Dating methods and their use.

[Klingan]

Dating Methods

 

One of the absolutely most important corners in the work of a historian or an archaeologist is the possibility to date their work. It’s such a natural feature of a museum, article or book that most common people don’t even thing about it. It’s just that way it is, the archaeologist knows when. Done. In all honestly though, dating is critical for all historical knowledge and research, it’s the frame on which we build all of our past. It is however not even close to as easy as it may seem, dating objects are a very complex work. There are even two primary dating systems, an important fact missed by most people; the absolute dating and the relative dating. This article is therefore meant as a base for you to consider what the dates you will get bombarded with really mean and to make it possible for you to draw your own conclusions on how reliable they are.

Another very important thing to keep in mind when talking about any dating is the time scale. Remember that not all cultures use the same timescale and the western world and not even we use the same scale at all times. The most commonly used are however BC/AD (Before and after Christ) and BP as Before Present where present is set to 1950 AD. This is mainly used for prehistoric periods before 8000-10000 BC, but it may of course be used for any other date too.

 

Relative Dating:

In theory it’s very easy to make out the difference between absolute and relative dating, I will start with explaining the relative dating and how it is used. As the name suggests it’s made up of relations between objects, a typological sequence where you can see how an object is changing its shape slowly through time. A classic archaeological example would be stone axes in the north or Athenian pottery in southern Europe. Even easier to see today is the changes in how a car looks . If you find pictures of an old horse cab and then get more and more pictures as it’s developed into the first automobiles and then modern cars you will see how certain features hangs on for a while to later disappear. The resemblances between a horse cab and a modern car may be virtually none, but if you have all the steps it will all unfold. This is exactly what relative dating is about, making a chain of objects that resembles each other and you can now say that this object is older than that object, you have a relative dating line. You will however, unless you can date one of the objects absolutely, have any idea about how old they are. The important part here is what objects to compare, it’s not enough that they are from roughly the same time frame, and they need to be from the same geographical area. Its often trickier then expected.

 

Absolute Dating:

To place an object exact in time you will need to use the other method, absolute dating and this is compared to relative dating a rather new phenomenon except or written sources. Before the emergence of modern physics and science the only absolute dating we had was from this kind of material. Absolute dating is the methods used to fix an object to a specific place in time. The scientific methods to do this are quite many and I will only mention the most important to save you a very long reading.

 

 

For what are the different methods used?

I am now to go through a number of relative and absolute dating methods and examples.

 

Stratigraphy:

This is probably the most common way to use relative dating today. When using stratigraphy you are practically digging a whole in the ground with very straight edges. What you’ll soon notice is how the difference earth layers will have different colours and content. In theory it’s really this easy, what’s in the higher layers are newer then the lower layers, and everything in the same layer belong to the same time. In practice there are a great deal of problems. First of all, these dig shafts obviously cuts away one part of the stratigraphy and shows another, in practice you can miss a whole layer like this if it ends in your shaft. Secondly it’s very rare to find a completely undisturbed stratification. In almost all cases it has either been disturbed by humans, as in when a new house is built on top of an older one and the founding are dug down, or by animals. This disturbance is called formation processes and it’s crucial for all archaeologists who are working with stratigraphy to recognize what process have affected what object . There are two main types Cultural and Natural formations where cultural are human made and natural basically everything nature have done to the object. Are the cut marks on a stick made by a stone knife of a beaver? These are important questions for the interpretation of an object. These stratigraphy layers may also be very deep, well over 10 meters in cities from the medieval times and even thicker in older cities. In the Middle East there is a phenomena called tells (Tel Aviv as an example) city mounds that may be very high since cities have been in the same place for thousands and thousands of years.

Now if you can date one object absolutely in a layer you know roughly from what time frame all objects in that layer are (Considering the above problems of course). The last thing to have in mind when doing this is that the date you get is the last used date not a date for when the object was originally made. Lots of objects can circulate for tens or even hundreds of years before it is finally lost or buried. In effect we know when the object was buried not when it was made or used and object of different age can therefore be mixed.

 

Radiocarbon dating:

The absolutely most well known absolute dating method is the radiocarbon dating method also known as the 14C method, which was invented by the American Williard Libby during the 1940’s. The method was an unexpected result of the intense atomic research of the day.

In earth’s atmosphere is a most unusual substance called coal-14 (One coal atom out of 1012) created when nitrogen (14N) is bombarded by neutrons from space in the form of background radiation. This gives 14N + n -> 14C + 1H, in other words the nitrogen and the neutron is transformed into this highly unusual coal-14 and hydrogen.

