In 1556 Georgius Agricola wrote, in his De Re Metallica, about the "colour of fumes" from various types of ores. This may be the earliest record available in which the color of a flame-excited metal sample was used to identify a substance. If so, it represents the start of flame spectroscopy as an analytical tool.
W. G. Schrenk, "Historical Development of Flame Excitation Sources for Analytical Spectroscopy," Appl. Spectrosc. 40, xix-xxviii (1986)