New technique to tell whether rocks are man-made or naturally occuring.

New technique uses infa-red to tell whether rock fragments are man-made or naturally occuring. This useful new technique has been tested successfully...
17 January 2011

Interview with 

Stefano Curtarolo, associate professor of mechanical engineering and materials sciences and physics at Duke University; & Kristin Poduska, associate professor of physics at Memorial University in Newfoundland


Diana -  Next up is a new development in the field of man-made or naturally occurring material identification.  An international team have put their heads together to find out what the differences are in the crystal structure of materials such as calcite.  Now calcite can appear naturally in rocks or as part of wall plaster and even ash.  So this team have come up with a simple infrared light method that can be done in the field to spot the difference.  I spoke to Christine Poduska and Stefano Curtarolo.

Christine -   Well I've always had an interest in materials and usually spent time trying to understand how to synthesise them to control properties, but what I did for a sabbatical leave was I worked with a group in Israel that was led by Steve Whiner and they are interested in applying these kinds of material characterisation techniques to archaeological materials.  And so, what brought Steffano and I to this project basically was trying to understand how the crystallinity of a material, how well ordered the atomic level structures in a material, affects how we interpret how that material was used, and there was a very need example related to calcite. Calcite forms as a rock but it's also exactly the same chemical composition that you'd find in plaster that you'd use to make a wall or a floor.  And so, we were basically trying to use material characterisation techniques that physicists and engineers might use both experimentally and theoretically to apply this to archaeological contexts.

Diana -   Right, so it becomes useful in trying to work out what's man-made and what's not?

Christine -   Exactly.

Diana -   Okay then.  So Steffano, how did you go about actually researching this to find out what the differences would be?

Steffano -   Okay,  we use our quantum calculation so we start from quantum mechanics and through quantum calculations, we can calculate and predict the effect of disorder in the vibrational spectrum. Aatoms at the nanoscale move.  Okay, so everything moves depending on temperature and light, and so on. when Christine, with her infrared laser, shoots light on these particles, all the atoms start moving, always a mode.  Everything starts oscillating.  The way they oscillate is correlated to the disorder that is left over inside these structures.  Since one of my expertise is to calculate vibrations with respect to disorder then we decided to try it.  So we created some artificial disorder. 

Based on the artificial disorder, we calculate how these vibrational modes are changed and then we discovered that the way they change is some sort of fingerprint of every material.  So every material, every crystal, has a set of variation of these vibrations with respect to the characteristic disorders that we can create.  In the case of archaeology, materials that have been made by man or materials that have been made by nature, , nature always have infinite time,  there's a lot of time of nature to find the proper crystal structure.  So all these materials, depending how fast they grow, how long they relax, and how long they stay under high pressure, they become more or less ordered and the left over disorder is responsible of the way this spectra changes with respect to size.  So what Christine is measuring which is the spectrum for different size of particles because she keeps grinding and then see how the spectrum moves is a fingerprint of each material. And the way they move is explained through our model.

Diana -   Okay and Chris, how did you go about actually proving that this was working?  I mean Steffano has just mentioned a little bit about you know, creating your own artificial crystals, but could you go into a little bit more depth about the tests that you ran?

Christine -   Sure.  Yeah, just to describe how the experiment works, it's neat because it's very simple and it's a kind of experiment that people might do in a chemistry lab, in a university kind of setting, but it turns out it's very easy to take this out and use this in the field which is why it's so good for archaeology.  So what we do is take a very small amount of our sample. Small means that if you drew a period or something on a piece of paper, that amount of material we use would basically fit in that, so it's a very small amount of material.  And we grind that up, just by hand with a mortar and pestle, and then we add another bit of material and it has sort of disperse our sample within that. Then what we do is we press that into a small little pellet and shine infrared light through it, and that's the same light that you would be using for example in your TV remote to change the channel.  That information we get, basically there are some frequencies that are absorbed and that tells us about how the molecules in that material are vibrating.  And from that, we can use that as a finger print to identify what type of material that might be chemically, so we can tell for example whether its calcite or whether it's quartz or some other kind of material like that. 

And so, what we found and what's a little bit different about our work is we were able to use some more specifics about those spectra that we get to differentiate how well ordered at the atomic level the calcite was.  And so, what this allowed us to do is it turns out that if you make a plaster, wall or floor for example, the atoms aren't very well ordered and on the other hand, if you have a rock that's been around for awhile, it turns out that's very well ordered at the atomic level.  And so, we were able to use this simple method to help us differentiate whether this was material that had been made by nature or had been made by humans.

Diana -   And have you been able to use this on any archaeological material yet?

Christine -   Yes and in fact, the group that Steffano and I were both affiliated with, Steve Whiner's group at the Kemmel Centre for Archaeological Science at the Weissman Institute has been using this technique for a few years now and they were the first to notice that you could differentiate a little bit about this spectra.  Basically, how wide the peaks were gave some information about whether it was plaster or it was rock.  And so, what Steffano and I did was demonstrate that this was indeed related to how well ordered at the atomic level these materials were.  So, we've been able to take this out on a site so we can take this instrument which is reasonably portable.  It does require an electric generator to be out there, but then we can run these measurements actually onsite and check right away to see what these materials are.  And we can look at things other than calcite as well and that's a little more standard. So for example I mentioned differentiating between calcite or quarts or sediments, and this sort of thing so we can also use that as a finger print for other different kinds of materials as well, and that's something that Steve's group at the Weissman Institute does quite well.

Diana -   Right and Steffano, where do you want to take this research next, now that you've got this new tool?

Steffano -   Well now, we can make fingerprints for a lot of other materials that have not been investigated in the same way.  So, our method which so far describes what is known, now becomes predictive because finally we have an explanation and there is no reason why other systems that grow and are formed is in a way than calcite should not have the same properties.  In the next months and in the next few years, we'll put a lot of effort in determining these shifts and these fingerprints for all those materials that we believe that might be of interest to be analysed on field.


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