On pp 462-463 of T.K.Rose's excellent "Metallurgy of Gold", 7th Edition, there's a short article on the "toughening" (purity raising) of gold bullion by melting and bubbling air or oxygen through the melt.
http://www.lmine.com/index.php?main_page=product_info&cPath=223_307&products_id=5931
The metals slag out in the following order,as oxides, : zinc, iron, antimony, arsenic, lead, bismuth, nickel, tellurium, copper. In doesn't mention indium, but I would guess it would oxidize, too. There is some overlap - for example, a little copper will come out before all the zinc is out. They ran it for 10 hours, skimmed off the slag once an hour, and the only thing left was essentially all of the gold and silver and some of the copper. You never get rid of all the copper. Some borax or borax plus sand (probably silica sand) was added as a flux, at the start. They used a clay pipe to introduce the air or oxygen into the melt.
A place I worked, occasionally used a very thick walled stainless pipe to bubble oxygen into molten silver, in order to remove the 1.5% tin (the only contaminant) alloyed with the silver. After an hour, or so, the silver was 999.8 Fine. The SS pipe slowly eroded after several batches. That's why clay pipe is used - same that they use in the Miller chlorine method.
I sure wouldn't want to breathe this stuff. I would think you would want to provide good exhaust
I've given you most of the info in that article but, the whole book is very valuable to any miner or refiner.
