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Silver & copper mokume billet failure


#1

Hi all,

I have a new problem that has popped up with my mokume and I’m
wondering if anyone out there with more experience might have some
advice for me.

So, I started making copper and silver mokume about a year ago and,
after the first billet being an utter failure, I’ve had good
success… until now. My past, successful billets were 12-14 layers
alternating copper and sterling 1" square sheets, 22-24 guage.
They’ve turned out great – not a single crack in the forging
process. Now I’m trying to make it 16 layers, alternating copper (24
guage) and sterling (26 guage) 1" square sheets with a thick sterling
piece on the bottom (12 guage). I’ve tried this twice and it has
twice failed in the forging process – massive billet failure with
multple tiny lines forming between layers with the first 2-3 forgings
and those develop into cracks before the billet reaches 7.5mm (starts
just over 9.0 mm). Here are the details I can think of:

  • sand down each piece with 400 grit paper

  • wash meticulously with perfumeless soap and then distilled water
    then spray with distilled water until no beading takes place, then
    dry with a blower…

  • place between 3/4" steel torque plates with four 1/2" bolts. Torque
    each bolt to 90 foot lbs.

  • place in steel tool wrap bag with activated carbon, fire at 1360
    for 10 hours.

  • remove billet from torque plates, linish down on all sides until
    no visible spaces/cracks, then linish off an additional.25 cm from
    all sides.

  • set kiln to anneal billet at 1100 F for 2 minutes (kiln gets up to
    about 1140 though, then stays around 1100).

  • reduce by .05-.01 mm by cold forging, then anneal and repeat over
    and over until it fits in my 5.5 mm rolling mill.

I think I’ve done everything exactly the same for the billets that
succeeded and for those that failed, except a couple things I can
think of that might have been different:

  • used less soap in initail washinge

  • copper may have been in worse shapee I see pits when looking
    through a 10X loop, but sheets are totally clean to the naked eye.

  • annealing at a different temeraturee I didn’t log the annealing
    temp on the successful trials!

  • I think I used to torque to 80 foot lbs instead of 90?

Any ideas as to what I once did correctly, but now am clearly doing
wrong?

Thanks!
Benjamin


#2

Mokume is spooky stuff. Sometimes things go wrong and you never
figure it out. The only variable I can think of that you didn’t
mention is the time between preparing the metal and firing the
billet. The longer you wait, the greater the chance of oxides
forming. Humidity can also accelerate oxidation. I always clean the
sheets immediately before firing so they don’t have time to oxidize.