Harnessing Heat

Some time around a million years ago, give or take, our early human ancestors (or cousins, maybe—we’re not sure) started playing with fire. Homo erectus undoubtedly came across natural fires at many points, ultimately gradually figuring out how to control and maintain a fire on their own… and then start them.

These fires were mainly basic wood fires, so they could get as hot as 2,000°F (1100 °C). This was more than enough to keep us warm, to provide a heat source for cooking, and to provide a little light for crafts when it was dark outside.

For a million years, 2,000°F was as hot as it got for us, and it was fleeting at best—open fires fluctuate a lot in temperature, and they’re not very reliable for steady, consistent heat. Our ancestors figured out how to smooth this temperature out with coals, but they only provided an approximately steady temperature.

Gradually, then suddenly, the invention of kilns allowed homo sapiens to take a huge leap forward. Ten thousand years ago, after Göbekli Tepe but long before writing, we discovered that you could dig a hole in the ground, start a fire in there, and then cover it up carefully.

This was an effective way to moderate the temperatures, but only compared to those open wood fires. The process required a lot of human expertise, and the temperature control was limited to stuff like allowing more air in by flapping the cover around, or inhibiting the air flow at times. Still, these kilns allowed us to start making more advanced pottery.

Eventually, these fire pits evolved into two distinct lines: kilns and ovens. Both moderated the heat much better than open fire, but kilns were much hotter. While our food required temperatures in the hundreds of degrees F, pottery demanded much higher steady temperatures.

However, it wasn’t pottery that led to the next huge leap upward in temperature. Instead, metalworking was our muse. In order to melt gold and copper, you need to get up close to that 2000°F (1100 °C) temperature, and you needed to keep it there for a while in order to manipulate the metal.

It must have been tantalizing to realize that these hard metals could be bent into particular shapes!

Curious people eventually started mixing metals together, and someone figured out that you could mix melted tin and copper together to make bronze. By 3300 BCE, people were experimenting with bronze, and by 2500 BCE, the Bronze Age was fully underway. Bronze was much harder than copper, and much more resilient than tin. It made the cutting edge weapons of the time for nearly two thousand years.

What about other metals, though? Could they be melted into other, even better weapons?

The answer to this came about around 1200 BCE, when the Iron Age began. In order to smelt (get iron out of rocks containing it) the iron ore, you need to get to 2800°F (1538°C). Some smart person came up with the idea of a bellow—an apparatus that could pump air into the kiln, creating more oxygen flow and allowing higher temps.

The Iron Age had officially begun, the last of the three metal epochs. Over the next 2500 years, give or take, the hottest sustained temperatures we could reach were around 3000°F (1648°C). The scientific revolution allowed for incremental improvements from the 16th century CE onward, but it wasn’t until electricity was harnessed for heat that we took another leap up.

The electric arc furnace is like an advanced version of a kiln, but very specialized, just for metals. By running an electric current into the apparatus, a little gap is created between electrodes (conductors) where there’s a constant spark, or plasma. This plasma can melt metal very, very quickly, and arc furnaces during the early 20th century could get up to 5000°F (2760°C), but their cores could get much hotter, probably around 10,000°F (5538°C).

I’ve had the opportunity to use an oxyacetylene torch to weld metal together. It’s awe-inspiring to slice through things with plasma, which can be tens of thousands of degrees. Far beyond this, though, the 20th century gave us nuclear fusion, reaching into the millions of degrees.

Modern nuclear detonations, very different from the controlled fires we’ve been talking about, can reach temperatures hotter than the Sun’s core, hotter than a hundred million degrees.

One big question for the 21st century is whether we can figure out how to harness this insane heat. There are signs of promise: for the first time in 2022, scientists were able to get out more energy than they put in. Still, we’ve got a long way to go if we’re going to harness fusion power. Whether we have the ability to make this economically viable is a real question.

It’s pretty wild to look all the way back, and consider how far we’ve come with harnessing heat. From a few thousand to a hundred million degrees, we’ve traveled far on our journey, and yet nearly all of those degrees are very, very recent. Where will this ability to produce very, very high temperatures take us?