During its life all living mater consume a small amount of 14C. After its death the consuming is done for and the coal-14 already in the object or animal is slowly falling apart, this because the 14C atom is unstable and is constantly trying to revert back to it’s more stable 12C state. Now we know how a 14C atom looks and by taking a sample we can know how much of the original atoms that are left and reverse calculate how long ago the object died, presented as BP, Before Present as in before 1950. However the method is far from perfect and rather complicated to use. The basis for the calculations for how old the object is are made by an estimated half life of 14C set by Libby to 5568 years, but since then a new more accurate half life have been calculated to 5730 years. Laboratories however still use the older less accurate number though since there are more problems to overcome before the dating can be used. One of those problems is that all dates are estimated, not exact. It’s accepted that the number has a 95% chance to be in about the range of 2.5-5% to the real date. This may seem as very little but for most objects before about the 1st millennia BC a relative dating is more exact, especially in the Greek and Roman world. The next problem is that when the first dates were coming out in 1949 they were all slightly off and too old. This was because the 14C have not always been constant and further calibration was needed. This is mainly done by dendrochronology (Tree ring dating). Ice cores and sea bottom cores have also been used in calibration. Further on wasn’t the first method to calculate the number of intact 14C atoms as accurate as those today and newer methods keep coming. There is also the problem of modern contamination, as soon as an object is taken up from the dig and comes into contact with modern organic material the coal-14 balance is changed. Very many objects are therefore impossible to date in this manner.

The last and maybe most problematic issue for prehistorians is the fact that the 14C method only allow us to date up to 40.000-50.000 years back, after that the number of 14C atoms are so low that it cannot be measured accurately and modern contamination is overwhelming compared to the original 14C.

 

Dendrochronology:

Another important method is dendrochronology. This is classed as an absolute dating method but it’s clearly an offspring of relative dating. The principle behind it is (as in most other dating methods) easy. Each year a tree grows a certain amount very much depending on how the weather have been. In the spring and summer the rings are thick and bright, then they get very thin and dark over the winter. This is constant to all trees in a regional area. If you can find a number of samples from different periods it’ll be possible to puzzle those together into a chronology. From this you may later date wooden objects. This method also has a lot of problems to consider though. The largest is that it does only work in temperate zones where the years have seasons, therefore it’s not possible to use in all jungle areas. Even in temperate zones it does only work for the same species as the one you have a chronology for. Some trees cannot even be used at all. The method is also highly regional, about 250 km in diameter very much depending on the climate and natural conditions and therefore very troublesome some to make. However can most places in Europe and Northern American be dated with dendrochronology up to 8000 BC. Beside dating of wooden objects is the absolutely most important use of this method to calibrate 14C and therefore making it more reliable.

 

Thermoluminicense:

Cheramics have been a problem since it cannot be dated with either coal-14 or tree ring dating (Unless the finder is in luck and a grain or piece of wood is stuck in the object). There is one method however called thermoluminicense that can date ceramics. The theory is that it’s possible to measure how much radiation the crystal structure have received since it was last burned. When heated up to more then 500 degrees C this radioactivity is released and the thermoluminicent clock is reset. What you do is that you take a sample and heat it up to 500 degrees C and it will release a faint light that can be measured. The stronger the light is the older the object is. Here the largest problem is that the margin for error is large, +/-10% that in all historic times is far too much to be of any use and almost never exact enough for any other pottery either. Therefore the largest use has been to identify fakes from real objects.

Closing thoughts:

All methods are problematic in one or another manner and must therefore be used with caution. Beside all methods individual problems there is one general point that must be taken very seriously. When an object is found you need to exactly understand what you are dating. It’s a common error to take a coal-14 test of an object and then say this is how old it is. This is wrong. That is how long ago the object consumed it’s last coal-14. Imagine a house dated in this manner. A house can be at the same place for several hundreds of years and be rebuilt any number of times. Therefore I want to finish this with; be critical to the dating you see, stop up for a second and think about what it is that is dated and I promise you, you will have a much greater understanding of the object that you are observing.

 

 

Sources:

 

Gr

Edited January 3, 2008 by Klingan

[Melvadius]

Allowing for some slight translation problems which are perfectly understandable this is a very good article covering all the major dating issues.

 

I would add in one important archaeological dating term, which may have been missed accidently, this is terminus post quem - the absolute date that is the earliest a stratigraphic layer can be.

 

The most obvious example is where you examine a structure (e.g. a wall or part of a building) and find a dated coin either built into the structure or else underneath it on an undisturbed layer. In such cases, although it can be later the structure cannot be earlier than the date on the coin.

