The effect of ambient temperature on the formation of microscopic (micro) glass beads was studied using a drop tube furnace in a weak reducing atmosphere, under conditions of controlled (staged) combustion. Indonesian coal, which has a high ash content, was used, and the experimental temperatures ranged from 1300 to 1500 degrees C. The fly ash was analyzed by an X-ray diffractometer and a scanning electron microscope, and the yield of cenospheres with density lower than 1 g/m(3) was determined by wet separation. The experimental results showed that the content of quartz in the fly ash decreased with increasing experimental temperature, while the content of the glass phase barely changed. The particles gradually assumed a spherical shape, with an average aerodynamic diameter of approximately 7 mu m. The proportion of cenospheres with a density less than 1 g/m(3) gradually increased with increasing temperature. On the basis of the above analysis, it can be qualitatively concluded that increased ambient temperature promotes the formation of microglass beads.