Using US silver coins to make karat gold...

[evanJ]

Hi! I am approaching the last stages in my jewelry project, and am ready to produce 18 karat shot. I have about 27.5 grams of refined gold, and have settled on a "green gold" alloy described here as it should be the right strength and color I am looking for. Doesn't hurt that it calls for 9:1 Ag/Cu, which I happen to have a lot of in the form of US silver quarters and dimes!

Or so I thought... But when looking for evidence of this ratio of silver to copper, I couldn't find much proof beyond the claims of the US mint.
This is the only XRF I could find, which on page 6 lists the copper content at 12% and the silver content at only 82.5%

So I'm wondering...

• Has anyone successfully made an alloy of 75% gold, 25% junk silver from US coins? If so, were its properties consistent with what you would expect from 9:1 Ag/Cu?
• Do you know of XRF reports that prove the 9:1 ratio?
• Or - has anyone recovered 90% silver and 10% copper and nothing else from US silver quarters and dimes? It's my understanding that even a little nickel or zinc will botch my alloy.

edit: I might add that I've also recovered enough silver from my refining to use instead, and that I don't mind buying a few cents worth of copper... but it would be really cool to use the junk silver if that doesn't botch everything.

Thanks so much!
Evan