Thermal mass, defined as the capacity of materials to absorb, store, and radiate heat, plays a critical role in improving the efficiency of biomass cookstoves. This study evaluates the integration of clay liners into charcoal cookstove designs using three methods: analytical modeling, OpenFOAM simulations, and electrical-analogy modeling. Radiation modeling between clay-lined combustion chamber surfaces and the cooking pot was validated experimentally by testing four Jiko-type cookstoves — which featured combustion chambers of different shapes but identical top inner diameters — against a traditional metal stove. ISO Water Heating Tests and Controlled Cooking Tests (CCT) were conducted. Results revealed that the net radiative heat transfer to the pot remained consistent (153 W ± 0.5%), very slightly affected by combustion chamber shape. However, chamber volume and fuel burn duration significantly influenced performance. Clay-lined stoves exhibited 32–59% higher ISO thermal efficiency than metal stoves and achieved 34–47% fuel savings during standard cooking tasks. Notably, inverted-truncated-cone designs achieved 20% greater thermal efficiency than right-circular-cylinder configurations; this improvement is attributable to their reduced volumetric capacity and their resulting fuel economy during CCT. Additionally, the clay lining’s radiative effect intensifies over time, resulting in an increase in thermal efficiency of up to 30% during the transition from cold-start to hot-start operational phases.