A substance called CREOSOTE, which is the residue left in the chimney after burning in a fireplace, which is the the source of many chimney fires. The Creosote heats up to over 2000 degrees, when it ignites and can crack chimney tiles which will open up even more areas with continued heat. The Cracks will allow heat to reach Flamables behind the chimney and allow the fire to spread through out the house.
Creosote leftover after a fire
Tile damage is the major result of a fire. After this type of damage a chimney needs to be relined and sealed, to avoid these damages and repairs have your chimney swept and cleaned annually.