My experience is that the soot that Don refers to is usually caused by the flame hitting the solder directly while the solder, pick and the piece are all at different temperatures, usually the solder and/or the pick is much hotter than the piece. It also happens most frequently with white gold (I hate white gold). The soot, even if it’s almost invisible, will definitely interfere with soldering. It causes fire scale to form inside the joint. Solder won’t flow there until the fire scale is removed, sometimes even requiring re-filing or sanding.
To avoid it, try heating the piece up closer to the flow temperature of the solder before putting the solder on the joint, and keep the flame on the other side of the joint as much as possible. In other words, don’t aim the flame at the solder, especially when starting cold,
Basic soldering 101. If solder won’t flow, it’s due to one or more of these three things -
- The joint or the solder isn’t clean.
- The joint doesn’t fit well, in other words there is a gap that the solder can’t jump, or that is causing the two parts to be at different temperatures…
- The joint and the solder aren’t hot enough. Both have to be at the flow point at the same time.
If the joint (not just the solder) is hot enough and it won’t flow, it’s one or both of the other two things that is causing the trouble. It really is that simple.
If the solder is balling up before flowing, it is reaching flow temp before the joint is, and you’re likely to get fire scale in the joint which will cause reason one to go into effect…
As far as the holes Fredman refers to, that’s most likely where the solder was placed, and when melted, it etched into the surface. That happens with almost all solder joints to one extent or another. The higher melt the solder is, the more likely it is to etch. The more heat applied and the longer it’s kept hot, the more it will etch. The main trick to solving that particular problem is in solder placement. It is usually best to apply many small pieces to many different places and place them as far out of sight as possible. The other thing to remember is to get in and out as fast as possible. If it’s not flowing when it should, don’t just put the coals to it and hold it, that won’t work. Stop and figure out what’s wrong.
In a case like Fredman’s decoration, I would apply the solder to the entire inside edge of the domed piece, file it flat and then place the piece on top of the sheet, making sure that is in full, tight contact with the sheet, flux it and then apply heat underneath the sheet, drawing the solder towards the sheet. Just like a blind bezel. You shouldn’t ever have to aim the flame at the top or front part of that piece, which helps to avoid fire scale too.