9kt gold is not considered gold in the US. It can't even be sold asgold here, so there would be no reason for him to mention it.
Daniel R. Spirer, G.G.Spirer Somes Jewelers1794 Massachusetts Ave.Cambridge, MA email@example.com
Steven Kretchmer wrote an article about his experiments in Mokumewith 18ct gold alloys. It's in Metalsmith spring 1983, Vol3 #2 pgs38-39. There are a series of mokume articles in that issue,including one about bronzes.