I have several one once Krugerrands that I would like to anneal
and run through my rolling mill.
Rex, while Krugerrands are work hardened, it's still 22K gold, and
there's a limit to how hard it can be. Unless you're trying to make
foil, you should be able to roll them without annealing if you wish.
Now if you want them as soft as possible, go ahead and anneal. but it
may not actually be necessary.
I have been told to bring them up to slight red and quench in
water, and I have been told to let cool in air.
Either will anneal them, but with the water quench, it will be
softer. Some rose golds will age or precipitation harden quickly,
even with just a slow air cool from annealing temps. I'm not sure if
22K has enough copper in it to do this enough to matter. But
quenching avoids the issue. Let the red glow just or almost disappear
before you quench. (You want to be quenching from around 900 degrees
F.) Quenching from too hot can sometimes crack the metal.
Can anyone tell me which method will produce the softest results?
Krugerrands are 22K and I believe the other metal in the mix is
copper. With copper in the mix I need to prevent fire scale. How
best do I prevent this?
22K has enough gold that while the copper may oxidize during heating,
it will remain a surface discoloration that will completely pickle
off if you like, unlike the deeper fire stain you see on sterling.
But if you wish to avoid the discoloration as well, dip the piece is
a thin slurry of alcohol and boric acid powder, and burn off the
alcohol to leave a thin dusty film of boric acid on the piece. then
anneal it. The boric acid will prevent the oxidation, and will pickle
off easily after.
I normally work with fine silver and either water quench or air
cool with equal results and I never have fire scale.
Fine silver, without copper, won't discolor, and won't age harden if
slowly cooled. So slow cooling and quenching are equal in softness,
though a slow cool might give you a very slightly coarser grain
structure. Of little consequence in most cases. When you add copper
to either gold or silver, age hardening (also called precipitation
hardening), becomes an issue, as do altered crystaline structures in
gold/copper alloys called ordered arrays, that can lead to extreme
brittleness, especially in 18K alloys. 22K is not so prone to this,
so don't worry about it. But quenching prevents that, too.