How are artifacts dated?

Dr. dig responds:
Archaeological sites are dated using a variety of techniques. Most archaeological sites and objects are dated by combining stratigraphy (how the layers of the past are piled up on top of each other - the oldest stuff usually being found at the bottom, and the most recent stuff at the top) and typology (identifying artifacts that are typical of different periods in each layer - a bit the way you can recognize that some cars are older than others because of their style).

Archaeologists use more scientific methods also. Radiocarbon dating is a technique which makes it possible to date organic remains and the archaeological deposits they come from. Most dating methods that archaeologists use are radiometric--that means they measure the rate of decay, or change, of particular radioactive elements that artifacts might contain.

Some artifacts made from wood can be dated using a relatively simple method called tree-ring dating. Tree-ring dating is a method that is less difficult to understand. Tree rings grow annually--in sensitive trees like oak, rings vary in thickness from year to year, depending on growing conditions. Using many trees, scientists have built up tree-ring sequences going back beyond 7,500 B.C.!

Bones can be dated using a number of methods, most of which are highly complicated scientific techniques.

The many methods of dating bones are: stratigraphy; flourine, uranium and nitrogen dating; Radiocarbon dating; amino Acid racemization; and electron Spin Resonance dating.

The age of bones from long, long ago can sometimes be determined by dating not the bone material itself, but the rock in which it is found. The methods used to date these rocks are called: Potassium-Argon dating; Fission Track dating; Paleomagnetism; Biostratigraphy; and Ocean Sedimentation.

dig magazine will be publishing articles from time to time about how objects are date. Be on the lookout for these great issues!

For more information on dating methods in archaeology, you will want to go to the website of the Mankato State University - it is really quite a good overview of the subject: emuseum.mankato.msus.edu/archaeology/dating

I would like to know more about how flourine dating works.

Dr. dig responds:
Flourine dating is often used in conjunction with two other dating methods that measure levels of uranium and nitrogen. The three techniques together are collectively known as F-U-N dating.

They are used to to establish relative dates for bone, teeth, antler and ivory when items of any of those materials are found together, but when it is not clear if they are contemporary with each other or not. This type of dating is most useful in archaeological deposits found in caves, for example, where fossilized human material if found with the bones of extinct animals. F-U-N dating does not provide exact dates for objects - what it does is to establish what objects are older than others - this is called relative dating.

Flourine dating relies on the incorporation into ancient bone mineral of flourine ions dissolved in groundwater, which is an irreversible process. Flourine dating, like uranium and nitrogen dating, depend on variables such as temerature and humidity. This means that they can be used to establish the relative dates of objects found together, because within the environment of a site like a cave, the rate of change of F-U-N levels should be the same for all contemporary materials.

Changes in flourine concentrations occur so slowly that they are only useful for material that is older than 10,000 years.

Do museums help you date things?

Dr. dig responds:
Some objects that archaeologists find are sent to be analyzed and dated more precisely in special science laboratories located at places like universities or even hospitals.

Objects that are sent to museums can be studied by scholars and museum curators who are able to date them by making careful comparisons to other artifacts in the museum's collection.

But the date of most artifacts are figured out by archaeologists at the time they are first discovered.

Does Carbon 14 testing do any damage to the piece being tested?

Dr. dig responds:
In order to date an object using the Carbon 14 method (also known as radiocarbon dating) a small sample needs to be taken from the original. The sample is then treated to remove any contaminants, and then reduced to pure carbon for testing.

Only the sample that is being Carbon dated is destroyed in the process, not the entire object itself. Taking a sample for carbon dating may not matter, say, if the object you want to date is a skeleton, mortar from a house or wood from a boat, where the damage is of little significance. But taking a sample from an otherwise undamaged ivory carving or other work of art may very likely destroy its aesthetic value no matter how small the sample.

If carbon dating can only date back to 50,000 years ago, then how can archaeologists know something is 2,000,000 years old? This was discovered by Richard Leakey.

Dr. dig responds:
Radio-carbon dating is an important scientific technique for dating organic remains to 50,000 ago. Archaeologists use many other scientific methods to date artifacts older than 50,000 years of age. Potassium Argon, Fission-track, and paleo-magnetism are other methods of dating non-organic materials that are far older. Please write me again if there is something more you would like to know about a specific dating technique.

Is it possible that an artifact can run out of carbon?

Dr. dig responds:
All living things take up C-14 during life, but the process ceases after death and the C-14 in the organism decays at a known rate.

Radiocarbon has a half life of 5,730 years (plus or minus 40): after that period has elapsed, half of the residual C-14 will have been lost by radioactive decay: after another 5,730 years, half of the remaining amount will have been lost: and so on. So, in theory, the C-14 never runs out, it just gets halved every 5,730 years.

Of course, dating an object using smaller and smaller amounts gets tricky, and the radioactivity of samples more than 40,000-50,000 years old is so weak that it is very difficult to measure at all. However, by passing the sample through thermal diffusion columns for several weeks, it is possible to concentrate the C-14 in the sample and extend the date range back to about 70,000 years. This process is usually considered too expensive for most excavations and it is rarely done.

Phew! If you can grapple this stuff you guys are definitely ready to go on and become archaeology laboratory technicians!