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My Gold is cracking, Is forging the answer?


#1

Was: how much cracking is normal

I recently made a post regarding my cracking gold. I am 99% certain that it wasn’t my copper source, I.e my copper was pure enough and it wasn’t an issue with impurities. I know this for a few reasons, one is that I also used it in green gold which experienced no cracking compared to the yellow. If the copper was impure I believe the green gold would have suffered the same problems though correct me if I’m wrong. Also the seller (rio grande) sold this wire as .999 pure copper. I re-poured both ingots and rolled some high quality wire. The difference this time was I forged them first. I never forge my silver and usually don’t have problems but it seems that gold is MUCH more prone to developing grain structure issues than silver which I highly suspect was what was causing my gold alloys to crack when I rolled them. It cooled very fast in the mould and probably had some dissolved oxygen among other issues all which could have caused cracking. By forging it seems I’ve alleviated these stresses and gotten workable metal although it certainly took gentler passes and more frequent annealing than silver.


#2

I’m no expert but I had an issue with drawing down gold wire which I had melted in my crucible. A bit of research tells me that the inclusion of oxygen through the melting process may cause splintering due to the crystalline structures in the metal being distorted. My understanding is that in my own case I probably melted for too long and achieved too high temperature. I would love to know more about this subject as well please. When I drew the wire through the rolling mill it virtually came out in long shards and was very brittle. I also now understand that as well I can use a covering of crushed charcoal across the top of the molten metal to further prevent oxygenation.


#3

Hey ArgentumMoon I was also wondering if you had success with remedying the same metal you originally had cracking with or did you use a fresh batch?


#4

It was the same metal, I poured a new ingot from it, forged and then rolled. It’s my understanding that forging the metal re-aligns the large crystals present from casting which are are brittle.


#5

Forging is a great help. But I believe that it breaks up and refines the crystals/grains, rather than re-aligning them.


#6

Yes thats sounds about right!


#7

Ok that’s good news! Thank you.


#8

To be precise, forging alone will not:

  1. change the crystal structure of a metal.
  2. break up and refine the grain structure of a metal.

What forging is known to do is to distort the grain structure by means of plastic deformation. In the case of a rod, forging all around the length of the rod will stretch/elongate the grains. The grains become narrow/thin and long and full of dislocations. The presence of excess dislocations inside the grain is what makes the metal harder to deform further. Metallurgists don’t call this grain refining. Grain refinement (making smaller grains) can be brought about by either (1) controlled heat treatment and/or (2) additions of grain refining agents like iridium or rhenium in karat gold.

But the basic premise of forging is still accurate - that it changes the as-cast grain structure (which, by nature of its formation, is prone to cracking during rolling). The subsequent careful annealing is what is going to recrystallize newer, smaller grains. This conditioning of the metal is what enables one to continue to cold work without edge cracking.

Since, manufacturers don’t have the luxury of forging tens of feet of rod or sheet in production, they depend on grain refiner additions to the alloy, which modifies as-cast grain structure, resulting in smaller grains which will prevent edge cracking during rolling.


#9

I stand corrected.


#10

So, why does forging aid in rolling? I never anneal after forging. I move directly to the mill.


#11

What would constitute careful annealing for yellow karat golds? I have heard conflicting info on how to anneal and how often. I have heard one should hear to a brighter red for gold than for silver, I have also heard to heat until a very low red or suffer from grain growth yet again. Also what exactly constitutes annealing “too often” as if I were to roll to a 50% reduction I would almost certainly experience severe cracking. Trust me, If I have a 6.4 mm ingot (which is the size I use) forging to a 50% reduction before rolling would be thinner than I need it, around 4 mm.

And rolling to a 50% reduction and I would have a SEVERELY cracked piece ingot.


#12

Andy,
I don’t know exactly how you are forging or what percentage reduction you are achieving by forging . If you are successful in going straight to rolling after forging without annealing, I can only assume that you did not stress the metal much during forging, perhaps just created a surface layer of material that is under slight compression. Compressive stresses at surface is known to aid rolling and prevent edge cracking up to a point.


#13

To get the full benefits of forging, what reduction must you give too? I only forged until I believed it was significantly work hardened but it didn’t significantly reduce the diameter. Since I tend to make thicker wire, would a I need to but am ingot mould that makes wider ingots and forge to a 50-60% reduction?


#14

Jonah,
I’m not a bench jeweler.
Annealing with a torch, in my opinion, constitutes “softening” of the metal rather than a proper annealing. The reason being, a “proper annealing” is usually done at a low temperature to prevent grain growth. Because the annealing temperature chosen is lower (about 1175F or 635C), the jewelry item must be kept at that temperature for at least 5-10 minutes for recrystallization to occur. As you know, at the bench, we don’t have such luxury for time. So we end up heating the piece rather quickly and hold it for a minute or two and be done with it. Such heating is difficult to be controlled depending on bench jeweler and is just good enough to continue to roll/draw.
Perhaps other bench jewelers may have better answer than mine.


#15

May I ask what your experience is? If not a bench jeweler, are you in the sciences? The industry ?Thanks.

Please excuse any typos-- curse my clumsy digits…


#16

Thank you for your insights.


#17

I’m a metallurgist working for Stuller.


#18

Thanks!