Chloe Adams
Candles have been used for over two millennia, and while they are no longer essential for illumination, they are still commonly used for functional, symbolic, and aesthetic purposes. But how do they work? The science behind candles involves elements of heat, light, and combustion. The candle's wick absorbs the liquid wax and pulls it upward by capillary action. The heat of the flame then vaporizes the wax, turning it into a hot gas that burns and releases light, heat, and fragrance. The vaporizing wax also cools the wick, protecting it from burning.
| Characteristics | Values |
|---|---|
| Candle Composition | Wax, tallow, or similar slow-burning material, with an embedded wick |
| Wick Composition | Naturally absorbent material, such as cotton, or strong capillary action material, such as glass fiber |
| Wax Composition | Hydrocarbons, primarily composed of hydrogen and carbon atoms. Common types include paraffin, soy, coconut, beeswax, apricot, and vegetable waxes. |
| Combustion Process | Heat from the flame melts the wax near the wick, which is then drawn up the wick by capillary action. The liquid wax vaporizes, forming hot gas that combines with oxygen to create heat, light, water vapour, and carbon dioxide. |
| Flame Characteristics | The blue base is oxygen-rich and where hydrocarbons break down. The dark orange-brown section has less oxygen and is where carbon particles form. The yellow region is where carbon ignites, giving the flame its yellowish perception. The outer blue edge is the hottest part, reaching up to 1400°C. |
| Historical Use | Candles were used as a significant form of indoor lighting for over two millennia until the invention of electric lighting. They also hold cultural and religious significance. |
| Production | Candle-making has evolved from ancient methods to modern machinery, producing a variety of colours, shapes, and sizes. |
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What You'll Learn
The science behind candles
Candles have been a significant source of indoor lighting for over two millennia. They are still commonly used for functional, symbolic, and aesthetic purposes, despite the invention of other light sources.
A candle is made up of three essential components: the wick, wax, and fragrance. The wick is a piece of string or cord that holds the flame. Commercial wicks are usually made from braided cotton, but wood wicks are also used for a rustic charm. The size of the wick matters—a larger wick produces a bigger flame and can lead to faster burning and potential soot.
The wick draws the melted wax or fuel upwards through capillary action. The liquid wax reaches the flame and vaporizes, turning into a hot gas that burns and releases fragrance. The vaporized molecules of hydrogen and carbon react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. This combustion process takes a few minutes to stabilize, and the flame will burn cleanly and steadily in a quiet teardrop shape.
The blue area at the base of the flame is oxygen-rich and is where the hydrocarbon molecules vaporize and break apart into hydrogen and carbon atoms. Above this is a small dark orange-brown section, where various forms of carbon continue to break down and form hardened carbon particles. The large yellow region at the top of the flame is where the carbon ignites, giving off a yellowish light. The fourth zone of the candle is the faint outside blue edge, which is the hottest part of the flame, typically reaching 1400° C.
Scientists have been fascinated by candles for hundreds of years, with Michael Faraday delivering a famous lecture series on the Chemical History of a Candle in 1860. Candle-making has also evolved over time, with improvements made to the wick and the use of different waxes, such as beeswax, paraffin, soy, and coconut.
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How candles produce light
A candle is an ignitable wick embedded in wax or another flammable solid substance. The wick is a piece of string or cord that holds the flame of a candle. Commercial wicks are made from braided cotton due to its ability to absorb liquids such as liquid wax.
When you light a candle, the heat of the flame melts the wax near the wick. This liquid wax is then drawn up the wick by capillary action. The heat of the flame vaporizes the liquid wax and starts to break down the hydrocarbons into molecules of hydrogen and carbon. These vaporized molecules are drawn into the flame, where they react with oxygen from the air to create heat, light, water vapour (H2O) and carbon dioxide (CO2).
The light produced by a candle comes from a chemical reaction known as combustion. In this process, the wax reacts with oxygen in the air to make a colourless gas called carbon dioxide. Water is also produced in the form of steam. Since the wax never burns perfectly cleanly, there is also a little smoke produced. The smoke is an aerosol (tiny particles of solid, unburned carbon from the wax mixed in with the steam) and it often leaves a black, carbon deposit on nearby walls or the ceiling above where the candle is burning.
The flame of a candle has multiple layers of different colours. At the very bottom, near the wick, there is a blue layer. This is the centre of combustion, where the hydrocarbons of the wax first meet oxygen and start to break down. Next, there is a dark brown/orange layer, which is where some solid carbon can be found. This carbon never combusted. Finally, there is the yellow/white layer, where the gaseous carbon that escaped the first two layers combusts just before it can escape for good. The outermost layer of the flame is the hottest layer and the only part that directly comes in contact with oxygen.
In the microgravity of space, with plenty of oxygen, candle flames are more spherical. Without gravity, there is no "up" direction for warm air to rise and create a convection current.
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How candles produce heat
Candles have been a source of light and warmth for thousands of years. They are made of wax, a substance that is largely composed of hydrogen and carbon atoms. When a candle is lit, the heat of the flame melts the wax, turning it into a hot gas. This liquid fuel moves up through the wick and vaporizes, burning within the flame. The combustion process involves breaking down the hydrocarbons into molecules of hydrogen and carbon, which react with oxygen from the air to create heat, light, water vapour, and carbon dioxide.
