The tetrahedron of fire

On Saturday, a 500 acre fire in Flagstaff, AZ was started by some guy, apparently lacking the foresight God gave pistachio nuts, who dumped his burning campfire coals on the forest floor instead of properly putting them out. The next day, some (presumably unrelated) equally responsible person left a campfire burning and just walked away. The first fire has stopped growing, but the second reached 10,000 acres in around the first 24 hours and is only 10-percent contained. As I blogged about a couple weeks ago, until June I’d been living in Flagstaff, AZ. If you follow the news you’ve probably seen it’s now surrounded by fires and burning in the wake of my absence (you can see a live view of the smoke plume from the fire in my alma-matter’s webcam).

Perhaps a public service announcement/review of fire physics will help future fire starters realize why hot things burn and to put out their fires like every girl/boy scout knows how.

Classically, the nature of fire is explained in a triangle that looks something like this. Heat is the bottom leg of the triangle, and fuel and oxygen compose the other two legs. If you deprive a fire of any of these things it will go out. Now that we’ve discovered that chemical retardants can put fires out, we know it’s not quite so simple.

When temperatures get sufficiently high for a material, something called pyrolysis takes place. This is where matter chemically decomposes through the action of heat. First, the combusting material starts emitting gasses such as water vapor. As the temperature increases, combustible gases start to ignite. Finally, when the temperature reaches a critical point we have ignition and the material burns.

So, to account for these chemical aspects, some geometry obsessed scientist added a fourth leg and called it the tetrahedron of fire. Combustion, aka fire, is actually a chemical chain reaction dependent on external factors. By using a dry chemical retardant like potassium or sodium bicarbonate (similar to a fire extinguisher), you can cut off the conditions necessary for combustion to occur. You can still have all the fire triangle elements present, but the fire is inhibited by stopping combustion.

What can you do to stop a fire? The fire must have one of its legs cut from it. You must remove the combustible material, lower the oxidation agent or cool the material beneath its ignition temperature.

How can you prevent fire? Ask Smokey, but basically he’ll tell you the obvious. Don’t leave combusting material lying next to other combustible material. Lightning and other natural causes of fire aren’t as frequent or problematic as the cause of these-human apathy.

Check out this guy’s blog for some incredible pictures of this fire.

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