These before and after pictures of a traditional masonry chimney flue. That ugly matierial you see in the before is creosote. Creosote is a byproduct of incomplete combustion of wood, typically most prevalent when first starting a fire and when burning unseasoned wood. Creosote is very flamible and where large deposits accumulate in a chimney, a chimney fire is very possible and extremely dangerous. The normal temperature in a chimney during a regular fire would be around 500 degrees. When a chimney fire occurs the temperature can go as high as 1800 degrees. When they occur, they can destroy the chimney and set the house on fire. Removing the hazardous creosote and inspecting the chimney for safety and integrity are the two main purposes of a chimney sweep. I get calls regularly from people who have had a chimney fire. It is commonly observed, when flame is shooting out the top of a chimney like a giant torch, with flame reaching 10 feet in the air. Quite a sight. Another call i get is people thinking a jet engine buzzed their house. When a chimney fire occurs it can make quite a roaring inside the house. I sometimes make the possibly poor joke, that a chimney fire can suck up small animals and children up the flue, so much air flow occurs to support the fire.