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We get very used, in working gold alloys, to simply assuming that the
resulting alloy will be a proportional mix of the properties of the
constituent metals. it's doesn't always work that way. In steels, and
various other metals, we're accustomed to the fact that metalurgical
properties can be complex, yet are surprised when we find that in
golds.
Silver and gold are completely intersoluable in each other, so
various mixes of silver and gold won't give you surprises. However,
copper is completely intersoluable in neither gold nor silver. This
is what lets it so effectively harden both metals, even in smaller
quantities. It's what lets you age harden a finished piece of jewelry
made with a copper containing alloy, by simply heating it carefully to
the right temps. Many jewelers are familier with only the effect of
annealing, or softening these metals with heat, and don't know that
both sterling silver and many gold alloys can be hardened quite a bit
with a heat treatment.
With 18K rose gold containing only or mostly copper and gold,
however, you have a unique alloy. Although there is 3 times the gold
by weight, this alloy has approximately the same number of gold and
copper atoms (gold is about 3 times the density of copper). At high
temps, and normally at low temps, the alloy forms the usual cubic
crystal system arrangement of a mix of crystals consisting of gold
with some copper dissoved in it, and copper with some gold dissolved
in it. However, there is a region around 700 degrees Farenheit, where
the alloy will rearrange it's atoms to form what is called an ordered
array structure. This amounts to a layer of gold atoms, then a layer
of copper, then another layer of gold, etc. Because it does not form
the usual high symmetry cubic crystals, with their great malleability
and ductility, but instead this completely different structure, it's
properties are different. And THIS structure has the general working
properties similar to glass. I mean, it's brittle as hell, and
unworkable. It's somewhat similar to the intermetallic compounds
formed by an 18K alloy of gold and aluminum, which gives us purple
golds that are also brittle and unworkable. Any attempt to work this
rose gold alloy if it's been slowly cooled through that 700 degree
temp range, will find that at least some fraction of it has formed
islands of this structured array, and will be cracky. Might be even
the whole thing, if you cooled it slow enough. I've seen castings
that literally shattered into lots of little pieces, just being
dropped a couple feet to a linoleum floor. Once cracks form, of
course, they don't heal, so a cracked piece must generally be
scrapped. However, you can, if the piece has not yet formed
microcracks, simply reheat to annealing temp, (low red heat), and
quench it from that temp. This then reforms the structure of the
metal, and restores workability. Quenching in alcohol will help to
prevent the quenching operation itself from causing cracks just from
the shock, while still cooling the metal quickly enough to prevent the
formation of that ordered array. You can also stop it by using a rose
gold that isn't entirely copper and gold, but instead still has
significant amounts of silver. This won't be a really red color, but
you can still get a decidedly pink cast, without risking this cracky
behavior. And with 14K rose golds, it's not a problem, as that atomic
ratio between the gold and the copper isn't right for that structure
to form.
Hope this helps.
Peter Rowe
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