The variation of viscosity with temperature has been determined over a wide range of temperature for a Muong Nong tektite material. The viscosity at the liquidus temperature of 1320¿C is about 2¿104 P. Crystallization heat treatments in the range of temperature between 900¿ and 1300¿C does not produce significant crystallization in the natural sample except when the sample is heated in contact with a synthetic tektite composition. In this case, cristobalite crystals grow from the contact surface into the natural sample. For comparison, two synthetic microtektite compositions with lower SiO2 contents than the Muong Nong material were also examined. These compositions could not be formed free of crystals in sizes as large as the Muong Nong sample. Heat flow calculations were performed for 2.5 cm and 10 cm spheres of tektite material cooling by radiation to their surroundings. The combined results of the crystallization study and the heat flow calculations indicate that the Muong Nong material is an excellent glass former and can be formed as a glass in bodies even larger than the natural samples. The results also indicate that the microtektite compositions require faster cooling rates to be formed as glasses than the Muong Nong material and that nucleation behavior is very likely important in determining the size of bodies found as tektites.