A tunnelled candle burns a narrow shaft straight down the middle and leaves a thick wall of wax stuck to the glass. You paid for that wax. You will never smell it.
The frustrating part is that almost nobody causes it deliberately, and almost everybody causes it the same way.
Wax has a memory
The first time you light a candle, the pool of melted wax spreads outward until you blow it out. Wherever it stopped is the edge it will return to on every subsequent burn. Light it for twenty minutes on the first evening and you have taught it to melt a twenty-minute circle — forever.
This is not a metaphor. Paraffin, soy and coconut blends all behave this way, because the wax that has already melted and reset has a slightly different crystalline structure from the wax around it.
The rule that prevents it
Burn until the pool reaches the edge of the glass. For most containers that is two hours on the first burn. A wide vessel can take three.
If you have twenty minutes, do not light it. Wait for an evening when you do. That single decision is the difference between a candle that burns cleanly to the bottom and one that gutters out at the halfway mark with half its wax welded to the sides.
If it has already tunnelled
You can usually recover it, and it is worth trying before giving up on the candle.
- Wrap aluminium foil around the top of the jar, leaving a gap over the centre for the flame.
- Burn for one to two hours. The trapped heat melts the wall of wax down into the pool.
- Remove the foil carefully — it will be hot — and let the surface set flat.
Repeat on a second evening if there is still a shelf of wax around the rim. Two attempts fixes most of them.
What else shortens a candle's life
A long wick. Trim to about 5 mm before every burn. An untrimmed wick produces a tall, unstable flame, soot on the glass, and a pool that melts unevenly.
Draughts. A ceiling fan or an open window pushes the flame to one side, and the wax melts to one side with it. Move the candle, or close the window.
Burning too long. Past four hours the wick begins to mushroom and the glass gets genuinely hot. Blow it out, let it cool, light it again later.
The short version
Two hours on the first burn. Trim the wick each time. Keep it out of a draught. That is nearly all of candle care, and it will get you the full burn time you paid for.