Is the wick of a candle what burns?
― What burns is the wax, turned into a gas
A candle wick is not the fuel. Solid wax will not burn as it is. The wick acts as a "pump": it draws up melted wax and turns it into gas. And the wick itself stands in the place in the flame where it is hardest to burn.
It is a power-cut night, or a birthday cake. You light a candle. At the tip of a thin wick, a flame sways quietly.
Many people think the thread-like wick is what burns, and the wax around it is just a stand to hold it up. But if so, the wick would be gone in a few seconds. In fact, one candle burns for hours.
What shrinks is not the wick but the wax. So what is going on inside the flame?
There are only two reasons the wick doesn't burn up
Solid wax doesn't burn when you hold a flame to it. Heat melts it into a liquid, it climbs the wick, and only once it becomes a gas does it burn. The fuel is always the wax.
The middle of the flame is full of wax gas, and almost no oxygen gets in. The wick stands in there, and it is kept cooled by the wax evaporating from it.
In other words, the flame burns only in the outer skin of a "cloud of wax gas" floating around the wick. Let's look at it step by step.
Why doesn't solid wax burn as it is?
Burning means a fuel combines with oxygen in the air and gives off heat and light. For that, molecules of fuel and oxygen have to meet.
Inside solid wax, the molecules are packed tightly together. Oxygen can touch only the surface. So the heat of the flame first melts the wax around the wick. That is the small pool of liquid at the top of the candle.
The melted wax climbs through the gaps between the wick's fibres. It works like a towel: dip one end in water, and the water soaks upward. Wax that is heated near the top of the wick turns into gas and drifts away from it.
Look at Figure 1. Right around the wick there is a dark region. It is where wax gas gathers, and it is not burning yet. Burning happens only at the outside of the flame, where the gas meets oxygen coming in from outside.
Why doesn't the wick burn up, even though it is inside the flame?
There are two reasons. The first is that there is no oxygen. The area around the wick is full of wax gas, and the outside air is used up in the flame's outer skin. With nothing to burn with, the wick cannot burn.
The second is that it is being cooled. When wax changes from liquid to gas, it takes heat from its surroundings. It is the same as sweat drying and cooling your skin. The wick always stands in a place that is cooled by evaporation.
The temperature also differs greatly from place to place in a flame. The outer skin is said to reach about 1400℃, while the dark part around the wick is thought to be far lower. "A flame is hottest in the middle" is another common mistake.
That said, the wick does not stay exactly the same. Today's candle wicks are made of braided thread. As it burns, the tip bends a little, and only the end that sticks out beyond the outer flame burns away bit by bit. In this way the wick's length is kept just right on its own.
Before braided wicks became common, candle wicks were straight twisted thread. The tip would be left unburnt in the flame and grow longer, and the flame turned dim and sooty. So people used special scissors to snip the end of the wick from time to time. If the wick burned, this tool would not have been needed.
Before it meets oxygen, the wax gas is broken down by the heat into tiny particles of carbon (soot). The bright yellow of the flame is the colour of these particles glowing with heat. A gas stove flame is blue because fuel and air are already mixed beforehand, so hardly any soot forms.
Summary
The fuel of a candle is wax. Solid wax melts and climbs the wick, turns into gas, and then burns in the outer skin of the flame, where it meets oxygen from outside. The wick stands in the centre of the flame, where oxygen can't reach and evaporation keeps it cool, so it hardly burns. The wick's real job is to keep drawing up wax and turning it into gas.
The wick is not fuel; it is a pump that turns wax into gas.
The flame burns only in the outer skin of a cloud of wax gas.
The way fuel burns only after it becomes a gas is closely linked to why adding fuel to a dying fire makes a pillar of flame shoot up. How the colour of a flame relates to the way it burns is also covered in the article on carbon monoxide.
- On a stable surface with nothing flammable nearby, light the candle and let it burn for about a minute so a pool of liquid forms. Keep a glass of water close by.
- Blow it out. A thin trail of white smoke rises from the wick. This white smoke is "what was wax gas", cooled into tiny particles.
- Straight away, bring a lit match or similar to the smoke 2 to 3 cm above the wick. The fire runs down the smoke, and the candle lights again without the flame touching the wick.
This is proof that what burned was not the wick but the wax that had turned to smoke. If you wait too long after blowing it out, the smoke thins and it won't work. Because you are using fire, always do this with an adult beside you.
For those who want to know more ― terms, formulas and links to textbooksFrom middle-school science to university courses, each part is marked with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry" and "Chemistry"
- High school+Advanced high-school material, or textbook sidebar content
- UniversityNot taught in high school; university-level courses (combustion engineering, reaction engineering)
- ResearchNot taught even at university as settled fact; questions researchers are studying now
Middle schoolTerms: this phenomenon has names
- Combustion: a chemical change in which a substance combines with oxygen and gives off heat and light. When wax burns, it makes carbon dioxide and water.
- Capillary action: liquid climbing up a narrow gap on its own. It is how the wick draws up wax.
- Heat of vaporization (heat of evaporation): the heat a liquid takes from its surroundings when it turns into gas. It is why the wick is cooled.
- Outer flame, inner flame, flame core: names for the outside, the middle and the part around the wick. The outer flame is the hottest.
