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How to date an archaeologist: Thermoluminescence Dating - Ep 31
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In this month’s Tea-break Time Travel Training, Matilda is looking at yet another scientific dating technique used in archaeology: Thermoluminescence Dating! What is thermoluminescence? How you can use it to date ancient pottery? How does it compare to previous dating methods covered on the podcast? And are dreams made of atoms? Listen in to find out!
Transcripts
- For rough transcripts of this episode go to https://www.archpodnet.com/teabreak/31
Links
- Chapter on luminescence dating
- Original article by Daniels et al (1953)
- Original article by Aitken et al (1964)
- Case Study example
- Paper on thermoluminescence dating
- Method description
- Dig it with Raven post on thermoluminescence
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Transcript
Introduction and Host's Tea Choice
00:00:00
Speaker
You're listening to the Archaeology Podcast Network. um You're listening to Tea Break Time Travel, where every month we look at a different archaeological object and take you on a journey into their past.
00:00:17
Speaker
Hello, and welcome to episode 31 of Tea Break Time Travel. I'm your host, Dr. Matilda Ziebrecht, and today I am savoring a peach and vanilla tea, which is just delicious. And interestingly enough, you're only supposed to stew it for like 30 seconds, but it's so potent and it's really nice and it's just sweet and refreshing and soothing and everything that one needs in a tea.
Purpose of the Episode
00:00:41
Speaker
Anyway, and there's no guess with me today because it's only the second Tuesday of the month, so that means it's time for our next instalment of Tea Break time travel training where we look at all of those theoretical and methodological backgrounds that every seasoned time traveller, and of course archaeologist, needs to know before they set off on their journey.
00:01:00
Speaker
So
Thermoluminescence Dating Explained
00:01:01
Speaker
today we're continuing with our theme of dating, as in checking how old something is, not dating. And we're going to be talking about thermoluminescence dating, which my goodness, when I was writing out the script for this, trying to write out thermoluminescence as many times as I needed to. Oh, it's a horrible word to write out. Anyway,
00:01:20
Speaker
So, thermoluminescence, what is it? So, basically, it's a dating method used to determine the time at which materials, including some kind of crystalline mineral, were last heated. So, let's go right back to basics. So, a few months ago, we looked at radiocarbon dating, and we talked about how you could date the point at which an organism died by identifying the point at which radioactive carbon-14 isotopes start to decay. So, radioactivity is also a very important part of thermoluminescence dating, but instead of looking at the release of radioactivity, so the decay of an isotope, we're instead looking at the accumulation of radioactivity over time. So,
00:02:01
Speaker
First, let's give a little bit of a background to the relevant information, those little pieces of physics that you need to know in order to understand thermoluminescence dating. So, right, right, right, right back to basics. An atom. What is an atom? So, an atom is basically the thing that we are all made up of. Although, funny story, I was recently watching my one of my favourite TV series called Outnumbered, and the little girl in it, Karen, is talking to her dad. She says, what's an atom? And he says, oh, well, everything's made up of atoms. You know, atoms are inside all of us. Well, even ducks. Yes, evenck even Yeah, yeah, even the spoon. Even dreams.
00:02:39
Speaker
And he goes, um, well, oh, that's, that's a good point. What about shadows? And he's going, um, well, it just made me think. But anyway, an atom basically is is what we're all made from. And it consists of a core nucleus, which has protons and neutrons. And we learned about this last time for radiocarbon dating, because remember, it's called carbon-14 isotope. And an isotope of an atom is basically different versions of the same atom with different numbers of neutrons.
00:03:05
Speaker
So anyway, so you have protons and neutrons in the middle, and then you have a little cloud of electrons, which you might remember from high school is generally depicted as lying in neat little rings kind of orbiting, almost like lots of little moons orbiting the central nucleus.
00:03:18
Speaker
So, that's important to remember. That is the general structure of an atom. So, another thing that is important to note is that all kind of minerals specifically absorb radiation from the environment over time. So, you're constantly getting radiation radiation coming in kind of on a steady stream. This ionises the kind of lattice matrix of the mineral, the actual structure of it itself. so What do I mean by the lattice matrix? I'm saying lattice matrix, by the way, because I read both words in different papers. I call it the matrix, but then that also makes me think of the matrix film. so Anyway, but you have the matrix, which is not a load of lists of lots of green numbers, but it's actually formed.
