Stage one: light soot

Dusty, light grey to brown, powdery. It brushes off easily, and a standard sweep removes it without difficulty. This is what a well-run fireplace burning seasoned wood produces, and it is not a cause for concern in itself. Regular sweeping keeps a flue at this stage indefinitely.

Stage two: hard flakes

Black, crunchy, flaky deposits, sometimes described as looking like cornflakes stuck to the flue wall. It forms when combustion is less complete — cooler fires, wetter wood, restricted air. It still comes off with a brush, but it takes more work, and it is the stage where an annual sweep stops being optional.

If you are finding black flaky chunks on the hearth, that is stage two creosote falling off the smoke chamber. It should not be there.

Stage three: glazed creosote

This is the one that matters. Shiny, tarry, almost wet-looking, like poured tar that has hardened onto the flue wall. It forms when smoke condenses heavily and repeatedly — the signature of wet wood, smouldering overnight burns, or an oversized or uninsulated flue that never gets warm.

Glazed creosote is fuel. It is what burns in a chimney fire, and it burns at temperatures the structure was never designed to contain. It also cannot be brushed off — a standard sweep will not touch it. Removing it needs a rotary head with chains or specialist cutting equipment, and sometimes chemical treatment first.

If you shine a torch up your cold flue and see something that looks like tar, do not light another fire until it has been dealt with.

The eighth-of-an-inch rule

The working threshold across the trade is about one eighth of an inch of accumulation anywhere in the flue. At that depth it needs to come out regardless of which stage it is at.

Why Campton Hills reaches stage three faster

Two local factors. Wood is a genuine heat source on a lot of properties here rather than a decorative one, so burn volume is high. And the tall chimneys on the larger houses, and the old flues on the farmhouses, are frequently oversized or uninsulated — exactly the condition where flue gases cool enough to condense heavily.

Prevention is the same three habits everywhere: seasoned wood, no damped-down overnight smouldering, and sweeping at the eighth-of-an-inch mark rather than by the calendar.

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