Hello,
Fischer in Germany has a well proved melting powder (based on
potassium nitrate) which I’ve been using for years. Add the powder 1
to 1 to your gold and let it melt in a furnace. Follow the
instructions and your gold will come out as a brandnew mix. The
product is rather cheap (about US25$ per kilo) and no further
refinning costs!
Have fun and enjoy
Pedro