That certainly sounds distressing and surprising, to have that sort
of discoloration. I am mystified as to why there was a difference,
but the prevention is the same as the cure that you used: Heating and
pickling after the final abrasive finishing creates a germanium oxide
surface, which prevents oxygen from tarnishing the silver (as well as
preventing oxygen from penetrating the surface to create firescale).
For a thicker germanium oxide, you may wish to heat in an oven or
kiln. 570 degrees F for 45 minutes or 350 degrees for two hours
hardens the Argentium Silver, as well as increasing the germanium
oxide layer. If there are delicate stones set, you may choose to heat
for 30 minutes or longer at a lower heat to increase the germanium
You can further increase tarnish prevention by using Goddards Long
Shine Silver Polish—either the liquid or cloth— or one of
Tiffany’s Silver Mitts after the heat treatment. These products
contain chemicals called thiols, which prevent tarnish on all
sterling silver, but seem to have an especially affinity with AS.
I hope this is helpful!