Wednesday, May 22, 2013

Let's do flotation!

On every archaeology project we collect samples of dirt to take back to the laboratory. These can include small samples which are processed for pollen, useful for identifying plants to look at the environment or presence of domesticated plants. Soil from canals sediments can be processed and examined for the presence of ostracodes- tiny molluscs that live in very specific types of water (hot vs cold, fast-moving vs slow-moving). And other soil samples that geomorphologists study to determine environmental conditions or

The primary soil sample we collect is for flotation. We are supposed to bring back six liters of soil from features.

Sample from our field school dig.

I am busy processing the samples. Normally we use our fancy flotation machine, but I have to save the heavy fraction, so I have to use the bucket method.

I fill out new tags and a line on a data form first. Then cut the bag open and pour it into a red bucket that has lines telling how many liters, I write down the sample size on the form and bags.

In the bucket.

In a 5-gallon "Homer" bucket from Home Depot I put about five inches of water in and then slowly pour the soil in.

This gets a little dusty.

I stick the hose in to agitate the water.

Charcoal floating.

I use a sieve to skim off all of the charred plant materials- bits of charcoal, corn cobs, and other plant tissues.

Light fraction.

I tap the sieve onto newspaper and keep repeating, stirring the soil in the bucket to make more charcoal float to the top. When I stop getting bits, I pour most of the water through the sieve and finish with the light fraction.

I then take the rest of the muddy dirt and dump it onto a screen with window mesh and hose out the bucket. I carefully wash off the dirt and then dump the rest of the material onto a screen as well.

Heavy fraction.

There are a lot of artifacts and animal bone fragments in the midden dirt.

The green piece is olive ware from Spain. Also Native American ceramics and animal bone.

Both samples are dried, re-packaged, and eventually analyzed. The light fraction will be looked at by a ethnobotanist who identified the wood, seeds, and plant tissues. You can learn about the local environment, agriculture, and wild plant gathering this way, and examine how these change through time.


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