Gold fingers turning dark during HCl solder removal...

[eprigge]

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]

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.