00:03:59
Speaker
from lots of atoms linked together in different structures depending on the kind of lattice or the crystal type. So you might again remember in high school chemistry, learning about for example graphite and diamond are both made up of linked carbon atoms but they have very different properties because the structure of their internal matrix is completely different. So while carbon is basically two sort of solid layers linked together by slightly more squishy links. That means that they move around a lot more. That means that graphite is a lot more malleable, whereas diamond is a very, very hard, very solid structure. So that means that the pattern of the matrix is is more solid and secure. And that means that diamond itself has very different properties to graphite.
00:04:38
Speaker
so that's So, we've learned about the structure of an atom, the structure of a mineral in terms of its matrix. So, we just talked about all minerals absorb this radiation and this ionises the mineral matrix. So, what is ionisation? Ionisation is basically creating something into an ion. What is an ion? Well, an ion is an atom or a molecule with a net electrical charge So what does this actually mean? Well, in extremely simplified terms, a non-ionized atom has basically the same number of negatively charged electrons, so the little moons orbiting the nucleus and positively charged protons, which are in the nucleus. So they sort of basically cancel each other out. So there's no negative or positive charge. They just kind of have a nice little equality, a nice little equilibrium going on. However, if you have more or less electrons than protons,
00:05:29
Speaker
then this makes it a form of ion of that atom. So there's if there's more electrons, then it will have a negative charge. If there's more protons, it will have a positive charge. Everyone's still with me. Okay,
Applications in Archaeology
00:05:40
Speaker
good. So what does all of this have to do with thermoluminescence dating? Well, if you break it down very specifically, thermo is heat, luminescence is light, or specifically the emission of light, so kind of light coming out of something, that's luminescence.
00:05:56
Speaker
So, when your radiation, which is constantly kind of, remember, accumulating over time, radiation is constantly being absorbed, it enters the crystal matrix. This leads to the ionisation. So, the actual way in which that happens is that the kind of little bit of energy that it gives kicks off a couple of the electrons from their lovely little orbiting rings.
00:06:16
Speaker
So these electrons detach themselves from their parent atoms, and they become sort of lodged in what's referred to as electron traps, which are basically kind of gaps or impurities or in the in the lattice matrix. And over time, this kind of continuous naturally occurring radiation from the environment continues to enter the matrix continues to ionize, so more and more electrons will be kind of pushed out and fall into these electron traps. So that's building up over time, this is what's known as a kind of radiation dose. So these electron traps get emptied in what is known as a clock reset when a very intense application of heat occurs. Because again, those of you who remember physics will remember heat equals energy. So you're pumping all of this extra energy into the matrix and all of these electrons which have just been kind of chilling in the electron traps will then get enough energy to push themselves out of the traps again and escape and go back to where they came from. so
00:07:09
Speaker
All of these electrons are escaping back to their spots and as they do this they kind of lose because it takes up a lot of energy so they emit energy in the form of photons which is a kind of light which in turn creates this luminescence because it's emitting light. So in in very simple terms that means that the object is heated And through this, all of the electrons going back to their places, a little bit of light is emitted. And that light is directly proportional to this level of accumulated radiation that's been going on over time, right? Because the more electrons you'll have being accumulated, the more light they will emit, so the more luminescence you'll have as they all return to their spots.
00:07:46
Speaker
So that means that by measuring this luminescence you can work out how many electrons have been freed, so how much accumulated radiation there has been, how much time has passed therefore since the last time the clock reset happened, so the last time all of these electrons were emptied. And by looking at that you can then basically see how long it's been since the last strong application of heat to an object.
00:08:10
Speaker
So, that's sort of how it works in principle. What you're basically doing is seeing the last time that something was heated. And this is, by the way, with thermoluminescence dating. You also have what's called optically stimulated luminescence, which looks not at when the last application of heat, but looks at the last application of light. Maybe we'll do another episode on that, or maybe, yeah, we'll see how that goes. I might not be able to stretch it out to a whole episode again. Well, we'll see.
