Today we have been getting some of the soil samples ready for study. Or, to be more strictly honest, Megan and Vanessa have been up in the lab doing the sample processing while I was either swanning about with the camera taking the pictures for this post or downstairs in my office doing completely unrelated things. The samples they have been dealing with come from the bottom of various of the pits, postholes and ditches. We took 5 litre buckets of soil in all these cases to allow us to look for a range of different kinds of evidence. To divide them up into their constituent parts we use a device called a flotation tank. This should not be confused with the thing 70s Californians used to bob about in to get back to the womb. In environmental archaeology a flotation tank looks something like this.
The flotation tank in all its custom-built glory. As you can see it is a modified rainwater butt wearing a bridal veil and plumbed into the mains water supply. Water goes in at the bottom and rises through the tank to flow through the soil sample which is spread out on the net. Then it flows out of a spout at the back and through a nested set of very fine sieves. The water then drains through a set of traps to catch all the silt and keep mud from blocking up the University’s drains. We built it ourselves (except the silt trap which we had to buy) about three years ago after a big shopping trip to Berry’s of Leyland and we are inordinately proud of our workmanship. As well as the rainwater butt and the net curtain, which is semi-disposable, there is a steel grill, 3 m of plastic piping, about a kilometre of duck tape and two whole tubes of bathroom sealant hidden away inside it.
Each soil sample is divided into four parts during the process. First, about a handful (precision is everything in archaeology after all) is taken out and put into a separate sample bag. This will go through a completely different set of processes to extract any surviving fossil pollen. This microscopic evidence for the past environment will need a post to itself to explain – and probably someone other than me to write the post.
The second stage is that all the rest of the sample is tipped into the top of the flotation tank. Here Vanessa is adding a sample, in this case from the bottom of one of the postholes, on top of the net. Under the water is the metal grill which supports the weight and under that is the water source. Note the stylish sequins on the net, which are an essential part of the whole process.
Once the water is turned on then the floatation process can begin. Megan is floating a sample by gently agitating it with her fingers. Macroscopic plant remains and charcoal which have been preserved in the soil float to the top (we hope) and are carried by the flow of the water out of the spout and into the coarse and fine sieves below. Stones and other heavy particles remain behind in the net and the rest of the sediment sinks down to the base of the tank: where it remains out of sight and mind until the sad day about once a year when the whole thing fills up and has to be drained down, emptied and cleaned out.
From this process we get three more fractions to add to the pollen sample taken at the beginning. From the two sieves we get a ‘coarse flot’ and a ‘fine flot’ – i.e. small and very small bits of preserved organic material that need to be parcelled up in lab towel and hung up to dry. Once they are dry then the will be examined under a microscope by an archaeobotanist and identified. The last fraction is the ‘flot residue’. All the stones and heavy particles are removed from the net curtain and put on more lab towel in trays to dry – we have a drying rack to aid this process – before being hand sorted. This involves a lot of patience, sharp eyes and tweezers. The reward is usually embarrassingly large fragments of worked stone, bone and sometimes even pottery.