According to Wikipedia, “A chimney fire is the combustion of residue deposits referred to as soot or creosote, on the inner surfaces of chimney tiles, flue liners, stove pipes, etc.”
So, what is creosote? When wood burns, certain by-products or substances are produced. There is the smoke you can see, as well as things you cannot see like tar in the smoke, water vapor, gases, unburned wood particles, and assorted minerals. These by-products are warm, and when they pass through the cool chimney, condensation forms. The resulting residue sticks to the inner walls of the chimney creating something called creosote. Creosote is highly flammable. If enough creosote builds up, and the temperature in the chimney becomes high enough, a chimney fire can erupt.
This chimney fire in Stevens Point caused a great deal of smoke damage to the entire home.