There's also the spark test where a clean grinding wheel is touched on the unknown and the resulting sparks observed. Different sparks mean different alloys. A keen, well-practiced observer using a magnet, file, sparks, the material's response to heat and acid, knowledge of the material's previous use, and a little black magic can make some shrewd guesses identifying unknown metals but they will still be guesses.
There are hundreds of stainless steel alloys some of whose constituent alloy proportions overlap. Identifying many from spectrographic tests can be difficuts without the full array (micrographic, hardness, physical properties, etc) can stretch even a metallurgical lab.
My rule is mystery metal is ignored as not worth the bother unless it can be identified with some certainty and there is enough at a good price to warrant reliable identification. Exceptions are rolled structural shapes, propeller shafts from lnboard engined boats (only a few materials used there all easily distinguishable), round stock from garden implements, etc.
Good home shop operators won't tolerate mystery metal in their shops. They have all their material identified in some way; by segregated storage, permanent marking, or paint coding.
Use mystery metal with caution and only on articles having no reliability requirements. For all other applications use material for whick you can honestly confirm its ID.
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