Candle making

Why your candle tunnels, and how wick sizing actually works

A tunnelled candle burns a narrow shaft down the middle and leaves a thick wall of unmelted wax on the glass. You lose most of the wax, the flame drowns in its own well, and the scent fades because the melt pool is too small. It is usually blamed on whoever burned the candle, when the cause was decided at the workbench.

Wax memory and the first burn

Wax does have a kind of memory. The first time you light a candle the melt pool spreads to a certain diameter and stops there; on later burns it tends to return to that diameter before spreading further. Start small and you have taught the candle to keep making a small pool.

So the standard advice is real. On the first burn, leave the candle lit until the melt pool reaches the wall all the way round — roughly one hour per inch (2.5 cm) of container diameter, so a 7.5 cm (3 in) jar wants about three hours. Never burn longer than about four hours at a stretch, or the container overheats and the wick mushrooms.

What gets left out is this: if a candle tunnels despite a proper first burn, the first burn was never the problem. Wax memory is real but small next to whether the wick can generate enough heat to reach the wall at all. A correctly wicked candle forgives a short first burn; an under-wicked one tunnels however carefully it is treated.

What the wick is being asked to do

A wick is a pump: capillary action draws liquid wax up the braid to the flame, where it vaporises and burns, and the heat radiating down melts more wax. The size you need depends on four things at once:

• Container diameter. The pool must reach the wall, so a wider jar needs more heat.
• The wax. A higher melt point needs more energy to keep the pool open; beeswax and pillar paraffin demand a much larger wick than container soy in the same jar, as covered in the wax comparison.
• Fragrance load. Fragrance oil is extra fuel with a different viscosity, and it changes how the wick draws.
• Dye. Pigment can partly clog the braid and reduce the draw, so a coloured candle sometimes needs a size up from the uncoloured version.

Change any one of those and the sizing is no longer valid, which is why swapping fragrance is not a free change.

Wick series and published diameter ranges

Wicks are sold in series, and within a series the numbers step up in size. The two families you will meet most are flat braided coreless wicks, which curl as they burn and partly self-trim, and cored wicks, which have a stiffening filament or a paper, cotton or zinc core and stand upright.

Flat braided and coreless

The usual first choice for natural waxes in containers. Some series curl strongly, which keeps carbon build-up down; others carry a stabilising thread that reduces the curl. They are gentler than cored wicks and more sensitive to a heavy fragrance load.

Cored

A core keeps the wick upright and gives a hotter, larger flame for a given nominal size, which makes cored wicks useful in wider containers, harder waxes and gel. They mushroom more readily if you go a size too large.

Whichever series you choose, the manufacturer publishes a recommended container diameter range for each size. Treat it as a starting point: those ranges are established in a reference wax with no fragrance and no dye, and your candle has both.

How to burn-test properly

A badly run burn test is worse than no test, because it gives a confident wrong answer.

1. Cure the candle first. Testing a soy candle the day after pouring tells you very little.
2. Make three test candles in the real container, with the real wax, load and dye: one at your chosen size, one up, one down.
3. Trim to about 6 mm (a quarter inch) and light all three in the same room, away from draughts.
4. Burn three to four hours, then measure and record what you see before putting them out.

What to look for

Measure the melt pool depth with a skewer or a ruler. Aim for roughly 1.3 cm (half an inch): enough to reach the wall and release scent, not so deep that the candle is consuming itself. Much deeper means the wick is too large, even if the top looks perfect.

Then read the flame. A healthy one is around 2.5-3 cm (1-1.25 in) tall, steady, teardrop-shaped and quiet. Too big: a dancing flame, soot on the glass, a black carbon ball (mushrooming) on the tip, a container too hot to hold, or a pool over 1.3 cm deep. Too small: the pool has not reached the wall after a full burn, the flame is short or keeps drowning, or a ledge of solid wax remains at the edge.

Stepping up or down

Change one size at a time, and re-test with a fresh candle rather than re-lighting one you have burnt, whose melt pool history now biases the result. Repeat for three or four consecutive burns before accepting a wick; plenty look fine on burn one and mushroom on burn three. If one size tunnels and the next up sooties, try a different series: the step between sizes in one family is sometimes larger than the gap you need.

Rescuing a candle that has already tunnelled

If the tunnel is shallow, the foil trick usually works. Wrap aluminium foil around the top of the jar, folding it over the rim so it grips, and leave an opening of a few centimetres over the flame. The foil reflects heat back down onto the wax ledge. Burn for one to two hours, checking regularly, until the ledge has melted flat and level, then remove the foil carefully — it will be hot.

If the wall is thick and hard, or the wick has burnt into a deep well, foil will not save it. The wick was too small, and the fix is a new candle with a larger one. Scenting candles explains why the cure step above matters as much as the wick.