How to Stop a Candle Tunnelling
A ring of wax left on the glass is easy to prevent, and not hard to put right.
Start with the wick
Most trouble with a candle begins at the wick. Trimmed short, it burns clean and steady; left long, it smokes and leaves soot on the glass.
Give it time
Wax has a memory. Let every burn run until the pool reaches the edge, and the candle will burn level to the end.