Candle flames have four distinct zones, each with unique properties that allow the flame to burn. Understanding the different zones and how they interact provides insight into the chemistry behind this familiar light source.
The 4 Zones
The four zones of a candle flame are:
- The wax zone
- The wick zone
- The inner flame
- The outer flame
Each zone plays an important role in sustaining the flame. Here is a closer look at what happens in each region.
The Wax Zone
The wax zone is the bottom region of the flame, closest to the candle’s surface. This is where the wax from the candle melts and vaporizes due to the heat of the flame above it. The wax acts as the candle’s fuel source – without melted wax, the flame would quickly die out.
In this region, the solid wax absorbs heat from the candle wick and surrounding air. As it is heated to its melting point of approximately 150-200°F, the solid wax transitions from a solid to a liquid state in a process called melting.
The melted wax then continues heating until it reaches its boiling point, vaporizing into a hot wax vapor. This vaporization occurs at around 400°F for typical candle waxes like paraffin or beeswax.
The presence of the liquid wax pool and wax vapors supports the combustion reactions occurring above in the wick and flame zones. The wax acts as the fuel that keeps the candle burning.
The Wick Zone
Above the wax zone sits the wick zone, centered on the candle’s wick. The wick is generally made from braided cotton threads that have been treated with products to help regulate burning.
Capillary action draws the melted wax up the wick, where it then vaporizes into gaseous molecules due to the heat. The vaporized wax molecules mix with oxygen in the air surrounding the wick. Meanwhile, the heat causes the wick fibers to begin pyrolyzing, breaking down the cotton strands into char, water vapor, and gaseous molecules like carbon monoxide and hydrogen.
This mixture of wax vapor, air, and wick gases gets heated to around 1100°F. At this temperature, the vaporized fuel molecules from the wax and wick mix with oxygen and undergo combustion.
The Inner Flame
The combustion reaction produces the visible inner flame zone, centered on the wick. This zone is bright yellow and bluish in color due to the reactions occurring.
The combustion of the gaseous wax and wick molecules with oxygen gives off energy in the form of heat and light. This exothermic reaction reaches temperatures of around 1400-1500°F.
The yellow color comes from glowing soot particles created from incomplete combustion. The blue color originates from intermediate radicals like CH, C2, and OH that form during the reactions.
In this oxygen-rich region, the hydrocarbon molecules from the wax and wick fully combust. The main byproducts are carbon dioxide and water vapor.
The Outer Flame
The faint outer zone surrounds the bright inner flame. This region is blue in color due to additional combustion reactions.
As it mixes with surrounding air, some of the heat from the inner flame helps ignite fuels that were not fully combusted. These remnants undergo additional oxidation here, creating more water vapor and carbon dioxide.
The cooler temperature of the outer zone, around 900°C, produces trace amounts of partially oxidized species like formaldehyde and acetaldehyde that create the blue glow.
The lower oxygen levels here also generate small soot particles that radiate in the yellow spectrum when heated. But the predominant blue color comes from the continuing combustion of incompletely burned carbon compounds.
How the Zones Interact
For a candle flame to burn, these different regions must work together in a self-sustaining cycle:
- Heat from the flame melts the top of the wax near the wick.
- The liquified wax gets drawn up the wick by capillary action.
- The wax vaporizes into gaseous fuel at the top of the wick.
- The vaporized wax mixes with air and is ignited in the inner flame zone, generating more heat.
- The heat vaporizes more wax to perpetuate the cycle.
- Simultaneously, the outer flame combusts any leftover remnants from the inner zone.
This sequence creates a continuous combustion reaction that provides enough heat to keep the wax melting and vaporizing. As long as there is adequate wax and wick to fuel it, the flame sustains itself through this closed loop.
However, several things can disrupt the zone interactions and cause the candle to go out:
- Insufficient oxygen to support combustion
- Buildup of excess wax on the wick, choking the fire
- Movement of air currents that detaches the flame from the wick
- Using up the available wax fuel
- The wick burning through completely
Avoiding these issues allows the four zones to continuously generate the heat, fuel sources, and combustion reactions necessary to maintain the flame.
The Chemistry of Candle Flames
The chemistry occurring in a candle flame involves complex reaction mechanisms. Here are some of the key processes:
Vaporization of Wax
The hydrocarbon molecules in the melted wax vaporize into gaseous alkanes and alkenes. For example, paraffin wax contains mostly straight and branched chain alkanes like octadecane (C18H38). Beeswax main components include palmitate esters (C15 to C36 acids with even number hydroxyl alcohols) and some monoesters.
Pyrolysis of the Wick
The cellulose and other materials in the cotton wick thermally decompose to produce combustible gases like hydrogen, methane, carbon monoxide, and aromatic hydrocarbons like benzene and toluene. Trace nitrates in the wick can form NOx gases as well.
Combustion Reactions
In the inner flame, the hydrocarbon fuels undergo complete combustion with oxygen to form carbon dioxide and water vapor. Some intermediate products like carbon monoxide and formaldehyde can escape, burning further in the outer zone. Soot particles from incomplete combustion radiate in the yellow.
Soot Formation
In regions of limited oxygen, small particulates of impure carbon called soot form. This gives the inner flame its yellow glow. Soot is hazardous if inhaled, one reason not to blow out candles.
Flame Structure and Shape
The structure of the flame zones produces the characteristic teardrop shape with a rounded tip:
- The narrow bottom is centered on the wick where vaporized wax and air mix.
- The cone-shaped center results from hot gases rising and reactants being drawn into the flame.
- The luminous ball shape forms as combustion reactions take place in the inner zone.
- The flame tapers off as remaining reactants consume.
This shape maintains the iterative zone process. It brings fresh oxygen and fuel up through the center while permitting hot products to radiate outward. Factors like airflow in the room can distort the shape slightly by bending the flame away from the wick.
Flame Height and Brightness
The height and brightness of the candle flame depend on several factors:
| Factor | Effect on Flame |
|---|---|
| Wax Composition | Softer, more volatile waxes produce taller, brighter flames. Harder waxes like beeswax make shorter, dimmer flames. |
| Wick Diameter | Wider wicks allow more wax to burn, increasing flame size and luminosity. |
| Wick Braiding | More tightly braided wicks sustain a brighter flame by reducing mushroooming at the end. |
| Air Currents | Breezes or drafts make the flame flicker and dance, affecting its shape and brightness. |
A properly burning candle flame will generally be tear-drop shaped, about 1-3 inches high, and bright yellow in the inner zone. The flame should not give off smoke or excess soot if the wick and wax are appropriately matched.
Safety Considerations
While beautiful, candle flames can be dangerous if not handled properly. Here are some important safety tips:
- Never leave burning candles unattended or near flammable objects.
- Trim wicks to 1⁄4 inch before lighting to prevent excess smoking.
- Use candle holders that are non-flammable and won’t tip over easily.
- Keep candles away from drafts to prevent rapid uneven burning.
- Extinguish candles when 1-2 inches of wax remains to prevent glass breakage.
- Consider using battery-operated flameless candles for unattended or long-term use.
With some basic precautions, candle flames can be enjoyed safely. But always exercise caution around open flames.
Conclusion
Candle flames provide ample fascination for their complex, yet familiar chemistry. The four zones work collectively to sustain the heat and combustion reactions.
The wax zone liquefies and vaporizes the fuel. The wick zone acts as the ignition point and fuel source. The inner flame provides the main visible combustion. And the outer flame burns any remaining remnants.
Understanding how these regions allow candles to burn gives insight into the physics and chemistry underlying this basic light source. Next time you light a candle, take a moment to observe the distinct flame zones that make the captivating glow possible.
