how to fix candle tunnelling

Tunnelling is when a candle burns straight down the middle and leaves a wall of wax around the edge. It looks like the candle is faulty. It usually isn't — it's the first burn that did it, and you can normally get most of the wax back.

Fixing one that's already tunnelled

Wrap a collar of kitchen foil around the top of the jar, leaving a gap in the middle over the flame. Light it and leave it for an hour or two. The foil holds the heat in and pulls the hard edges down into the pool. It looks daft and it works.

If the tunnel is deep — more than a centimetre or so of standing wall — do it twice rather than once for four hours. You want the wax to melt, not the jar to get hot enough to be a problem.

If it's gone too far, a hairdryer on the surface will level it off. You'll lose a bit of scent doing this, but you'll get the burn time back.

Why it happened

Soy wax has a memory. The first time you light a candle, it will only ever melt out as far as it got on that first burn — every time after that, it follows the same line. Blow it out after twenty minutes and you've drawn the tunnel yourself.

So the fix is the first burn. Light it when you've got a few hours, and leave it until the melted pool reaches the edge of the glass. In a 170ml jar that's usually two to three hours. After that, it'll behave for the rest of its life.

The other things that cause it

• A wick that's too long. Trim it to about 5mm before every burn. A long wick makes a tall, unstable flame that burns up rather than out.
• Draughts. A candle by an open window or a radiator burns unevenly because the flame is being pushed sideways.
• Short burns, repeatedly. Twenty minutes here and there never lets the pool reach the edge.
• The wrong wick for the wax. This one's on the maker, not you. If a candle tunnels badly despite a proper first burn, it was underwicked.

How long is long enough?

A useful rule: one hour of burn for every 2.5cm of jar diameter, on that first light. Most 170ml jars are around 7cm across, so about three hours.

Don't burn for more than four hours at a stretch, though. Past that the jar gets hot, the wick mushrooms, and you get soot rather than scent.

I wick-test every scent I make before it goes out, because different fragrance oils behave differently in the same wax — a heavy amber needs a different wick to a sharp citrus. It's the least glamorous part of making candles and it's the part that decides whether yours tunnels.