Fire is a rapid oxidation process, a chemical reaction that releases both heat and light. The visible light emitted from a flame is generally related to the temperature of the combustion. Understanding this relationship requires looking at the underlying physics of how heated matter emits energy. This knowledge allows us to determine which colors represent the lowest thermal temperatures in a typical fire.
How Temperature Determines Flame Color
The color seen in most common fires, such as a wood fire or a candle, is caused by thermal radiation, also known as incandescence or blackbody radiation. This light originates from tiny, unburned particles of soot—essentially carbon—that are suspended and glowing within the flame. As these solid particles are heated, they begin to emit light across the electromagnetic spectrum.
The temperature of the particles determines the peak wavelength of the emitted light. According to the laws of thermal radiation, as the temperature of an object rises, the color it appears to glow shifts toward the blue end of the spectrum, which corresponds to shorter wavelengths. A lower temperature means the peak light emission is at longer wavelengths, which our eyes perceive as red.
This principle is why a piece of metal first glows a dull red when heated, then transitions to orange, yellow, and eventually white as the temperature continues to climb. In a flame, the transition from red to white is directly tied to the increasing thermal energy of the soot particles. Blue light, which is associated with the shortest wavelengths in the visible spectrum and the highest energy photons, indicates the greatest heat and maximum combustion efficiency.
The Lowest Temperature Colors
The coldest colors in a conventional thermal fire are deep red and orange, reflecting the lower energy output of the glowing soot particles. These colors are typically found at the outermost edges of a flame or near the base, where oxygen mixing is incomplete and heat is lost to the surrounding air. The very first visible glow, often a deep or dark red, begins to appear when the burning material reaches approximately 500 to 600 degrees Celsius.
A dull red color can indicate a temperature around 700 to 800 degrees Celsius, which is common in smoldering wood embers or the cooler parts of a large bonfire. Moving slightly higher on the temperature scale, the color shifts to orange, which represents a thermal temperature range of roughly 1000 to 1200 degrees Celsius. This red-to-orange progression is a reliable indicator of increasing temperature in fires where the color is generated primarily by incandescent soot.
In stark contrast, the hottest parts of a thermal flame are yellow-white, indicating temperatures above 1200 degrees Celsius. The hottest flames of all are blue, which is often seen near the base of a gas burner or in the inner cone of a candle flame. This blue color signals temperatures that can exceed 1500 degrees Celsius, representing the most complete and energetic combustion of the fuel.
When Flame Colors Are Misleading
The relationship between flame color and temperature holds true only for thermal emission from incandescent soot particles. However, not all flame color is produced by this process; some colors are generated by a different mechanism called atomic emission spectroscopy. This occurs when certain chemical elements are heated and their electrons become excited, jumping to a higher energy level.
When these excited electrons fall back to their original, lower energy state, they release the absorbed energy as a photon of light at a very specific wavelength. The resulting color is unique to the element and is not dependent on the fire’s bulk temperature. For example, sodium atoms will always emit a strong yellow light, and copper compounds produce a vibrant blue or green color.
This means a green flame from a copper-containing compound, such as in fireworks, may not be nearly as hot as a pure yellow-white fire from a wood log. These chemically-induced colors can therefore be misleading if one attempts to judge the fire’s temperature based on color alone.