1. Removing Some Air Made a Candle Flame Round and Blue
Students at Nara Women's University Junior High and Senior High School discovered that when they removed a small amount of air from a container holding a candle, the flame became round and changed to blue within a dozen seconds [8]. They confirmed that convection had stopped by observing the movement of incense smoke, which explained why the flame became round [8]. While they suspected the blue color was due to a drop in temperature, their study reported the reason for the color change as "unknown" [8]. Since the flame stretches upward and is yellow on Earth but becomes round and blue when air is removed, this experiment shows that the upward shape is created by air flow (convection). The yellow color depends on the soot inside the flame. To explain this difference in detail, we must first understand what is glowing inside the flame.
Safety Note: Do not try this at home. Experiments that reduce pressure in a container while it is burning carry risks of fire, burns, and container damage. They require special equipment and strict safety management.
2. What a Flame Is and What Glows in It
According to encyclopedias, a flame is the part where gas is burning and emitting light and heat. Even solids and liquids like wood or oil can release gases that burn when they are decomposed by heat or evaporated, and this gas burns to create a flame [1]. The word “flame” is said to come from a word meaning “blazing fire” [1]. You may have seen explanations calling flames "plasma." Plasma is a gas that carries an electrical charge because electrons have been stripped away. According to a science Q&A from a US university, ordinary flames are not hot enough to carry enough charge to act as plasma [4]. The maximum temperature of a candle flame is about 1500°C, while an acetylene fuel flame reaches about 3100°C, which may be considered plasma, according to the source [4]. The red and yellow light of a flame is not because it is plasma. So, what exactly is glowing?
3. Hot Soot Particles Make a Candle Flame Bright Yellow
The reason a candle flame is bright and yellow is that particles of carbon that did not burn completely—soot—get hot and glow [2]. When objects get hot, they emit light, a process called thermal radiation. A column from the Osaka City Science Museum explains that as objects are heated, they begin to glow red, and as the temperature rises, the color changes to orange, yellow, and then whitish [3]. This color is determined almost entirely by temperature, rather than the material itself [3]. For example, light bulbs with bamboo filaments look orange because their temperature is relatively low [3]. In 1860, the British scientist Michael Faraday demonstrated through experiments in a Christmas lecture for children that the brightness of a flame is due to solid particles [2]. These lectures were later published as the book *The Chemical History of a Candle* [2]. The true source of a flame's brightness is unburned soot. So, what is the blue part of the flame?
4. Why Parts of a Flame Glow Blue Instead of Yellow
A candle flame is divided from the outside in: the outer flame, the inner flame, and the flame core. The outer flame has plenty of oxygen, is the hottest part, and glows blue. This is explained as molecules in the process of burning receiving energy from heat and releasing that energy as light [1]. The inner flame lacks oxygen. Therefore, soot is produced there, and it glows orange [1]. This means there are two types of light in a flame: the yellow-to-orange light from hot soot, and the blue light emitted by burning molecules. The next question is about shape. Why does a flame stretch upward like a teardrop?
5. Hot Air Rises and Stretches the Flame Upward
Hot gas becomes lighter and rises. This air flow caused by buoyancy is called convection. According to NASA, the teardrop shape of a candle flame on Earth is due to this convection [5]. Convection carries soot toward the flame's tip, which makes the flame appear yellow [5]. Below the flame, surrounding air is drawn in, and hot gas is pushed upward. This causes the flame to stretch long and upward. This explanation matches the experiment by the junior high and high school students mentioned earlier. When they removed air from the container, convection stopped, and the flame became round [8]. So, what happens to a flame in places where there is no convection?
6. What Candle Flames Looked Like in Space Experiments
Reports from experiments conducted on the Space Shuttle (USML-1) summarize about 70 candle tests. In microgravity, the flame shows soot only for a brief initial period, then becomes a hemispherical blue flame [6]. The soot becomes invisible, and the distance between the flame and the wick widens significantly. It is concluded that if there is enough air and space, the flame can continue to burn [6]. NASA says that in its microgravity candle comparison, convection is absent, so the flame is spherical, lacks soot, and turns blue [5]. In 2023, a lesson where astronauts lit a candle on the Chinese space station was broadcast live. It was reported that the flame was not vertical but nearly spherical [7]. Experiments are also being conducted on space stations using round flames to investigate how soot is formed. According to Washington University in St. Louis, the goal is to find ways to create blue flames that do not produce soot [9]. Soot-free flames may be useful for future combustion research. However, the sources do not yet conclude whether the blue flame in space and the blue flame in the classroom container are blue for the same reason [8].
7. Comparing the Color and Shape of Flames at Home
Observe a candle flame from the side with an adult. Where are the blue parts, and where are the yellow and orange parts? Next, ask an adult to light the candle and hold the back of a metal spoon near the yellow part of the flame for just 2 to 3 seconds. See if a black substance forms. If it does, it is likely unburned carbon, or soot. The spoon gets hot quickly, so have an adult hold the long handle, and place a cup of water nearby. Be careful not to let hair or clothes get close to the flame. If you can see a gas stove flame, you can compare it with the candle to look for differences in color and shape. The sources for this article do not explain these differences, so you could investigate them further. If you are interested in the original texts, Michael Faraday's *The Chemical History of a Candle* is available in full English text on Project Gutenberg [2]. Japanese translations are also published. NASA's comparison images of flames [5] are in English but show the difference between flames on Earth and in microgravity.
Safety Note: When observing, look at the candle flame from the side with an adult to confirm the position of the blue and yellow/orange parts. Because the spoon gets hot, have an adult hold the long handle, keep water nearby, and keep hair and clothes away from the flame.