There are some things you have to see to believe.

Yes, this looks as unappealing and formidable as you might be thinking. It is creosote, the No. 1 nemesis in the world of fireplace and wood stove lovers. Chimney sweeps certified through the Chimney Safety Institute of America employ all sorts of pole-mounted brushes and rotary sweepers designed to knock out this very unwanted object.

The Chimney Safety Institute of America wants to show you some samples of creosote that were removed from the inner walls of a chimney.

MORE: CSIA’s education director gets a hand on creosote, showing consumers how the black chunks formed. [VIDEO]

Creosote is residue, a by-product of combustion – the substances produced when wood burns. [These include smoke, water vapor, gases, unburned wood particles, hydrocarbon, tar fog and assorted minerals. As these substances exit the fireplace or wood stove, and when they flow up into the relatively cooler chimney, condensation occurs.]

Creosote is black or brown in appearance. It can be crusty and flaky…tar-like, drippy and sticky…or shiny and hardened.

“I’ve been asked a lot of questions about creosote,” said Ashley Eldridge, CSIA Director of Education. “And while creosote has a bunch of different forms, most of them look much like this.”

Screen Shot 2015-01-25 at 11.14.53 PM

“It could be originally very dense and collecting in the chimney from incomplete combustion or especially high moisture content,” Eldridge said. “As it is exposed to heat, it puffs up, like this …

Screen Shot 2015-01-25 at 11.05.57 PM

“… It bubbles and boils and then turns to a wasp-nest like consistency. All of this material could have been consumed in the firebox if combustion were more complete. Instead, it’s being deposited in the chimney and this is what supports chimney fires,” Eldridge said.

MORE: Creosote has many forms, many levels of misery.

“This is what happens when the smoke cools and condenses inside the chimney,” says Eldridge, holding up still another piece of creosote. “All the oils and all the moisture and all the smoke turn solid.” [VIDEO]

Screen Shot 2015-01-25 at 11.20.10 PM

“A lot of people think that only wood burners have to worry about having a chimney fire. This material (pictured below) was actually recovered from an oil flue, from an oil-fired appliance burning poorly,” Eldridge says. “Subsequently it caused the chimney to catch on fire.”

creosote chimney fire

Creosote from an oil flue / chimney fire

“So it’s important to reduce the volume of creosote that collects in your chimney, and certainly have it removed through sweeping on a regular basis,” Eldridge says.

Certain conditions encourage the buildup of creosote. Restricted air supply, unseasoned wood and, cooler than normal chimney temperatures are all factors that can accelerate the buildup of creosote on chimney flue walls.

Does that sound like that’s what has happened to your chimney? [Creosote can catch fire but may be difficult to extinguish.] You can find a CSIA Certified Chimney Sweep at Use our free zip-code locator. You can also find a bevy of resources on that page.

MORE on CSIA’s homeowner resources page: “The facts about chimney fires and why creosote builds up inside.”