Environmental sampling is an important part of many archaeological excavations. Although much of the work is undertaken post-excavation, it can simply be thought of as part of the excavation, but carried out under laboratory conditions.
Why?
During an excavation artefacts and ecofacts will be recovered by hand. However, many of these (particularly the ecofacts) would be impossible to spot by eye. Sampling therefore allows archaeologists to recover small, key pieces of evidence that would otherwise be missed. This is generally ecofacts (such as charred seeds or charcoal) which allow us to learn about the local environmental conditions of the site in the past.
What?
We predominately sample dated or dateable contexts as its no use knowing about the local environmental conditions of a site at a particular time, if we then don’t know when that time is. Of course artefacts are often recovered in environmental bulk samples, so a few sherds of pottery can suddenly date a context and often a sample will be taken in order to fid dateable artefacts (such samples would be dry/wet sieved as opposed to the method described in the How? section).
Generally speaking, archaeologists take a series of bulk samples from a range of feature types in each phase/period of a site in order to characterise them. Sampling is undertaken to help us better understand the site and help us answer our excavation aims.
As sampling is an excavation under laboratory conditions, certain finds 9such as a cremation urn) will be sampled whole to ensure full retrieval.
How?
40 litres of soil from a context (or 100% of the context if it is smaller) will be put into buckets. These are then brought back to the palaeoenvironmental facilities in Bury St Edmonds where they are put through a device called a ‘flotation tank’. Simply put, this is a barrel into which a constant supply of flowing water is pumped.
First, the sample is emptied onto a one millimetre mesh suspended just below the surface of the water. The flow of the water, coupled with manual working, breaks down the soil which passes through the mesh and settles at the bottom of the tank. What is left in the mesh after all the soil is washed away is called the heavy fraction and is basically all the inclusions in the soil that are too heavy to float, such as brick and bone. At the run off point, a 500 micron sieve is placed to catch all the floating material. This is called the light fraction, or flot, and is where the bulk of the palaeobotanical evidence will be found, such as charred seeds and charcoal (though other typical floating items include small bones, and snails). The two separate fractions are then sorted with all relevant artefacts and ecofacts being extracted and assessed.