Hey guys, today I am back with another
archaeological dating methods video and
today we're talking about
thermoluminescence. DAMN that's a long
word! So we're gonna be the really cool
archaeologist that we all know we are
and use the term that all the
professionals do: TL. Archaeologists
have given this dating method a very
complicated name, and I have yet to find
a very easy to follow, easy to understand
resource for this so this is my attempt
to kind of clear the waters a little bit
and explain it as best as I can. So... here
we go! Let's start with the word:
thermoluminescence. It can be broken down
into two separate words: Thermo meaning
heat, and Luminescence meaning to give
off light or to emit light. It
essentially means that materials that
have accumulated energy over a long
period of time will emit light when
exposed to high heat. So thermoluminescence dating is used mostly on
pottery and other inorganic objects such
as burnt flint. It's a very popular dating
method in archaeology because it can
date the one thing that we find the most
on site: pottery!
We're literally drowning in pottery. That,
and it can also go further back than
fifty thousand years ago which is very
good because carbon-14 dating only goes
to about fifty/ sixty thousand years ago.
So this is much more versatile for older
sites as well of course, the cincher
to why this is such a great dating
method is that because it is cheap! Yes,
in comparison to other dating methods
this is a very affordable one to do so
it really does help if you're on a
budget. Let's get into it.
Thermoluminescence is another form of
radioactive dating, except this time
we're measuring the amount of radiation
that is accumulated over time instead of
the amount of radiation that's been lost
like with carbon-14. Materials like
ceramics and things like that, they're
all made from geological materials and
the atoms of these materials, when
they're joined together, form these
crystalline lattice net-type structures.
When the atoms in this lattice get
exposed to nuclear radiation, the
electrons get all hopped up on this
energy and they break free from the
lattice. Some of the electrons will then get
trapped inside of the lattice
deformities which are caused by either
missing atoms or impurities within the mix. This
is why we call them electron traps! Of
course this isn't just a one-time thing-
over time more radiation is gonna be
absorbed into the object and more
electrons are gonna break free and get
trapped inside of this lattice work. If
the absorption of radiation happens at a
constant rate something that we call the
annual dose, then the electrons will
accumulate uniformly over time. The size
of the population of these electrons can
then be measured and directly related to
the amount of radiation that the object
has been subjected to over time. This is
what we call the total dose. This of
course directly relates to the total
time that an object has been subjected
to radiation and we can calculate it
*in theory* with a simple equation. This
equation: age equals the total dose
divided by the annual dose. The elements
that we get the annual dose from are
uranium thorium and the radioactive
isotope of potassium which is
potassium-40.
These isotopes emit all the things. We're
talking alpha particles beta particles
and even gamma rays... yeah the stuff
superheroes are made out of. Oh THOR-ium, I get it. I'm not a Marvel person. Alpha and
beta particles can't penetrate the
surface of an object very well though,
and they can't give us a proper reading
of the date so what we really need to
look at are the gamma rays because gamma
rays can penetrate about 20 centimeters
into an object's surface, which is
excellent news for us. These three
isotopes have super long half-lives, and
their emissions are assumed to be at a
constant rate, which is not like the
carbon-14 which can kind of fluctuate
over time. This makes it really easy to
measure the annual dose part of the
equation just by looking at emission
levels today. Now we need to get to the
total dose, which is done by measuring
the trapped electrons inside of the
object. This is where thermoluminescence
comes in. When pottery and other ceramic
objects are made they are fired, which
means that they are exposed to very very
high heat. This makes them stronger, more
durable. The heat that's needed for this
also serves a purpose for those
electrons that are trapped inside the
lattice. The heat provides the electrons
with the energy needed to break free
from their very sad prisons.
This means that the electron count gets
set back to zero and the accumulation
process begins all over again. So if the
pot was fired once and then just used
for cold storage or any other sort of
circumstance that did not bring it into
an area where there was high heat, when
we would date this object we would
actually get the date of what it was
mad,e when it was produced. But, if the pot
was used for cooking or other sorts of
activities that used high heat, the
electron count would have then been
reset every time that it was exposed to
heat. Which means, when we date an object
like this, like a cooking vessel or
something like that we actually get the
date of the last time that it was heated-
maybe even used. This method of heating and
releasing electrons from their traps is
how thermoluminescence works. We reset
the clock to find out how long it's been
running for. First, you shave away the
first few millimeters of the pottery or
the sample that you're gonna be testing
to get rid of all those alpha and beta
particles because they're not gonna give
you the right information. They're gonna
give you some false information. We want
to get to the gamma rays. Then the
material is heated to about 500 degrees
Celsius or higher in a
very controlled setting. What happens is
the energy that's lost from the
electrons as they make their great
escape gets emitted as light radiation.
This luminescence is measured and is
directly proportional to the amount of
electrons that were trapped inside
before it was heated. And this gives us
the total radiation dose. Tada! You then
plug in all of those numbers into the
equation and you get your age. Now of
course you can't just sample one
specimen or object and assume that it's
a date for the entire site. You usually
have to do other things that are very
vital like taking soil samples and
dating those as well to make sure that
the radiation matches the object that
you pulled out of the ground. You also
need to measure multiple samples to make
sure that your calculations and
everything was correct. This is just
proper science guys. The precision of
this dating method is plus or minus ten
percent, which is very very good. But of
course if you're dating some very very
old pottery, you're gonna get a larger
margin of error. Let's not forget that
once you heat that object, once you heat
that sample or that specimen-
it's clock gets reset back to zero. So
you can't really use it
for further testing and it can't be
properly dated anymore. This can cause a
lot of problems when you're identifying
the authenticity of objects. I've heard
some stories of people in China using
cat scans and x-ray machines to actually
put in more radiation into their objects
so that way when it's dated, it
actually dates from much
older time. Which is very sneaky, I must
say, that's very smart I wouldn't have
thought about doing that. But don't do
that! Just... this is what happens, this is
the reality of what I'm telling you. As a
conservator I also have to note that
this method is damaging to the object,
right. It's destructive. It's a
destructive invasive process. You have to
take a sample, you have to shave it away...
You also have to think about the
ethics for that. You have to make sure
that this object is the only object that
you can take the sample from. You need to
make sure that it's worth interfering
with the object to get this date.
That being said, pottery is super
abundant in archaeological sites so I'm
sure there's gonna be one or two pieces
that are not very diagnostic that you're
able to take samples from. So that's it!
Hopefully a simple explanation for a
complicated word. If you want a full
write-up with some other resources and
things like that there's a link to my
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And as always, stay dirty, my friends
