I can't quote a book reference off hand and my knowledge of the
metallurgy involved tends to be empirical. I use mainly hard solders,
older joints just don't seem to remelt or move without a lot of
effort. Easy and even medium solders are different, too much low
alloy in the mix I believe. Usually easier and better to avoid using.
If you want heavy wall copper tube one source is to pull down BC"
tubing (local hardware store) A boat winch draw bench makes the job
easier, and you might need an internal mandrel to reduce the wall
thickness. With some care it should be possible to make telescoping
Analog/Digital Modeling & Goldsmithing