Heat hardening is made possible by the mechanism of “precipitation
hardening.” This is possible when some copper is present, as in the
case of sterling silver. Copper tends to crystallize out of solid
solution when sterling is heated to the correct temperature, and the
tiny copper crystals naturally precipitate out along the grain
boundaries between the larger silver crystal grains that constitute
the bulk of the structure of sterling silver. These “lock” the grain
boundaries and prevent the slippage along the boundaries that makes
malleability of fine silver so pronounced.
Since fine silver contains no copper, heat hardening of this kind is
not possible.
As an aside, fine silver does undergo “work hardening” as a result
of distortion of the individual crystals when hammered or drawn. This
is true of most metals, and is linked to overall malleability of the
metal as well as its “transition temperature.” Annealing takes place
when a work hardened metal is heated to at least the transition
temperature, which allows the crystal lattice to re-form and internal
stresses to relax. Some metals (lead for example) enjoy a transition
temperature at ordinary room temperature, so do not work harden.
HTH.
Dick Davies