There are several things that could cause the gold to crack. The
primary one is oxygen built in during melting. The second thing
is not compressing it properly and annealing it after casting
before rolling to create a small crystal structure. Also, if
there are indentations in the sides of the ingot these may foster
cracks as well.
Silver and gold alloys containing silver love to absorb oxygen
while at high temperatures, where it is dissolved just like CO2
in soda pop-you can’t see it until you change the pressure
conditions when the gas comes out of solution like when you take
the top off the soda bottle. (in Lab conditions 1 kg of fine
silver can dissolve 20kg of Oxygen by weight-luckily once you add
copper in the mix it is not as bad.
If you have allowed oxygen to be dissolved in the metal while
molten then it comes out as the temperature drops and the metal
solidifies, tending to occur and be sqeezed towards the
gate/sprue button area of the model or ingot thus resulting in
The other thing that happens is that dissolved oxygen bonds with
copper contained in the alloy and forms brittle compounds that
are deposited at the grain boundaries. When you roll these they
try and slide past each other and can cause cracks. Large
crystals are worst. Large crystals are made by heating too high
when annealing, not compressing heavily and annealing after
making an ingot, air cooling and so on.
Avoidance: in industry melts are done in vacuums, under ammonia
atmospheres and so on. Sometimes an induction melting crucible
is used, load the crucible, put a thin layer of charcoal powder
over the metal (eats oxygen), put a lid on it until molten. In a
small studio one has to use other methods.
Do all you can to limit oxygen access at high temperatures to
your metal. Keep the time it is hot/molten and able to absorb
oxygen to a minimum. Preheat the crucible intensely (like a long
long time) and use a slightly gassy flame-note the slightly-if
you are leaving black soots around you need way more oxygen in
the flame, what you are looking for is a slightly yellowish
flicker at the end of the inner blue cone-assuming an
oxy-acetyelen or oxy-propane rig.
Watch the edges of the crucible, they should glow evenly, if it
darkens in one spot then oxygen is sliding into the crucible.
Use a bushy, large flame and slant it towards the crucible so it
makes a lid to protect the melt, a sheet of flame as a shield. In
Germany the actual cast is sometimes done through a pure (yellow)
gas flame as well.
Roll your metal that you are going to melt out as thin as it
will get and roll it up into coils like mini rolled rugs for
melting. This increases the surface area and reduces the melting
time dramatically (the time your metal is hot and exposed to
oxygen). Keep the melt covered by the gassy flame at all times
and consider adding a chunk of charcoal to the crucible to
consume oxygen. The shorter the melting time the less oxygen can
be absorbed by the metal. Do not add to much oxygen to the flame
to shorten melting time-it is counterproductive-you need that
Do not remove the flame to add flux, if you have to add flux
then insert an iron or graphite rod into the flame to warm, dip
it into powdered flux and then insert the rod through the flame
to add flux-do not remove the flame which is protecting the melt.
If you have melted very well you may not need to make any flux
additions, the melt will be smooth and shiny like a mirror-in
that case you did well and just cast with it.
Scrap cast metal may have dissolved oxygen locked into it from
previous exposure at high temperatures, it seems to be
Some people add a ‘flux’ to the melt, ammominum chloride and so
on to ‘degas’ it, I feel this is not useful. In refineries they
sometimes add the teeniest smidgion of zinc to a melt just before
casting to ‘degas’ it, often with silver alloys.
After casting an ingot which you are going to work you need to
recrystalize it for best results, to make a lot of fine crystals
or grains in the metal. To do this compress (do not forge out,
but compress) the ingot with the flat side of a forging hammer.
Compress up to 15% to 40% or so thinner than when you started.
Then anneal (but not too high! Just when the flame leaving the
metal turns orange, if your metal is glowing red you are too
hot!), then quench it (everything except nickel white gold).
This resets the crystal structure. Then file and sand (I like a
small belt sander called the ‘Zippee’) so that the sides of the
ingot are smooth, any small indentation can turn into a crack.
Roll until you have reduced the thickness by at least 30% (there
are actual percentages for each alloy, perhaps Marcus and Peter
Rowe can enlighten us, examples are sterling is 45%, 18 k gold
can be 75% reduction before annealing). If you do all this then
this should reduce the cracking problem.
Lets say you did all this and your gold is still cracking
(probably oxygen built in-made brittle copper oxides at the grain
boundaries) then you could try a remelt to fix it. You need very
good ventilation for this fix-the fumes are dangerous. Melt the
metal. Add a heaping tablespoonful of potassium nitrate
(saltpeter), this will foan up and make a spongly mass which then
collapses down to a pool of metal again. Pour this as an ingot
and it often fixes the problem. You amy expereice some loss of
weight in the ingot.
When melting metal: Oxygen is your enemy.
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