RE: Sig Smith Miniplane build
Actually, copper does have a "temper" (example as in hard drawn water pipe) and if using a small butane torch, you'll probably see the copper wire turn red . . . but the silver solder will be much stronger than the copper and it gives the solder something to "hold on to" and "fill the joint".
Probably not an experts description, but best I can offer.
T-man49