Creosote is a black grimy substance formed by carbons and oils in smoke. When oils in wood and fossil fuels are burned in your chimney, it releases them into the smoke. While the smoke is hot, the carbons and volatiles remain as gasses. But as the smoke travels higher up your chimney,

 it starts to cool. When the smoke gets cooler, the volatiles start to take physical form as that of the black oily residue known as creosote.

Creosote is not only gross looking but it is an extreme fire hazard. This black slime is highly flammable and can build up inch upon inch in an unkempt chimney flue. On e the creosote has formed a good sized bulk inside your chimney, it becomes very dangerous. The hot oxygen of a current fire can reach the residue left behind from previous fires and ignite your chimney in a dangerous and out of control blaze.

Routine cleaning can help cut down on creosote buildup but your best option is to take a preemptive measure and not have to worry about it at all. A good advance plan is to look into a stainless steel chimney liner. A stainless steel liner covering the interior of your chimney will keep creosote from forming.

The stainless steel lining is more consistent when it comes to heat. Since the smoke is being exposed to a warm temperature all the way up the chimney, the volatiles never get to cool and form creosote.