Chloe Adams
Candles have been the subject of scientific inquiry for hundreds of years, with scientists like Michael Faraday giving lectures on the chemical history of a candle. Candle flames have also been studied in microgravity conditions by NASA. The combustion of a candle involves the wax acting as fuel and reacting with oxygen in the air, with the heat of the flame melting the wax near the wick. The flame of a candle has three main parts, each with distinct characteristics: the dark inner zone, the bright yellow middle zone, and the blue outer zone. The blue outer zone is the hottest part of the flame, reaching temperatures of about 1400°C, while the yellow zone is responsible for generating light, ash, and soot. The size and rate of burning of a candle flame are influenced by the candle wick and the type of wax used.
| Characteristics | Values |
|---|---|
| Shape | Elongated or teardrop-shaped |
| Colour | Dark orange-brown, yellow, blue |
| Temperature | The outer zone is the hottest, reaching 1400°C. The dark or orange/brown middle zone is approximately 1200°C. The innermost zone is 1000°C. |
| Composition | The blue zone is oxygen-rich. The yellow zone is luminous and is caused by incomplete combustion. The dark zone is cooler and is caused by unburnt wax vapours. |
| Wick | Made of braided cotton, the wick draws melted wax up to the flame through capillary action. |
| Wax | Made of hydrocarbons, largely composed of hydrogen and carbon atoms. |
| Self-sustaining | Yes, the heat of the flame melts the wax, which is then drawn up the wick and continues to burn. |
| Smoke | Caused by unburned soot particles escaping from the flame due to incomplete combustion. |
| Size | The size of the flame is controlled by the candle wick and the type of wax. |
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What You'll Learn
The chemical composition of a candle flame
The specific chemical composition of a candle flame can be influenced by several factors, including the type of wax used, the presence of additives, and the characteristics of the wick. The kind of wax can affect the burn rate and temperature of the flame, with certain waxes, such as beeswax and coconut wax, burning longer and at higher temperatures than paraffin or soy wax.
The wick also plays a crucial role in the chemical composition of the flame. Its diameter, stiffness, fire resistance, and tethering characteristics can impact the burning behaviour. Commercial wicks are often treated with chemicals like ammonium nitrate and ammonium sulfate to modify their burning properties, including the afterglow of the wick after extinguishing the flame.
A candle flame consists of distinct zones or regions, each with different colours and temperatures. The outermost zone, known as the blue zone, has a temperature of approximately 1400°C and is characterised by complete combustion, resulting in a non-luminous flame. The middle zone, with a temperature of around 1200°C, exhibits incomplete combustion, emitting a yellow or orange luminous flame due to the limited supply of oxygen. This zone is responsible for the generation of ashes and soot.
The innermost zone of the flame is further divided into the dark zone and the blue zone. The dark zone, with a temperature of about 1000°C, is formed by unburnt wax vapours and is relatively cooler. The blue zone, the coolest part of the flame, exists at the base and is where the hydrocarbon molecules vaporise and break down into hydrogen and carbon atoms. This zone is rich in oxygen, and the hydrogen atoms react with oxygen to form water vapour.
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The colour of a candle flame
At the base of the flame is a blue area, known as the oxygen-rich zone. This is where hydrocarbon molecules vaporize and break down into hydrogen and carbon atoms. The hydrogen reacts with oxygen to form water vapour, and some of the carbon burns to form carbon dioxide. The blue colour is caused by the incandescence of fine soot particles formed inside the flame. With a sufficient oxygen supply, more complete combustion occurs, resulting in less black body-radiating soot and a brighter blue appearance.
Above the blue zone is a small dark orange-brown section. This area has a relatively low oxygen level, and it is where various forms of carbon continue to break down and form hardened carbon particles. These particles are then heated to approximately 1000 degrees Celsius as they rise.
The third and most prominent region is the large yellow region that we typically associate with candle flames. This area is dominated by the yellow portion of the spectrum when the carbon ignites, giving it a yellowish appearance to the human eye. At the bottom of this zone, the formation of carbon soot particles increases, and they continue to heat up as they rise. Near the top of the yellow region, these particles oxidize at a temperature of around 1200 degrees Celsius.
The fourth and outermost zone is a faint blue edge that extends from the base of the flame up the sides of the flame cone. This is the hottest part of the flame, reaching temperatures of approximately 1400 degrees Celsius. It appears blue due to its direct contact with the oxygen in the air.
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The temperature zones of a candle flame
A candle flame has three main parts: the inner zone, the middle zone, and the outer zone. Each part has a different temperature, colour, and burning process.
The inner zone is dark, and the coolest part of the flame exists at the base. The oxidation of vaporized wax produces carbon monoxide, which burns to create a bluish flame with a temperature of about 800°C. This zone is where the hydrocarbon molecules vaporize and start to break apart into hydrogen and carbon atoms.
The middle zone is the most luminous due to the depletion of oxygen. The burning of molten wax creates a yellowish flame with a temperature of about 1000°C to 1200°C. This zone is responsible for generating ashes and soot.
