One can add borax to a gold sand then heat it up until it melts and then just let it cool to grab the pure gold. This is just strange. How is borax lower the melting point of something it only touches and not even form a solution?
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A short answer, because there's nothing much to add really.
Borax works because it doesn't so much lower the melting point as it does help insulate the melt, and acts to allow the heat you do apply to achieve maximum effect. As I am sure you know, it also acts to float impurities away from the gold as they are released in the melt and helps keep everything liquid.