18K yellow gold developing rose patches with cracks

Jewelry Discussion
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I’m trying to melt 18K yellow gold (United 790 alloy, consisting of 53% Cu, 44% Ag, 3% Zn and maybe a trace of some ‘proprietary’ metals). On my first attempt I treated it very similar to the Stirling silver I’m used to and the ingot started cracking like crazy, so I read a bunch of different resources and threads on the matter and remelted and started working the bar according to this process:

-Melt with a reducing flame, stir once molten with a graphite rod for 45 seconds or so, then pour into iron ingot mold (4.5mm thick) pre heated to 300C. Quenched as soon as I could get it out of the mold.

-Sprayed with firescoff and annealed for 20 minuted in a electric furnace set to 704C, as per the spec sheet, quenched in water pretty much immediately. Here I’ve seen some conflicting info, but I figured annealing immediately couldn’t hurt, especially since it probably took me a minute or so to actually remove the ingot from the mold. I’ve also read about quenching in alcohol, but this is a small bar (~20g) so I doubt that’s the issue. I then pickled the bar.

-Forged the bar to ~3.5mm, annealed again with the same process, forged again down to 2.5mm. At this point there were some small cracks on the edges which I filed, and the metal had some slight patches that had a more coppery rose like color.

-Annealed again with same process, then went to the rolling mill. Rolled to 2.25mm, and annealed again.

It is at this thickness that I noticed some pretty large patches with a coppery color, I would compare it to Russian 14K, and significant surface cracks, most of the cracks in the coppery areas. They are not yet super deep so I filed them all out, but sanding the whole thing down gently, the rose color patches stay, and I’m certain it will continue to crack if I try to roll further. Besides the fact that the color differences already make it unusable.

My first attempt I simply torch annealed and less often, and went to the mill straight away, result was the same, just happened earlier.

The coppery patches to me indicate that the issue is likely with my casting, but what should I do differently? I’m thinking of keeping it liquid for less time, but otherwise I really don’t know what else to try.

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I’m thinking it is not homogenous, too little stirring.

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This appears to be the case, but I’m not really sure how it could possibly have happened given that I’ve melted it twice now and stirred extensively both times. Can wrong melting temperature cause the metals in the alloy to come apart so quickly? I plan to use a sulfuric acid pickle to make sure the issue is not surface level only before melting again, though I feel like warm sparex should have been enough.

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You should call United and talk to one the tech guys , they are always very helpful

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In case anyone has had the same issue and finds this, I was able to (mostly) fix the color issue by adding hydrogen peroxide to the pickle, so I think the color was mostly just on the surface. I’m not sure how exactly actual copper is getting to the surface (When united saw a picture they assumed it was completely not mixed), but regardless, this seems to have somewhat reduced the amount of cracking during rolling.

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‘similar to other comments, it sounds as though your melt wasn’t thoroughly mixed.

I recently had a similar problem with surface copper patches while alloying reticulation silver. In hindsight (rushing) and I circumvented, what I believe is the most important step when rolling in studio plate.

In my past, we always make the first ingot pour, sacrificial, rolling the ingot to almost paper thin plate (anneal as required) to make sure the alloy is thoroughly mixed, then we would cut plate into small pieces, remelt and pour the final ingot.

I also borrow a steel technique, and lightly radius the corners of the ingot before the initial roll through the mill, it helps to reduce cracking.

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I remelted the whole thing with a little bit of fresh gold, and made sure to mix for an entire minute before pouring. What I find strange is that for there to be straight up copper unmixed, it would have to first separate itself from the master alloy.

Do you guys anneal freshly cast ingots before rolling/forging? United advised I can go straight to the mill if the bar was quenched, but it probably takes me upwards of a minute to remove the ingot from the iron mold before I can quench it.

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I had problems once and there ended up being a small amount of lead in the melt. Just a thought.