Lab Work in the Field, 2017
Following the sad departure of Debora, Tom set to work processing the dry sieve and flotation sediments collected from La Milpa. Tom and his students only focused on three of the units (two on the terrace and one in front of the terrace) due to the sheer volume of sediments that were collected and the limited amount of time remaining in camp. 93.7 liters of soil were dry sieved using nested geological sieves with 4 millimeter, 2 millimeter, 1millimeter, 0.425 millimeter sized mesh. 87.6 liters were floated using a float tec machine for units BV, BM, and BQ.
Dry Sieving and Flotation
Dry Sieving
The volume of each sample was measured using a liquid measuring cup, recorded, and transferred to a plastic bucket. Approximately 5 liters of sediment were contained in each sample. Once measured, small amounts of sediment were poured into the top sieve and gently brushed using a large paintbrush. The brushing caused the soil to spread out with the smaller components passing through each successive mesh starting with the 4mm. The entire nested sieve collected was gently rocked from side to side to also facilitate size separation. After enough material had built up on the mesh, the materials from each sieve fraction were then placed into separate labeled ziplock bags. Fresh sediments were placed on the 4mm and the whole process is repeated until all of the sediment from that sample was separated. Dry sieving takes approximately one hour to complete per sample from start to finish. The weight and volume of each dry sieved fraction (4mm, 2mm, 1mm, and 0.425mm fraction) from each sample was then measured and recorded. Samples were then placed back into storage for further analysis.
Flotation
Approximately five liters of sediment sample was measured and recorded using a measuring cup and then transferred to a 5 gallon plastic bucket. Water and one third of a box of baking soda was added to the plastic bucket with the sediment sample. This solution was stirred with a wooden dowling rod and let sit for 15 minutes. The baking soda was added to the soil sample because it helps to break up the clays in the soil that clump together and prevent the recovery of botanical remains. After 15 minutes had passed, the sample was then gently poured into the flotation machine. The buoyancy of the organic materials, such as archaeological seeds, caused them to float and be carried over the sluiceway into the cheesecloth. This cheesecloth is called the “light fraction” and is where we will find the majority of archaeobotanical remains. The light fraction is gently tied with string and hung to dry on a clothes line.
The other materials sank to the bottom of the machine where they were caught in a 1/8th inch mesh. This is referred to as the “heavy fraction” and contains anything that does not float. The majority of materials were rocks but this is where you can find lithics, ceramics, and bone. Some botanical materials do not float which is why it is also important to examine the heavy fraction. The heavy fraction was removed from the bottom of the float machine box and transferred to a clean mesh cloth where it was laid out to dry on the ground. Flotation takes about 30 minutes per sample from start to finish.
Once dry, the heavy fraction was transferred to labeled ziplock bags and placed in storage. The light fractions were sieved through 4mm, 2mm, 1mm, and 0.425mm meshes using the same technique that was applied to the dry sieving. The volume and weight of each size fraction for each sample was taken. All of light fraction materials that were processed this summer were set aside for export back to the United States.
2 comments
3 comments
4 millimeter, 2 millimeter, 1millimeter, 0.425 millimeter sized mesh