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Dear gold smelter. I am sorry for my late reply on your problem But I
just saw it. The thing what is happening in the white gold, containing
palladium and nickel, is an intercristaline hardening process. This can
happen when the metal is changing from liquid into solid state.. When
the metal is cooling down from the liquid phase into the solid phase,
the alloys are pushed to the crystal edges.The nickel and palladium
are well fitting into each other and are walking together to the
crystal edges. Here they form wedges between the crystals. The metal
is not ductile anymore and has a high strength. To prevent this
walking a fast cooling from the liquid to the solid state is
necessary. This also happens after you heated up the metal above the
recristalisation temperature (for gold approx. 650 degrees C) and let
it cool down slowly the alloys then have time to move. But in
metallurgic we now also an other process happening at low
temperatures, this is called precipitation hardening. The alloy(s)
are creeping over time to the crystal edges. This is a process of time
and can be speeded up on a higher temperature.(remember the hardening
of 14 K gold alloys at 250 degrees Celsius). In the industry aluminum is
often treated this way. High strength means that you can use less
material and build lighter constructions. Aluminum rivets in airplanes
for example are alloyed with copper. They are heated up above
recristalistion temperature and sqeeched*. The rivets are now soft.
But after 6-10 hours the hardening will occur. If you want to rivet
them after this time they will crack.
Now you will say what does this means for my gold. I don't know, it
is one of the three things, ore a combination mentioned above. You have
to try to get the alloys back into the crystal of the gold again that
for shore. Casting a sheet you can do in a metal form (fast cooling)
or in sand like the Delft casting method (slow cooling). If neither
effects the structure of the metal you have a problem with the second
two points. I have had also some alloys with give shit gold but the
way to get at least a sheet out of it is like this: Hammer after
casting it until to build up some internal tension. Not so hard that
cracks occur. The build up internal pressure by hammering, brakes down
the crystals at recristilisation temperature. Then glow it to dull to
cherry red (shortly) and let it cool it down in air below 400 degrees
C (until you see now glowing in a dark room) and then cool it down
fast in water. Not in acid, because acid eat up in the crystal edges
copper and zinc residues. This will give a starting crack which will
grow during rolling. If you heat it up for a long time the crystals
will grow again and the alloys will settle more to the edges. The
smaller the crystals, the more ductile the material will get, and the
less problem you will have with the wrong alloys.(because there is
more edge surface). Next start rolling it only a few % until you feel
it hardens again. Glow and sqeeche again. This can take a lot of
steps. It is better to do it once to often, than to go over the edge
and have cracks again. If you are lucky and get the thickness reduced
to approximately 35% you will see that it gets ductile again. You will
have now fine crystals. The material will never be as good as new
bought gold, but it will be workable. It also helps before work on
this material after so time, to glow it first to get it softer again.
If everything will not work the only thing to do is cast some jewelry
out of it. This is my way to get rid of shit gold. Good luck and if you
have some questions don't hesitate to ask.
Martin
martin.niemeijer AT hetnet.nl
* to sqeeche : I don't now if I spell this right, but I mean very
fast cooling down
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