I was looking for the perfect solder for copper and finally found it. This is a solid copper-phosphorus solder Felder 94, no silver content.
I conducted an experiment and to solder a few rings, using different types of solder.
1 - Copper-phosphorus solder paste. On the one ring I put more paste to another less.
2 - Solid copper-phosphorus brazing with silver content.
3 - Solid copper-phosphorus solder Felder no silver content.
And here's the result: solid solder does not leave silver trace. Especially if you use the optimal amount.
This is a video where I took the whole process of using solid solder. Copper-phosphorus solder very hard. Use very large wire cutters to chop solder into small pieces. If you get a big piece of 3-4 mm it can be cut in half.
It is important to choose the best size for each solder joint. Place the solder near the seam. Because the phosphorus content in the solder, we can work without flux. Use a butane torch to heat the copper. Solder is melted by hot metal and will flow into the place with the highest temperature. This way you can control it. If too much solder can distribute it on the surface of a metal with a flame. After heating the copper became dark - use citric acid to clean it up.
Use solder-paste is more convenient (it's easy to apply and it melts more quickly) - but it always leaves a dark mark. It is very upsetting to me. Another advantage of solid solder - it does not have a pungent smell like solder-pasta. I have been working under the air hood, but still lack of smell - is of great importance to me. And solid solder convenient to store, it does not deteriorate with time and is not toxic.