A similar question was raised in I Forge Iron Forum some years back - this might help a little although it does not refer directly to coins. I have copied one post but worth reading the discussion
I am a goldsmith and have done what you are asking but I added iron to the gold, a small amount of iron will turn the gold blue but it is real brittle and not useable in any way. I alloyed the gold with 25% iron to make an 18k alloy I used about 5 grams of gold. The blob that was left was like a anodized blue black color I melted it in an electric furnace covered in flux in an oxygen free envioronment. I used to piece as an inlay on a pendant the stuff could be broken by hand it was very week.