NDAS Flotation Tank

Following the NDAS digs at Holworthy in 2003/04, we ended up with several bags of bulk sample material from post holes and gullies etc. The purpose was to extract any organic materials from the fill of these various holes. The Society had only a couple of sieves, which were not suitable to carry out the complete sorting of the samples. What we needed was some way of floating organic matter out. Having read descriptions of something called a ‘flotation tank’ I bravely (or foolishly) volunteered to have a go at constructing one. A couple of basic diagrams were found in leaflets and library books, but alas, no dimensions or details. The item most often used was a 40 gallon steel drum. This at least pointed me in the right direction, but these drums are not found lying about, nor do they grow on trees.

At this time, the North Devon District Council began delivering ‘Wheely Bins’ to our area. That’s it, I thought, forget the steel drum, a wheely bin would be ideal, light in weight, and with wheels and a handle for pushing around. A phone call to the N.D.D.C. was very decisive, they cost over £200 and are definitely not for giving away, even to a highly respected NDAS. So it was back to a steel drum.

Following a bit of prospecting in the boatyard at Watermouth harbour, I was put in touch with Andy Clark at ‘Alpharma’, a Chemical Company in Barnstaple. A steel drum was kindly donated to the Society, along with a plastic drum and a polythene tank, the latter very useful for a number of purposes, especially if you have a three stone baby to bath!

After a bit of cutting, drilling, bending and hammering and a lot of time spent thinking, something resembling the sketches I had seen was produced, and with a splash of paint here and there it was completed (See diagram). The big day arrived when six members of NDAS turned up at our house to have a go. After processing two sample bags we already had good results, but the method we used was somewhat inefficient, basically because of lack of experience. After some trial and error however, we arrived at what appears to be the most effective method of operation: the drum is filled almost to the top with water, the supply coming in at the bottom with the overflow through a spout near the top.

The sample is passed through a 5mm mesh sieve whilst being immersed just below the surface, which basically washes the sample, the silt dispersing in the water and out through the spout, the coarsest material being caught in the sieve and and finer non-floating material being caught on a 1mm mesh suspended about one third of the way down the tank, while any sand or silt not washed away falls through this mesh and is deposited at the bottom of the tank to bedrained off later. Hopefully floatable material (charcoal and carbonised items) will remain on the surface, to be scooped off using a kitchen sieve. Any floating material escaping through the overflow spout is caught in a sieve suspended on the outside of the tank below the spout, this sieve being lined with 1mm mesh. The material caught on the suspended mesh inside the tank may contain organic matter that has failed to float because waterlogged. This is therefore lifted out, dried and immersed in water again to float out organic matter. All organic matter recovered is dried and later sorted using hand lens, microscope and forceps.

By these methods a good quantity of organic matter was obtained from the samples including many small fragments of charcoal plus a few carbonised grains of cereal, possibly barley or wheat (expert analysis will tell) as well as other seeds and plant fragments. Among these was what appears to be a ‘glume base’, this is the little thingummy that holds the cereal grains on to the ear of whatever plant it is.

One lesson learned at the first session:
February is not the best time for flotation! Though after ten minutes with your hands in cold water, fingers go numb so you don’t feel the cold anyway. After a few modifications to the tank and the donation of very useful nylon mesh from ‘Alpharma’ a second session was carried out in the middle of March. The water in the tank was more tolerable, but this time the heavens opened up and we ended up being soaked. Nevertheless, this second attempt was a great success: we found cereal grains, seeds and possibly very small berries, and also possible fragments of hazelnut shell. The third session was carried out in late March on a day of perfect weather with nine NDAS members and our grandson taking part. Two sample bags were processed, and we found over 70 whole or parts of seeds in an extremely small portion of the samples as well as 3 fragments of pottery and what might be a piece of clay loom weight.We had an excellent day with a good exchange of ideas, opinions and plenty of chit -chat.

What has been learnt from these three sessions?
Apart from the general method of separating the material the most important thing by far, has got to be the care needed to avoid mixing up various bits and pieces of different sample bags. The only way to avoid this confusion is to have just one sample bag to hand, the one you are working on. Keep all others under lock and key until the one completed is bagged, labelled and locked away. If the chance of confusion is allowed to arise, then it definitely will!