Effects caused by exposure of large-core, fluorine-doped, step-index silica optical fibers to high temperatures were studied experimentally in controlled laboratory conditions. A fiber was located partially inside a temperature-controlled electric tube furnace and irradiated from the end by a light source. The light source was either an incandescent halogen lamp or a blackbody radiator. The influence of fiber temperature on the angular distribution of the radiation in the fiber and the coupling and propagation of thermal radiation in the fiber was studied. With increasing temperature the profile of the angular distribution of the radiation was transformed irreversibly from a Chinese hat profile to a much flatter one. This effect was pronounced at 1000 degrees C and above. Radiation from the furnace was found to propagate in the fiber as an attenuating mode at temperatures above 800 degrees C. Most of the radiation exited the fiber at angles beyond but close to the acceptance angle. Calculations show that thermal self-radiation of the fiber is negligible. The physical explanations for these effects are discussed and practical conclusions are drawn.