00:08:34
Speaker
So, what does this have to do with archaeology and how it applies? Well, so, thermoluminescence itself was already pretty well known before it was applied to archaeology, so it was quite commonly used to investigate kind of defects in solids and materials, because remember, these electron traps are caused by defects in the lattice matrix. So, you'd think the more luminescence that you have, the more defects you might have as well. It was also widely used to measure the radiation dose given to cancer patients, so a very different form of application, but also extremely useful.
00:09:03
Speaker
And then, Farrington Daniels and a bunch of his fellow researchers, back in 1953, they wrote a paper where they first noted the potential of using thermo-luminescence dating to date things, to date these big events. And they specifically looked at it in terms of kind of geological, but also archaeological research. So they looked at dating and correlating geological events that happened in quaternity stratigraphy, so specifically lava and eruptions and volcanoes and things, because that's looking at this intense heat, right?
00:09:32
Speaker
And then from there, in the early 60s, it was developed even further as a means of dating fired pottery, most notably by Martin Aitken and his team in 1964. Martin Aitken has then written loads of really great papers and books on the topic as well. So he's kind of the father of thermoluminescence dating in archaeology.
00:09:51
Speaker
And so that was kind of the real start of it being used in archaeology. It was used to date pottery because you're looking at an object that at some point in its past, when it was first created, as you know, hopefully, in order to fire pottery, you have your kind of clay that you mold into something and it will dry and it will be solid. But the only way that you can really strengthen that material further is by firing it. So putting it in usually a kiln, it used to be also in just a fire pit. And the point is, you want to have a very, very high heat, which then kind of merges all of the lovely molecules and matrices and everything together and makes it a really nice solid material. So this heating event would have been the last kind of big firing event.
00:10:34
Speaker
in this object's life. Because of course, you might have pots and things that are then heated on the fire, you you know, to cook and all of that kind of stuff, but they will never be heated up to the same temperature as when they were originally fired. So that's the sort of emptying the the the clock reset event, which then empties all of these electron traps would have been the firing of this material. So you think you have a bunch of clay, you gather that clay up,
00:11:01
Speaker
over time, you know it's gradually getting more and more radiation accumulation, lots and lots of electrons are falling out into these electron traps, you have this clay, you're doing stuff with it, you're putting it up, more and more electrons, more and more electrons falling into the electron traps, you make it into a little carbon, more and more radiation, more and more radiation.
00:11:18
Speaker
And then you've created this lovely cup, it's dried, maybe you've glazed it, I don't know, and you put it in the kiln or the fire pit or something like that and it fires. And that fire, that application of heat is what creates the clock reset. So all of the electrons go woohoo and they go back to their original spots in the lattice matrix. And then It's taken out of the fire, it cools down, whatever. and Throughout its use then, as a pot and throughout its burial, throughout its yeah just life in a museum, whatever, it's again constantly accumulating more and more radiation, more and more electrons and more things. so
00:11:54
Speaker
Then, at some point, scientists, archaeological scientists, will come along, will take a little sample of the pottery, will put it on a really high level of heat, again, equivalent to the firing heat, and that will then empty it again. And by measuring the luminescence, then, that comes out during that kind of heating event and during that clock reset event, they can see, ah, this is the amount of time that's passed since the last time it was heated to this higher temperature. So this is how long it's been since this pot was fired. So you can see when the pot was made.
00:12:24
Speaker
which is pretty exciting. So yeah, and since that sort of initial use being referred to so pottery dating, it also is used to look at, for example, heated flint tools. So quite often flint tools in the past would have been heated to make them easier to work. This is kind of one of the theories. So that's a sort of research sort of avenue that's happening at this point. um You can also look at, for example, slag, which is the leftover material that you get when you do iron smelting. So it's all the kind of impurities and all of the stuff that comes out of of the iron when you're trying to get the pure iron um smelted out of your your sort of ore, your iron ore, you get the iron and then you get the slag, which is sort of the leftover stuff.
