How creosote forms, and why the glazed kind is a different problem
Wood does not really burn. Heat drives volatile gases and tar out of the log, and those gases burn above it if they get enough heat and enough air. Whatever fails to burn goes up the flue as smoke (unburned gas, tar droplets and water vapor), and the moment that smoke touches a surface cooler than its condensation point it stops being smoke and becomes a deposit. Everything else about creosote follows from that one sentence. A hot flue keeps the smoke moving as gas. A cool flue collects it.
The deposit turns up in three conditions, and they are not the same job.
- Loose and dusty (Stage 1). Light gray-to-black flakes and soot that a brush lifts straight off. This is what a flue that gets swept every year looks like.
- Hard and granular (Stage 2). Crunchy black flakes packed against the flue wall, sometimes with a shine on them. A brush still moves it, but it takes work, and it says fires have been running cool or the flue has gone more than a season.
- Glaze (Stage 3). Tar that ran while it was liquid and then set, fused to the flue in a hard black skin, sometimes rippled like candle wax. This is the one that changes the conversation, because a brush polishes glaze rather than removing it. It is treated chemically so the coating turns brittle and can be broken down, usually across more than one visit, and on a badly glazed flue relining is a more honest answer than cleaning.
Glaze is also the stage that burns. It is concentrated fuel bonded to the flue wall, and once it is there brushing alone will not shift it. The argument for sweeping every year is not that one season of soot is dangerous in itself. It is that soot left in place is the raw material glaze is made from.
What you burn changes what ends up in the flue
Two properties of the wood decide most of it, and moisture is the larger one. Wet wood spends the first part of every fire boiling water out of itself, which holds the firebox and the flue cool at exactly the moment they need to be hot, so more of the smoke condenses on the way out instead of leaving. Wood is not seasoned by time. It is seasoned by being split, stacked off the ground, and covered on top with the sides left open to the air. That last detail matters more here than in most places, because a stack sealed under a tarp in Palm Beach County humidity does not dry. It composts.
You can check a piece without any equipment. Split it and touch the fresh face; cool and damp means it is not ready. Knock two pieces together and listen for a sharp crack rather than a dull thud. Look at the end grain for radial cracks, and at the bark, which loosens as wood dries.
Species is the second property. Resinous softwoods light easily and burn fast, which makes them useful kindling, but they carry more volatiles and leave more behind when they are burned slowly. Dense hardwoods burn longer and steadier and are the better choice, with the caveat that they need far longer to season. Oak in particular is often sold and burned well before it is ready. Then there is the list that never goes on a fire at all: anything painted, treated, glued or manufactured, and, this being the coast, driftwood. Salt-soaked wood gives off chlorides as it burns, and chloride is precisely what attacks stainless liners and caps.
Why a damped-down overnight fire builds more deposit than a hot one
The instinct is to close the air down at bedtime so the fire lasts until morning. It is the hardest thing you can do to a flue, and the mechanism above is the reason. Choking the air does not stop the wood releasing gas; the wood is still hot, so it keeps releasing. What it removes is the oxygen those gases need in order to burn. So the flue receives unburned tar at the same moment the low fire stops keeping it warm: maximum deposit and minimum flue temperature, held there for hours. One banked overnight burn can put more into a flue than a whole evening of brisk fires.
The better pattern is smaller loads burned hot, adding wood rather than starving what is already in there, and letting the fire go out instead of banking it. You can audit it from the yard without going near the roof. A fire burning properly shows a heat shimmer above the terminal and very little visible smoke once it is established. A steady plume of gray or blue smoke is deposit going up the flue, and a share of it is not leaving.
