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Forging fine silver got a crack


#1

I am hammering a bangle out of a section I cut from a 10oz silver
bar. I did this before several days ago, but have since learned I
was holding and swinging the hammer incorrectly. My hammer blows
are more effective now and the silver moves much more. The thin
edges of my little bar were very rough due to my crude methods of
cutting the 10oz bar into strips. ( I gave up on the jewelers saw
and switched to more aggressive cutting tools) This rough edge
didn’t cause any problems in the first bangle, due to my wimpy
forging and frequent annealing, and I think it looks good in the
finished bangle. Anyway, now I am using the hammer better and the
rough edge turns out to be not such a good idea. A very small crack
developed on the edge in the center of my unfinished second bangle.
I am aware that a hole can be drilled in front of the crack to stop
it, and the crack is on the edge near the center of the bar, so I
could cut it into two and make two thinner bangles. Of course I did
neither, I heated it up to red and brushed the torch at the crack
until it turned shiny and wasn’t a crack anymore. I have been reading
the archives, and suspect that I have damaged the grain structure of
the silver. The work is not at the end of it’s plastic working,
that is I want to hammer longer. To anneal I am using a magic
marker and trying to watch the flame color coming off the metal and
quenching. But I know that some Orchid people are heating to cherry
red and quenching to anneal. What I have done with regard to the
crack isn’t far from what they do to anneal. I forged it a bit more
and there seems to be no harm, but, it’s not like I can actually see
what is happening to the grain structure.

My options seem to be: Continue on, pretend the crack is all better
now. ( I suspect this would work, but is it the “correct” thing to
do?) Cut the work at the crack and forge both pieces in the direction
away from the crack, but I have overheated the metal already. Melt it
all into a shiny line of silver and start again. But I don’t
understand why re-melting and re-forging is different from just
re-melting a tiny little section in the middle and continuing to
forge.

What really bothers me is that yesterday I spent hours filing my
anvil. Today I put 7 little round dents in it. I’m not going to
file it again until I am good enough not to miss the silver.

Oh, by the way, I am using charcoal blocks for annealing, sort of.
Actually they are sections of 2 x 4 lumber. But the tops turn to
charcoal right quick! Is there anything wrong with this?


#2
But I don't understand why re-melting and re-forging is different
from just re-melting a tiny little section in the middle and
continuing to forge. 

Welding the crack as you did allows the grain to grow enormously in
size. if this is fine silver, you might actually get away with it,
since other than grain size, you won’t get differences in
composition. But what you may have to contend with would be some
residual weakness where the crack was, either from where it might not
have actually fully fused After all, you got the surface to flow, but
can you be sure it penetrated? You didn’t, after all, melt the whole
edge and section of the bracelet to a puddle… If it DID fully
penetrate, and if it didn’t form stress cracks or other problems from
shrinking again when it cooled, then you’d likely be OK. but the now
much coarser crystal structure won’t be as strong. So go a bit more
gently in forging for the next couple courses. Once you’ve worked it
a bit, which compacts it again, and annealed in properly again, once
or twice, you should be just fine.

   What really bothers me is that yesterday I spent hours filing
my anvil.  Today I put 7 little round dents in it.  I'm not going
to file it again until I am good enough not to miss the silver. 

If you’re anvil is soft enough to dent, then it’s also soft enough
to planish. Just as you can move the metal in your silver to flow it
around, you can also planish out dings and dents in the iron. think
of the stuff as clay. Harder than your silver, but just as
malleable. Use a nice smooth flat planishing hammer, and gently
planish around the ding and along it’s edge. For there to be a dent,
metal had to be displaced. That means it’s still there, usually
raised up a bit from the original surface like the rim of a crater.
You can move it back with a bit of practice. now, it won’t likely be
a perfect job, but you sure can fix most of such dings, then needing
just a bit of touch up with some emery, not the major work with a
file. This is even easier on a curved stake, but you can do a
decent job with flat ones too.

I happened to be sitting in this morning, for the beginning of one
of Charles Lewton-Brain’s fold forming workshops after the SNAG
conference, at the Revere Academy, and he happened to be touching on
just this subject. And proceeded to demonstrate quite nicely, how to
planish out a ding in a stake or anvil. It was enough of a dent to
put an objectionable mark in ones work, and it took him less than a
minute to planish it quite smooth again, to the point where a few
moments with, say, 400 paper would have removed all evidence of the
ding. Or one could have just used the surface as it was. One
could tell it had been planished, but it no longer left any mark on
the work.

   Oh, by the way, I am using charcoal blocks for annealing, sort
of. Actually they are sections of 2 x 4 lumber.  But the tops turn
to charcoal right quick!  Is there anything wrong with this? 

You smoke up your shop pretty quick as the wood burns. And as it
heats, it can exude various resins, which can catch fire, or deposit
on your work (more a mess than anything else)

But other than that, I can’t think of anything really wrong. It
won’t hurt your metal. But what’s wrong with a nice fireproof fire
brick? then you wont’ burn your studio down… You DID say this is
fine silver, right? So oxidation isn’t an issue, and you don’t need
the reducing environment of a charcoal block…

Peter Rowe


#3
    Oh, by the way, I am using charcoal blocks for annealing, sort
of. Actually they are sections of 2 x 4 lumber.  But the tops turn
to charcoal right quick!  Is there anything wrong with this? 

