Microscope

Paisley Caves – a view from the microscope

photo

Working on the microscope

So this is my second year taking part in the Day of Archaeology. It’s great to look back at my post from last year, when I was working as a research associate on the Feeding Stonehenge project – lots of new things have happened since then, including a new position for me as a research fellow at the University of Edinburgh. Since joining Edinburgh I’ve been working on a lot of new and exciting projects, which you can read more about in my blog, Castles and Coprolites. This week I’ve mostly been sat in front of a microscope, analysing thin section samples from the site of Paisley Caves, Oregon USA, directed by Dr Dennis Jenkins. Paisley provides evidence for the earliest dated human occupation in North America, famously in the form of ancient human DNA recovered from coprolites, aka fossilised faeces. The samples I am working on are known as thin section micromorphology samples – perhaps not as well known as animal bones and charred plant remains, thin section samples investigate the actual sediments in which archaeological materials are found.

The way in which sediments are deposited can actually tell us a great deal about the environment and human activity in the past, and are also useful in helping interpret the artefacts that are found within the sediment. For example looking at soils and sediments under the microscope, we can tell whether they were deposited by wind or water action, or whether they were trampled by humans or animals. With the Paisley samples we are looking at the formation processes in the cave environment, to see whether this can help understand the activities that were occuring in the cave.

Paisley 3

Thin section slide of sediments from Paisley caves. Photo by Julie Boreham, Earthslides.com

Thin section micromorphology is quite a specialised technique, and requires laboratory processing. We cut out blocks of sediment from profiles during excavation, wrap them very carefully to avoid any disturbance, then take them to the lab where they are set in resin and cut into slides for viewing under the microscope. I have been working with Earthslides to process the Paisley samples, and we will be presenting a poster exhibiton of the thin sections at the European Association of Archaeologists conference this September.

Friday is actually the day I do the least.

In a typical week, I am shattered by Friday morning. I’ve spent the week staring down a microscope, collected a lot images, and/or passed a couple afternoons staring at a green and white screen of the SEM. I have learned that it’s not good to spend too much time in the microscale. It’s important to remember that while the work I am doing is very detailed and requires a good bit of microanalysis, there also is a wider cultural component that is important on the macroscale. This morning, I have been reading some archaeological theory about the spread of ancient technology, operational chains, and the cultural importance of material production. This is the part I enjoy the least, but, as I’ve been reminded on several occasions, is just as important as the analytical details. In fact, there is little reason to even begin the analysis without a Bigger Picture reason for doing so.

 

Figure 1. This is my desk in its clean state.  That stack of papers is what I’ve been reading for the last 2 weeks. Usually there is a stone cold half-cup of coffee sitting there as well.

 

 

 

Actually, this is where the real work is done. All of the sample analysis and data collection would mean nothing if I didn’t spend many more hours pouring over the results, trying to tease out some sort of meaning from black and white images and long series of numbers. This is the part where I get to make charts! And data tables! This is the part I enjoy most.

In the afternoon, I was able to escape back to the lab and do some initial prep work for the petrographic work I am planning for next week. Here is the rig I am using:

 

Figure 2. I’m using a polarizing microscope with an attached video camera. There are four lenses from 4X to 40X, and the homogeneity of the concrete samples and variable particle sizes of the inclusions  mean that all lenses are useful at some point or another.

 

Before starting a quantitative point count, I take some time to get the know the sample. That involves having a look to see what the components are, if there is anything unusual about the sample, any unexpected components, etc. I take notes on a high resolution scan of the thin section and capture images of what I’m seeing down the eyepiece (you can sort of see an image on the screen there). For these initial examinations I usually work with the 4X objective lens and move up in magnification as needed. When I feel like I have a good idea what I’m dealing with, I set up the parameters I’ll need for the point count.

Figure 3. Here, I’ve mounted a computerized microstepper to the microscope stage, which moves the slide a specific distance. This way, I can be sure that the step distance between each is point is standardized and precise. The microstepper and PETROG software have been generously loaned to me for the duration of my research by the manufacturers.

 

At each point I identify and record what I am seeing with the 10X objective lens. The software allows me to record the data in a database for subsequent analysis. (I also keep paper records because the computer I am using is rather old and prone to crashes) What I’ve done this afternoon is program in the area of the thin section I want to count, the number of points I am aiming for (500-600), and the step distance (1 mm). Now I’m all set to do the full point count on Monday, which should take me about 6 hours.

When I’m finished with the microscope work, I will have generated a good description of the concrete fabric, recorded the size and nature of voids and cracks, and quantified the types of aggregate. I’ll also have a good idea which areas I want to target with finer detailed analysis with the SEM. All of this will (hopefully!) go a long way toward determining the original mix designs of the concrete, which I will compare across the site.

So that’s a typical Friday for me in the lab. A bit of reading, a bit of playing with data, a bit of exploration, and some planning for the week ahead. I have a few more samples to get through this summer and then I’ll be collecting more samples at the end of the summer. I am hoping to have everything analyzed by this time next year and to have my thesis submitted by the end of next fall. Fingers perpetually crossed.