00:13:07
Speaker
but very, very simply. And so inside that slag, you'll have things like quartz crystals and that kind of stuff. You can also then look at ah when was it last heated to in this furnace to create this smelting event kind of thing. That's what you can also look at in terms of that. You can also look at, for example, volcanic rock from glass production. So there's all kinds of things basically any time an object A mineral object in the past was heated, so an ah inorganic object was heated at a particularly high temperature for a particular event. You should be able to date that using thermoluminescence dating. So hopefully that's sort of clear. We're going to have a quick pause while you can take it all in and I'll be back in a moment.
Advantages and Challenges of Thermoluminescence
00:13:50
Speaker
Welcome back, everyone. So hopefully, you kind of understand the concept of luminescence dating at this point. But so how it actually works in practice, we sort of talked about, it's a bit harder to explain than the previous dating methods, because it basically involves a lot of calculations, which we're going to get into again in a little bit.
00:14:11
Speaker
But let's look at sort of the pros of it first. So actually, in comparison to a lot of other scientific dating methods, especially those that we've already looked at on this podcast, it's not as limited when it comes to time period, for example. So I mean, dendrochronology actually you could argue you could go all the way back into the dawn of time, because you just have to create that sort of never-ending little reference kind of visual. Radiocarbon dating, as you know, can only go back to sort of 40,000 to 60,000 years ago. But actually, thermoluminescence dating, you can go back as far as you want, really. And you can basically go all the way back to the start of human history, for sure. And as I say, they were using it to to measure these geological events. So you can even go back even further.
00:14:53
Speaker
so Also, what's really great about thermoluminescence dating and the dating of kind of inorganic materials in this way is because remember, what's really important to look at when you look at dating techniques and dating technologies is to consider, okay, but what actually is this date that I have? Like, what actually am I looking at when I look at this date? So remember with things like dendrochronology and radiocarbon dating, because you're looking at these organic materials, what you're essentially looking at is the point at which that organism died. So you're looking at when the tree died. You're looking at when the animal that was used to make the leather died. You're looking at the bone. um You're looking at when the animal died, all of that kind of thing.
00:15:33
Speaker
And so sometimes you have, especially when it comes to dendrochronology, you have these issues where you have wood that is reused. You have things that may have died, but then we use later, you have, it's it's harder to get a kind of specific idea of exactly what it is you're dating and exactly what that date means. But thermoluminescent staining is of an actual event, like an activity, because it's a heating event. So what you're dating is the point that when that ceramic was heated, or when that when that piece of pottery was fired into a ceramic, you're looking at that point at which someone heated up that flint tool in order to work it, you're looking at the point that someone fired up that furnace to smelt their iron ore to create iron. So it's yeah,
00:16:16
Speaker
ah a bit more accurate in that respect. And it's actually in general considered one of the most accurate methods of scientific dating because there's a lot of different ways that you can allow and sort of calibrate for errors. And it's sort of known what the errors are a little bit more as well.
00:16:32
Speaker
which we'll get into now too, so in terms of the issues of thermoluminescence dating. so one of the main problems well There's a couple of main problems, but one of the main problems is that different minerals, so different kind of rock types or whatever you want to call it, have different levels of sensitivity to the application of heat.
00:16:50
Speaker
So we already talked about right differences in the in the sort of lattice and matrices of different mineral types, so graphite versus diamond, et cetera. So of course, sometimes that also means that the sensitivity will be different depending on which sort of structure, internal structure they have.
00:17:08
Speaker
So for this reason, generally when you're looking at things like thermoluminescence dating, you want to always focus on one mineral type, which is generally what people do. They might like focus on a couple of different mineral types, but the problem is that the point is that you want to have each kind of dating process focus on one one, a mineral type. And then you can have multiple dating processes of multiple mineral types from one side. But yeah, you don't want to have like, oh, let's just add in like, the quartz and the feldspar and the thing and you know, from from this and and compare them all together because they might have very different levels of sensitivity. And in relation to that, you also need to always calculate what's called the radiation dose rate. And again, I don't want to get too much into all this because it gets a bit complicated, especially if you can't see the measurements and the calculations written down. I've included, by the way, a bunch of case studies in the show notes. So if this is something that's of interest to you, or if this is something that you need to know more about, do go and have a look at them. They give sort of some nice examples of the different graphs that you get, because basically what happens is you
00:18:07
Speaker
the thermoluminescence comes out and you measure it in what's known as a glow curve, which is basically kind of seeing how much is coming out related to the time. And the actual kind of measurements that you're doing are also very much related to the the amount of thermoluminescence versus the radiation dose.
