The C-14 Method or Radiocarbon Method is the oldest physical method, which allows to determine the age of an object, if it contains carbon. The method is named after its principle, it is based on the natural radioactive decay of the carbon isotope C14. It was developed in the 1950s by a team of scientists under Professor Willard F. Libby of the University of Chicago. Libby received the Nobel Prize in Chemistry "for his method to use Carbon-14 for age determinations in archaeology, geology, geophysics, and other branches of science." in 1960.
First a word on how the name of this method is written. The C14 is a isotope of carbon, which is otherwise C12 or C13. The C means carbon, the number gives the atomic weight rounded. There are various ways to write it, C14, C-14, 14C or most accurate 14 C. Unfortunately the most accurate way to write it is not very practical on HTML page, as it influences the line height. It is impractical even for books and papers, so the C14 notation is most commonly used, and we will do the same on this website.
So now about the way it works! Carbon is a very common element. The atmosphere contains a certain amount of carbon dioxide, a gas which is composed on carbon and oxygene. But there are three different kinds of carbon, which differ in a way that all are still true carbon, but their atomic structure is different. Two are called C12 and C13, carbon with an atomic weight of twelve or thirteen respectively - which are the normal and stable carbon - and the last one is C14, an isotope of carbon, which is subject to a very slow and harmless radioactive decay. This decay results in radiation and a stable isotope. So if you take some carbon with C12, C13 and C14 and wait long enough, you will only have C12 and C13 left.
The dating using any kind of isotopes is based on determining the ratio between stable isotopes and non-stable isotopes. If we know where we started, and if we know the half-life (amount of time it takes for half of the isotopes to break down) we can compute the necessary time to reach todays state. This is the theory. Now lets have a short look at archaeological daily life.
Live on earth is based on carbon, which is gathered from food or from the air. The carbon dioxide is reduced by plants into carbon and oxygen in a process called photosynthesis. So the plants contain carbon from the atmosphere. In an excavation we often find things which contain some carbon. Charcoal in furnaces or fire places is common, also charcoal used for drawing cave paintings. Other organic remains also contain carbon, like leather, wooden tools, seeds or other food and sometimes bones. So it should be rather easy to find something with carbon in a layer we want to date.
But one second, we do not know one thing: where did this carbon start? Which ratio did we originally have? Probably it was similar to today, with 98.89% C12, 1.11% C13 and 0.0000000001% C14. Unfortunately it is not that easy, but it was possible to make a timetable of this ratio. And so we are able to determine the age of carbon if we measure the amount of C14.
C14 dating is a very usefull dating method with some important drawbacks. It works only if we have some carbon and know its origin. Unfortunately there is a much bigger drawback: the half-life of C14 is only 5730±40 years. So the amount of C14 vanishes rather fast, and if it is too little to be measured exactly, we can not determine the age any more. So this method is only usefull for archaeological dating. It works very well for the last 30,000 years, but becomes more and more inaccurate for older samples.
And there is a last important drawback. We must know the original ratio to calculate the age. It works well if we have a plant, so we know it grew up using carbon dioxide from the air. Speleothems are also often based on carbon dioxide, so it is possible to determine