600-650 degrees Farenheit (almost 350deg C) is the
temperature at which fine silver anneals. So what happened was
predictable.
Not to me. My research suggested that 600F was the correct
temperature to achieve full hardness in fine silver. Obviously my
research was badly mistaken. I already know that the silver has
been annealed; what would be really helpful is if somebody could
offer the correct procedure for hardening fine silver.
Pretty simple, Michael. I’m afraid your research is mistaken.
Heat treatment to harden silver requires an alloy, generally of
silver with copper, such as sterling silver, in which the two or more
alloying metals are not completely soluable in each other at room, or
other lower than annealing temperatures. Copper is fully soluable in
silver when they are molten, but increasingly less so once solid, and
as the temperature gets lower. (Look at a silver/copper phase diagram
and you’ll see a zone at each end that shows the percentage of copper
than can be held in stable solution in silver, or vice versa, at a
given temperaqture)
Sterling silver can be hardened by heating (and yes, 600-650F is the
correct temperature range) because at these temperatures, although
the structure does not fully recrystalize (anneal) the copper is
able to come out of solid solution in the silver crystals, migrate to
and recrystalize along the grain boundaries between the silver rich
crystals. This makes the grain boundaries much less able to stretch
and deform when the silver crystals themselves are deformed, thus
making the whole mass much harder and stiffer. Even very slow cooling
after annealing will do some of this, and this is why quenching
sterling silver from above about 750F after annealing, makes the
silver softer than if allowed to fully air cool.
This effect, called precipitation or age hardening, does not occur
with fine silver, because there is no structural change that can
occur from simple heating (to any temperature) that will make
crystals or the boundaries between them, less deformable. Annealing
fine silver at a higher temperature or for extended periods of time
will cause grain growth, which will reduce the metals strength,
malleability, ductility, etc, but that just makes it easier to break
or tear. It does not increase the hardness at all. The ONLY way to
harden fine silver is cold working, such as rolling, hammering,
twisting, or the like, in ways which deform and stress the crystal
structure. There is simply no heat treatment that can do it to truely
fine silver.
There MAY, however, be so-called “micro alloys” of silver, where the
silver is alloyed with tiny percentages of odd materials (such as
calcium, among other elements, in one micro alloy example I’m aware
of), which MIGHT be able to be heat treated. But I’m not sure off
hand if this is the case. I’m aware of such alloys marketed because
they can, due to the tiny percentage of alloy, still legally be
marked as “fine”, or .995 pure, but are still harder and more
durable than actual pure silver. What I don’t know is whether such
alloys offer heat treatments to increase hardness. It seems
reasonable to think such a thing might be possible, and it would be
worth asking your refiner or metals supplier if they know of such
alloys. No doubt some other kind Orchidian can suggest a supplier.
I’ve got a bit of the micro alloys in both silver and gold, and
they’re nicely similar in color, etc, to normal pure metals, only
harder (the silver works much like sterling for hardness, while the
gold works much like 18K, yet both are almost pure) However, I
don’t, right off hand, have the name of the supplier in my head… If
you wish, email me and I’ll try to see if I can look it up.
Hope that’s of use.
Peter Rowe