How do historians know about the exact time of an artifact?
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There are several ways that artefacts are dated but it is rare to date one exactly.
The first thing that archaeologists will do to date and artefact is put it in a rough date category. These time periods are quite broad. They will be the standard ages such as bronze age, iron age, Roman etc. From this the pottery will be sent to specialists who will narrow down the age of the artefacts which will fall into categories like this:
Pottery tends to have dates assigned to it by type. These typologies have been built of many decades of research. They are constantly updated when the pottery is found with other items. Due to the scientific dating methods these dates are fairly accurate which allow us to assign date brackets to work from.
How do we know the age of pottery? Well there are several ways. Commonly this is by association with other artefacts that can be dated specifically especially any charcoal that is directly linked to the pottery such as Cremations inside cremation urns. Residues from cooking etc. there are some methods for dating ceramic objects such as New ceramic dating process unearthed or Thermoluminescence dating.
Only a few of the objects need dating as typologies group the pottery into quite tight groups that look very very similar.
A lot of finds can be dated by associated dating as well. This is where an artefact (or more commonly a group of artefacts) is dated by something precise within one archaeological context. the closer the artefact is to the thing being dated the better. cremation urns as mentioned above are a good example of this as they contain charcoal that was placed within the pot and are left untouched afterwards.
This associated dating can be spread further due to stratigraphy. This is where the soil layers are documented to show which ones came earlier or later. This is a simple logic exercise. Say a posthole can be seen cut into the fill of a ditch the posthole is clearly later and had to have been dug once the ditch was filled up. As we know how long it takes a ditch to fill up in certain environments we know the gap between the two this we can use terminus post quem and terminus anti quem to put a feature with no artefacts into a date range. Obviously the more this is done the more accurate a date can be. For this we use a Harris matrix.
Now there are a few other things that can be done (do also note I haven't even touched on Carbon dating yet). The first of these is finding coins. Coins often have dates on them. Modern ones are the exact date of minting but, say for example Roman coins, have the date of the emperor often dated by their consulship. As these are dated by looking at the written evidence we have we can securely assign a date to these. Due to the changes in emperors being violent and coins being recalled we can often give a good time due to the coin. For example if someone randomly drops a coin and we find it on its own then it is likely to have been 'deposited' at the time of that emperor. If there are coins in a hoard we can assign a date by looking at all of the coins. No coins after a certain date will generally mean that the coin horde was placed there at that date. This means through several periods we can get a good associated date range.
Then there is the big one carbon dating. (here is Wikipedia's run down Radiocarbon dating). Essentially the amount of carbon in the object is counted and the rates of decay are known these dates can then be correlated and give a date range. The more dates you do from a single sample the more accurate the date as always. This means that if pottery is associated with burning frequently then it's date will be more secure. So those cremation urns can be dated pretty closely at this point. there are some issues with carbon dating but the prices are falling which means that more dates can be done. The larger amount of data giving more accurate results. The reason I have this last is because all of the other methods are used first. As funds are limited trying to identify the most important places to carbon date is important.
Another interesting one I'll stick in is Archaeomagnetic dating this is a method fir dating hearths and kilns. This is incredibly important as the data from this and the carbon dating data can often be done from the same feature allowing us to get a more accurate date. then you have to realise that pottery is fired in these kilns. Pottery known to be from that kiln or kiln site can then be accurately dated.
There are a few more dating methods but essentially using various methods the finds are grouped. Those groups are then dated by whatever available method we have. The more dating we can get for one site or one group the more accurate it is which then affects the associated dating. Do remember that dating to a very specific point is almost impossible. There is usually a bracket and combining artefacts and sites with written records of dates can be very difficult. Although have a look at Tephrochronology Tephrochronology and its application: A review a relatively new and interesting dating method.
