For instance the heat hardening process for regular sterling is generally accepted to be a two-step process that requires a kiln. See James Binnion's post
http://www.ganoksin.com/orchid/archive/200503/msg01337.htm
for details. In fact that whole thread is worth reading, partially because you'll see that some folks report hardening benefits using a simplified one-step process.
I attended the Santa Fe Symposium again this year and had a chance
to speak with Dr. Jorg Fischer-Buhner of FEM (Research Institute for
Precious Metals and Metals Chemistry), Schwebisch Gmund, Germany.
About age hardening of sterling silver as I remembered our
conversations here on Orchid about the process and I knew he had
done some work on this. He kindly sent me a paper he had written and
presented at the 1st Int. Conf. Goldsmith Production Technology,
Montegrotto Terme/Italy about age hardening silver titled “Hardening
Possibilities of Sterling Silver Alloys”. In this paper he has a
series of tests of different solution (homogenization) temperatures
(650, 700, 730 and 800 degrees C) on standard sterling alloy
(silver, copper) and the resulting hardness from precipitation
treatment at 300C. The results are that there is some minor hardness
increase if you do a homogenization treatment at 650C (1202F) for one
hour and then apply the precipitation treatment at 300C for 30
minutes to 1 hour on cast material this increase is on the order of
20 points on the Vickers scale and on rolled sheet the increase is
only 10 points after a 1 hour precipitation treatment neither of
which are probably worth the time and energy required for such a
minor improvement. At 700C (1292F) the hardness increase is 40-60
points which is starting to be significant. But you must hold it at
700C for 1 hour then quench followed by 30 minutes at 300C (572F) to
achieve this. The big problem with this is that a typical medium flow
silver solder melts at 1265F (685C) so you would need to use a hard
solder to make this work with soldered jewelry.
It is obvious from his work that there is some increase in hardness
achievable when homogenizing at lower temperatures but his tests did
not look at the area between 650 and 700C so it is not known if the
increase is somewhat linear or if it rises rapidly as you approach
700C. But it does support the idea that there is some benefit to be
gained from age hardening standard sterling silver at temperatures
lower than the optimum homogenization temperature of 745C. It is
still necessary to hold the homogenization treatment temperatures for
a significant amount of time so you will not see a hardness increase
of any appreciable amount if you simply torch anneal prior to the
precipitation treatment at 300C (572F)
Dr Fischer-Buhner had also presented an earlier version of this
paper at the Santa Fe Symposium in 2003 titled “An Update on
Hardening of Sterling Silver Alloys by Heat Treatment” this is
available from the http://www.santafesymposium.org/ . The paper
presented in Italy is available from
http://www.jtf.it/index.php?page=main
Jim
Jim Binnion
James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160
@James_Binnion
Member of the Better Business Burea