As long as they were not exposed to direct heat, sunlight, moisture etc they should be reusable. Most chemical we use are quite stable. Some chemical such as kh2po4 and kno3 decompose if expose to heat above 400 degree Celsius. Radiation from sunlight might cause some reaction. Not sure about iron and micro though. Ferrous may be oxidised to ferric?? Need to do some extensive study about their stability . Maybe someone can give more detail.
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