Where creosote comes from
Creosote is what wood smoke leaves behind when it cools against the walls of a flue before it can get out. Every wood fire makes some; how much, and which kind, depends on the wood, the air and the temperature of the chimney.
Smoke is unburned fuel: tars, water vapor and fine particles that left the fire before they could burn. A hot flue carries them out. A cool flue, a smoldering fire or wet wood lets them condense, and the deposit that builds up is flammable, which is why the trade treats it as the fuel of a chimney fire rather than as dirt.
Our creosote removal page covers booking the removal of a heavy or glazed deposit; a routine chimney sweep handles the first degree. This guide is about telling the three apart from your own firebox and understanding what each one says about the way you burn.
First degree: the dry soot that brushes out
First degree creosote is a dull black or gray-brown dust with soft flakes, like fine ash that has stuck to the wall. It is the normal result of a hot fire with dry wood, and it is what a routine sweep removes.
Run a poker tip through it and it falls away in a powder, leaving a clean line. There is no shine and nothing sticky. It collects on the smoke shelf, in the smoke chamber and in the first few feet of flue above the damper, and in a fireplace that burns dry fir on weekends it builds slowly toward an eighth of an inch.
The Chimney Safety Institute of America’s Level 1 procedure tells a sweep to clean, or recommend cleaning, any flue with an eighth of an inch or more of buildup, and first degree is the deposit that rule was written for. A poly or wire brush sized to the flue, rods from the top or the bottom and a vacuum at the firebox take it off in one visit.
Second degree: crunchy flakes and chips of tar
Second degree creosote looks like black cornflakes or chips of dried roofing tar, shiny on one face and dull on the other, and it crunches rather than powders. It is the sign of smoke that cooled further than it should have.
You see it most in stoves and inserts, because a closed appliance with the air turned down sends cooler, slower smoke up the pipe than an open fire does. Overnight burns, a load of wood that was not quite dry, and a liner that is a size too big for the appliance all push a flue from the first degree into the second.
A soft brush slides over it. Removal takes stiffer wire brushes, more passes, and often a drill-driven rotary head on flexible rods that whips the flakes off the tile. It comes out, but it takes longer than a routine sweep, and a flue that shows second degree every year is telling you to change something about the wood or the air before the deposit hardens further.
Third degree: the glaze
Third degree creosote is a hard, shiny, tar-like coating that looks like black glass or dried asphalt poured down the flue. It can run and re-harden into drips and bubbles, and no brush will move it.
It forms when smoke condenses faster than it can dry, layer on layer: green or wet wood, fires left to smolder for hours with the air shut, a large cold flue behind an insert with no liner, or an exterior masonry chimney that stands in rain on the north side of a house and never warms through. Portland winters supply all four.
Glaze is concentrated fuel. It burns hot, it is the deposit behind most chimney fires, and when it does burn it swells into a puffy, honeycombed crust that is the classic evidence of a fire nobody noticed. If your flashlight finds a shiny black coating, leave the fireplace cold and read the black shiny creosote page before you do anything else.
Removing it is a different job from a sweep. The usual sequence is a chemical treatment, a powder or liquid the sweep applies to the glaze that turns it brittle, followed by rotary chains or heavy-duty heads that chip the softened glaze off the tile. Stubborn flues take more than one visit, and a camera look before and after tells you whether the tile underneath survived.
The three degrees side by side
The table puts the three degrees next to each other, with the cause behind each and the tool that takes it off, so you can match what your flashlight shows to the right column.
| Degree | What it looks like | What usually caused it | What removes it |
|---|---|---|---|
| First | Dull black or gray-brown dust and soft flakes; powders under a poker tip | Hot fires with dry wood; the normal product of burning | A routine sweep: poly or wire brush on rods, vacuum at the firebox |
| Second | Crunchy black flakes or tar chips, shiny on one side; crunches, does not powder | Cooler smoke from a stove or insert run with the air turned down, overnight burns, wood not quite dry, an oversized liner | Stiff wire brushes, repeated passes, a rotary head on flexible rods |
| Third | Hard, shiny glaze like black glass or asphalt; drips, bubbles, runs | Wet or green wood, long smoldering burns, a cold or oversized flue, an insert with no liner, an exterior chimney that never warms | Chemical treatment to make it brittle, then rotary chains or heavy heads; often more than one visit |
Why glaze changes the job, and sometimes the chimney
A glazed flue is priced, scheduled and inspected differently from a sooty one because the tools, the time and the risk to the tile are all different. The chimney sweep cost guide explains what that does to the number.
The risk to the tile is the part homeowners do not expect. Glaze bonds to clay tile and to the mortar joints between tiles, and chipping it off can expose cracks the glaze was hiding or open a joint that was already weak.
That is why a camera run comes before and after removal, and why a flue with heavy glaze on cracked tile often ends up relined with stainless steel instead of getting another round of chains. The chimney liner installation page covers that decision.
A classic glazed flue in an older Portland house sits behind a wood insert that was pushed into an open fireplace years ago without a liner. Smoke from the small firebox enters a large, cold brick flue, cools at once and glazes the tile from the smoke shelf up. The fireplace insert page explains why a full-height liner is the fix as well as the prevention.
Burning habits that keep a flue in the first degree
The way to keep creosote in the first degree is to burn dry wood hot, with enough air, in a flue that is the right size and stays warm. Each of those is a habit you can change this season.
- Burn wood below 20 percent moisture. That is the EPA Burn Wise figure; a moisture meter on a freshly split face settles the question in seconds. The seasoned firewood guide covers getting Douglas fir, alder and maple there in a wet climate.
- Season for at least six months, split and stacked. Also EPA guidance; rounds left in a heap under a tarp do not count, because air has to move through the stack.
- Build hot, bright fires. The same EPA page says a smoldering fire is neither safe nor efficient for most appliances. Small, frequent loads beat one big load choked down.
- Open the air before you load and leave it open longer than feels thrifty. Turning a stove down to its lowest setting for the night is the habit that turns a clean stove flue into a glazed one fastest.
- Give an insert a full-height liner sized to it. A liner keeps the smoke warm and moving instead of letting it spread out and cool in the old flue.
- Never burn trash, treated or painted wood, plywood or plastic. The EPA lists them, and Oregon law bars burning garbage, treated wood, plastic, rubber, petroleum products and paint in a solid fuel device; they also leave corrosive deposits.
- Have the flue inspected every year. The how often to sweep guide explains the eighth-inch rule that decides when the brushes come out.
What a sweep’s report should say about creosote
A useful sweep report names the degree, estimates the depth, says where in the system it was found and shows a photo, because those four facts tell you whether anything about your burning needs to change.
Location matters as much as degree. Glaze in the top third of an exterior chimney points to a cold flue; tar on the smoke shelf behind an insert points to a missing liner; crunchy flakes in a stove’s connector pipe point to overnight burns. Ask for the before and after photos, and ask which degree the sweep found, in those words.