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Annealing issues in gold leaf production


#1

Hi Everyone,

I’ve been trying to manufacture 24kt and 23.75kt gold leaf (Au, Zn
or Au, Ag) for some time now. We already manufacture silver (0.999)
leaf without any problems. The problem that we are facing, as you
will see below, is for gold and its alloys.

Basically the process is:

  1. Continuous Cast metal in 50mm Wide * 3 - 5 mm thick strips

  2. Roll metal down to 10 - 20 microns (4-hi rolling mill)

  3. Hammer metal to 0.2 - 0.4 microns thick (singnificantly thinner
    than a human hair)

Now…here’s the problem…it seems that after rolling AND annealing
the gold still seems to be hard and has very minimal expansion in
the hammering process.

I have a very good feeling that we are not annealing correctly. Our
annealing process is:

Till the gold is above 0.50 mm we anneal at 250 degrees C for 30
minutes, and than air cool it every time the reduction is around
75%. After the metal goes below 0.50 mm we anneal at 600 degrees C
for 3 - 5 minutes and air cool it for every 75% reduction.

We had tried to water quench the metal, as a cooling process after
annealing, but that caused significant pin holes in the final
product. ( we are using basic tap water)

I would really appreciate if anyone with any knowledge on this
subject can help me out.

Looking forward to your input.

I realize that this product might be a new subject for many of you
but please don’t hold back your opinions. We are very willing to try
any ideas you may have.

Thanks in advance for your help.
Warm Regards,
Ashish


#2

Hi Ashish:

Good god, what are you making? .4 microns will be thin enough to see
through, as well as being fragile enough to shred from a misplaced
thought. Are you sure plating wouldn’t be easier for whatever it is?

I did a fair bit of gold foil production in grad school. Both 24K and
23.5K (Au/Ag) Final end product was about .0006" thick. (about 150
Microns if my math’s right.) My experience was that 24K does work
harden, albeit very slowly.

I annealed it with a torch, but used a pre-oxidized piece of copper
as a shield. Put the gold on the copper, hit the copper from
underneath with the torch, get the copper red, and away you go. My
temp was probably a little higher than your 600C, but that’s the
right ballpark. The soak time was measured in seconds though. Once
any given spot was red, I moved on. So it was a very quick thing,
under a second probably.

My memory of later learning tells me that pure gold anneals fully
down around 350C, so you may well not need to get it all the way up
to 600C. My suspicion is that your long soak is letting the grains
grow, which is boosting your apparent hardness. Try reducing your
soak time as low as your gear will let you, and see what that does.

I’d strongly recommend picking up a copy of the new release of Mark
Grimwade’s “The Metallurgy of Precious Metals”. He does a very good
job of explaining proper annealing procedure for pure and micro-
alloyed golds.

For whatever that all’s worth.
Regards,
Brian Meek.


#3

Hello Ashish,

Try a distilled water quench after heating for your annealing
process. Also, make sure the metal is clean before you heat it. It
sound as if there if something on the gold or in it.

Good luck to you.
Tom Arnold


#4

Hello Ashish

try to quenched in blue Alcohol

take care