but how can I know whether this gold has picked up impurities and
are there any ways around the hardness problems, for example can I
try holding the metal at annealing temperatures for many
minutes..... etc....
Chris, the problems you’re having are almost certainly due to
impurities or base metals now mixed with your scrap. It can come from
foreign, non gold parts of the jewelry, or most commonly solders,
including low melting tin/lead and other types. There are several
such additives to a gold alloy that will make it almost unworkable,
as you describe, including tin, iron, lead, and others. Sometimes,
just mixing white gold with yellow gold results in unfortunate
ratios of various metals in the alloy resulting in such behavior.
The fix? Well, the real one is to have the metal refined in trade
for new, properly workable metal, but that may not be acceptable for
this use and customer.
But try this. Sometimes it at least helps. Sal Ammoniac, or Ammonium
Chloride, is one of what are called refining fluxes. When added to
molten metals, they facilitiate the removal of undesirable parts of
the metal. Sprinkled onto molten gold, Ammonium Chloride does not
melt to a liquid glaze the way boric acid or borax would do, but
rather, just goes straight to a gas phase. Chlorine gas, mostly. The
chlorine is highly reactive, and will tend to react with metals to
form chlorides. It will do this most preferentially to the most
reactive of the metals in your mix, which are also generally the
undesirable contaminants in your alloy. This pulls these metals out
of the melt, since the resulting metal chlorides are insoluable in
the melt, and instead, mix with the borax (etc) flux, and slag off.
It’s not very efficient, nor will it remove all the contaminants, but
you can sometimes reduce their levels enough to be able to use the
metal.
BE SURE TO DO THIS WITH GOOD VENTILATION. The fumes generated are
toxic.
Ammonium Chloride, as noted above, seems to be fairly effective at
helping trashed gold alloys. But as I said, it’s nasty, generating
some noxious bad fumes. A similar refining flux that’s also a
traditional addition for this purpose, is the less reactive sodium
chloride. Table salt. It too does not melt to a glaze on the melt,
but similarly, helps to pull some of the base metals from the melt.
It’s less effective, but also less noxious.
Using a long anneal on your metal, by the way, will damage it, not
make it more usable. Too cool, and you’ll be age hardening it instead
of softening it. Above the actual annealing temp so it can soften,
doing this for extended times tends to give you grain growth if
you’re above the ideal temperture, and this makes the metal weaker
and more prone to crack. That’s the risk when torch annealing, where
you likely are bringing the metal to at least a low glow, at which
temp annealing is very quick, and extending it doesn’t help. If
you’re doing furnace annealing, so you have precise control over the
temperature, then the best anneals are are what will seem to a bench
worker as too cool a temp, since it’s not glowing, but it’s high
enough to be in the annealing range. Done right, this can give the
least grain growth. But you need some decent data on the proper
temps, or need to hit it by luck. And it sounds like your metal
won’t be fixed just by annealing.
Good luck.
Peter Rowe