Have you looked at the Copper Development Association website? There is all sorts of technical information there.
They have a publication search (although the reference I found to 'Modern methods of welding copper' was dated 1943. Modern, I suppose, compared to Modern machine shop practise by Joshua Rose, from 18-something).
I believe the alternative (and original) gas is helium. This gives a hotter weld pool. Hence the original name for TIG, heliarc. Argon is used now as it is way cheaper.