00:18:29
Speaker
I think it was thermoluminescence measurement divided by radiation dose. I think that was what it was. Again, this is all in the case studies, so do go and have a look. but So the radiation dose rate is is the amount of radiation accumulated per year. And this is actually yeah a surprisingly simple process, but you do you need to measure the radioactivity of the sample material. You need to measure the cosmic ray dose of the environment it was found in. And then through combining kind of those calculations with a couple of other calculations, you reach something known as the radiation field.
00:18:59
Speaker
And then that's your base that you have to divide the thermoluminescence measurements by the radiation dose to then find the years or the amount of time. So yeah, so so you see in the GloCo curve, you see kind of the older the sample, the higher the thermoluminescence, which then makes sense. But then the curve itself is also shown in the kind of a graph that includes all kinds of other calibrations to make sure that you know exactly what it is you're measuring and to kind of compare those against almost control measurements. So it's it it looks more complicated than it is, basically, but it's sort of you're almost testing yourself as you do it. So you're making you're making sure that it's all sorted as you're doing it, which is quite nice.
00:19:39
Speaker
Another thing that you have to keep in mind is that the intensity of the heat actually has a direct influence on the amount of radiation or the amount of kind of extra energy i mean sorry that enters the matrix. So, you'll have a difference in thermal luminescence or radiation sensitivity in minerals heated to different temperatures as well. So, the same mineral heated to different temperatures will show different things. And this is ah particularly important to be aware of If, for example, you're trying to date something like, say, the segment of a kiln or a furnace wall, the mineral samples that are taken from different layers of that wall will have different thermoluminescence measurements because they'll have been heated to different temperatures. Obviously, the inside of the wall will be super hot, right, compared to the outside of the wall.
00:20:26
Speaker
It might only be like an infinitesimal difference, but it's still going to be different. And it's still going to then technically show different ages, even though they were all involved in the same heating event. And again, there's a way to counteract this. It's known as the foil technique, where basically you account for those different measurements.
00:20:43
Speaker
in the different layers and create almost like an average measurement to allow for kind of the range of sensitivity. But this is something that's still being kind of researched and still being kind of classified and coordinated. And like I say, actually, thermoluminescence is one of the most accurate ones in that respect, because although you do still have some influence from external kind of contexts, like environmental conflicts, actually, this is significantly less, like there's significantly less influence in thermoluminescence dating than in other methods like radiocarbon dating. So things like the Industrial Revolution, all of that kind of stuff, things like environmental differences, don't actually have as many detrimental effects on the efficiency as a dating method, as we discussed in previous episodes. Because as long as you take the correct calculations and the correct measurements, you'll be able to calibrate it fairly easily. um So that's quite nice about this this method as well. Another thing that is important to keep in mind is that of course if there have been other heating events, so I mentioned already that usually like if you're cooking a pot on the fire that won't be enough to empty the electron trap so that won't be enough to kind of reset the clock but maybe it was refired, maybe it was caught in a fire, maybe it was thrown in a lava pit, I don't know but like the point is
00:21:56
Speaker
There's, yeah, maybe it it had other heating events or maybe it wasn't actually fired enough the first time to completely empty the electron trap so there were still some there so it wasn't really a full clock reset.
Ensuring Accuracy in Dating Methods
00:22:07
Speaker
Again.
