What creosote is
When wood burns incompletely, the unburned gases condense on the cooler surfaces of the flue. What settles there is creosote, and it is flammable. A chimney fire is creosote igniting inside the flue at temperatures that can crack masonry and clay liners.
It forms in three recognised stages, and the stage determines the method, the price, and sometimes whether cleaning is possible at all.
Stage one: soot
A fine, flaky black powder. This is what a healthy, well-burned fireplace produces, and it comes off with a standard brush. This is the annual sweep, and it is the cheapest outcome.
You get stage one by burning seasoned wood with enough combustion air.
Stage two: hard flakes
Shiny black flakes with a tar-like feel, often found where a stove is connected to an oversized flue or where the chimney runs cold. A brush alone will not shift it; it needs rotary equipment with chains or whips.
Stage two usually means something in the setup is wrong — an unlined flue, a stove too large for the space so it gets damped down, or persistently wet wood.
Stage three: glazed creosote
A hard, shiny tar coating fused to the flue wall. It is highly flammable, it is what most serious chimney fires are made of, and a brush will not touch it.
Removing it takes either chemical modification — a product that changes the deposit into something brushable, applied over several burns — or rotary power equipment, and in some cases the liner has to be replaced instead. This is where the four-figure quotes come from, and where they are legitimate.
Why the inspection comes first
Nobody can price a chimney sweep honestly over the phone, because nobody knows which stage is in the flue until they look. That is what a Level 1 inspection is for: the technician examines the firebox, the damper and the accessible portions of the flue, and tells you what is there before any work is quoted.
If a company quotes a flat price sight unseen, they are either assuming stage one and will renegotiate in your living room, or they are not planning to look.
Prevention, which actually works here
Burn seasoned hardwood at 20% moisture or below — a moisture meter costs about twenty dollars and pays for itself in one winter. Give the fire enough air rather than damping it down overnight. And do not burn softwood, construction scrap or anything painted or treated.
A flue that gets used properly stays at stage one, and stage one is a routine appointment.