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Carbon Dating

Radioactive Carbon14 as a method to tell how old something is

Just about everyone has heard of carbon dating, but not many people know how it works. So, I'll explain.

Carbon dating is a scientific method to tell how old things are: Ancient artifacts made of organic materials such as wood, cloth, paper, bone, etc.

Here's how it works:

* Carbon in the environment is mostly Carbon12 with a small amount of stable Carbon13, but there's some which is Carbon14. Carbon14 is a naturally-occurring radioactive isotope which is formed because cosmic rays bombard the Earth's upper atmosphere.

* The different types of carbon are all freely mixed-up and so the CO2 at ground level has the same average amount of radioactive Carbon14.

* When plants and trees grow, they too have that standard amount of Carbon14 in them.

* When trees are cut down and turned into furniture etc, the amount of Carbon14 doesn't get recycled around the air anymore and becomes fixed in the objects.

* Carbon14 is a naturally-occurring radioactive type of Carbon which has a radioactive half-life or 5,700 years. That is: Every 5,700 years the amount of Carbon14 in the artifacts goes down to half what it was. This works for wooden items such as furniture, cloth items (which are made from natural fibres), books made of paper, etc.

Or, to put it another way, here's a table of antiquity age and amount of Carbon14...

Artifact Age Amount of Carbon14
as a percent of normal
Brand New


5,700 years old


11,400 years old


17,100 years old


22,800 years old


So, as you can see, if you measure the amount of radioactive Carbon14 in an ancient artifact, it gives a good estimation of how old it is.

Even an object that's 100 years old has only 99% of the amount of Carbon14 that a brand new item has. It's a sliding scale of antiquity versus amount of radioactive carbon.

Also, carbon dating isn't purely based on theory. The scale is calibrated versus known historical events. For example, we know the Battle of Hastings took place in 1066, so items which are from the battle and are stored in museums can be checked against the date to make sure it's correct. There are trees with rings going back about eight thousand years, and so wood samples can be compared with the tree-ring age and the carbon-date age matching up.

Admittedly, carbon dating is pretty tough luck for people wishing to believe some "sacred" artifact is contemporary with some legendary event, if it turns out to be the wrong age. That is sad, but surely it's better to know it's a fake than to believe it's genuine when the evidence shows to the contrary?

Carbon dating works best for things that contain carbon, as might be expected. Things that are organic, ie they were alive and then they died and were turned into historical pieces, are good on this.

Gold can't be carbon dated because it doesn't contain any carbon. Dinosaur bones can't be dated using carbon because they don't contain carbon either. (They have been turned to stone). However, dinosaur bones can be dated by considering the rock strata layers they are in. The bones are in sedimentary rock, and then if some volcanic rock has fallen on top of that, it's possible to date the volcanic rock by other radioactive isotopes in the rock, and it's a reasonable assumption that strata arrive on top of other strata, giving an order to the arrangement, with the more recent stuff on top.

There are other isotopes of various elements which can be used in a similar style to carbon dating, to give reasonable estimates of the age of things that are many millions of years old. The trick is to use isotopes with half-lives that are long enough to be within an order of magnitude of the age range being measured.

You can't carbon-date your grandma while she's alive, but interestingly you can tell how old some people are by the fact that they lived through the era when nuclear tests were being conducted. These tests have left tiny amounts of radioactive material in people's bodies.

Here's an allegory of understanding how carbon dating works: Suppose you went into a room where someone had poured out a glass of lemonade earlier that day and abandoned it. You could make a reasonable estimate of how long ago they poured out the lemonade by how flat it now was. The lemonade would have had 100% of the normal amount of fizz when it was first poured into the glass, and then after half an hour it would have less, and after an hour even less than that. So, by counting how many bubbles were popping in the lemonade per minute now, you could determine how long ago the lemonade was poured.

The difference with carbon dating is that it's atomic and doesn't depend on weather, temperature, chemical reactions, etc. Carbon14 fizzes away at a scientifically known rate, such that every 5,700 years there's half what there was before. Also see half life

Not to be confused with other types of dating , where age is only one of many factors to consider!