What causes candle soot?

Incomplete Combustion -The primary reason is that the wax doesn't fully vaporize and burn in the flame.

Long Wicks  - A wick that is too long draws too much fuel, or liquid wax, up the wick, leading to an over-fueled flame that can't burn it completely.

Mushrooming Wicks - A mushroom at the tip of the wick is a sign of incomplete combustion and a precursor to sooting.

Drafts - Air currents from fans or open windows can cause the flame to flicker and become unstable, interrupting the combustion process and producing more soot.