metal at soldering temperatures is easily marred or distorted. Remove the torch, allow the piece to cool slightly, and then place in pickle.

Strip solder joint. Edge-to-edge surface

STRIP SOLDERING. The typical strip solder joint is used in soldering a bezel, a peg for a pearl or bead, or wire decorations onto a base. After making the fit as close as possible and thoroughly cleaning all the parts, flux the smaller part to be soldered and position it on the base, wiggling the smaller piece around a bit to flux the base. (Because the joint is usually small, liquid flux is my choice for most strip solder joints.)

When choosing the size of solder pallions and where to place them in a strip solder joint, there are several things to consider. In general, I prefer to use smaller pallions and more of them, rather than larger and fewer pieces. Solder flows by capillary action, and smaller pieces placed closer together don't have to flow as far. Because solder leaves tracks wherever it flows, using smaller pieces also cuts down on clean-up afterward, even if it means spending more time placing the solder beforehand.

Where to place the pallions depends on the design of the piece and how the heat will be applied. Solder should be placed where it will be either least conspicuous or most easily cleaned up. For soldering a bezel, for instance, this usually means placing the pallions on the inside of the bezel, where the stone will hide any solder tracks. But for soldering a raised frame to a base, it might mean placing the solder on the outside, on a small “shelf” left for this purpose when cutting the base. Afterward, the shelf and the solder tracks can simply be filed away.

The main consideration, however, is how the heat will be applied. Remember that solder flows toward heat (have I mentioned this before?). Place the solder on the side of the joint opposite where you intend to apply the heat, and the heat will draw the solder through the joint.

Commonly, in strip soldering, the pieces to be joined are of very different sizes and so will heat unevenly. The larger pieces will need more and longer heating to bring them up to soldering temperature, and the smaller pieces can easily be overheated and melted. The solder pallions will be the smallest pieces of all, so: never heat solder directly, but instead, heat the areas where you want the solder to flow because - that's right - solder flows toward heat.

For a typical bezel, I'll use solder pallions of about 1mm square, placed about 3mm apart, around the inside of the bezel. The heat is directed first on the base, from underneath if it is placed on a mesh screen on a tripod. Only when the flux turns glassy, indicating that the melting temperature of the solder is being approached, will I bring the torch flame up on top and then circle the outside of the bezel until the solder flows and the heat draws it out through the seam.

To solder on a wire peg for a pearl or bead, I usually prefer to drill a hole in the base for the wire, so it will stand on its own and won't need to be held in place. This also increases the contact between the wire and the base, strengthening the joint.

A small pallion of solder where the wire joins the base is all that is needed; again, heat the base, from beneath if possible, and direct the torch flame at the joint only when the flux has indicated the correct temperature. If drilling a hole for the wire is not feasible, I usually premelt a small bit of solder on the end of the wire, which is held in crosslock tweezers, and then heat the base to the melting temperature of the solder before placing the wire down on the base. (Technically, this last is a sweat solder operation.)

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Butt solder joint. Edge-to-edge surface

BUTT SOLDERING. A butt joint is produced by soldering the ends of a bezel together, joining sheet metal edge to edge (for “marriage of metals”) or joining the ends of a wire to make a frame or ring. Again, the fit is critical. Keep the edges of a bezel perfectly level, not rounding the corners, or there will be a gap. If possible when using larger gauges of metal, increase the surface area of the joint by beveling each edge to create a diagonal cross section. This is more difficult and time consuming but will result in a stronger joint.

The best arrangement for butt soldering is to place the solder on the backside of the seam, place the pieces on a mesh screen on a tripod, and then heat from underneath. However, if you want to join multiple pieces or if they are not level, you may need to place them on a soldering pad or coil to hold the parts in place while soldering.

In this case, control of the heat becomes critical, because you will be heating with the solder side up, and since solder flows toward heat, the solder will have a tendency to spread out rather than flow downward through the joint. You may need to reheat the piece from the front to draw the solder through the joints; if you do, make sure that all parts are held securely in place so they won't shift, and don't forget to reflux as well.

To solder a bezel closed, I usually hold it in a pair of crosslock tweezers on the side opposite the joint, place the solder on the inside just across the joint, and turn my torch flame vertically so the upward-pointing flame heats the entire bezel evenly. Using your smallest pair of pointed crosslock tweezers, just barely hold the bezel in the tips. Especially on small bezels, using a broad-tipped tweezer may cause the bezel to spring open when heated, separating the solder joint. The same technique for bezels applies to soldering frames or rings: hold in crosslock tweezers in a vertical flame with the solder on the inside of the joint.

For soldering jump rings closed, I place the solder pallion on a soldering pad, then (after fluxing both ring and solder) place the jump ring joint on top of the solder. This prevents the solder from shifting during the heating process, and since solder flows toward heat, heating from above will draw the solder upward through the joint. When making a chain, solder half the links closed first, on a flat surface, then join these links with the remaining jump rings.

Whenever a solder job is not going well, stop and rethink the process. Do I have the right solder for this job? Have I used the appropriate kind of flux? Do I have the right kind of heat source, and am I applying the heat in the best manner (i.e., am I remembering that solder flows toward heat)? Is the fit as close as possible, and are all the parts, including the solder and flux, clean?

Usually, upon a little reflection, the answer becomes evident.

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 in Lapidary Journal so far includes: “Solder,” December, 1998; “Flux,” April, 1999; “The Heat Is On,” July, 1999; and “Fit and Cleanliness,” September, 1999.

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