Western sources describing applying gold to steel and copper using this procedure mention as a colour/temperature indicator that the metals oxidize bright blue before the gold will stick (Diebeners, p. 72, Wilson, p. 472). Experimentation with a cleaned piece of steel heated over a low flame as a heat transfer to the silver showed this to be true; blue appeared when the gold stuck. This temperature lies between 650-950oF or 350-510oC (Andrews, p. 50).