The outer zone is the hottest part of the flame, with a temperature of about 1400°C. It is bluish in colour and non-luminous in nature. This zone is where the various forms of carbon continue to break down and small, hardened carbon particles start to form.
The size of a candle flame and its burning rate are largely controlled by the candle wick. The diameter, stiffness, fire resistance, and tethering of the wick are important characteristics that influence how the candle burns. Commercial wicks are often made from braided cotton and may be infused with chemicals to modify their burning characteristics.
The shape of a candle flame is typically elongated or teardrop-shaped due to the convection current created by the upward movement of warm air around the flame. However, in microgravity conditions, such as those created in NASA experiments, a candle flame takes on a spherical shape due to the absence of gravity's pull.
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The structure of a candle flame
A candle flame has three distinct regions, each with its own temperature, colour, and characteristics. The outermost zone is the hottest part of the flame, with a temperature of approximately 1400°C. It appears blue due to the incandescence of very fine soot particles formed inside the blaze. This zone has an unlimited supply of oxygen, allowing for complete combustion. The middle zone, with a temperature of about 1200°C, is the largest part of the flame and is characterised by varying shades of yellow and orange. Incomplete combustion occurs in this region due to a limited oxygen supply, resulting in the emission of light and the formation of ashes and soot. The innermost zone, with a temperature of around 1000°C, is further divided into the dark zone and the blue zone. The dark zone is the cooler segment, generated by unburnt wax vapours, while the blue zone at the base of the flame is the coolest part.
The size and burning rate of a candle flame are largely influenced by the wick, with factors such as diameter, stiffness, fire resistance, and tethering playing a role. The type of wax used also affects the burn rate, with beeswax and coconut wax lasting longer than paraffin or soy wax. The flame's elongated or teardrop shape is a result of convection currents created by the upward movement of warm air around the flame. However, in microgravity conditions, such as those studied by NASA scientists, a candle flame takes on a spherical shape due to the absence of gravity's upward pull.
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The history of candle flame research
The study of candle flames has captivated scientists for centuries, with research spanning various eras and disciplines.
Medieval Europe
In medieval Europe, candles were primarily used in Christian churches and later made their way into the households of the wealthy. They were typically made from tallow (rendered from beef or mutton fat) or beeswax, with the latter being more expensive and sought after for its clean burn and lack of unpleasant odour.
19th Century
The 19th century witnessed significant improvements in candle technology. M. Cambacérès, a French inventor, introduced the concept of "self-trimming" or "self-consuming" wicks in 1825. These wicks were plaited and soaked with mineral salts, allowing them to curl and trim themselves. Around the same time, Michel Eugène Chevreul and Joseph Louis Gay-Lussac made advancements in candle composition by separating stearin from animal fats, resulting in harder and brighter-burning candles. The production of candles became an industrialized mass market during this period.
Michael Faraday's Lectures
In 1848, Michael Faraday delivered a series of six lectures titled "The Chemical History of a Candle" at the Royal Institution. These lectures were intended for young beginners and aimed to explore the chemistry and physics of candle flames. Faraday used the candle as a symbol to explain the nature of combustion, highlighting the roles of oxygen, hydrogen, and carbon in the process. He also discussed the different zones of combustion within the flame and the presence of carbon particles in the luminescent zone.
NASA Experiments
In the late 1990s, NASA scientists conducted experiments on candle flames in microgravity conditions. They discovered that without the influence of gravity, candle flames took on a spherical shape instead of the familiar teardrop shape observed on Earth.
Today, candle flame research continues to be a subject of interest for scientists and students alike, with ongoing experiments exploring combustion, emissions, and the underlying physics and chemistry of these deceptively simple sources of light.
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Frequently asked questions
A candle flame is the light produced by the combustion of a candle. This combustion occurs when the wick of a candle is lit, melting and igniting the wax, which vaporises and combines with oxygen in the air.
A candle flame has three distinct regions, each with its own colour and temperature. The inner zone is dark, the middle zone is bright yellow, and the outer zone is blue. The yellow colour is due to the incomplete combustion of wax, which generates soot particles that ignite and emit a full spectrum of visible light. The blue outer zone is the hottest part of the flame, reaching temperatures of around 1400°C.
Candles are typically made from wax, which acts as fuel for the flame. Different types of wax include paraffin, soy, beeswax, coconut wax, microcrystalline wax, gel wax, and plant waxes such as palm, carnauba, bayberry, or soybean wax. The type of wax affects the burn rate of the candle, with beeswax and coconut wax burning longer than paraffin or soy wax.
A candle wick is typically made from braided cotton and works through capillary action, drawing the melted wax or fuel up to the flame. The diameter, stiffness, fire resistance, and tethering of the wick influence how the candle burns. The wick's capillarity determines the rate at which the melted wax is conveyed to the flame, and wicks are often infused with chemicals to modify their burning characteristics.
