What weight leather do you use for backing?

Generally bows are backed with rawhide, not processed leather.  I haven't backed a bow with rawhide, so I can't comment - I can check the TBB tonight, but someone with experience will come along before then  

The only bows I know of that were backed with leather are horn composite bows and it was more for decoration and protecting the sinew from the elements than for strength.  That was fairly thin leather 1-1.5oz to keep the weight down.

The only bows I seen backed with leather are horsebows.  In fact, many have limbs that are covered completely in leather.  However, leather doesn't add performance, insurance, or speed, and on a long bow will just slow it down. I'd use thin rawhide.

I was reading last in a bow building book that they use "clarified calfs hide"...almost translucent. It is used by banjo builders.

Clarified calf hide is raw hide and not tanned leather. I agree with 4est. Also leather will soak up moisture like a sponge and that is not good for bows.

Rawhide! Not leather..sorry. Got my terminology wrong. Any sources for rawhide?