In this paper, we present a basis for rigorous experimental measurements of the thermal diffusivity α of metals and hence their thermal conductivity K in 3D geometric material objects rather than the 1D objects used in current literature. The proposed experimental technique is based on the numerical method known as the Cairo technique. The key point of this research is that the new numerical method anticipates an exponential cooling curve for material objects and describes the exponent control equation as a function of thermal diffusivity and body dimensions. We measured the time-dependent 3D temperature field during the cooling curve in standard 10 cm cubes of pure Egyptian aluminum and high-grade Russian lowcarbon steel. The experimental results obtained confirm the exponential cooling curve and presented precise values of thermal diffusivity of aluminum and steel in good agreement with those of the thermal tables. The experimental measurements of the timedependent 3D temperature field confirm the validity of the proposed experimental technique and the accuracy of its basis in the numerical method called Cairo technique.