No, there is nothing wrong with using lumber as an annealing block.
I would suggest, however, that you char the top of the block before
you begin the annealing process.

Cheers from Don at The Charles Belle Studio in SOFL where the sun is
out this AM and where simple elegance IS fine jewelry! @coralnut1


#4
    I am hammering a bangle out of a section I cut from a 10oz
silver bar.  ...  A very small crack developed on the edge in the
center ... heated it up to red and brushed the torch at the crack
until it turned shiny and wasn't a crack anymore. .. suspect that I
have damaged the grain structure of the silver.    ... I forged it a
bit more and there seems to be no harm, but, it's not like I can
actually see what is happening to the grain structure. 

I speak as a metallurgist who is doing a PhD on
annealing/recrystallisation/grain growth, but has NO specific
experience of silver. I hope other readers will not hesitate to say
"but you can’t do that with silver!" if it needs saying.

Here is what will have happened inside your silver:-

You have formed a crack by working the metal more than its available
ductility and/or by causing internal stress from localised working
(made worse by the stress concentrating effect of a notch in your
rough edge).

You have heated the metal in general and locally melted it at the
cracked area, thus welding the crack shut again - but you have
probably caused grain growth from heating too high and/or holding
too long at high temperature. Grain coarsening is dependant on time
and temperature (though not in any simple and easy to predict way).

You can regain a fine grained structure in three ways-

1- re-melting and starting again (an unattractive prospect) 2- hot
working (forging with the metal just above its annealing
temperature) - I don’t know if this is practical with silver or with
your equipment, probably has a bunch of other disadvantages too,
like poor surface finish, which is a problem in hot work with most
metals. 3- possibly by very carefully cold working and annealing.

The problem with the coarsened grain structure is that the metal
will now have a lower strength than before and a lower ductility -
so you cannot work it so far before annealing and will have an even
higher risk of cracking it than before. However if you can cold work
it some more, when you next anneal you will get a finer grain
structure (possibly back to where you started) - if you have cold
worked it enough.

You are aiming at a narrow (possibly non existent) gap between two
bad situations - if you cold work it just a little and then
anneal, you run a big risk of “abnormal grain growth” in which just
a few grains grow and consume all the others - giving a grain
structure so coarse you don’t need a microscope to see it. If you
cold work it too much you will simply crack it again, and with a
coarsened grain structure this is now an increased risk. You have to
hope that the amount of cold work needed to get a proper
recrystallisation is less than the amount needed to crack it - it
probably is, but possibly not by very much. You then have to guess
when to stop working it and anneal - I can’t help you there, though
you could try heating a small test coupon just as you have heated
your work and using that to get a feel for just how far you can now
push it.

I can think of two other things that will improve your chances.

1 Do your cold work as evenly as possible. Maybe those less
effective hammer blows weren’t such a bad thing. Work it all over in
several passes, don’t get one part to where you want it and then
move on to the next - the mismatch between neighbouring areas of
highly-worked and less-worked metal generates large internal
stresses that the metal will tolerate even less well now than it did
before. 2 Smooth off those edges. Any notch (even a microscopic one)
is the starting point for a crack, because a notch will locally
magnify the applied stress from the hammer and the internal stress.
If you like the rough edges, rough them up again when its at
finished shape. But for now grind them back to a smooth (not
necessarily straight) finsh, ending up with a fine grinding paper,
working along the length of the piece - definitely not across
the thickness of the piece. Your ideal is a rounded edge (because
corners concentrate stresses too, though not so much as notches)
with all of its remaining scratches as shallow as you can manage and
running along the length.

You could polish the edges after grinding if you’re feeling
paranoid, but that’s probably going too far.

Iain Fielden
Sheffield UK


#5

Thank you for your post. It helps me to understand better what is
going on with the metal. I ended up forging the piece lightly then
annealing then forging with more and more effect, annealing whenever
the metal seemed not to move as much. I finished it into a closed
bangle and soldered it closed. I need work on soldering, but light
peening made the solder seam invisible on the outside, and filing
and brass brushing made the seam less obvious on the inside. The
bangle is not up to the level of quality that I want to achieve, but
after finishing it is too good to simply scrap. So I gave it to my
oldest son. I may color it with the patina recipe recently posted,
in fact I colored several pieces of scrap silver today to see how it
would go. I will watch this bangle to learn how fast fine silver
wears and what effect the welded crack may demonstrate over time.
My son wanted it more oval shaped so I hammered it into an oval from
round, purposely stressing both the area of the solder joint and the
area where the crack was.

As for hot forging silver, I have read that it can be done with fine
silver but there is normally no reason to do so. It is possible
that this would be an example of a circumstance where it would be of
advantage. It would be a bit of a challenge, I think hot forging
would have to be done on a wood block as a steel anvil would cool
the silver so quickly.

In the future I will cut my materials with more care so that I can
avoid this as much as possible.

Best Regards