Middle schoolHigh schoolChecking with a formula: how much heat does one candle give off?
Once we know the fuel is the wax and not the wick, we can work out the heat the flame gives off from "the amount of wax burned per second".
| In symbols | P ＝ ṁ × Q |
| In words | Rate of heat from the flame ＝ mass of wax burned per second × heat released when 1 g of wax burns |
| Where it comes from | Conservation of energy. The chemical energy the burned wax held goes out as heat and light, with nothing lost |
| P | Rate of heat from the flame. Unit: watt (W, joules per second) |
| ṁ | Mass of wax burned per second. Unit: grams per second |
| Q | Heat of combustion: the heat released when 1 g of wax burns completely. Unit: joules per gram |
| Heat of combustion of paraffin wax | About 46000 joules per gram |
| Wax a household candle burns in one hour | About 6 g (varies with thickness) |
| Seconds in one hour | 3600 seconds |
| Mass of wax in one candle (a small one) | 20 g |
| Heat released in one hour | 46000 × 6 ＝ 276000 joules |
| Per second (＝ P) | 276000 ÷ 3600 ≒ 76.7 watts |
| How long a 20 g candle keeps burning | 20 ÷ 6 ≒ 3.3 hours |
One small flame gives off about 77 watts of heat. That is more than an old 60-watt incandescent bulb, and close to the heat given off by one adult sitting still (about 100 watts). It lasts more than three hours because there are 20 g of wax fuel. The thin wick weighs only a few hundredths of that, and if the wick were the fuel, the flame would be gone in a few seconds.
High schoolHigh school+The reaction of burning wax, and how a flame glows
High schoolParaffin is a mixture of long chain molecules (alkanes) made only of carbon and hydrogen. Taking a molecule with 25 carbons as an example, the burning reaction is as follows.
C25H52 ＋ 38 O2 → 25 CO2 ＋ 26 H2O
Each molecule needs as many as 38 oxygen molecules. Because that much oxygen has to arrive little by little from outside the flame, burning is limited to a thin outer skin.
High school+The yellow light of a flame is not light from the chemical reaction itself. It is "thermal radiation": hot soot particles glow in a colour set by their temperature. It is the same mechanism that makes charcoal or an electric heater glow red. The light at the blue base comes from molecules formed midway through the reaction giving off fixed colours, which is an entirely different mechanism.
UniversityDiffusion flames: how fast oxygen "seeps in" sets how it burns
A flame like a candle's, where fuel and oxygen come from different directions and meet and burn at the boundary, is called a diffusion flame (diffusion combustion). It is the counterpart of the premixed flame, as in a gas burner, where fuel and air are mixed first and then burned. In a diffusion flame the reaction itself is fast enough, and the burning rate is set by how fast fuel and oxygen mix by diffusion. So the flame clings as a thin sheet to the surface where fuel and oxygen are in exactly balanced proportion. This idea is known as the Burke–Schumann flame sheet model. The amount of wax the wick lifts is set by capillary action and by the balance of heat returning from the flame to the pool, so the size of the flame keeps itself steady.
📖 Derivations and further reading: Diffusion combustion (Wikipedia) ／ Capillary action (Wikipedia)
ResearchWhat is still not well understood
- How is soot born? The stage where the first particles, about a nanometre across, form from gas molecules is still studied in detail. It also bears on air pollution and health.
- Flames where nothing has weight. On the space station, hot gas does not rise, and candle flames are reported to be round and bluish. Experiments continue on how far gravity decides the shape and brightness of a flame.
- Flames of candles side by side sway together. Japanese researchers and others have reported that the flames of bundled candles pulse about 10 times a second, and that two flames brought close together fall into the same rhythm. The details of how this works are still being studied.
So what this article says is also "an explanation of what we understand so far". The candle, which became the subject of a lecture more than 160 years ago, is still an object of research today.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science Year 1 "Changes of state", Year 2 "Chemical changes (combustion)" | Wax melts and turns to gas before it burns |
| High school | Basic Chemistry "Chemical equations", Chemistry "Heat of reaction" | The combustion equation; working out the rate of heat from the heat of combustion |
| High school+ | Physics: advanced "thermal radiation" | The yellow light is thermal radiation from soot |
| University | Combustion engineering "diffusion flames", interface chemistry "capillary action" | Why the flame clings to a thin outer sheet |
| Research | Soot formation, microgravity combustion, flame oscillation and synchronisation | What is still not understood |
| ― | Links to daily life | Handling candles; telling how something burns from its flame colour |
- Commentary on Michael Faraday, The Chemical History of a Candle (1861; Japanese translations include Iwanami Bunko and Kadokawa Bunko) (ロウソクの科学)
- Diffusion combustion (Japanese Wikipedia) (拡散燃焼)
- Heat of combustion (Japanese Wikipedia) (燃焼熱)
- The Japan Society of Mechanical Engineers (日本機械学会), ed., Combustion Engineering Handbook (燃焼工学ハンドブック), Maruzen
- Kitahata, H. et al. "Oscillation and Synchronization in the Combustion of Candles", The Journal of Physical Chemistry A (2009)
※This article is a general-audience science explainer. The numbers given are rough guides to help you understand how things work. When using candles or other fire, keep them away from anything flammable and never leave them unattended.