Soil flotation: A DIY approach

Soil flotation: A DIY approach

Post-excavation analysis is the underappreciated sister of excavation–less glamorous and exciting–but absolutely fundamental to making sense of a site. Here at the Achill Field School we like to give our students a taste of all aspects of the archaeological process, so this morning we rolled up our sleeves for some soil flotation. Soil flotation is the first step in processing soil for environmental and other analyses. It works by separating out small light buoyant components –such as charcoal or burnt seeds–from the rest of the soil.

There are different ways of doing soil flotation ranging from the basic ‘bucket technique’ to the use of special custom-made flot tanks. The ‘bucket technique’ works perfectly well, but can be quite slow, so this year as we have a big backlog of samples we decided to build a d.i.y. flotation tank. The system is fairly simple. It consists of a big flexi-bucket with an inserted plastic spot. We used a recycled plastic bottle for this but you could also use a short length of piping. We attached it with duct tape and glue. The tank is filled up with water using a hose and the soil is added and agitated (using hands) to release the flot, which flows through the spout and into the three stacked environmental grade soil sieves. We have a bucket under the sieves to catch the overflowing water. In the picture on the left you can see a piece of muslin cloth pinned inside the tank. This is to collect the retent–which can contain important small artefacts and ecofacts like bone or pottery. There are many guides to building a d.i.y. flot tank on various archaeology blogs. I’ve posted a few links below.

Today we’re flotting samples from a prehistoric site on Slievemore known as the ‘Cromlech Tumulus’. The site is a Middle Bronze Age structure and was extensively sampled during excavation. The interior of the site was covered in a dense profusion of pits and post holes, some of which contained some significant deposits included cremated (human?) bone. We’re also on the look out for charcoal for wood identification and/or carbon dating.Once the soil samples have been flotted the content of the three graduated sieves can be bagged and dried for later analysis.

Soil flotation in action

The muslin cloth can be used to catch the retent.


Charcoal, seeds, and other light ecofacts are trapped in the fine gague sieves.


Retent can be collected in the muslin. Depending on the deposit this might contain small bone, pottery, or other artefacts.

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