An ember is a piece of glowing, hot material remaining after a fire, representing a stage of combustion that has moved past visible flames. This appearance often hides intense heat held within the remaining solid fuel. The ember is actively undergoing flameless combustion, or smoldering. Understanding the true heat of an ember requires looking beyond the visual glow to the underlying chemical and physical processes.
Measured Temperature Range
The temperature of a glowing ember is much higher than people might expect, often rivaling the heat of the initial flame. Typical surface temperatures for actively glowing embers range between \(750^\circ\text{C}\) and \(950^\circ\text{C}\) (\(1,382^\circ\text{F}\) to \(1,742^\circ\text{F}\)). This heat is concentrated at the surface where the reaction with oxygen occurs. Under optimal conditions, especially with increased air movement, maximum surface temperatures can reach up to \(1,100^\circ\text{C}\) (\(2,012^\circ\text{F}\)).
The lower end of the range is seen in slow-burning, dark red embers nearing the end of their activity, where temperatures might be closer to \(500^\circ\text{C}\) to \(600^\circ\text{C}\) (\(932^\circ\text{F}\) to \(1,112^\circ\text{F}\)). These measurements generally represent the surface temperature, which is the location of the greatest heat output and light emission. While the internal temperature of a larger ember may be slightly lower than the glowing exterior, the entire mass remains thermally energized. This high-temperature range allows embers to ignite new fuel sources long after the main fire has disappeared.
Variables that Adjust the Heat
The temperature an ember reaches is not constant and depends on several variables that govern the smoldering process. Airflow, or the availability of oxygen, is the most significant factor influencing an ember’s heat output. Scientific measurements show that increasing air velocity from \(1~\text{m/s}\) to \(4~\text{m/s}\) can elevate the mean surface temperature by as much as \(200^\circ\text{C}\). A greater supply of oxygen feeds the chemical reaction on the ember’s surface, accelerating the rate of combustion and releasing more thermal energy.
The nature of the fuel also plays a substantial role in determining how hot and how long an ember will burn. Hardwoods, such as oak or maple, are denser and contain less volatile material than softwoods like pine. This higher density means they produce a more substantial, longer-lasting char that retains heat more effectively and sustains the oxidation reaction for an extended period. Similarly, materials that are pure carbon, like charcoal, produce a consistent and hot ember because the pre-pyrolysis process has already removed most of the non-carbon volatile compounds.
The physical size and mass of the ember influence its thermal capacity. Larger embers possess greater thermal mass, which allows them to resist cooling and maintain a higher internal and surface temperature for a longer duration. This increased mass provides a larger reservoir of hot material to sustain the necessary heat for the surface reaction to continue. The amount of insulating ash that forms on the surface can also affect the heat, as a thick ash layer can slow the rate of oxygen delivery and reduce the heat loss through radiation, resulting in a slightly cooler, slower burn.
The Physics Behind the Glow
The heat and light produced by an ember are the result of a scientific process called smoldering combustion. This is a slow, flameless form of oxidation that occurs directly on the solid fuel’s surface, specifically the carbonaceous material known as char. Unlike flaming combustion, which involves the rapid burning of released gases, smoldering is a heterogeneous reaction where oxygen reacts directly with the solid carbon.
The visible light emitted by the ember is a consequence of its heat, a phenomenon called incandescence. The solid material becomes so hot that it begins to emit electromagnetic radiation in the visible light spectrum. This process is essentially black body radiation, where the color of the glow serves as a visual thermometer for the material’s temperature. Dark red indicates the lowest visible temperature, transitioning to a brighter red-orange or even yellow-white as the temperature increases significantly.
The heat generated by the smoldering reaction is primarily transferred to the surroundings through thermal radiation. This is why a person can feel the warmth from an ember bed even when standing a short distance away. This constant radiative heat loss is balanced by the internal heat generation from the continuous oxidation reaction, allowing the ember to persist in a state of sustained, flameless heat until the fuel is consumed.