Jessie Taylor
When candles are lit, the heat from the flame melts the wax near the wick. This liquid wax is then drawn up the wick and vaporized, where it combines with oxygen in the air to create heat, light, water vapour, and carbon dioxide. The liquid wax that does not get drawn up the wick and vaporized can be observed as a puddle around the base of the candle. Interestingly, the liquid wax that does not evaporate can also be seen to become clear as it melts. This is because air or water was trapped in the wax during the creation process, and maintaining the wax at a temperature of 85-95°C will allow any trapped moisture to evaporate.
| Characteristics | Values |
|---|---|
| Candle wax becomes clear when melted | Soy wax |
| Reason for cloudiness | Air or water trapped in the wax during the creation process |
| Solution to cloudiness | Maintain wax temperature at 85-95°C to allow trapped moisture to evaporate |
| Reason for rough surface | Wax cools too quickly, too slowly, or contains small air bubbles |
| Solution to rough surface | Gently tap the container after pouring to help air bubbles escape; experiment with pouring temperature |
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What You'll Learn
Candle wax is made of hydrogen and carbon
When a candle burns, the heat of the flame melts the wax near the wick, which then travels up the wick and evaporates. The wick, usually made of cotton, also burns, but it's the wax that provides most of the heat. The melted wax is not always visible, which is why candles sometimes seem to melt invisibly.
Paraffin wax, made from crude oil, is the most frequently used candle wax worldwide today. However, this is a relatively recent development, as paraffin wax was only introduced in 1830. Before this, candles were typically made from animal fat (tallow), whale fat, olive oil, cinnamon, and beeswax. Beeswax was used for candles in China as early as the Tang Dynasty (618-907 AD) and in Europe during the Middle Ages, although its expense meant that it was rarely used in homes. In ancient Egypt and Rome, tallow rendered from animals was the primary ingredient of candles. In early Japan, candle wax was made from tree nut extracts, while in India, the fruit of the cinnamon tree was boiled to create wax.
Today, candle waxes are also made from soy, palm, gels, synthetic waxes, and synthesized waxes, as well as various wax blends and customized formulations. The development of new waxes for candles has historically depended on the availability and ease of processing of raw materials, as well as the desirability of the wax compared to other options.
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When lit, the heat of the flame melts the wax
The liquid wax is then drawn up the wick by capillary action. The heat of the flame vaporises the liquid wax, turning it into a hot gas. This gas, which is essentially made up of hydrogen and carbon atoms, rises up into the flame, where it reacts with oxygen from the air. This creates heat, light, water vapour, and carbon dioxide.
The heat radiated from the flame is enough to melt more wax and keep the combustion process going until the fuel is used up or the heat source is removed. It takes a few minutes for this combustion process to stabilise when a candle is first lit.
To prevent tunneling, it is recommended to melt the entire surface of the candle before blowing it out for the first time. This usually takes at least an hour or two, depending on the size of the candle. If the candle is not allowed to melt evenly across its surface, the wax surrounding the tunnel will remain harder and more challenging to melt the next time the candle is lit.
Additionally, the wick size should be considered. If the wick is too small, it might not generate enough heat to melt the wax at the edge of the candle, leading to tunneling.
Soy wax, in particular, tends to develop a rough surface due to the formation of solid crystals during cooling. Maintaining a temperature of 85-95°C can help evaporate any trapped moisture, resulting in a clearer appearance.
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Liquid wax is drawn up the wick by capillary action
When a candle is lit, the heat from the flame melts the wax near the wick. This liquid wax is then drawn up the wick through capillary action. Capillary action is the process by which liquid moves through narrow spaces due to adhesive and cohesive forces. In the case of a candle, the liquid wax is drawn up the narrow spaces between the fibres of the wick, which is typically made of braided cotton.
The adhesive forces between the liquid wax and the wick can be stronger than the force of gravity acting on the liquid, allowing the liquid wax to move upwards. As the liquid wax rises through the wick, it is vaporised by the heat of the flame, turning into a hot gas. This gas then breaks down into molecules of hydrogen and carbon.
The vaporised molecules of hydrogen and carbon react with oxygen from the air, creating heat, light, water vapour, and carbon dioxide. The heat generated is sufficient to melt more wax and sustain the combustion process until the fuel is depleted or the heat source is removed. The liquid wax continues to be drawn up the wick through capillary action, ensuring a steady supply of fuel for the flame.
