Creosote 101: What It Is and Why Cortland Wood-Burners Watch For It

Creosote is the tar-like residue that wood smoke leaves behind as it cools inside your flue. It is the reason chimneys need sweeping at all, and understanding how it forms is the key to keeping a Cortland chimney safe.

How creosote forms

When wood burns, it releases smoke made of gases, water vapor and tiny unburned particles. As that smoke rises and cools against the flue walls, those particles and vapors condense and stick, building a deposit over time. The cooler the smoke and the wetter the wood, the more condenses — which is why slow, smoky fires and unseasoned wood are the worst offenders.

The three stages

Creosote builds in progressively worse forms. Stage one is a dry, dusty soot that a brush clears easily. Stage two is a flaky, hardened buildup that needs more aggressive rotary cleaning. Stage three is a shiny, glazed layer of concentrated, hardened creosote — effectively fuel coating the inside of your chimney. Stage-three glaze is both the hardest to remove and the most likely to sustain a chimney fire, and severe cases sometimes require replacing the liner.

What speeds it up

Several habits accelerate creosote: burning green or unseasoned wood, which is high in moisture; damping a fire down to a slow smolder instead of letting it burn briskly; an oversized or cold flue that lets smoke cool too fast; and simply going too long between sweeps. Restrictive or poorly drafting chimneys compound the problem because the smoke lingers longer against the walls.

How to slow it down

You control more than you might think. Burn only seasoned hardwood dried below about twenty percent moisture, build hot, brisk fires rather than slow ones, and keep the flue clear so it drafts strongly. These habits noticeably reduce buildup between visits for Cortland wood-burners — though they do not replace the annual sweep.

Why removal matters

An annual sweep keeps creosote from ever reaching the dangerous stage. When we clean and inspect, identifying which stage is present also tells us how the chimney has been used and what it needs. Clearing it is the single most important step in chimney safety, full stop.

Recognizing a chimney fire

Not every chimney fire is dramatic. A fast-burning one roars, cracks loudly, and pushes dense smoke and sparks from the top — unmistakable. But many are slow-burning and nearly silent, quietly cracking the liner while you sit by the fire unaware. Afterward, the signs are subtle: puffy or honeycombed creosote, warped metal, cracked flue tiles, or a discolored cap. If you ever suspect your Cortland chimney has had a fire, stop using it and get a Level 2 inspection before the next burn — the liner may no longer be safe, even if the fire seemed to go out on its own.

Published September 23, 2026 · Imperial Chimney Sweepers, DeKalb, IL