The temperature of a candle flame varies depending on which part of the flame is being measured. The hottest part of the flame is the blue area at the base, where the flame has the most oxygen and combustion occurs, creating temperatures between 1400-1800°F. The yellow part of the flame is cooler, at around 1200°F, and the outermost part, the luminous mantle, is the coolest, with temperatures ranging from 800-1000°F. The average temperature of a candle flame is around 1000°C.
The heat produced by a candle radiates in all directions, melting more wax to sustain the combustion process until the fuel is depleted or the heat is removed. This self-sustaining process ensures the candle continues to burn, gradually becoming shorter as the wax is consumed. The heat generated by a candle can vary depending on the materials used and the environment, and it is influenced by factors such as insulation and air currents.
While candles produce heat, they may not be the most effective way to warm up a room. The heat produced can be influenced by various factors, such as the size and insulation of the room, the presence of other heat sources, and the number of candles used. Additionally, the heat generated by a single candle is relatively small compared to other heat sources, and it may be challenging to retain the heat within a larger space. However, in certain situations, such as being trapped in a car during a snowstorm, even a single candle can provide valuable warmth.
To enhance the heat produced by candles, some people use terracotta pots or insulators like teapots to concentrate the heat in a small area. However, it is important to exercise caution as the combination of open flames and stacked objects can pose a fire hazard. Additionally, the wax itself can reach its flash point, igniting and leading to accidents. Therefore, while candles can produce heat, it is crucial to prioritise safety and explore alternative heating methods for sustained warmth.
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How candles are made
Candles are made up of three essential components: the wick, wax, and fragrance. The candle-making process has evolved over the years, from hand-rolled candles to the use of moulds, and the materials used have also changed, with beeswax, tallow, and various types of wax being popular choices.
The wick is a crucial component of a candle. Commercial wicks are typically made from braided cotton, which is absorbent and allows the wick to draw up the liquid wax through capillary action. The wick's capillarity determines the rate at which the melted wax is conveyed to the flame. Wicks are often infused with chemicals to modify their burning characteristics, such as ensuring the wick does not glow after the flame is extinguished.
The type of wax used can vary, with paraffin, soy, coconut, beeswax, apricot, vegetable, and other options available. All waxes are essentially hydrocarbons, composed primarily of hydrogen and carbon atoms. The choice of wax influences the burning rate, temperature, and fragrance of the candle.
To make a candle, one common method involves dipping the wick into molten wax or pouring wax onto the wick to build up the candle. In the 14th century, Sieur de Brez introduced the technique of using a mould, which was further improved in the 19th century for more efficient production.
The process of burning a candle involves the heat of the flame melting the wax near the wick. This liquid wax is drawn up the wick through capillary action, and the heat vaporises the wax, turning it into a hot gas. The vaporised molecules combine with oxygen in the air, creating heat, light, water vapour, and carbon dioxide. The flame then melts the top of the wax, which is continually burnt, maintaining a constant flame.
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History of candles
The history of candles dates back to ancient civilizations, with the primary use of providing light. They were also used in cultural ceremonies and spiritual worship. The earliest use of candles is often attributed to the Ancient Egyptians, who made rushlights or torches by soaking the pithy core of reeds in melted animal fat. In China, textual evidence suggests that candles may have been made from whale fat in the Qin dynasty (221–206 BCE). Objects that could be candle holders have been found in Babylonian and middle Minoan cultures, as well as in the tomb of Tutankhamun.
In the Roman period, candles were primarily made from tallow and beeswax in Europe. Beeswax was a byproduct of honey collection, and it was collected after honey was extracted and purified by boiling it in seawater several times. The early candles were produced using a variety of methods, including dipping or drawing the wick in molten fat or wax repeatedly until it reached the desired size, or pouring fat or wax onto a wick to build up the candle.
In the Middle Ages, beeswax candles were introduced in Europe, offering a cleaner-burning alternative to animal-based tallow, which produced a smoky flame and an acrid odour. The use of moulds for candle-making was introduced in the 14th century.
The first major change in candlemaking since the Middle Ages came in the late 18th century with the growth of the whaling industry, which made spermaceti, a wax obtained by crystallizing sperm whale oil, available in large quantities. Spermaceti wax produced a brighter light than tallow or beeswax and was harder, so it wouldn't soften or bend in the summer heat.
In the 19th century, large-scale industrial manufacturing techniques were introduced for the mass production of candles. During this period, French chemist Michel Eugene Chevreul discovered how to extract stearic acid from animal fatty acids, leading to the development of stearin wax, which burned cleanly and was durable.
In the first half of the 20th century, candles enjoyed renewed popularity due to the increased availability of byproducts from the US oil and meatpacking industries, such as paraffin and stearic acid, which became basic ingredients in candle-making. In the 1980s, consumer interest in candles as decorative items, mood-setters, and gifts began to increase, leading to a surge in popularity in the 1990s. Today, candles continue to be valued for their ability to bring light, fragrance, and comfort into people's homes.
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