Creosote is the residue from burning wood which travels up the chimney with the smoke. Like plaque on your teeth, creosote forms a layer on the inside of your flue over usage. When creosote sits in the flue over long periods of time, it hardens into a glass-like substance. This is the creosote that is flamible and causes chimney fires. Even though the homeowner has only burned a few fires each year over a few years this glaze can become so hard and thick that the flue now needs to be treated with a chemical in order to penetrate the glaze and make it non-flamible and easier to clean.