A candle flame is a visible manifestation of a continuous, controlled chemical reaction, specifically the combustion of vaporized wax. The phenomenon known as “flicker” describes the rapid, observable changes in the flame’s height, color, and intensity. This instability occurs when the delicate equilibrium required for steady combustion is suddenly disturbed. The dance of the flame is a visual indicator that the balance between fuel delivery and oxygen supply has been momentarily broken.
The Science of a Stable Burn
A stable candle flame relies on a precise cycle of heat, fuel, and oxygen, often referred to as the combustion triangle. Initial heat melts the solid wax, which is drawn upward through the wick via capillary action. This liquid fuel vaporizes at the wick’s hottest point, turning into a gaseous fuel that sustains the flame. This continuous process creates a characteristic, elongated tear-drop shape maintained by a natural convection current. Hot combustion gases rise rapidly, drawing cooler, oxygen-rich air in from the sides and base. The stable flame structure features distinct thermal zones based on oxygen availability and temperature.
The innermost zone, surrounding the wick, is a dark, non-combustion region consisting of unburnt wax vapor because it has the least access to oxygen. Above this is the luminous yellow zone, where partial combustion occurs, and the heat causes soot particles to glow, producing the majority of the candle’s light. The outermost region is a barely visible, blue zone where complete combustion takes place, making it the hottest part of the flame due to the ample supply of surrounding atmospheric oxygen.
How Air Movement Causes Instability
The primary cause of flickering is the disruption of the flame’s self-regulating convection current. In still air, the rising column of hot gases forms a protective, stable envelope around the flame, ensuring a consistent, laminar flow of oxygen to the combustion zones. External air movement, such as a draft from an open door, an air conditioning vent, or a person walking by, introduces turbulence. This external airflow breaks the stable convection envelope, causing an erratic shift in local oxygen concentration. The flame tip will bend, jump, or momentarily be starved of the necessary oxygen.
When the oxygen-fuel balance is abruptly altered, combustion becomes incomplete, and the flame temperature drops quickly. This results in unburned carbon particles, or soot, escaping the flame before they can fully combust, often visible as a wisp of smoke accompanying the flicker. The visual “flicker” is the rapid fluctuation in light intensity as the flame struggles to re-establish its stable convection current against the external draft.
Wick and Fuel Influences
Beyond external air currents, the candle’s physical components can internally contribute to instability. The wick’s condition is a frequent cause of flickering. A wick that is too long or has developed a carbon buildup (“mushrooming”) delivers an excessive amount of fuel. This over-fueling creates a larger, less focused flame that is less stable and more prone to flickering and smoking. Trimming the wick to approximately one-quarter inch before each burn helps regulate the fuel supply and maintains a smaller, steadier flame.
Impurities within the wax, such as dust, debris, or high concentrations of fragrance oils, can also interfere with combustion. These contaminants can clog the wick’s microscopic channels or burn inconsistently, leading to momentary sputtering or an unsteady supply of vaporized fuel.
The container itself can indirectly affect stability, particularly in jar candles. As the flame heats the glass walls, a boundary layer of warm air is created. This can generate minor, internal turbulence that subtly restricts oxygen flow or creates small, uneven air currents contributing to flicker.