00:22:09
Speaker
there's sort of a test to check for this. It's known as the plateau test, and it's always incorporated into thermoluminescence dating measurements. So you'll have that glow curve showing showing the luminescence, so showing the light that it's emitting out. But you'll also have the plateau test, which uses very similar calculations, but is much less prone to error because it uses sort of controllable data. And again, quite complicated calculations that I can't really explain or describe properly, but I'll leave those case studies in the show notes, and you'll be able to see it there. It's basically kind of creating a little plateau that shows the point at which the glow curve would show the thermoluminescence. So it's kind of a nice second check, almost, or side of a part two check. So that is thermoluminescence dating. I think I covered all of the main points that I needed. Funny story, I actually wrote an essay on thermoluminescence dating back in my masters. So i I had a look at that, I admit, when I was writing the script for this episode.
00:23:06
Speaker
But there were still quite a few things that I didn't really get, so I had to look up more stuff.
Host's Reflections and Audience Engagement
00:23:10
Speaker
But I'm really enjoying doing these Tea Break time travel training episodes. They're really good fun for me, personally, because I really enjoy being able to look up all of this stuff. And I know kind of the basics of each one, but it's fun because this time I really get to be like, wait, but why is it like that? And then I follow that rabbit hole and I look it up and I'm like, oh, that makes way more sense.
00:23:31
Speaker
So I'm learning lots. I really hope that you're learning lots. I hope that you enjoy these ones. Please do let me know. It's really nice to get feedback from listeners, even if it's just, I don't know, a comment on an Instagram post or, you know, I share i share this stuff on Instagram. You can follow me at the archaeologist teacup.
00:23:50
Speaker
on all social media platforms. I also have a website. Do get in touch and let me know what you think about the podcast because, I mean, I really enjoy doing it like I say, but I don't want to just be kind of speaking into the void. It takes quite a lot of time to research this stuff.
00:24:05
Speaker
and then record and then you know a lovely editor Rachel at the APN has to go through and edit it and it has to be published and it has to be publicized and all of this kind of thing so it does a lot of work for one episode and you know it's free for you guys to listen to.
00:24:20
Speaker
So it would be nice to hear that there are people actually listening to it. Do let us know. Do let me know if you're enjoying these and if you think that it's worth me keeping doing them, because if you don't, then then I don't have to. That's fine. That'll save me an hour or two a week of of work. And ah yeah, that would be, so yeah, do get in touch via social media. You can find me at the archaeologist's teacup on Instagram, on Twitter, on Facebook.
00:24:49
Speaker
I'm not as active on Twitter and Facebook, I admit, mainly Instagram. You can also find my website, thearchaeologistteacup.com. You can also comment on the APN, which is, yeah, Archaeology Podcast Network on pretty much all social media platforms.
00:25:03
Speaker
and my content details are in the show notes. So, yeah, do get in touch. Keep an eye out for this month's guest episode as well. I chat to Dr. Sachiya Frandefard for Shroff all about the Sword of Oz, which is very exciting. So, yes, keep an eye out for that on the fourth Tuesday of the month, so in two weeks' time.
00:25:24
Speaker
I hope that you enjoyed our training session today. Again, please do get in touch and let us know what you think of these podcasts. Also, if you want to help support the show, please do like, subscribe, follow. Wherever you get your podcasts, leaving a review is also extremely useful because it helps others to find us. And if you want to support all the other amazing series that form the Archaeology Podcast Network, you can also follow all of them. They you can find them all on the website or on the social media.
00:25:49
Speaker
You can also become an APM member, which will be helping us to create even more amazing content, free content. You'll also have it exclusive access to ad-free episodes and bonus content like our quarterly online events. So for more information on that, do check out the homepage at archaeologypodcastnetwork dot.com. And yeah, I look forward to hopefully hearing from you.
00:26:10
Speaker
Bye! I hope that you enjoyed our journey today. If you did, make sure to like, follow, subscribe wherever you get your podcasts, and I'll see you next month for another episode of Tea Break Time Travel.
00:26:22
Speaker
This episode was produced by Chris Webster from his ah RV traveling the United States, Tristan Boyle in Scotland, DigTech LLC, Cultural Media, and the Archaeology Podcast Network, and was edited by Rachel Rodin. This has been a presentation of the Archaeology Podcast Network. Visit us on the web for show notes and other podcasts at www.archapodnet.com. Contact us at chrisatarchaeologypodcastnetwork.com.