Hide glue

I have a question–is the hide glue you are referring to the kind
you use in woodworking and marquetry? Can you tell me a likttle
more about its use in metalsmithing?<<

The very same!

It performs 2 jobs. 1st, it holds the granules in place; 2nd,
when the piece is heated to fuse/solder the granules to the
backplate, the glue becomes a sort of charcol & provides a
reducing atmosphere to aid in the fusing/soldering action that
takes place where the granule contacts the backplate.