An employee observed a flash originating from the bag dump station, which attractedthe attention of several workers in the area.2. At about the same time, workers heard a large thumping sound that they describedas coming from overhead, accompanied by a pulse that shook the entire building,drawing more workers from their respective workplaces to investigate.3. After about 2 minutes, seven workers observed an approximately 1-foot flamedirectly over T-306. The flame then gained additional energy from the powderymixture of accumulated carbon black, Gilsonite, and clay in the ductwork of the dustcollection system. The mixture acted as fuel, and the fire flashed over the assembledworkers in the doorway of the pre-mix room.