Gold fingers turning dark during HCl solder removal...

eprigge · Oct 6, 2024

I am doing my first Au refine from gold fingers. I haven't gotten far and I'm already confused... I have 250g of fingers in 730ml of 10% HCl. I boiled for about 20 minutes and the small amounts of solder that were present dissolved but some of the fingers turned a dark grey color. Others have changed color a little. I have attached a before and after picture of one of the pieces. What would cause this?

Also, on a related note, can someone point me to a tutorial thread here for this process? I had planned to use HCl to remove the solder, nitric to liberate the foils and then aqua regia to dissolve them, sulfamic acid denox and SMB precipitation.

Thank you!
Eric

Yggdrasil · Oct 6, 2024

eprigge said:
I am doing my first Au refine from gold fingers. I haven't gotten far and I'm already confused... I have 250g of fingers in 730ml of 10% HCl. I boiled for about 20 minutes and the small amounts of solder that were present dissolved but some of the fingers turned a dark grey color. Others have changed color a little. I have attached a before and after picture of one of the pieces. What would cause this?

Also, on a related note, can someone point me to a tutorial thread here for this process? I had planned to use HCl to remove the solder, nitric to liberate the foils and then aqua regia to dissolve them, sulfamic acid denox and SMB precipitation.

Thank you!
Eric

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The recommended way is to use Cupric Chloride (aka AP) to release the foils.
And there is no need to use AR to dissolve it, just HCl /bleach , HCl/Peroxide or HCl/pool Chlorine.
The greying might be Tin cementing out again.

AuggieDog · Oct 7, 2024

This always happens to me when I use HCL to remove solder, it is just some bit of metal cementing onto the gold. If you take a tiny bit of dilute nitric and wash the surface, your gold is still perfectly fine. However, I would suggest next time you just clip any residual solder from fingers and save the cost & time of doing this. The yield from 250g of fingers doesn't justify it. It is apparent you have done some research as your plan is solid, but as Yggdrasil stated, though it might take a bit longer using AP and going to HCL/bleach is certainly going to be cheaper. But you are fine to proceed as you see fit, and I never worry about the foils turning dark, I just move on to the next step.

Marcel · Oct 7, 2024

Some HCl should remove it

Liquidau · Oct 7, 2024

Fingers (well-trimmed) should be done in AP as Yggdrasil noted above. I’m doing 15 kg in batches of 1.5kg per 5 Gal pail; just pour 2 Gal of HCl (35% Muriatic Acid from hardware stores) into a bucket, add a splash (100 ml) of hydrogen peroxide from a pharmacy, pour in the fingers, and add a bubbler tube (rigid PVC or polyethylene 1/2” or so) attached to an aquarium pump at the bottom of the pail. Let it bubble away for a week, maybe stir it up every few days, and it’s done—-all the foils will be at the bottom of the pail. Trust me, I’ve tried nitric and AR, and the way to go is AP. The leftover liquid is reused over and over (maybe add a bit of HCl every few batches). Read up to understand why this is so.

eprigge · Oct 7, 2024

Thanks for the information guys. I had planned on doing a small batch with AR since I don't quite know what I'm doing yet and I will want to know how to go that route for other gold scrap I will be working with (jewelry). I had avoided the AP process due to how much time I thought it took, but that sounds like the obvious choice with fingers for cost, waste, and simplicity. I will go that way and experiment with AR later. Thanks again!