When you think of a typical controlled fire, such as a campfire or bonfire, many of the adjectives that come to mind probably concern heat and temperature: Hot. Roaring. Roasting. On the other hand, you may have a number of visual impressions, too: Sparkling. Shimmering. Dancing.
Just as colors appear in a variety of hues, intensities and on physical media such as painting and clothing, they can also present the same apparent range of visual "flavors" when the medium is what you know as fire. This makes sense, since fire is just . . . really hot light. Or is it?
As it happens, the colors you see in fire do correlate with the temperature in fire, so that you can expect to see certain colors more often in hotter flames and others when things are just getting cooking or dying out. But the situation is more complicated than that because exactly what is burning in a given fire also influences the display of colors in the flaming mix.
How Are the Visible Colors Produced?
What you see as light is actually electromagnetic radiation (EM), visible light being one of a number of types of EM and occupying only a small fraction of the entire EM spectrum. EM waves are characterized by a wavelength, the distance between corresponding points along a graphed EM wave, and a frequency, the number of wavelengths per second passing a fixed point.
The product of wavelength (λ) and frequency (ν) of an EM wave is always the speed of light c (3
× 108 m/s) no matter the EM wave type.
The range of wavelengths below about 440 nanometers ( 4.4 × 107 m) includes radio waves at the low end, then microwaves. Above about 7 × 107 m, X-rays and gamma rays appear; these have high frequencies and are associated with higher energy as a result. This has implications for the colors seen glowing in flames.
The visible light spectrum itself (4.4 × 107 to 7 × 107 m) includes radiation perceived by the human eye as, in order, red, orange, yellow, green, blue, indigo and violet (the famous "Roy G. Biv" of elementary-school science classes). As you'll see, this order carries over into fire, albeit with incomplete fidelity.
What Is Heat in Physics?
The reason most fires you're likely to see on Earth burn is that some kind of material is undergoing combustion, and this requires the presence of oxygen gas (O2).
Various factors can influence how hot the flame burns, including the nature of the material (obviously, gasoline burns very well; water, not so much) and whether it is being "fueled" with more material and oxygen as the fire grows.
Heat has units of energy and can be conceived as a quantity that moves from regions of higher density to regions of lower density, as with the simple diffusion of molecules. Light and heat are both (generally desirable!) products of fires, and as noted above, light waves are associated with energy in proportion to their frequency. These faster oscillations result in a greater liberation of heat, and this in turn is associated with higher temperatures within and near the flame.
Types of Flame
Many materials produce characteristic colors when burned. For example, the element sodium, which combines with chlorine to form ordinary salt (NaCl), produces a bright orange color when burned. Sodium is found in most kinds of wood, so it would be unusual to assemble a fire from the usual branches and sticks and have it not display at least some orange or dark yellow color.
The blue often seen in wood flames comes from the elements carbon and hydrogen, which emit light in the upper end of the visible light spectrum, and thus create blue and violet hues. The metal copper is known to turn green if exposed to the air for long enough; copper compounds create green or blue colors when burned. The metal lithium, to effectively round out the whole rainbow spectrum within this one section, burns red.
At the center of a very hot fire, you may see a dull orange glow or even curious dark space. This is known as blackbody radiation, and is characteristic of very high temperatures (for example, it's a feature of stars). Metals that can heat up even more progress through other colors of this type of radiation (that is, toward the violet end of the visible spectrum).
What Is The Temperature of Fire?
Now, you're cooking! So, before getting a look at just what colors to expect of fires burning at a given temperature, it's helpful to know the range of temperatures produced in the sorts of fires you're apt to encounter and scan for colors. After all, this isn't information most people keep inside their heads or someplace handy on their smartphones.
The flame of a typical candle has an outer core that burns at close to 1,400 °C (about 2,500 °F) while the core of the flame burns at 800 °C (1,450 °F). These are extraordinary temperatures for such a small flame! The walls of a household oven, meanwhile, can reach temperatures of around 500 °C (900 °F); that means the baking or broiling temperature only reaches about half of that in the metal in the walls.
If you have a fireplace in your home that you like to warm your hands over at a discreet distance, the flames providing the heat are roaring away at about 600 °C (1,100 °F). A bonfire stoked with charcoal and wood can get up to 1,100 °C (2,000 °F), as can a laboratory Bunsen burner. Of course, the sun's inner temperature of 2,000,000 °C (3,600,000 °F) makes all of these values seem rather trivial.
Are Temperature and Flame Color Directly Related?
As you have learned, both the type of material being burned in a fire and the temperature of a fire influence the colors you see produced. Also, as the example of the two vastly different candle temperatures illustrates, any one fire is almost certain to have a range of temperatures within it (explaining a large amount of the color variation sometimes observed).
When something is heated, it first turns to gas (something you typically cannot observe). These gas molecules then react with the oxygen if they are in fact combustible molecules. It would be typical to see a fire consisting of a uniform material and heated in a controlled way show reddish, then orange and finally bright yellow flames, demonstrating increasing energy and heat released.
If you light and closely study a candle, you will probably note that a sizable portion of the outer core is blue, something not usually seen much in, say, fireplaces. Considering the differences in temperatures given for these fires, this isn't surprising at all.
Flame Color Temperature Chart
While sources vary somewhat, it is possible to construct a reliable enough chart showing the relationship between flame temperature and flame color across the visible light spectrum.
Dark red (first visible glow): 500 to 600 °C (900 to 1,100
°F) * Dull red: 600 to 800 °C (1,100 to 1,650
°F) * Bright cherry red: 800 to 1,000 °C (1,650 to 1,800
°F) * Orange: 1,000 to 1,200 °C (1,800 to 2,100
°F) * Bright yellow: 1,200 to 1,400 °C (2,100 to 2,500
°F) * White: 1,400 to 1,600 °C (2,500 to 2,900
°F)
Temperatures high enough to produce blue flames are unusual in campfires, which is why they are more often seen when metals are used, as in welding,
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