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One of the best-known ways to figure out how old something is, is radiometric dating. Which includes carbon dating, though we can use other elements too.

Basically, the less radioactive material you find in whatever you're trying to date, the older it is. But not everything can be dated that way. Certain handmade crafts, for example.

If you're trying to find out how long ago some ancient person made a pot, it doesn't help to use radiometric dating and find out how old the clay is. Because that doesn't tell you when it was shaped and fired. So can we tell not how old something is, but when it was shaped by humans?

Well, actually, we can. And it involves heating that ancient pot right back up. Certain minerals have a property called thermoluminescence.

When you heat these minerals to a sufficient temperature, they emit electromagnetic radiation, specifically, visible light. And the amount of the light they emit is proportional to how long it's been since they last got really hot. Which is exactly what we need to learn how long ago some ancient crafter made their pot, or any other ceramic wares.

This phenomenon was first described all the way back in the mid-17th century, but we didn't have the technology to apply it towards dating artifacts until the 1950s. And it works well when other lines of archaeological evidence might be unclear or poorly preserved. Like, it's been used to date artifacts from the Indus Valley civilization, one of the world's most ancient urban cultures.

Here's how it works. All minerals, including the ones in materials like clay, have their atoms arranged in patterned crystal structures, or lattices. But there are imperfections, or flaws, in that structure.

And that's the key, here. See, throughout its lifetime, a mineral gets exposed to radiation, maybe from radioactive materials in the mineral itself, or maybe from the soil it's buried in. Some of that radiation has a lot of energy, and if it hits one of the mineral's atoms, it can shake one of the mineral's electrons loose.

And sometimes the radiation is actually a brand new electron altogether. Either way, these electrons can end up getting trapped inside spots within the mineral's imperfect lattice, called electron traps. And over time, more and more trapped electrons accumulate until the spots fill up.

Different traps have different depths, meaning it can be easier or harder for the electrons stuck in them to gain enough energy to escape. So how can they get loose? Well, heating them up is one pretty reliable way.

If an entire chunk of mineral gets too hot, say, because an ancient potter just stuck it into a kiln, all of its trapped electrons will escape, and as they do, they shed all their excess energy in the form of light. Basically, the mineral has a stopwatch that had been ticking up the more electrons got stuck, and heating the mineral zeroed it out. Then, over the thousands of years since the clay pot was made, the electron traps started getting filled again.

The clock started ticking upward again. And to read out that number, scientists simply have to heat it back up. I mean, it's not that simple -- but that's the basic idea.

So, we can measure how old a ceramic pot is, even if the clay the pot is made from is way older. The amount of light emitted indicates how much time it's been since it had been heated previously. Of course, that brings up an important caveat:.

The pottery may have been heated up in between the kiln and the lab. Like, maybe it was a family heirloom, and a few hundred years into its life it was caught in a house fire. You can't really account for that.

It could also be that the ambient radiation coming from the environment you found your pot in changed significantly in some unexpected way at some point in the past. But you can account for other things that affect the amount of light you measure. For instance, electrons can also get trapped by absorbing energy from cosmic rays coming down from space, which can penetrate several tens of meters down into the earth.

So you have to know how deeply your pot was buried, if it ever was. But assuming you've gathered as much data from your archaeological site as you can, here's how you date your sample. You break a piece off and crush it up into a bunch of separate samples.

Some of them you steadily heat to around 400 to 500 degrees Celsius to measure how much light they emit as the trapped electrons reset. For some other samples, you heat them to reset their clocks, then expose them to an artificial radiation source so you can figure out how readily the electrons get trapped in the first place. So you'll have the total number of trapped electrons, and the rate at which they build up over time.

And then it's basically just division. Thermoluminescence doesn't just work on pottery, of course. You can use it on bricks and soil that was part of an ancient hearth.

Or even stuff that had nothing to do with humanity, like meteor impact sites, or volcanic rocks, because it works for aging stuff up to about 200,000 years ago. The crystal structure can only store so many trapped electrons, so rocks that are any older than that will all release the same amount of light, assuming they're the same type. So at that point, we have to resort to other dating methods.

But it's pretty cool to know that we can tell when some ancient potter thousands of years ago first fired an artifact, just by heating the daylights out of it all over again. Thanks for watching this episode of SciShow, and thank you to SR Foxley, our President of Space, for helping to make it happen. And you, too, could be President of Space one day.

If you wanna throw your hat in the ring, you can check out patreon.com/scishow. [♪ OUTRO].