If you have a finished piece like a necklace and only want to work harden certain parts, or it is too large to fit in the tumbler, then hammering with a rawhide or nylon mallet will be the better option; each of these will harden the metal yet not mark it. Using a mandrel to maintain the shape of the piece while hammering is also recommended. After hammering, you can work the wire with a burnisher to harden even further with the extra advantage of shining the silver.
Tip: Starting your design with half-hard wire can reduce the need to further harden your sterling silver.
- Michelle Wood, Jewelry-Making Expert