We're not told exactly when Eöl devised galvorn, but we know that it emerged from the smithcraft he learned of the Dwarves of the Blue Mountains. That hardly helps to narrow down the timescale, however, as the Dwarf-cities of Nogrod and Belegost had been standing for 2,500 years or more at the time of Eöl's death in I 400. The timeline above assumes that Eöl kept its secret until his death, which seems to be the case - certainly the substance is never mentioned after this point.