Creosote is a highly flammable tar residue that can form when wood is burned incompletely. A smoky fire without enough oxygen or a malfunctioning stove can increase the opportunity for creosote buildup. There are several forms of creosote buildup: the first, a fine, dusty soot that can be cleaned very easily; the second, a grainy black sootthat can be cleaned fairly easily with a chimney brush; the third; a thick, tar-like buildup that is more difficult to remove and requires special tools; and the fourth, a shiny, glazed coating that is the most difficult to remove. If your chimney technician finds a creosote buildup even one-eighth of an inch thick or with a glazed coating, it’s time for a cleaning as soon as possible.