Creosote
Creosote is condensed wood smoke, it is the fuel in a chimney fire, and how fast it builds is almost entirely about how the fire is burned.
Short answer
Creosote forms when wood smoke cools below its condensation point inside the flue. Wet wood, a smouldering fire, an oversized or cold flue and an uninsulated exterior chimney all accelerate it. It builds through three stages, and the third — hard glazed creosote — is difficult to remove and is the dangerous one.
How it builds
Wood smoke contains unburned volatile compounds, and when it cools below its condensation point inside the flue it deposits on the walls.
Burning wet or unseasoned wood is the single largest driver, because the water has to be boiled off and that cools the fire and the flue.
A smouldering fire — damper closed down, air starved — produces far more creosote than a hot clean burn.
An oversized flue never warms up, so the smoke cools and condenses.
An exterior chimney on a cold Toronto wall stays cold, which does the same thing.
And softwood burns faster and cooler in a damped-down appliance, though well seasoned wood of any species burned hot is fine.
The three stages
Stage one is a light flaky soot that brushes off easily. Stage two is a harder, flaky tar-like deposit. Stage three is a hard shiny glaze that cannot be brushed off and needs chemical treatment or rotary tools, and it is the one that produces a serious chimney fire. The stages are cumulative and the cure is to stop making it.
What reduces it
Seasoned wood, under twenty percent moisture content, which means split and stacked under cover for a year or more. A moisture meter costs very little and settles the argument.
Hot clean burns rather than long smouldering ones. A small hot fire produces far less creosote than a large damped-down one.
A correctly sized and insulated liner, which keeps the flue gases above their condensation temperature.
Annual sweeping and inspection, and more often with heavy use.
Never burning treated wood, painted wood, pallets, cardboard or garbage, which produce worse deposits and corrosive compounds.
And a top-down fire lighting method, which gets the flue warm faster and reduces the smoky start-up phase.
If there has been a chimney fire
Call the fire service even if it appears to have gone out, because fire can continue inside the structure.
Do not use the appliance again until the chimney has been inspected.
Expect cracked clay tiles, which is the standard consequence, and expect relining to be the outcome.
Expect damage to the crown, the cap and the masonry from the heat.
Get a higher-level inspection with a camera rather than a visual one, because the damage is inside.
And tell the insurer, because a chimney fire is a reportable event and the subsequent relining is frequently a claim.
Common questions
What causes creosote to build fast?
Wet wood and smouldering fires, plus an oversized or cold flue.
What is glazed creosote?
The hard shiny third stage. It cannot be brushed off and it is the dangerous one.
After a chimney fire, can I use the fireplace?
No. Not until it has been inspected, and expect cracked tiles.