Age hardening or heat treating is a vastly under utilized
characteristic of precious metals. It becomes important when you
are making something that needs springiness after you have
assembled it, as when making a tension setting, money clip, pin
mechanism, or even hardening a prong setting to increase its
wear resistence. In essence, what you are doing when age
hardening is allowing the grains to grow in size so that the
metal becomes less malleable.
When researching precious metallurgy for my book, Professional
Goldsmithing, I compiled from the Alloy Data Sheets
supplied by the World Gold Council in 1990. Up to that point,
the WGC had issued this as part of the individual
data on each alloy, but not as a table comparing them, which is
what I did. The tables on page 206 and 207 in my book are as
complete as possible and offer procedures for age hardening 13
alloys of 14k and 18k gold in a range of colors, plus sterling
For example: The most common 14k gold alloy is yellow gold. It
is composed of 585 parts gold, 205 parts silver and 210 parts
copper and has a Vickers hardness of 190 when fully annealed.
After completely work hardening (by hammering, rolling, drawing,
forming, etc), it reaches 260 Vickers which we know from
experience is much harder. However it can be age hardened even
further, to 270 Vickers by heating to 360 degrees Centigrade for
60 mintues after annealing. That means you can make it harder by
heating than by working!
The same holds true for most 18k gold alloys, although the exact
procedures vary according to the specific composition of the
alloy. Depending on the application, you might want to select an
alloy that can be worked easily and then age hardened to a very
For example: If you wanted to make a tension setting in 18k
gold, the best alloy would probably be a rose color (pink) with
a composition of 750 gold, 45 silver, 205 copper. Fully annealed
it has a Vickers hardness of 165 so that you can move the metal
in place during setting. After maximum working (which is not
achieved during setting) it can reach a maximum of 240 Vickers.
However after all the work is done and the stone is set, you can
increase the hardness significantly, to 325 Vickers by annealing
and then heating at 280 degrees Centigrade for 60 minutes.
This is not as complicated as it might sound at first, and the
knowledge of how precious metals behave can be very helpful when
applied by the bench jeweler.
Revere Academy of Jewelry Arts