you can braze with gold, silver and brass, and other non ferrous metals. For an full fusion to occur, yes you do need clean metal, but dirty metal will adhere to brass VERY tenaciously in little spots throughout the surface, I tried casting barstock out of scrap brass in some steel troughs, that stuff was stuck GOOD.
Brass has zinc added to make it bronze.
bronze is copper and tin, brass is copper and zinc
, but you are right about one thing, I think brazing rods have zinc in them, at least they release that white smoke if you over heat it.
Molten brass is Red hot, you also are risking scaling occurring on that machined mold, I wonder how barnes makes those sledgehammer bronze/brass? solids.
If your willing to sacrifice a mold, post pics please