How do scientists determine the age of ancient artifacts?
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They use a variety of methods because it’s only with redundancy that they can gain confidence.
One of the most important methods is “context.” Were was it found? In what state was it found? What was around it when it was found? If the context says “this belongs to *this* already dated ruin/find” then there ya go.
Another fantastic method, though one that is not always applicable, is radiometric dating, usually carbon dating. When cosmic rays hit the atmosphere it converts some of the nitrogen into an unstable form of carbon called Carbon-14. Over a predictable period of time it decays into the much more stable Carbon-12 or Carbon-13 isotopes. Because it’s constantly being created and constantly decaying, the atmosphere has a relatively consistent amount of carbon-14 in it. As plants grow they take carbon from the atmosphere and incorporate it into their structure, meaning that living plants have a fairly consistent amount of carbon-14 in them. But when they die they stop incorporating new carbon-14 into their structure and the clock starts (this is also true for the animals that eat those plants and the animals that eat those animals). As more and more time passes the amount of carbon-14 gets lower and lower. By measuring the percent of carbon-14 they can determine, to a reasonable degree of accuracy, when the organism died. There are limitations, of course, which is why they use a multitude of methods, but it does mean that if they find a tool that incorporates organic elements (such as a wooden handle) then they can radiocarbon date the handle and narrow down the range for when that handle was made – or at least when the wood used to make it died.
There’s also certain key indicators that can be used. For example, if the thing is buried in a soil layer that includes markers from certain events of a known date — such as a flood, a volcanic eruption, things like that — and the context of the find suggests that it wasn’t buried there later, then we have a pretty good idea of when that thing was buried because we already know when that particular flood happened.
There’s lots and lots of different methods used that have varying degrees of accuracy. By using many methods they can increase the confidence of the age.
Carbon dating is a process that merges chemistry and physics to determine the age of organic (once living) material. For example wood, or bone.
Living organisms absorb carbon 14, once they die they immediately stop absorbing carbon 14.
Carbon 14 will decay at a very predictable rate, and turn into other isotopes. By measuring the remains carbon 14 and other isotopes we can measure with great accuracy when those organic materials died (for example when a tree was cut down to be carved into some object).
This works to about 60,000 years.
We can use similar methods to measure other types of radioactive decay for inorganic material as well. For example if we find a Dino bone in volcanic ash many millions of years old, we can measure the devay of the materials in the ashe to deitmine its age. Radioactive materials decay at very predictable rates and turn into other materials so by measuring the material we can determine dates.
Another way is by a contextual study. So for example where was the object found?
Was it found at a specific depth in sedimentary Rock, which will allow us to understand against other known objects found in that same layer of rock and dirt.
We can study for example the writing or symbols on a Roman coin to understand against the historical record to know under which emperor it was minted.
There is a technique called thermoluminescence which is used for items like pottery. Some crystalline materials – such as the minerals in clay can trap energy from radiation. When the clay is fired, all the trapped energy is released as light. Then, when the item is abandoned in an archaeological site it begins to trap more energy from the environment, the longer it is buried, the more energy is stored in the pottery. If the item is then recovered and heated, it releases light – the longer it has been buried, the more light it emits. The age of the item can then be calculated.
For items made of wood, dendrochronology is possible. Trees lay down rings of growth each year, the thickness of the ring depends on how well the tree grew in a given year. As the weather changes from year to year, the rings produce a unique pattern of growth. It’s possible to compare the pattern of rings in a wooden object with reference samples of known age to determine their age. For places like Western Europe where there are detailed records of tree growth going back a few thousand years, this technique can be used to date objects with incredible accuracy.
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