Creosote is a naturally occurring substance that is produced when wood is burned. Aspen is a type of wood that is known to produce high levels of creosote when it is burned. The reason for this is because aspen contains a high level of tar and other chemicals that are released when the wood is burned. When these chemicals are released, they combine with the oxygen in the air to form a sticky substance called creosote. Creosote can build up on the inside of your chimney and cause problems if it isn't removed regularly.