Hydrothermally synthesized ZnS nanopowders comprising small and large particles were used to synthesize ZnS ceramics. Small particles (200 nm) existed in the gaps between the large particles (0.7 mu m) and assisted the densification of the ZnS ceramics. ZnS ceramics sintered at low temperatures (<1000 degrees C) exhibited small grains with large grain-boundary areas that provided diffusion paths for carbon ions from the graphite mold, resulting in carbonate absorption bands. ZnS ceramics sintered at high temperatures (>= 1000 degrees C) for a long time (>= 2.0 hours) exhibited a dense microstructure with very large grains (>500 mu m). The ZnS liquid phase, which was formed at approximately 980 degrees C, assisted the densification and grain growth of the ZnS ceramics. A 3.0-mm-thick ZnS ceramic sintered at 1000 degrees C for 16 hours showed a high Knoop hardness (321 kgf/mm(2)) and a high transmittance of 71% in the wavelength range 6.0-12 mu m without carbonate absorption bands.