Silver ingot flaking

[CatarinaN73817]

Hello everyone!

Once again, I’ve been surprised by metal.
I’m used to doing ingots, I’ve been doing them since I’ve learned jewellery making, and I’ve never had this happen before, at least not as severe!

I casted an ingot of sterling silver, that was checked by the assay office and they evaluated it as 931,8 purity, so higher than 925 silver. This was because I joined all my scraps, and as I had a couple with bits of solder, I joined a bit of clean pure silver to it, and I wanted to be sure of the purity of the metal I would be working with.

Anyways, I poured an I got to make wire, everything seemed fine after the pour.
When I started rolling it in the mill to pull it down for the drawplate some flakes started appearing, I clean them, file them and sand them before rolling more. I started to anneal more often, but they kept on coming…
I thought, as these are only at the ends of the wire, I’ll continue and go with it, maybe I’ll be able to salvage the middle section of the wire.

To melt the ingot I use a torch with Romassgas, could this be the issue?
I only have either Butane torch - ORCA or this one I bought at Leroy Merlin.
The Butane doesn’t have the strength to melt these ingots, I’ve tried.

Does anyone has any idea why this happens?

Thank you in advance for your help!!

[rmeixner]

When you go from a just poured ingot to the mill or draw plate, you should first sand or grind it to remove any small sharp edges or flashing from the parting line. Then forge the ingot all over moving in the same direction on all sides. Otherwise, the little bits of flashing and sharp edges will roll over and look like they are a part of the larger mass only to stick up as soon as you bend the metal at that point. The forging breaks up the crystal structure of the metal. You will have to consult a metallurgist to learn why, I just know that it makes a difference. You can also introduce these small flakes if you are first rolling in square grooves to reduce the size of the ingot. If you get things too tight, you can cause small flashings where the metal squeezes out between the rollers. They then get rolled down the next time you pass the wire through the mill. They may not show up again until you draw through a draw plate…Rob

[wldlzrd1]

Hi,

one possibility is that trapped gas(?) can form blisters on, and pockets in the metal…when the surface of that blister becomes thin enough thru drawing or rolling, it can flake off…

julie

[jeffg-moderator]

This does look like classic pushing the metal too hard through the drawplate (or rolling mill).

A friend of mine told me one of the best explanations of why this happens. It’s two-fold.

First, cast silver, gold, etc is kind of like hyper-annealed metal. The grain structure is spread so far apart that the metal is actually brittle. With a cast ingot to bring it to a malleable state, you have to do what Rob says. File and sand off any parting lines or imperfections. Then forge, anneal and pickle at least 5 times before touching the rolling mill or drawplate. (Some people say to forge, anneal, pickle enough to reduce the thickness by 20%. I’ve found that approximately 5 times works for me.) The idea is to compress the grain structure and strengthen the metal, so it can withstand the stress of rolling or a drawplate.

Second, my friend said to think about the metal like an ocean wave where the water on the surface moves at a different rate then below the surface. That’s exactly what happens with milling out an ingot. The outside of the metal moves at a different rate than the inside of the ingot.

That’s what it looks like here. The outside was moving more than the inside which created stress fractures on the surface.

In addition to doing the forging, annealing pickling that Rob suggested, you also probably need to anneal more frequently when drawing or rolling, especially at first.

Sure, there’s other possibilities, especially since you said there’s solder in your mix, which probably is a mixture of silver, copper and zinc. Zinc can cause all kinds of unexpected problems sometimes.

Personally, I don’t see any problems with your gases or torch set up. If it’s melting the metal, it’s melting the metal.

This is hard to diagnose from a picture, but that’s where I’d start.

Hope that helps!! Maybe others have some different useful tips.

Jeff