It sounds like you are taking different karat gold jewelry and just melting it together into a bar. Is this the case? The borax as a flux is not going to remove the various metals used to alloy the gold. I always remove all gemstones, bits of cardboard filler, epoxy, tiny iron springs found in clasps (I hate those things, but a strong magnet will catch them) before processing. I also hit the material with a propane torch to incinerate/remove anything else. It's painfully time consuming, but helps before I inquart with silver and cornflake to then chemically refine.
I suppose the question I have for you is what is your end-state? Are you planning on chemically refining, selling your bars as is, or just holding the gold. By mixing all the different karat gold together, you may have a harder time selling it, it will also make it a little bit more complex to process. Are you testing your scrap jewelry before melting? A couple pieces of fake costume jewelry will really contaminate you gold bar with various metals.