Archaeologists use a variety of techniques to date the artefacts based on the material from which the objects are fashioned and the chemicals they contain. Sometimes the context in which the artefact was found will help date the object or written documents will give you an idea of when this occurred. Read about some of the dating methods below.
Three Age System: Prehistory is divided in three “ages” based on the material used to fabricate tools. Everybody knows about the “Stone Age”, the time where people made tools and weapons from stone, but there is also the “Bronze Age” and the “Iron Age.” This division is somewhat arbitrary because each age is different from country to country based on their technological advances. Some countries, even skip an age!
Dendrochronology: Also called “tree ring dating”, dendrochronology is the method by which wooden artefacts and buildings can be dated. It uses the variations in the thickness of the annual growth rings of a tree to date an artefact. Each year, a tree will have a new “ring” that will vary in size from previous rings based on fluctuation in climate. Heavy annual rainfall will make the ring much larger, a year of drought will result in thinner rings. Take a look at the cross-section of a log and you will see these rings. The time span of dendrochronology is from the present to 332 BC in the American Southwest and a bit earlier than 3000 BC in Europe.
Radiocarbon Dating (C14): Carbon-14 dating is used to determine the age of living things on earth and is based on the radioactive decay of the Carbon-14 isotope. Each living creature/organism absorbs Carbon-14; when this creature/organism dies, it decays at a gradual and steady rate. Radiocarbon dating measures the amount of this C14 isotope and indicates how much time has elapsed since the creature/organism died. Therefore, you can date bones (animal or human), charcoal from a burnt piece a wood found in a hearth, or timber used in construction. You cannot date rocks as they are not living things and do not produce carbon. The time span of radiocarbon dating is about 50, 000 years ago to AD 1500.
Thermoluminescence: This method is used to date pottery and other objects made of fired clay. It can be used to date burnt flints as well. It is a dating method that relies on the effect of low-level radioactivity within the pottery. Electrons trapped inside the clay are released as light when the clay pot is fired in an kiln. In a laboratory, scientists will re-heat this clay pot and they will measure the amount of light emitted in order to determine the time that has elapsed since this pottery was first fired.
Potassium Argon Dating: Scientists will use a spectrometer to measure the concentration of argon 40 that has accumulated since a volcanic rock has formed. Argon 40 is released when radioactive potassium (40K atoms), which is contained in potassium (in its natural form), decays. For 100 atoms of 40K that decay, 11% becomes argon 40. This method is very useful to prehistoric archaeology, making it possible to date sites with lava flows. With potassium argon dating, it is possible to date a mineral or rock of 100,000 years or older.
Historical Records: Written documents can be very useful to date a particular event or to sort out a sequence of events. Writing was invented around 3200 BC in Mesopotamia and soon spread to Egypt. However, many countries and regions of the world did not have their own writing system until much later (some as late as the 19th century)! Earlier documents are much harder to date because the civilisations from which they come did not use the same methods of recording time (they used different calendars) and these documents must be correlated with other findings. The time span of historical records is from the invention of writing until now.