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Toothbrushes & microscopes: tools for studying tools

One of the things that is wonderful about archaeology is the breadth of different fields of enquiry it spans, and the fact it is both a science and humanities subject. Trying to uncover ancient human behaviour and experience is about knowing our predecessors better as well as our present (and future) selves. As an archaeologist I try to follow a scientific approach, thinking carefully about how I collect, analyse and interpret my data. And who can not be excited about all the fabulous techniques we now have to examine and understand our human past through the materials we left behind?

However, even within archaeology there's a little bit of "science snobbery", with certain of those who receive BScs/ MScs degrees (i.e. Bachelor/Masters of Science) holding themselves slightly superior to those with BA/MAs (Bachelor of Arts) like myself. Whether it's only in jest or not, I sometimes wished I'd followed a specialism that's a bit more "white-coat", despite knowing full-well that good science is about your attitude and approach much more than if you wear a lab-coat and get to play with large rooms full of expensive machines.

Digital calipers, used for measuring various aspects of stone tools. These are carbon fibre, better than traditional metal ones as they don't scratch the lithics. Also a mm and cm scale used for photographs.

Until my postdoc, I didn't have much in the way of "sexy science kit" in the area I chose to work in- the tools I use to study lithics are pretty old-fashioned- primarily measuring calipers, for recording lots of different metrical values from the obvious like maximum length, to the more arcane, such as the lengths of scars (previous removals) on the back of a flake. Even if the tool I use is similar to that wielded by lithics specialists decades ago, the types of data I collect are quite different: as well as much more detailed measurements, I also record many other variables that tell me about how the artefacts were made, such as different technological categories describing parts of the tools.

Yes, even exciting science involves tedious chores! In this case, having to hand-wash all my lithics which were collected from surface survey. Depending on size of the stones and tenacity of dirt, you can use a toothbrush or handbrush, but warm water is ALWAYS recommended. Doesn't make it go faster, but is much more civilized!
 Beyond this however, I've not expanded into the many other "sciencey" methods for analysing stone tools, such as studying use-wear (macro- and micro-polishes that occur in specific ways according to a tool's use) or chemical sourcing. I've always fancied getting a bit more familiar with microscopes, and as part of the career development aspect of my Marie Curie postdoc, studying types of flint up close and personal has given me the chance to do that.

As I've explained in previous posts, part of the project is about looking at the use of one particular stone type, silcrete, from a large open-air source in the Massif Central region. But we also want to know if other stone types were brought to the site, and therefore being able to identify the silcrete from "exotic" stones imported to the site is important. Luckily (as I only have a 2 year postdoc!) much previous work has been done on the geology of the region, as well as this tricky stuff silcrete. Because IDing stone isn't always a simple thing- even before you take into account all the chemical and physical changes that can happen to a stone tool (it erodes off an outcrop, falls down a slope, into a river, is picked up, knapped, dropped and re-buried), some rocks are highly variable to begin with. Silcrete is one of these, and in the case of the material at St Pierre-Eynac (the site I'm working on), it's not always even the same texture or colour. This is because of its complex formation (see here), but it does mean you need to 'get your eye in', and to be really sure, you have to get inside it- well, almost.

Paul Fernandes' (very expensive!) binocular microscope, for examining the structure of stone samples.
As part of my training, I spent a few days over the summer with flint and silex (siliceous stones) expert Paul Fernandes, who works with a French commercial archaeological unit, but is also involved in many research projects. His specialism is in studying how stone -specifically silex- is transformed throughout its life, from formation to erosion to human use and discard. This is done by studying samples from many different contexts microscopically. For my first time working with Paul however, I was there to learn the basics- what different types of flints as well as silcrete look like through microscopes, and how we identify and record different characteristics.
I had a great time learning with Paul who was really patient with my many questions (we managed to get by in a mix of French and English), and I discovered just how complex and diverse flints are when you look at them REALLY closely. So here's a few photos showing the methods he uses to examine lithic samples.

First up, this is the (very expensive!) binocular microscope, showing two eye-pieces, and the black tubes are two positionable lights that curve down to the samples. The pieces of stone themselves are in a plastic box, filled with water, as this shows the colours and surface much better than if they are dry. There's also a water-spray bottle out of shot for re-moistening the stone surface if it's sticking about the level of the water, and the paintbrush is there to gently move away air-bubbles (actually quite tricky sometimes!).