 

This system can be expanded with pottery or other items where they can be similarily dated through association with other finds that can be dated using one or more of the the other investigative techniques. An example would be the date range of particular types of pottery where the pottery kiln site(s) has been found and through kiln residue both the last firing date of the kiln (and ideally also the earliest known firings from nearby kilns) and the types of pottery being made can be determined.

[Klingan]

Ah I knew I would miss something, terminus post quem really does deserve it's place in the article. Thanks for mentioning it Melvadius!

 

On the translation errors, I do believe my English will improve in time.

 

Edit: The references wouldn't be shown here, if you are interested just open the word document.

Edited January 3, 2008 by Klingan

[Gaius Octavius]

I guess that life is made easier for the Klingon and Melvadius types when they find a building with a corner stone with a date and/or a name on it. Ditto, for an object.

 

Thanks guys.

 

P.S.

Oh!, tyrant of the posts, I can't help this, it's in my DNA: And for many English speakers, English is not their first language!

 

Edited January 3, 2008 by Gaius Octavius

[Primus Pilus]

Well done Klingan. I personally enjoy the casual approach in an introductory piece such as this, and the language issue is quite minor. Continued practice will help smooth out the rough edges.

 

Out of curiosity, was this written purely out of personal interest?

[Gaius Octavius]

Out of curiosity, was this written purely out of personal interest?

 

I think, that earlier, Klingon told us that he would post a synopsis of some work that he was doing for school.

[Klingan]

Out of curiosity, was this written purely out of personal interest?

 

I think, that earlier, Klingon told us that he would post a synopsis of some work that he was doing for school.

 

Actually it's rather much a combination. I had a paper (Written in Swedish) for my university course on the subject limited to two pages, basically I completely rewrote it and added to that (twice as much now). I tell you, going through this subject in two pages was a nightmare. As you might guess it's quite a deal of work rewriting/translating but I really enjoyed it! I somehow in a strange way find it funny to write this kinda stuff.

Edited January 3, 2008 by Klingan

[Melvadius]

I guess that life is made easier for the Klingon and Melvadius types when they find a building with a corner stone with a date and/or a name on it. Ditto, for an object.

 

Thanks guys.

 

<SNIP>

 

 

Finding a nice little stone with an inscription on it - most of the Roman period archaeologists working in Britain today would love to find such an item. As much of the UK has little in the way of precisely dated inscriptions we have to make do with the methods indicated above. Mind you on my last dig was at a major Romano-British multi-phase probably religious site, which dates from the late Pre-Roman Iron Age through to at least the fourth Century AD. It has been frustrating in its total lack of inscriptions, so has necessitated reliance on dating of artefacts for feature dating.

 

In my final week I was asked to clean up a corner section of a trench by removing a layer of cobbles to confirm that everything under it was part of the same layer only to find the start of a totally unsuspected wall feature cutting into the trench from the direction of what we believe is 'semi-amphitheatre' probably used for religious activity including plays and/or possibly some form of gladiatorial contests. (There have been a lot of votive items such as remains of sacrifices and deliberately broken pottery found around this part of the site).

 

After planning the wall I had to take a section through it which was done for part of the time with the help of the deputy trench supervisor who got the section which contained the fragments of about a third of a Roman pot which had been built into the wall layered as a small stack. OK I was a bit miffed about not getting the section with the pot, however that pot along with a piece of a different pot that I found at the base of the wall may well provide the terminus post quem for that part of the site was very pleasing to know.

[Gaius Octavius]

I wouldn't dare to so much as insinuate that my betters are in error. I thought that I was adding to the ways something might be dated.

[Melvadius]

I wouldn't dare to so much as insinuate that my betters are in error. I thought that I was adding to the ways something might be dated.

 

A good thought but to be honest a dated inscription is another example of terminus post quem, as with a few minor exceptions you are generally dealing with an item which cannot be earlier than the date on the inscription.

 

The exceptions tend to be with details rather than broad dates. Fairly modern large public buildings often have foundation stones built into the structure early on and I remember hearing of at least one Victorian period instance where the person named did not dedicate the building and a year later than the date inscribed.

 

Roman dedications tended to be at the top of buildings or on public monuments and often were in better quality stone than locally available. In large parts of Britain where good quality building stone is fairly rare you can have some dating problems due to dedication stones being moved and re-used for other purposes so not in-situ with the buildings you would like to identify.

[Ursus]

Thank you for posting this. Well done.