The lightweight aggregate industry employs the rotary kiln as the primary device for high temperature processing of shale, slate or clay. The product, an expanded shale aggregate, is used in many masonry applications including high-rise buildings, bridge decks, etc. Like all rotary kiln processes, achieving product uniformity is a challenging task despite the typically long residence times. A mathematical model has been formulated to describe the expansion of shale, slate or clay in the high temperature region of the rotary kin. The model predicts transient temperature distribution within a shale particle and simultaneous density changes due to the expansion kinetics (bloating) of the material. A major application of the model is found in the process control depending upon two predicted non-dimensional parameters, the Peclet and the Damkohler numbers.