No on the denim, I mean, you could use it- it would be done same as with silk/linen- but cotton is no good for bow backing, too much stretch to offer much protection. Go to a thrift store and get some cheap old silk ties, or even a silk dress. Silk is my go-to for when I am unsure of the bow's back (iffy grain especially). If you are not sure something is silk, test burn a strand with a lighter- if it stinks a bit like burnt hair, its silk. If it curls up and smells plasticky?- its likely a poly blend.
I prefer to get silk from things other than ties, since alot of ties are bias cut, which is the direction in silk fabric that "gives" the most. It is more preferable to get your length cut with the direction of least stretch (you can determine this by pulling the fabric in different directions).
Mike
