I always thought that the flexible tubing was purer than the ridgid lengths, but I see from that reference that both are the same except the soft is annealed.
Back in 1988 when I was playing with electroforming I used ridgid tubing as my anode and noticed that I got some carbon residue that I didn't get when I used wire. From Harold's comment I conclude that a little carbon isn't a problem. I was using cheap hardware store 1/2 and 3/4 type M tube.