Binocular microscope and stone sample, with paintbrush for bubble elimination.
One can look through the eyepieces to initially get a view of the sample, but we actually then use a digital camera attached to the microscope to look at it on-screen. This is easier on the eyes (avoiding strain). You can move the sample by hand, and it's shown instantly on screen. Then you use an automatic function within the computer program to take a still image in very high resolution (about 30MB) when you have something interesting in view. You can then adjust things like contrast, add a scale and save the files.

The computer program that allows images from the digital camera attached to the microscope to be displayed and edited.
So, what did we actually look at? This first set of photos below are of the surface of silcrete samples from St Pierre-Eynac, the site I'm working on. Some were pieces I picked up, others date back several years (even to 1962!). In the first picture, the scale is 400 microns, about the same size as a dust mite (1 micron is 0.000001 meter), but in the other images it reduces to 100 microns (equivalent to magnification of x100).
What you can see in this photo is that the silcrete surface starts to look highly textured at magnification, and pits barely visible to the eye on the sample become large voids. The base of the pits is out of focus here because we chose to focus on the surface and top edges of the pits, but by changing the microscope setting we could have looked at the bottom of the pits in sharp focus.

Silcrete under magnification, showing geodes
If we go back to thinking about the minerals that make up silcrete (see previous post), it's important to remember that flint and chert (generally referred to together as silex in France) are really both forms of quartz, which is basically derived silica. Silcrete forms (and here I'm simplifying!) when water loaded with silica infiltrates pre-existing sediments, often quartzites, and cements them. Often the cementing silica-heavy material is opal or chalcedony- both of which are also different 'flavours' of quartz (with added seasonings of other silica-related minerals).
However, geology is tricky, and although rocks might seem immutable most of the time, they undergo long processes of evolution depending on factors like heat, pressure, presence of water and other minerals. Because all of the types of rock I've mentioned (flint, quartz, quartzite, opal, chalcedony) are chemically and mineralogically related, they can evolve into these different forms in the same rock. There is a sequence to this, with opal turning into chalcedony and finally into quartz.

You can see in the next photo of a different sample another smaller void within a very fine matrix, at much greater magnification. This photo is cool because you can actually make out (in the outlined box) the natural blobby form of chalcedony that has formed inside the geode. The dark colours are probably from oxidation.

Chalcedony spherules in silcrete
 The next photo (zoomed in to twice the magnification) on a different piece of silcrete (still from St Pierre-Eynac) shows more voids, known geologically as geodes, within a matrix. The much darker colour of this silcrete is obvious, and is referred to locally as "resinite"; it can look dark red or black to the eye. The colour variation depends on many aspects of formation and subsequent taphonomy (what happens to the stone after first formation).
Here, the dark colour may be related to oxidation, but the matrix is still chalcedony. Around the edges of the geodes, you can see a pale margin, this is also chalcedony. Inside the geode is more uneven, glittery material: these are quartz crystals. This implies that this silcrete sample may have had multiple stages of evolution: the surrounding chalcedony matrix has turned into quartz within the geodes.

Resinite silcrete with quartz inside geode
The next photograph demonstrates another kind of silcrete story. You can see running across the image a grey line. This is a fissure cutting through older chalcedony matrix. Paul speculated that it may have fractured as part of the faulting or even volcanic activity present at St Pierre-Eynac. After this crack split the silcrete rock, a later phase of silicification happened, when more silica-rich water infiltrated the rock (perhaps some of the silica even came from partially dissolved matrix), and a new cementation within the fissure happened. This is the milky-gray translucent material, probably chalcedony.

Silcrete with fissure filled by chalcedony
The last photo, zoomed in on a different sample, shows a cryptocrystalline (i.e. very fine) matrix, with some cool mineral structures within it and a bit of oxidation on the surface. The mineral structures are called "framboids"- they are basically tiny groupings of iron pyrite spheres and sometimes cuboids, that end up looking a bit like raspberries- see the large one on the left. They're called framboids after the French word for raspberry, framboise. These are not particular to silcrete, and can form in many different geological settings. But they do give some clue to the mineral make-up of this particular silcrete, and are often seen within the St Pierre-Eynac samples, alongside other forms of iron pyrite (aka "fools gold").
Once again if you were looking at the image live on the screen, it would be possible to shift the focus down into the matrix, which here is a bit like looking at frozen leaves in the ice on top of a pond.

