Degrees of separation – a brief guide to the flotation process

The recovery of large artefacts, such as stone tools and pottery, is just one part of the excavation process. What about the tiny items and organic material that are a vital part of the Ness of Brodgar story?

For that, we turn to flotation.

During excavation, soil samples are taken from each context across the site, such as new layers, pits or other diagnostic features. For greater detail on an area, several samples might be taken from different gridded plots of one context, e.g. a floor.

These are transferred to the Archaeology Institute UHI, in Kirkwall, where environmental archaeologist Cecily Webster runs each sample through a flotation unit.

Flotation processing. (Cecily Webster)
Flotation processing. (📷 Cecily Webster)
Cecily and Travis sieving the flotation tank residue. (Sigurd Towrie)
Cecily and Travis sieving the flotation tank residue. (📷 Sigurd Towrie)

The soil sample is placed in a fine gauze inside a tank.

The tank is filled with water and the soil carefully broken up, releasing organic materials like seeds and charcoal, which float to the top of the tank and flow over the lip of the tank to be collected in a sieve.

The dried flot sample is then passed to the plant experts for study, interpretation and, sometimes, radiocarbon dating.

The unfloating residue left at the bottom of the tank, including rock, bone, pottery and often many tiny finds, is then removed and dried.

This is then painstakingly inspected, using a series of progressively finer sieves, revealing tiny pieces of pottery, charcoal, flint, bone, burnt daub and more.

Each type of find is passed on to the experts in that field for further study.

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