NASA has considerable interest in the growth of strategically important materials in microgravity. Bridgman-Stockbarger directional solidification within a sealed ampoule are planned for CdZnTe, HgCdTe, HgZnTe and GaAs. In order to optimize the thermal parameters for the growth conditions, it is necessary to characterize the furnace and understand its behavior. This entails the use of appropriate thermal modelling based on the known physical properties of the material, the ampoule material and the furnace itself. A suitable test is the determination of the shape and location of the solid-liquid interface relative to the furnace. Electrical and chemical etching techniques have been used to locate this interface for certain materials. One of these, germanium doped with gallium, a material with well-known properties, has been used to characterize the furnace model. Demarcation has been extended to cadmium telluride by post-growth examination of the precipitation of tellurium inclusions at positions of abrupt thermal changes in the furnace regime. Deliberately imposed mechanical vibration of the melt can also produce evidence of interface location. This work was done with infrared microscopy. Other techniques, available in termary solid solutions, involve the effect of change of lower zone furnace temperatures on the compositional profile. Examples of demarcation are shown in gallium-doped germanium, cadmium telluride, and mercury zinc telluride.