I'd already seen a missing brick in this chimney with a camera from the inside so I cut a hole in the top to access it for repair. The missing brick enabled smoke to leak through into a disused flue in a bedroom. Replacement of the brick solved the problem.
A combination of not burning hot enough and a lack of sweeping enabled this liner to become tarred up to the point where it no longer drew properly. (Photo courtesy of Dave O'Donnell)
It was suggested this might have been a witches mark, to keep witches away, on the side of a chimney. Research suggests its more likely the 'makers mark' of the brick manufacturer from a couple of centuries ago.
When the tar in a chimney catches fire it aerates & expands changing texture from like hard toffee to like a meringue. This stove was full of aerated and partially burnt tar when I finished sweeping the residue from a chimney fire above it.