ECR ion sources (ECRIS) of CAPRICE-type, working at 14.5 GHz, are in use at the High Charge State Injector (HLI) of the accelerator facility at GSI for beam production and at a test bench for development work. The ECRIS is mostly used to produce ion beams from rare isotopes because of its high efficiency and low material consumption. Depending on their material properties beams of rare isotopes are produced from gases, gaseous compounds, solid materials or solid compounds. Gases can be used directly, while solids have to be transformed into the gaseous state for the ECR plasma which is achieved by using resistively heated ovens. As enriched materials are produced by isotopic separation processes their composition including contamination by impurities can be of importance for the handling in the evaporation process and can be detrimental for the beam user if the ion beam contains additional ion species. Characteristics and suitable treatment of materials and production processes are described. Experimental investigations with different sample materials and operational experiences are reported.