You light a candle, settle down, and twenty minutes later something looks wrong. The flame has sunk into a crater, there's a ring of untouched wax around the rim, and the wick is disappearing down the middle. That's tunnelling, and it's one of the most common complaints from candle makers and candle burners alike.

The good news is that tunnelling almost always has a clear cause, and in most cases you can correct it before the candle is ruined. Work through the possibilities below, starting with the most likely culprit.

The first burn sets the pattern

Wax has a kind of memory. It melts and re-solidifies into whatever shape it was last encouraged to take, so if you light a new candle for twenty minutes and blow it out, the melt pool never reaches the edge of the container. The wax that hardens forms a shallow bowl. On the next burn the flame sits inside that bowl, melting downwards instead of outwards, and each short session carves the crater a little deeper and the surrounding wall a little taller.

Fix it by giving a new candle one long, patient first burn, long enough for the melted wax to reach or nearly reach the rim. A small tin might take an hour. A wide three-wick jar could take three. Don't push past four hours in one sitting, though; past that point the wick mushrooms, the container gets hotter than it should, and you gain nothing.

What a good first burn looks like

• A melt pool that reaches within a few millimetres of the rim
• A flame roughly 2–3cm tall, steady, with no sooting or dancing
• A container that feels warm but is still comfortable to pick up

A wick that's too small for the jar

If the flame is small and feeble, keeps drowning in liquid wax, or gutters out on its own, the wick simply isn't producing enough heat to melt the wax around it. The melt pool stays narrow, the wax builds up at the sides, and the tunnel forms quickly.

For makers, this is a testing problem rather than a burning problem. Wick charts are a starting point, not an answer: the correct wick depends on your wax blend, the container diameter, the fragrance load and the dye. A wick that burned beautifully in a plain soy test may struggle once you add 8 per cent fragrance oil. Wick up a size or two, then run a fresh test burn in the actual container with the finished recipe. For shop-bought candles, there's little you can do about the wick itself, so head to the rescue section below.

A wick that's too big

An oversized wick causes the opposite set of symptoms: a tall, restless flame, black soot on the glass, a mushroom-shaped tip, and a container that gets uncomfortably hot. In a wide jar, one hot wick can also race down the centre, boring a deep hole while the outer wax stays solid because a single flame can't reach that far across.

If your container is wider than about 7–8cm across the top, one wick is usually not enough. Two or three smaller wicks, spaced evenly, melt the surface far more evenly than one large wick ever will. If you're wicking down, trim to about 5mm and test again. If the jar is hot enough that you can't hold it comfortably, stop using that candle; overheating can crack the glass.

Draughts, radiators and moving air

A candle that tunnels on one side only is almost always sitting in moving air. The flame leans, that side of the wax melts first, and the pool never balances out. Look for doorways, open windows, extractor fans, ceiling fans, air conditioning vents, radiators and even a laptop fan or a television stand nearby.

Move the candle to a still, level surface away from anything that pushes air around. A heatproof coaster or tray helps too, because a cold stone surface pulls heat out of the base of the candle and can make the outer wax set faster than the middle. If the candle has already tunnelled to one side, the uneven rim will keep guiding the melt pool in that direction, so you'll need to level the top before it burns cleanly again.

Wick height and placement

Trim the wick to roughly 5mm before every single burn, once the candle is completely cool and using proper wick trimmers or sharp snips. A wick left too long gives a big, smoky flame that heats the wax unevenly. A wick trimmed too short may barely manage a flicker, and no flicker means no melt pool. Remove the trimmings from the wax rather than leaving them to float.

Placement matters as much as length. A wick that leans towards the edge will melt a channel down that side, and in a glass container repeated off-centre heat can eventually weaken or crack it. When you make candles, hold the wick upright while the wax sets and check it again once the surface has firmed up.

Wax, fragrance and colour

Overloading a candle with fragrance oil or dye can clog the wick, so the flame shrinks and the melt pool narrows. Very hard wax blends can behave the same way if the wick isn't matched to them. Stick to the fragrance load your supplier recommends, usually somewhere between 6 and 10 per cent for soy, and add dye sparingly.

Air pockets and uneven cooling can also leave a candle that burns down one side. Pour at the temperature your wax supplier advises, and let candles cool slowly at room temperature rather than in a cold room.

Quick diagnosis

• Tunnels on one side only: a draught, or an off-centre wick
• Deep crater with tall hard walls: first burn too short, or wick too small
• Tiny flame that keeps going out: wick too small, trimmed too short, or blocked by too much fragrance or dye
• Tall flame, soot, mushrooming, hot jar: wick too big, or not trimmed before burning
• Hole bored down the middle of a wide jar: not enough wicks for the diameter

How to rescue a tunnelled candle

You can often bring a tunnelled candle back with a bit of patience. Never leave a burning candle unattended, keep it away from children, pets and anything flammable, and stop burning when about 1cm of wax remains at the bottom.

1. Trim the wick to 5mm and pick out any debris from the crater.
2. Burn for a long session, two to three hours, and watch whether the pool spreads outwards at all.
3. If the walls are still stubborn, try the foil method: wrap the top of the candle loosely in foil, poke a hole directly above the wick, and burn for about an hour while you stay in the room. The trapped heat helps the surface melt evenly.
4. If that fails, level the top instead. Warm the surface with a hair dryer on a low setting, or carefully pour the liquid wax into a lined tub rather than down the sink, then soak up the last of it with kitchen paper. Once the top is flat, the wick has a fair chance again.
5. If the candle was badly wicked to begin with, don't fight it. Scoop the wax into a wax warmer, or use it as a melt, and put the experience into better wick testing next time.

Tunnelling is rarely a mystery. It's a short first burn, a wick that doesn't match the jar, a draught, or a wick that needed trimming. Get those four things right and your candles will burn cleanly to the edge, every time.

Photo: Pexels / Pixabay