Iron pyrite framboids in chalcedony matrix
If you want to take a more detailed look at the structure and minerals present in samples, you can get some "thin sections" made- these are what they sound like, extremely thin slices of the rock, that allow light to shine through. Because different minerals are distinctive in form, and especially in varied types of light, you can identify what your stone is made up of. Thin sections are used in many other sciences, including archaeology- for example you can take slices of pottery sherds and determine what types of clay were used. The magnification used here is actually not as much as for the other microscope, only up to 40x, but what you're seeing is different.

In our case we were looking at flint thin sections rather than silcrete. You can change the type of light shining through your thin section by adjusting the microscope, with startlingly different results. Normal light shows up some parts as black on a white background- these are more weakly mineralized. Towards the middle of the cross-hairs you can make out two small cuboid black shapes- these are probably iron pyrite.

Normal light through flint thin section
Polarised light shining through the same sample produces a much more colourful display, and allows you to see the minerals better. In this case, the chalcedony that makes up most of the rock shows up as blue, yellow and deep pink. The photo below is exactly the same view as before, but with the light changed; you can still see the small cuboids, but now the white 'background' is revealed as a chalcedony matrix.

Much more colourful!
Looking at another sample shows in more detail the actual structure of a chalcedony spherule, which is pretty cool. As a mineral it has a fibrous structure (as opposed to quartzite which is more like a chain), and depending on how the fibres are oriented, the colour of the light changes, giving the yellow, blue and red/pink. You can see towards the centre of the image there is a vague circle with blue and yellow quarters; this is the spherule showing its microstructure. You can see a clearer example in this website.

Chalcedony spherule at the centre
After we looked at the mineral structure using the thin sections, we went back to the binocular microscope to try and identify some "mystery" flint samples that my colleague on the project Vincent Delvigne had sent for me to get some experience in learning about the structure and palaeontological inclusions in flint, which is quite different from silcrete.
This first image shows what one type of flint looks like close-up; to the eye it just looked pretty grey. Here you can see how it's very textured, and made up of lots of little bits. Most of these bits are probably tiny fragments of fossils, as well as some quartz and chalcedony.

The next photo is of a different sample, but at the same scale, and it's obvious that the matrix is much finer. There are still fossil inclusions, but they're within a less 'bitty' matrix; it's possible to change your focus and look progressively deeper to see the inclusions more clearly.

The next sample has a less fine matrix, but you can clearly see that this inclusion looks biological; it's a bioclast. Paul identified this as a probable radiolarian, tiny organisms with siliceous internal structures that are part of the plankton zoosphere in oceans. If you're lucky and have some in your sample that aren't fragmented, it's possible to identify them to groups, which helps in ageing your flint samples.

Radiolarian in flint sample
Finally, other kinds of bioclasts exist which can also assist in ageing flint and suggesting which formations your sample or archaeological tool originally came from. In the photo below is another very obviously organic clast within the fine cryptocrystalline matrix. This sample had radiolarians, small fragments of bivalve fossils and also foraminifera, another type of micro-fossil that have shells. These are especially good for dating flints, as their fossil record is quite detailed. The one in the picture looks similar to several here, and that link also shows you the variety of foraminifera.

Foraminifera in flint sample
Taking on all this information in just a couple of days was a lot of fun but also challenging, and I've only scratched the surface. To get the skills that Paul has in identifying silcrete and flint structures, and then tying these to particular geological formations in the landscape, takes decades. My whole postdoc is only 2 years, and the training element of that much less. So this experience has been more about introducing me to the methods and tools involved in this kind of work.
I'm hoping to apply some of this knowledge in separating out any non-silcrete material that has been imported to the raw material source I'm studying (St Pierre-Eynac). At the least it will give me a grounding in a skill that I could potentially take further in my career, perhaps through collaboration: certainly the geological understanding in France of flint varieties and their evolution in relation to archaeology is light-years ahead than in Britain, and we could really do with something like the massive rock databases that Paul has complied.

Next steps in the project are recording and analysing the surface survey samples that were collected at the end of this year's field season, to get an idea of what kinds of artefacts are present, of what age and in what spatial distribution. That will be keeping me busy for a while, but more updates to come. Thanks for coming into the micro-world of silcrete and flint with me!


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