The design of candle wicks has evolved over time. In the past, wicks required regular trimming to maintain an optimal length. Modern wicks, however, are designed with a flat braid that causes the tip of the wick to curve outside the flame, providing it with enough oxygen to burn itself and maintain a constant length. This self-trimming mechanism ensures a more consistent burn and reduces the need for manual wick trimming.
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The liquid wax is then vaporized and turns into hot gas
When a candle is lit, the heat from the flame melts the solid wax near the wick, causing it to become a liquid. This liquid wax is then drawn up through the wick by capillary action. As the liquid wax rises through the wick, it encounters the intense heat of the flame, which vaporizes it, transforming it into a hot gas. This process of vaporization is crucial for the combustion of the candle.
The vaporized wax molecules, now in a gaseous state, mix with the oxygen in the surrounding air. This mixture of hot gas and oxygen is drawn into the flame, where combustion occurs. The combustion process breaks down the wax molecules, which are primarily composed of hydrocarbons, into their constituent atoms of hydrogen and carbon. These atoms then react with the oxygen to produce carbon dioxide and water vapour.
The heat generated by the combustion process is what keeps the candle burning. Approximately one-fourth of the energy created by the combustion is released as heat, radiating in all directions from the flame. This heat is essential for sustaining the melting and vaporization of more wax, ensuring the candle continues to burn until the fuel source is depleted or the heat source is removed.
It is important to note that the liquid wax does not instantly vaporize upon melting. The melting process merely transitions the solid wax into a liquid state, making it easier for the wick to absorb and transport the wax upwards through capillary action. It is only when the liquid wax reaches the flame that it is subjected to sufficient heat for vaporization to occur, transforming the liquid wax into a hot gas.
To prevent issues like candle tunneling, it is recommended to allow the entire surface of the candle to melt evenly during the initial burn. This helps to avoid the "'memory'" effect of wax, where it hardens and becomes more challenging to melt with subsequent burns. By ensuring a uniform melt pool across the candle's surface, you can minimize tunneling and maximize the burn time of your candle.
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The wax vapour burns, reacting with oxygen to create heat and light
When a candle burns, it's not just the wick that's fuel for the fire. The wax, too, is consumed as it melts, rises up the wick, and evaporates. The subsequent vapour then reacts with oxygen in the air to create heat, light, water vapour, and carbon dioxide.
Wax is made of hydrogen and carbon atoms, which are hydrocarbons. When a candle is lit, the heat of the flame melts the wax near the wick, and this liquid wax is drawn up the wick by capillary action. The heat of the flame then vaporises the liquid wax, turning it into a hot gas. This vapour then rises into the flame, where it reacts with oxygen from the air.
The oxygen-rich blue zone at the base of the flame is where the hydrocarbon molecules break apart into hydrogen and carbon atoms. Hydrogen is the first to separate and reacts with the oxygen to form water vapour. Some of the carbon burns here to form carbon dioxide. The dark or orange/brown region of the flame has relatively little oxygen. This is where the various forms of carbon continue to break down and small, hardened carbon particles start to form.
As the carbon particles rise, they are heated to approximately 1000 degrees Centigrade. At the bottom of the yellow zone, the formation of carbon (soot) particles increases. As they rise, they continue to heat until they ignite to incandescence and emit the full spectrum of visible light. The yellow portion of the spectrum is the most dominant when the carbon ignites, so the human eye perceives the flame as yellowish.
The combustion process in a candle is extremely efficient, mostly releasing water and carbon dioxide, with minimal soot. About one-quarter of the energy from combustion is emitted as heat, which maintains the reaction by vaporising more wax so that it can burn, thereby melting more wax to keep the candle burning.
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Frequently asked questions
When candles melt, the wax usually becomes clear. However, it may sometimes become cloudy due to trapped air or water during the creation process.
Candle tunneling occurs when only a small portion of the wax surrounding the wick melts. This happens when the wick is too small to generate enough heat to melt the wax at the edge of the candle, or when the candle is blown out before the entire surface of the candle has melted.
To prevent candle tunneling, ensure that you burn the candle long enough for the entire surface to melt evenly before blowing it out. This usually takes at least an hour or two, depending on the size of the candle.
When a candle burns, the heat of the flame melts the wax near the wick, which then travels up the wick and evaporates. The wax vapour combines with oxygen in the air and combusts, producing heat, light, water vapour, and carbon dioxide.
