Tag: fire

Cutlass -> Fuel

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My first thought was to go from fuel through fire to blacksmiths, but I’ve gone down that road already. Let’s flip it.

A cutlass is a sword, and swords have to be sharp. (It’s also a kind of car, which has to be sharp in a different way to cut through the air.) But we’ve been sharpening things far longer than we’ve had swords. Members of our genus have sharpened tools for millions of years. We’ve gotten good at it by this point. We’ve gotten so good at it that the Ancient Greek philosopher Democritus used a knife to reason his way to a fledgeling atomic theory.

Here’s what he said: When you cut into an apple, some of the apple ends up on either side of the knife. But the only way the knife can get between the two parts of the apple is if there was already space between them. So there must be bits of matter with empty space between them. QED—or whatever the Ancient Greek version of QED was. It’s not quite the argument we’d use today, but it’s neat, even when summarized as fleetingly as I just did.

Modern atomic theory came from the dual directions of physics—where atoms explained temperatures and sounds and a bunch else—and chemistry—where atoms explained reactions. Chemical reactions seemed to consume the input elements in certain ratios, and we now know it’s because a certain number of atoms of one element would always react with the same number of atoms of the other: Two oxygens and one carbon became carbon dioxide, say.

Fire, as we know from the other day, is a complex collection of chemical reactions involving oxygen. But fires need two other ingredients. They need heat and, of course, they need fuel.

Chimney -> Spoon

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Chimneys only work because hot air rises, which it does for reasons that I’m sure I’ll explain in more detail someday. But today is not that day. For now, we can just content ourselves with that statement and move on: Hot air rises. That’s not where my interest here lies, anyway. My interest lies at the bottom of the chimney. The fire in a fireplace can get pretty hot, anywhere from 500 to 1,100 or so degrees Fahrenheit. (That’s 260-600 Celsius, if you’re into that sort of thing.)

There’s a lot of complicated chemistry in a fire, but this is a physics blog. We can imagine the whole thing as reactions that combine oxygen from the air with carbon—making carbon dioxide—and hydrogen—making water—in the wood. Those reactions give off energy (loosely speaking), and that energy both sustains the fire and heats the surroundings. It can seem unbelievable at first that fires give off water, but you can prove it to yourself the same way Michael Faraday did: Put a piece of glass above a burning candle and, among the soot, you’ll see condensation. The soot is evidence of the chemistry I’m avoiding; the condensation is evidence of water.

Anyway, fireplace fires can be hot, but they’re nowhere near hot enough to melt most metals. Silverware used to be silver, sensibly enough, and silver melts at 1,700 degrees. Today’s utensils are often made of metals like aluminum instead of silver, but the melting point (and the broader point) is effectively the same. You’re not getting there in your home fireplace, no matter what kind of fancy system you’re running. Getting an oven that hot requires reinforcements. You often need to burn the remnants of ancient wood and other plant matter—what we’d more conventionally call coal. (Or propane. Or something else that burns that hot.) Once your oven is that hot, you’re able to melt all sorts of metals, pour them into molds, and form them into whatever shape you want.

The other option, of course, is to make your spoon out of something that melts at a low temperature, like gallium. Gallium spoons are a pretty popular gag among chemists, although it’s not an element you want sitting around at the bottom of your tea.