Soldering can seem like alchemy at times. You apply heat, the solder runs, and the pieces still won’t stick together or you end up with a joint that’s bumpy and undermines your work. The issue usually isn’t with the solder. Almost every failed joint is the result of either inadequate surface preparation or uneven heating. Solder follows the heat, so if one side of the joint is cooler, the solder will actually pull away, rather than flowing across to fill the joint.
It’s before you turn on the flame that you really win or lose a soldering battle. The pieces should really be as close to fitting as if they’d hold on their own without solder. If you’re new to this, you might think that solder will magically flow in and fill the voids, but not only will larger joints be ugly and show, but they’ll be weak. What works better than applying more solder is just filing the two surfaces that need to come together until they actually do fit. Another problem is dirt. Finger grease, lacquer, etc, will cause joints to refuse to solder. If you take a piece of 600 grit sandpaper to it and clean off any oils, the joint might just suddenly decide it wants to behave.
The most frequent error is to heat the solder itself and not the metal. This causes the chip of solder to ball up and not spread out as it should, resulting in an ugly bead attached to one side of the seam. The trick is to get both pieces of metal up to the temperature where they pull the solder into the joint. You want to get to the point where the color starts to change a little and you see a tiny glow, rather than trying to make the solder melt. The goal is to make the solder disappear as it runs into the joint.
If you have a few minutes, practice. For 15 min, butt a couple of little pieces of the same material together, flux them, add a tiny bit of solder, and try slowly and evenly moving the flame back and forth along the joint. When cool enough to handle, try bending the joint to see if it is sound. If the joint doesn’t break and the seam is not fat, you are doing it right. If the joint breaks or the seam is fat, make the fit tighter or change how you are applying heat.
With joints, you learn more by waiting and watching than by muscling them into place. Every joint shows you where the heat goes in a piece of metal, how the solder flows when it gets there, and how to control it. And eventually it doesn’t seem random anymore. Eventually it’s one of your favorite things about jewelry making.