Huge thanks George NM for calling out some of the biggest godawful flaws in my hare brain scheme. Having an expert finding faults in the story premise is not only super valuable -it's the only way forward to get to a workable plot that I can use.
And also thanks to Frosty. I got a got a kick out of your plot -you have a very active imagination.
Please understand that the idea is not set in stone. The process to obtain the contaminated cast ingots doesn't have to go down the way I wrote. I was just throwing it out there, see if it sticks. It's fiction after all, so anything can happen, right? The thing is I cannot go off the rails too hard. I still have to make this bit somewhat believable, or I won't be able to use it at all. I can certainly make it improbable, but not impossible.
So back to the drawing board. I have to work with the following constraints in the story:
-It happens in Europe in 1944. So no modern furnace tech will do.
-Whatever the process, it somehow has to start with 24 karat bullion bars and end up with the production of cast gold ingots with purity degraded to around 21 karats. The compound doesn't have to be 10% Fe, 90% Au (the steel mix was just an idea). But the new ingots must contain a distinct (preferably unusual) combination of impurities that act as a sort of unique fingerprint. This is something that will help identify the provenance of the gold later in the story.
-The contamination in the re-smelting process of the original 24k gold ingots cannot be a willful act. It must be the consequence of an unintended action at the foundry. Something like a mistake, sabotage, or some kind of oversight or accident (you can leave these details to me, Frosty).
On the other hand, I'm free to end up with whatever product you might think possible -a homogeneous alloy or some very different compound (a two-phase material?) -anything, just as long as the ingots can be later reheated to extract/refine the gold.
Perhaps my original idea can be improved? Or maybe it's just so bad it cannot be salvaged. Maybe got to find a completely different way to get to the intended result. Unfortunately, I don't know a thing about furnaces, or metallurgy or alloys.
I really appreciate your creativity. So if you can cook up a better idea of how to end up with the contaminated ingots, please tell.