I'm surprised that no one has posted on this question before so I'll
give it a try now. First of all, copper is more often silver
soldered (brazed) than it is welded. Copper and silver are both very
heat conductive and so the whole piece has to be brought up to
temperature before solder can melt and fuse to the metal. I'm
surprised that you can weld it at all. I'm not familiar with your
torch. An air/gas torch that makes a big soft flame is usually used.
I'm surprised at your comment about no firescale. When I heat copper,
there is usually a black "crust" that flakes off if allowed to air
cool. I quench mine in water before pickling.