I'm really surprised. Willow bark twine is very strong indeed, and especially so when damp.
Ehm, usually I strip it down to 2 or 3mm thick and either make string or use multiple strands to make bootlace thick cordage.
If I want thicker I ply that up.
I wouldn't go more than 5mm for spun and laid, though that might just be me, iimmc. It plaits quite well too.
I strip it and soak it for a day or so....it kind of goes sweet, certainly tastes sweeter when you nip off the sticky out plys (yeah, my dentist gives me hell
), and I don't boil it.
I've got some here that I made ten years ago and it's still sound.
cheers,
Toddy