Why is Fire Blue (& Is It Hotter)? Answered

Last month, I lit my fireplace and turned on one of my favorite shows, Avatar the Last Airbender. After watching a fight scene, and then looking at the fire in my fireplace, it got me wondering if blue fire is hotter and why. So, I did some research. Here’s what I found.

Most fires are red, orange, and yellow because that’s the color that carbon, such as wood, emits when it’s burned. On the other hand, gas flames, such as propane and natural gas, burn blue. Because gas fires have more oxygen and complete combustion, their flames are usually hotter than wood fires.

So, while blue fire is generally hotter than red, when is it normal to see blue flames, and how much hotter is blue fire? Let’s take a closer look.

Blue Fire vs Red Fire

Blue Fire	Red Fire
Usually from a gas fire	Usually from a wood or carbon fire
Complete burn	Incomplete burn
Hotter	Cooler

We know that fire requires oxygen, heat, and fuel in order to burn. But if you simply bring those pieces together, nothing happens. Fire requires a chemical reaction between those elements, which then produces heat and light.

And the color of the light depends on what exactly is burned.

Most of the fire we know is usually red because it’s the wavelength that carbon emits when burned.

The blue flames that we see are usually due to hydrocarbon gas burning (often propane or natural gas). When hydrocarbon molecules become excited (heated), they emit light in the blue part of the spectrum.

So, the material burned affects the wavelength of light emitted.

Is Blue Fire Hotter than Red?

Blue fires are usually hotter than red. This is because the gas from blue fires has a higher supply of oxygen and more complete combustion than fires that use solid fuels, such as wood (red) fires.

Due to this increase in combustion, gas flames are often about double the heat. For example, gas fires from propane and natural gas can reach up to 3600ºF (1,980ºC), while the average wood fire is between 1000-2000ºF (538-1093ºC).

For more context, fireplaces are commonly around 1100ºF (593ºC) and bonfires are about 2000ºF (1093ºC).

Since wood fires have incomplete combustion, they often leave behind materials such as charcoal. They also create more particulates and emissions than gas fires.

Natural gas produces 71% less CO₂ than coal and a reduction of nearly all air pollutants for an equal amount of output.

There are also times when gas fires turn red instead of blue. Commonly, this is because there’s another fuel mixed in, or there’s an insufficient amount of oxygen.

You may see this in a gas burner that has debris or is clogged. The material lodged in the burner likely has carbon that’s burning, and/or is restricting the amount of oxygen the burner receives.

Do Blue Fires Have Less Light?

Blue fires have less light because they’re a clean and efficient burn, leaving behind little to no glowing specks in the flame. On the other hand, red fires (especially from wood fires) have plenty of burning soot and carbon residue in the flame, which causes a brighter glow.

So, by burning propane or natural gas, there’s complete combustion and less carbon that glows in the flame, meaning less light.

On top of this, we also perceive blue light to have less light because of how our eyes work.

The 3 cones in our eyes (red, green, and blue) perceive yellow and lime colors to be the most intense, while purple and blue are more muted. This is not only because blue light is the shortest wavelength on the spectrum, but our blue cone is actually the least sensitive, making the color seem duller initially.

After the blue signal leaves our eyes, our brain has to amplify it for us.

Can You Change The Color of a Fire?

As we covered, fire isn’t just red or blue—it can be all colors depending on the chemical that is burned.

Keep in mind, the fire needs to be hot enough to ignite the chemicals.

While the image above is helpful, I added a few items I found in my research and put together a more comprehensive table of the different colors of fires and their sources below.

Fire Color	Source
Blue	Hydrocarbons (Propane, Natural Gas)
Red	Carbon, strontium
Orange	Carbon, calcium
Yellow	Carbon, sodium
Green	Copper, boron
Purple	Potassium, lithium
Invisible	Methanol

With a wood fire, you’ll likely only see red, orange, and yellow due to the carbon being burned. Wood also has some sodium, which might explain why some fires have more of a yellow tint.

Occasionally, you might see blue flames in a wood fire, which is usually caused by the presence of copper or other metals in the fuel.

For gas fires, the hydrocarbons from propane and natural gas are mostly or entirely consumed and show a blue flame, unless it’s dirty or clogged.

Final Thoughts

Many people think that blue fires are always hotter than red fires, but the color of a fire comes down to the chemical that is being burned. It just happens that blue fires are typically from gas fires, which have a much more efficient and complete burn (leading to higher heat).

So, it’s a case of “correlation does not equal causation”.

If you’d like to see the different colors of fire in action, check out this YouTube video by d’Art of Science.

Sources

• https://sciencing.com/colors-fire-hot-8631323.html
• https://www.eia.gov/energyexplained/natural-gas/natural-gas-and-the-environment.php
• https://askabiologist.asu.edu/rods-and-cones
• https://sciencenotes.org/colored-fire-spray-bottles/