What Is a Cold Fire? Also, Invisible Fires

If you’ve never heard of a cold fire before, the title of this post probably seems like nonsense to you. At the very least it seems oxymoronic in a way that there’s no way the fire is anywhere near the temperature of like… an ice cube. But hopefully that’s not where your head was going and you’re asking “what is a cold fire?” with us. If not, maybe the fact that invisible fires are a thing will do something for you?

We discovered them by accident

The idea of a cool flame was discovered in 1810 by a guy named Humphry Davy. Davy didn’t set out to create a cool flame, though. He was sticking a heated platinum wire into air and ether trying to make a slow combustion of ether. What he ended up observing was a weirdly pale light. As he recreated this weird new fire he found that some of the flames didn’t burn him, and didn’t need something to ignite them. 

It wouldn’t be until 1929 when the term “cool flame” got adopted though. The term was coined when the emission spectra of cool flames were first analyzed.

By definition, cool flames are still pretty hot, their temperatures peak at 400 degrees Celsius, which is four times greater than the temperature water boils at. In Fahrenheit, that’s 752 degrees. For comparison, a candle flame’s average temperature is 1,000 degrees Celsius (1832 degrees Fahrenheit). While the lower barrier of what constitutes a “cool flame” doesn’t have a concrete definition, the benchmark is “can be seen with the naked eye when it’s dark.” In daylight, a cool flame is functionally invisible. Here’s an example of a methanol fire which, in the daylight, is invisible. 

If you saw the fire, you’ll also see that when observed the methanol fire was blue, which is pretty common for most cold fires when it’s dark enough to observe them. 

Why are they cold?

You’re probably unsurprised to find out that what’s going on under the hood of a cold fire isn’t the same as the summer campfire you and your friends had. Your average fire sees small fragments of molecules combining with oxygen. In a cold fire, the fragments are both larger and can combine with each other instead of exclusively oxygen. This is why cool flames last so much longer. You can sort of see it as a very slow burning fire. 

We’ve even done cool flame experiments in space, since they’re actually a lot easier to create outside of Earth’s gravity. In 2012 it was discovered that cool flames could be made non-premixed—which means the fuel and oxidizer of the flame didn’t have to start together. This upended our previous understanding of cool flames, which were originally thought to occur because of something that happened pre-ignition and because of pre-mixed gasses. They ended up making spherical, blue, and cool flames. 

Cold Fire Was Also a California Wildfire

In 2016 the northern region of California saw a nine day wildfire that has since been named Cold. Also it took place in Yolo County. The Cold Fire was one of roughly 30 1,000+ acre wildfires in California, in 2016 alone. Also, there’s a Wikipedia page for just 2016 California wildfires. 

Fun. 


See if you know your fire-related vocabulary here.

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