Although candles look and sound simple (light a wick and the wax melts), it is actually a lot of chemistry behind how candles work. Scientists across the world are still experimenting with candles to learn more about candle flames and the combustion process.
The burning of a candle requires a fine balance of fuel, oxygen, and heat. Without sufficient amounts of each of these, the candle will either burn out, or will not light at all. Think of them as mini campfires. Without the key ingredients, the fire dies out.
How Candles Burn
Candles don’t burn by themselves initially. They need energy to kick-start the chemical combustion reaction that makes the wax melt on the top of the candle. The initial energy you need to start the chemical reaction is called activation energy. This can be provided by using a burning match or a lighter.
Candles need three key components: a wick, wax, and a flame. Each one plays an important role in keeping the candle burning.
A candle combines an ignitable wick with wax and a flammable substance. All waxes are hydrocarbons, meaning they are made up of carbon and hydrogen atoms. Wax has a low melting temperature, making it perfect for candles.
Depending on the type of wax, the melting point will vary. All types of wax are formed from hydrocarbons but different types of wax have different formulations that affect the way they burn. Soy wax candles generally have a longer burn-time and a cleaner flame than paraffin candles, provided you take care of the candle (trim the wick, keep it away from draughts etc.)
The wick must be made of an absorbent material, usually twine, that will easily set alight. It must have a strong capillary action. If you’re wondering how to make your own candle wick, check the article I wrote named ‘How to Make a Candle Wick’.
Where Does the Melted Wax Go?
This is a common question people have about candles.
When you light a candle, the heat of the flame melts the wax around the wick. This liquified wax is then drawn up the wick by something called capillary action. The heat from the flame causes the liquid wax to vaporise, turning it into a gas. This breaks down the bonds between the carbon and hydrogen in the candle wax and the gaseous molecules get drawn up into the flame. At this point, they react with oxygen in the air to produce heat, light, water vapour, and carbon dioxide.
Around one quarter of the energy created by the candle’s combustion is released as heat. This heat helps to melt more wax and continue the combustion process until you blow the candle out, or until the candle reaches the end of the wick. You can also deprive the flame of oxygen by placing the lid on the jar or covering it, and the candle will burn itself out.
You can tell if the combustion process is working well by looking at the flame. If the flame is flickering and smoking, the flame is not burning properly and it is undergoing incomplete combustion. This can happen when you first light the candle for a few minutes until the process is stabilised, or when there isn’t enough oxygen getting to the candle. Incomplete combustion can cause soot to be produced, which is the black smoke coming off the flame.
When you blow out a candle, there is a stream of white smoke that rises from the wick. This is vaporised paraffin wax that is condensing and it continues as long as the wick is hot enough to vaporise the wax.
How do Scented Candles Work?
You might be wondering how scented candles seamlessly produce such a lovely aroma. When the vapour reacts with oxygen and ignites, combustion occurs. Scented candles use this process by placing the fragrance oils directly into the wax. When the wax liquifies, it carries the scent along with it, forming scented vapour. The presence of fragrance oil particles in the vapour is what we can smell when we burn our favourite scented candles.
Colours of Candle Flames
Candle flames are mostly yellow but if you look more closely, you will see that the base of the candle is actually blue.
The blue part of the flame is full of oxygen and is the hottest part. It is where the hydrogen and carbon bonds first break and the atoms start to separate. As they vaporise, the atoms bind with oxygen in the air to form water vapour (hydrogen and oxygen) and carbon dioxide (carbon and oxygen).
Around the sides of the flame, there is an area called the veil. This forms when the flame directly meets the oxygen in the air and is the hottest part of the flame at 1400°C (2550°F).
There is also a small orange region of the flame between the blue and yellow parts. There is little oxygen in the area. Hardened carbon particles start to form here, and travel up the flame to become vaporised.
At the bottom of the yellow part of the flame, soot is produced from carbon atoms. The soot particles rise in the flame until they ignite to emit light across the full spectrum of colours visible to the human eye. Yellow light is the most dominant colour that is produced, so we perceive the flame to be yellow.
The Shape of the Flame
As a candle flame heats up the air directly around it, the warm air starts to rise and cooler air rushes in below it towards the bottom of the flame. This process repeats over and over, creating a convection current, giving the flame its characteristic teardrop shape.
If we did not have gravity on Earth, then the flame would not appear as a teardrop, but instead it would have a spherical shape! This is because there is no ‘up’ or ‘down’ in space, so there is no ‘top’ and ‘bottom’ of the candle. Convection currents aren’t created, so the air doesn’t flow around the candle in the same way as it would under the influence of gravity. The oxygen is able to reach all parts of the flame, giving it a spherical shape.
What Part of the Flame is the Hottest?
Sometimes candles are split into zones. Each zone has a different average temperature. Here is a breakdown of each zone and the average temperatures of each part of the candle.
Zone I (400°C (750°F)). The non-luminous, coolest part of the flame just around the wick. There is insufficient oxygen for the fuel to fully burn.
Zone II (800°C (1470°F)). The blue zone around the base of the flame just by the wick where the wax starts to melt. The oxygen supply is sufficient so complete combustion can occur.
Zone III (1000°C (1830°F)). The orange/brown area that contains unburnt wax.
Zone IV (1200°C (2190°F)). The luminous yellow flame that most of us associate with candles. Oxygen levels are not sufficient for complete combustion to occur so unburnt carbon particles (soot) are sometimes produced.
Yellow part of the flame 1200°C (2190°F)
Orange part of the flame 1000°C (1830°F)
Melted wax at the top of the candle 60°C (140°F)
Zone V (1400°C (2550°F)). The non-luminous outer edge of the blue part of the flame known as the veil. Complete combustion occurs here.
Have you ever had one of those candles that relight themselves after you blow them out? No, this isn’t magic!
Paraffin wax is derived from crude oil so it requires fossil fuels. When a wick is coated with wax, it will burn on its own if it gets hot enough, similar to cooking oil or coal. The tiniest amount of wax on the wick will be hot enough to vaporise and burn when it meets oxygen in the air.
Many re-lighting candles work because there is magnesium in the wick. Magnesium is a metal that can combine with oxygen in the air to produce light and heat, just like a candle. It catches fire at 430°C (800°F), which you will notice is similar to the average temperature of a burning wick.
Whilst the candle is burning, the magnesium is shielded away from liquified wax and kept cool. When the candle is blown out, the wick cools down and stops drawing the wax upwards. This means the magnesium now comes into direct contact with the liquid wax which is still hot enough to cause the magnesium to ignite and the candle flame to come back to life!
Candles and Pollution
Candles make a small amount of pollution, some more than others. The release of vaporised hydrocarbons into the air produces small amounts of carbon monoxide and nitrogen dioxide. If combustion is incomplete, it can cause production of soot due to the unburnt carbon atoms.
The type of candle affects the amount of pollution. Scented candles produce up to twice as much pollutants as non-scented candles. Paraffin wax candles release more chemicals into the air than soy and beeswax candles.
However, compared to many other daily activities such as travelling and heating our homes, candles contribute very little to air pollution.