Here's how to heat-harden sterling silver to increase the strength of the metal and reduce its ductility.
To harden the metal, you will be applying heat to the metal; whenever you apply heat to sterling, surround it with nitrogen, argon or forming gas or cover it with flux to prevent the metal from oxidizing. Note that fine silver cannot be heat-hardened.
Check the sterling for any solder joints that may already be present.
Heat the sterling to 1292°F–1346°F (700°C–730°C) for 30–60 minutes; adjust temperatures if solder is present (if low-temperature solder is present, heat the piece only to 1000°F–1200°F). Quench in water.
Heat the sterling again, this time to 572°F (300°C), holding at that temperature for 30–60 minutes. After cooling, Vickers hardness will range between 120–140dph; if lower temperatures are used, the sterling will not achieve this level.
This tip is offered here courtesy of Jörg Fischer-Bühner and is reprinted from Santa Fe Symposium® Proceedings, 2003