Candles have been used for lighting and rituals for thousands of years across many cultures. But how exactly do candles work from a scientific perspective? Here we will explore the chemistry, physics, and engineering behind candles and how they produce light.
The Chemistry of Candle Wax
Candle wax is primarily composed of hydrocarbons, chemical compounds made up of hydrogen and carbon atoms. The most common types of wax used in candle making are:
- Paraffin – A byproduct of petroleum refining, paraffin wax is a mixture of straight chain alkanes.
- Beeswax – Produced by honey bees from wax glands, beeswax consists mainly of esters of fatty acids and long chain alcohols.
- Soy wax – Made from hydrogenated soybean oil, soy wax is composed of triglycerides.
- Palm wax – Derived from palm oil, palm wax contains esters as well as hydrocarbons.
These waxes burn cleanly and have different melting points, burn rates, and flexibility when hardened. This allows manufacturers to create candles with different properties by blending waxes.
The Wick
The wick is the component of the candle that enables capillary action to draw fuel (melted wax) to the flame. Candle wicks are typically made from braided cotton, but can also be made from paper, wood, or plastic fibers. An ideal wick material has a high capillarity and low ash content. As the melted wax travels up the wick, it vaporizes and combusts at the tip, producing a flame.
| Wick Material | Pros | Cons |
|---|---|---|
| Cotton | High capillarity, low cost | Prone to mushrooming (ragged tips) |
| Wood | Stiffness maintains shape | Brittle, burns unevenly |
| Paper | Softens wax clogging | Low combustion temperature |
The thickness of the wick also impacts the flame – a thinner wick will produce a smaller flame, while a thicker wick will create a larger flame. Wick size needs to be balanced to generate enough heat for liquefaction while avoiding excessive smoking.
How the Candle Burns
The candle flame is the result of a controlled combustion reaction between wax vapors and oxygen. As heat from the flame melts the top layer of wax, liquid wax moves up the wick via capillary action. The heat vaporizes the liquid wax molecules which then mix with oxygen in the air. When the vapor/oxygen mixture reaches the ignition temperature (~140°C), it combusts and releases energy in the form of light, heat, and water vapor.
The visible part of the flame consists of hot combusting gases like carbon dioxide and water vapor. The blue-yellow color comes from carbon particulates (soot) incandescing from the heat. The hottest part of the flame is the colorless zone just above the wick tip, where wax vapors continuously ignite.
The wax pool melts outward as heat travels downward from the flame. Molten wax moves toward the flame via convection currents, creating a self-sustaining “heat engine.” The wax liquefies, travels up the wick, vaporizes, ignites, heats more wax – perpetuating the candle’s burn.
Factors Impacting Burn Quality
Many variables influence how evenly and brightly a candle burns, including:
- Wax composition – The type of wax affects melting point, viscosity, and burn rate.
- Wick material – Fibrous wicks like cotton promote capillary flow of melted wax.
- Wick size – Wick diameter impacts rate of fuel consumption.
- Wax additives – Dyes, scents, and opacifiers can alter burning behavior.
- Wax quality – Contaminants and air bubbles create an uneven burn.
- Fuel-air mix – Insufficient oxygen makes the flame smoky and dull.
Candle manufacturers must balance these parameters through testing to create clean, long-lasting burns. They may tweak wick size, wax blends, fuel load, candle shape, and additives to optimize candle performance.
The Candle Flame Reaction
The overall reaction occurring in a candle flame is the oxygen-fueled combustion of paraffin wax, which contains long alkane molecules such as C25H52 and C30H62. Here is the basic reaction scheme:
C25H52 (vapor) + 38 O2 (g) → 25 CO2 (g) + 26 H2O (g) + Energy (light/heat)
The alkane vapor reacts with oxygen gas to generate carbon dioxide, water vapor, and roughly 100kJ of energy per mole of paraffin consumed. As heat from the flame vaporizes more wax, the reaction is continuously fueled until the wax is depleted.
For beeswax and soy-based waxes, the chemistry is more complex, as they contain a mixture of hydrocarbon and oxygenated compounds. But the same basic principles apply – vaporized organic molecules are oxidized to CO2 and H2O by atmospheric oxygen, releasing energy.
Kinetic Concepts
There are several key physics concepts that describe the various processes occurring in a burning candle:
- Capillary action – Intermolecular forces allow melted wax to rise through the wick against gravity.
- Vapor pressure – Wax molecules evaporate based on their volatility, wick temperature, and air circulation.
- Convection – Hot gas and air currents transport heat and mass through the system.
- Conduction – Direct contact transfers heat down the wick and outward in the wax pool.
- Radiation – Infrared energy is emitted from the flame zone, heating surrounding objects.
- Activation energy – The minimum energy needed to ignite wax vapors is provided by the flame.
Mastering these kinetic processes allowed candlemakers to develop effective fuels, wick materials and candle designs.
Engineering Design Aspects
Candles are a marvel of chemical engineering, carefully designed to control the complex interactions between solid, liquid and vapor phase wax. Engineers utilize the following tactics to optimize candle performance:
- Managing melt pool shape/size by tweaking wick diameter, wax thermal properties, and candle diameter.
- Matching wax viscosity, capillarity, and wick absorbency for ideal capillary flow.
- Blending waxes to balance melting point, hardness, and burning rate.
- Adding opacifiers like titanium dioxide to radiate more heat outward.
- Designing candle shapes/sizes to fine-tune air circulation and heat dissipation.
- Positioning multiple wicks to evenly heat large or irregular candle bodies.
The goal is to create a system where wax melts at the ideal rate as it travels up the wick, evaporates at the tip, and sustains a steady, soot-free flame until the fuel is gone.
Conclusion
While conceptually simple, candles are complex systems with competing thermodynamic and kinetic factors. By leveraging principles of chemistry, physics, and engineering, candlemakers are able to balance these scientific forces to create safe, high performance candles. The underlying molecular dance of phase change, capillary motion, evaporation, diffusion, convection, combustion, and heat transfer contains a surprising amount of nuance and still holds some remaining secrets.
Understanding the science illuminated centuries of candle progress – from rudimentary torches to the advanced candles we enjoy today. And there are still new frontiers to explore, as researchers apply nanoscience and catalysis to wick and wax development. The synergy between art and science will undoubtedly lead to even more candle innovations in the future.