Thermal, mechanical, and optical properties of Cleartran® multispectral zinc sulfide were measured. Heat capacity, thermal conductivity, and thermal expansion were measured over the temperature range 213 to 643 K. Young's modulus and shear modulus were measured at 289 to 473 K. The characteristic four-point flexure strength for a 1-cm2 area was 75.4±0.5 MPa at 289 K and 89.5±0.4 MPa at 473 K, with Weibull moduli of 11.0±0.7 and 19.3±1.5, respectively. All 24 specimens tested at 289 K conformed to the Weibull curve. However, 2 of 23 specimens tested at 473 K had low, outlying strengths. The fracture origins of the outliers were surface flaws that were not detected by microscopic examination prior to mechanical testing. The only reliable means that we are aware of to identify weak material is a mechanical proof test. Infrared transmittance and reflectance at 297, 373, and 473 K were measured with an integrating sphere. The transmittance of s- and p-polarized infrared light was measured at angles of incidence of 0 to 60 deg. By fitting the transmittance with the Fresnel equations, the absorption coefficient was computed for wavelengths of 8 to 14 μm. Bidirectional transmittance and reflectance distribution functions (optical scatter) were measured at 3.39 and 10.6 μm.