Creosote is a residue that builds up in fireplaces as fuels, especially wood, are burnt. The smoke that rises from burning fuel contains many contaminants and unwanted gases. While these pass through the chimney and into the air, some bond to the inner surfaces of the chimney and stay there, slowly building up a thick, oily layer over time. Creosote can be a serious fire hazard and if your fireplace has creosote problems, you should clean it and your chimney as soon as possible.
Creosote is bonded to the metal and stone surfaces of your fireplace and will not come off easily. Water, however, can wash off the top layers of dirt and oil, making it easy to start on the tough inner layers of baked carbon. You can also spray creosote with water so that some soaks through and begins to loosen up the residue enough to remove it.
Benzene is a strong solvent that is derived from coal tar. If you have any around your home in the form of a cleaning solvent or motor fuel, then try using it to break down the bonds the creosote has created. Benzene is a toxic substance, so protect your lungs, eyes and skin when you prepare it (wear protection when directly cleaning creosote at all times, for the same reasons).
Scouring powders and filler materials
While creosote may be tough, you can often scrub it away with the right type of cleaning products. Using a scouring powder like baking soda or a similar substance to give your cleaning some traction. You can also use filler materials like hydrated lime or talc, which can help break down and remove some of the creosote deposits.
To remove creosote you will have to do a lot of scrubbing. This may be easier if you can dismantle your chimney into smaller pieces---otherwise you may need to use a long chimney brush for scrubbing. Find brushes with tough or metallic bristles that can scour deep into the creosote, and prepare to use elbow grease.