The three stages of creosote

Stage one: light soot

A fine dusty layer that brushes off with a standard sweep. Normal byproduct of burning wood.

Stage two: flaky tar

Black or brown flakes clinging to the flue walls. Does not brush off cleanly and needs rotary cleaning tools. The flue needs more than an annual pass at this point.

Stage three: glazed

A hard glossy coating that looks like tar-glass. It is the most flammable deposit a chimney can hold and does not come off with standard brushing at all. Creosote removal at this stage takes rotary tools plus treatment, and the flue needs a camera check after to confirm it is actually clear.