Environmental Archaeology Crash Course: Flotation

Environmental Supervisor and archaeobotanist Alice Wolff gives us some insight into her work on site:

Today’s blog is going to be a more in-depth look at the environmental archaeology activities here at Bamburgh. Last weekend, we had three open-day sessions where members of the public helped us process some samples through flotation. In this blog, I’m going to break down what exactly we did and what it helps us learn about the site!

A wooden tank labeled “HMS Floaty McFloatface."

What is flotation?

Flotation is a method of processing bulk soil samples using water. Essentially, the different materials in the sample (such as the soil, the rocks/bones/artefacts, and the charred material) have different densities. When you put the whole sample in water, the soil and the artefacts sink while the charred material floats. This allows us to extract fragile and hard-to-see objects such as charred seeds or fish bones that are essential to our understanding of diet and environment at the site but are nearly impossible to excavate in the trench.

Flotation at Bamburgh

After recording pertinent information about the sample – i.e. where it came from in the trench, its volume and weight, what the soil looks like – we dump it into a 500µm mesh that lines the flotation tank.

Two students with their hands in the flot tank.

Next, we raise the water level until it covers the sample completely. We then shut off the water and gently massage the dirt with our hands.

Two pairs of hands submerged in muddy water in a flot tank.

Once most of the soil has fallen through the mesh to the bottom of the tank, we turn on the water again and let it flow through the spout, catching any floating material in a 250µm mesh bag.

A stream of muddy water flowing into a white bag held in place with clips.

In order to conserve water, we use two settling tanks and a pump. This allows us to recycle water and avoid flooding the castle at the same time!

Two students at the flot tank in front of two black bins filled with muddy water.

Once flotation is finished, the heavy fraction of the sample (the bones/rocks/artefacts) is dried in trays while the light fraction bag is hung up on a line indoors out of direct sunlight to dry slowly. After drying, both fractions are sieved, sorted, recorded, and stored in our archive for future researchers to look at! The heavy fraction can be picked over by students, but the light fraction requires the use of a microscope to separate out and identify the charred seeds.

The members of the public only spent a few hours in enviro, but our students spend at least one full day per week doing flotation and/or sorting. On particularly sunny days, enviro is a welcome break from the trench! After spending so much time carefully studying the various materials we find in samples, students return to the trench with a better understanding of what they are digging up and why environmental samples are so important for filling in the picture.

 

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