Creosote or “soot” is caused by simply burning materials, such as wood, in your fireplace. There is no way to prevent buildup of creosote. Creosote is unburned fuel that gathers in the form of black powder, flakes, or baked-on glaze, depending on the degree of buildup. This is the main cause of chimney fires. When there is excess buildup of creosote in the chimney and the temperatures rise, the soot is ignited and an actual fire is started inside your chimney, showering your roof with sparks.