How archaeologists date historical artefacts

Before modern science, archaeologists used general ways to date objects. 

The simplest way was to compare artefacts with each other. 

If archaeologists knew which items were older or younger, they could arrange the artefacts in order of age. 

This system did not give exact dates. It showed which items were older or younger in relation to others. 

Because this method relies on the relationship between artefacts, it is called relative dating. 

Here are some ways archaeologists tell which objects are older or younger than others:

Stratigraphy is the study of the soil layers where artefacts are found. 

Geologists have observed that layers of soil have different colours. These layers formed at different times. 

A single layer of soil is called a stratum. Multiple layers are called strata. 

This type of relative dating is called stratigraphy. 

This technique works because of the rule of superposition. 

The rule of superposition states that the soil layer closest to the surface is the youngest. Layers below are older. 

Artefacts found below other artefacts are therefore older. 

Typology is another relative dating method. It compares similar objects over time. 

For example, to know which iPhone model is older, we compare different models over the years. 

Archaeologists have noted that object types change gradually over years, decades or centuries. 

We can arrange objects from youngest to oldest based on changes in shape, decoration or style. 

Since this method focuses on object 'type', it is called typology. 

Typology is often used for pottery or ceramics because pots and vases last a long time in the ground. 

Ceramic typology works worldwide because many cultures used specific pottery styles and decorations.

The second type of dating method is called 'absolute dating'. A modern scientific approach allows dating objects by measuring internal elements. 

Absolute dating aims to find an exact age for an object. To do this, researchers measure a chemical or physical element inside the object itself. 

The exact element can vary depending on the object type.

The best-known and most commonly used absolute dating method is radiocarbon dating. 

Radiocarbon dating measures a natural element called carbon-14. Carbon-14 exists in all living things on Earth and is taken in throughout an organism's life. 

After an organism dies, carbon-14 begins to fade. The rate at which it fades is very consistent. 

Half of the carbon-14 remains in the organism’s remains after 5,730 years, which is the 'half-life' of carbon-14. 

Because this rate is reliable, researchers can take a sample from an artifact, measure how much carbon-14 is left, and then work backward to find out when the organism died. 

Radiocarbon dating only works on items that were once alive, so it can date bones, shells, and wood.

Another absolute dating method is dendrochronology. This approach finds an exact date for an object by a method that uses tree ages. 

The Greek word for tree is dendros, so this method is called dendrochronology. 

Scientists saw that when you cut down a tree, you can see rings inside its trunk. 

Each ring represents one year of growth. However, on closer inspection, each ring looks a bit different. 

Some rings are wide, some are narrow, and some have different shades. 

Growth rings show environmental conditions during each year. Changes in rainfall, sunlight, temperature, and soil nutrients create a specific ring. 

All trees in the same region share the same ring patterns. Because each area has a unique pattern, the sequence of years in that area forms a distinct ring pattern. 

When wood is taken from a tree to make buildings, tools, or other wooden items, it keeps this pattern. 

Researchers can compare wood samples and build a timeline for the years in that place. 

Since each ring equals one year, dating wood with dendrochronology can often give the exact year a wooden object was made. 

The last absolute dating method measures radiation from minerals in pottery. 

When clay is heated to make ceramics like pots and vases, the heating changes the minerals in the clay. 

In nature, some minerals like quartz and feldspar slowly collect radiation from the environment over time. 

When these minerals are in heated clay in a kiln, they release this radiation as heat and light. 

By the end of firing, these minerals are emptied of radiation and must collect it again.

Radiation builds up at a steady rate and researchers can find out how long it has been since the pottery was made by measuring how much radiation it has gathered. 

They take a piece of pottery, heat it again, and measure the heat and light released. 

Because it measures heat, which is thermo, and light, which is luminescence, it is named thermoluminescence dating. 

Once the radiation is measured, archaeologists can work backward to estimate when the pottery was originally fired in a kiln.