The fundamental building blocks of light absorption in photothermographic imaging systems are the nanosized silver particles formed from the thermally induced reduction of silver ions. The silver particles tend to be filaments or clustered agglomerations of nanospheres (dendrites). While the silver particle size is well known to be important for light absorption properties, it is shown here that the physical proximity between particles is also a critical controlling factor in obtaining a neutral black image tone. It is further shown that the ideal form of metallic silver in any thermally developed black and white imaging system comprises dispersed, tight (physically separated by 0–30 Å) clusters of 10–30 polydisperse, spherical silver nanoparticles, each having diameters in the 5–30 nm range. The minimum cluster aggregation size should be in the 50–200 nm diameter range, depending on shape. This understanding provides a target for silver efficiency, tone, and optical density for photothermographic media constructions.