Gray Matter: Fire In The Hole

Slow Burn Wood treated with stump remover burns with a purple-blue flame at and below its surface. Mike Walker

When you need to remove a tree stump, you have several options. Sissies call a tree service. Tough guys loop a chain around the stump, hook it to the bumper of their truck, and find out which one is stronger. Others use gunpowder to blow them up, though this is not advisable in most jurisdictions (unless your cousin is the sheriff and you let him watch). But my favorite method is to convert the stump itself into gunpowder and then burn it up. That is the secret behind how chemical stump remover works.

You might think you could just light stumps on fire and let them burn until they disappear. But since they’re underground, there’s no source of oxygen to sustain the flame. Even with kerosene soaked into the wood, the part of the stump under the surface won’t burn. Gunpowder, on the other hand, burns even inside a sealed space because it contains its own source of oxygen in the form of potassium nitrate, or KNO₃, better known as saltpeter. Get saltpeter into the stump, and it supplies oxygen to combust the wood.

Better Landscaping Through Chemistry: Saltpeter changes the composition of a tree stump, allowing it to burn underground after it's been soaked with kerosene  Mike Walker

The stump’s altered chemical composition—potassium nitrate combined with organic carbon to produce heat and gas—is similar to gunpowder. That explains the unusual flame. The burn is slower, though, taking minutes instead of milliseconds to complete.

It might be surprising to discover that you can buy the key ingredient in gunpowder at any garden center. But here’s the kicker: The other two ingredients are readily available as well. If you want to find out what those are, and read about my adventures making gunpowder, you’ll just have to check out next month’s issue.