since most salts contain a metal and a salt, how can one be sure that it is the metal atoms that emit photons when the salt is burnt (and analyzed under a spectroscope) and not the non-metals?
I don't fully understand your question. Manufactured metals? or are you referring to ore minerals? If you are referring to manufactured metals, iron tends not to rust so much - not just because it seldom rains but because the humidity (water vapour present in the atmosphere) is so low. Cars left abandoned in the desert will remain in a useable condition years later. If you are referring to ore minerals, you find similar minerals in the desert to what you find in places with wetter climates. Evaporites, especially gypsum halite, are frequently found in a desert environment but these are not metals. Sulphide minerals of copper, lead and zinc would tend not to oxidise and hydrolyse so much but that said, I have personally seen oxidised copper minerals in the Atacama desert which is one of the driest places on Earth.
