A candle needs a wick

Capillary action feeding a flame, and why the wax alone will not burn.

A candle needs a wick because solid wax itself will not burn, only wax vapour will, and the wick's job is to draw a continuous supply of melted wax up from the pool at the base of the flame to where it can vaporise and finally react with the surrounding air.

Why a lump of wax is hard to light

Solid wax touched directly by a match takes real effort to ignite and burns poorly even then, because a flame needs fuel in gas form to react quickly with oxygen, and a lump of solid wax offers almost no vapour at its surface for a flame to catch hold of. What burns in a candle is wax vapour rising from a thin, exposed film, and keeping that film in existence continuously, right where a flame can reach it, is the wick's entire function. A block of wax with no wick at all can be persuaded to burn only with real difficulty, needing an unusually large, sustained heat source to keep even a shallow surface pool melted and vaporising, which is precisely the demonstration that convinces a sceptical audience that the wick is doing the work of feeding the flame.

How the wick lifts liquid wax

Once a candle is lit, heat from the flame melts a small pool of wax at the base, and the wick, a fibrous braided cord running up through the middle of that pool, draws the melted wax upward through the narrow gaps between its own fibres by capillary action, the same surface tension effect that pulls any liquid up through a sufficiently narrow channel against gravity. By the time that wax reaches the height of the flame, sitting in the hottest part of the wick, it has been heated enough to vaporise, and that vapour, arriving continuously and in the amount the flame is currently consuming, sustains the burning.

The arrangement regulates itself without any moving parts or controls. A flame burning through vapour faster draws more heat down into the wick, which vaporises wax faster, which supplies vapour faster, roughly keeping pace with demand on its own. Trim the wick too short or leave it too long and the balance shifts, either starving the flame of vapour or flooding it with more than it can cleanly burn, with nothing changed except the length of fibre exposed above the wax pool.

How much wax a wick can lift in a given time is set by its diameter and by how tightly its fibres are braided, so a wick sized correctly for its candle keeps pace almost exactly with the flame and holds a steady flame size. A wick too thick floods the flame with more wax than it can vaporise and burn cleanly. A wick too thin for the candle's diameter produces a different symptom over the course of a burn: only a narrow well directly around the wick liquefies, and the candle tunnels straight down through its own centre, leaving a solid rim of wax around the outside that never gets hot enough to melt at all.

A paper towel on a spilled drink

A paper towel touched to a puddle of spilled juice on a kitchen counter draws that liquid steadily upward into itself, against gravity, without anything mechanical pumping it there. The gaps between the towel's fibres are narrow enough that the liquid's own surface tension pulls it into and along them, wicking the puddle up and away from the counter through nothing more than the geometry of the material itself.

A candle's wick performs exactly the same trick with liquid wax, pulling it upward through the narrow spaces between its braided fibres because those spaces are narrow enough for capillary action to do the work. The fibres provide the channel, and the liquid climbs on its own, continuously resupplying the flame exactly as the paper towel continuously draws the puddle away from the counter.

A towel already soaked through shows the limits of the effect just as usefully as a dry one. With its channels already full it draws up very little further liquid, in much the same way a wick flooded with more melted wax than it can move upward gains nothing by drawing up still more. In both cases capillary action has a fixed capacity, moving liquid at a rate set by the size and packing of the channels, however much liquid happens to be waiting at the bottom.

Trimming, tunnelling and extra wicks

Trimming a candle's wick before lighting it matters more than it might seem, since an overly long, untrimmed wick draws up more wax than the flame at its tip can properly vaporise and burn, producing a larger, smokier, less controlled flame than the same candle would give with a wick trimmed to the right length. Throughout the burn, the wick is quietly deciding how much fuel reaches the flame at any given moment, exactly as Faraday spent an entire lecture demonstrating to an audience who had almost certainly never thought about their own candles this carefully before.

The same fixed capacity explains why a candle much wider than a standard taper is often made with two or three wicks. A single wick can only melt a pool out to a certain radius around itself, and on a broad candle that pool never reaches the edges, so the wax at the rim is left behind as the flame burns downward. Several smaller wicks, each melting its own pool, together cover a surface that one wick was never sized to manage alone, and the pools merge into a single even surface that burns down level instead of tunnelling.