Archaeology is a meticulous science. We try to collect the most information possible at a site, especially during excavation where we destroy some evidence as we collect others. One of the techniques that archaeologists use to collect some of the smallest samples is called flotation. There are many reasons to take samples and many strategies on how one samples, but the primary reason that archaeologists employ flotation is that it allows for the collection of the most miniscule datasets that could not be recovered in any other way. Most commonly, this is the best way to collect any remains of food production through seeds, small bones, and charcoal. This method can also recover micro-flakes from stool tool making, beads, miniscule charcoal bits that can help in dating or establishing a fire history for the area, and all the other tiny artifacts that would otherwise be lost in traditional excavation methods.
In this methodology, a sample of soil of consistent volume is taken from the excavation. This sample is then placed in a container of clean water. In some places, archaeologists have constructed specialty tubs to speed up this next step of removing the dirt from the micro-samples. In these wonderful machines, the sample is put in a mesh container that fits into a large tub of clean water. Agitators (tubes with holes along the length that shoot water upwards like a dishwasher) on the bottom of the tub spin and gently clean the artifacts. The heavy artifacts, intrinsically called the "heavy fraction", sink to the bottom of the interior basket while the lightest materials, called the "light fraction", float to the top. The light fraction is skimmed off the top of the water and caught in a light fabric square to dry. The heavy fraction is taken from the water in the interior mesh basket and the micro-artifacts let out to dry. Although having an agitator helps speed up the flotation process, sometimes the dirt can still be hard to break up, especially in soils heavy in clay. Depending on the size of the tub, large sample sizes can be processed at once. This is convenient for large, ongoing projects that may need to process many samples of large volume. However, the tubs do cost a fair amount of money to construct, and a good supply of water in order to run (which also costs money).
After the samples have dried, the micro-artifacts are often sent to specialists for analysis and further processing. A lithics (stone tools) expert may look at the micro-debitage; a paleo-botanist (floral analyst) may look at the seeds and plant remains; a faunal expert may look at any tiny bones or scales recovered. These experts may be able to look at seasonality, food consumption, tool production, and where different materials that peoples in the past were using. In the next post, I'll tell you how we used flotation techniques at Shabbona Grove and what it tells us about the past!
