This article presents laboratory measurements of the radiative properties of ash and slag deposits that have been extracted from combustion devices. The measurements were made by a technique employing a sample heating device that is coupled to a Fourier Transform Infrared (FT-IR) spectrometer to measure the sample’s emission and directional-hemispherical reflection. By this technique, the temperature at the measurement point and the spectral emittance of the surface are both obtained. These measurements are then related to the physical and chemical properties of the surface to determine what controls the radiative properties. The measurements have shown that the physical state of a deposit (i.e., fused, sintered, or packed particles) greatly influenced the measured spectral emittance. The results were in agreement with mathematical models that account for the physical properties.