chemical composition

Roman tapped iron-smelting slag

Roman tapped iron-smelting slag. The field of view is approximately 2.5mm. The horizontal line across the centre is the chilled margin of an individual flow lobe.

Following the morning’s excitement of a delivery of new material, it is back to the interpretation of a large dataset collected on the SEM last week. Some of the collections of archaeometallurgical residues that get examined require detailed analysis to reveal their secrets. Various techniques are used to analyse for chemical composition, mineralogy and microstructure. One of the most commonly used tools is the analytical scanning electron microscope.  The analytical SEM allows chemical microanalysis from precise locations in a sample.

From this information the analyses can be converted into chemical formulae, allowing the detailed mineralogy can be established. Analysis of regions of slag also allows the overall chemical composition of the slag determined.Processing of the microanalyses is time-consuming

Spreadsheet of chemical data

Processing microanalytical data, to convert the microanalyses into mineral formulae.

In this example, the chemistry of the slag clearly indicates that the smelters were using iron ore from the Forest of Dean. This ore is generally very pure and produces a slag with a rather simple mineralogy. Here, however, the slag has reacted with the ash of the charcoal fuel, levels of calcium and potassium have been increased, and additional phases formed.

Detail of Roman iron smelting slag

Detail of Roman tapped iron-smelting slag. Field of view is approximately 0.17mm. The image shows the minerals wustite (FeO, white), fayalite (Fe2SiO4, pale grey), kirchsteinite (FeCaSiO4, mid grey) and leucite (KAlSi2O6, dark grey).

So, analysis has, in this instance, clarified not only where the ore was mined, but also provided some subtle indicators that may help with understanding the details of the smelting technique employed.

There are, however, lots more numbers to crunch before the full significance of the material can be understood…



Archaeometallurgy in the university – a PhD student’s day


My name is Ruth Fillery-Travis, and I am an archaeometallurgist. That is, I use scientific analytical techniques to examine metal objects and the evidence of their production. I’m in a sub-discipline of a sub-discipline, as archaeometallurgy is a part of archaeometry/archaeological science, which is often considered a sub-discipline of archaeology. Luckily enough all the sub-sub-sub stops there, because I’m in the UK – if I were in the US archaeology itself might be considered a sub discipline of anthropology!

It might seem strange to be in such a niche subject area, but it fits my interests perfectly – I wanted to be a physicist right until I started studying it at university! After that I switched to Classical Archaeology, which had zero science and a lot of critical analysis of art, architecture and archaeological objects. Archaeometallurgy allows me to combine those two areas – I get to look at sometimes quite stunningly beautiful classical objects and not just analyse their physical appearance but use scientific techniques to analyse what they were made of and how they were made – like the snake ring adjacent.

Snake head of gold Roman finger ring

I’m currently reading for a PhD at the Institute of Archaeology, University College London. I started in 2009 – before that I worked in local council archaeology as a records assistant on the ‘Sites and Monuments’ or (in more modern terms) ‘Heritage Environment Records’ of Norfolk and then Greater London. Nothing to do with archaeometallurgy – but then that’s a common problem with studying archaeology. If you can get a job in the discipline at all – which is tough – then it often isn’t in the area you originally trained in. But the advantage of that is that you can gain some fantastic experience – certainly my time working on the Greater London records was fascinating.

Between those jobs (more…)

Ancient concrete? Really?

Yes, really.  I first fell in love with old buildings in Pompeii, where I spent summers working as an excavator from 2002-2008. Every day it struck me that I was in a place that still looked and felt like a real city. To my mind, this was down to the fact that the buildings are still standing. After more than 2000 years. Someone did something very, very right when making those buildings and I want to know more.

For my D.Phil research, I have landed in an opportunity to study structures in Ostia, Italy, which is also a preserved city-sized site.  The structures I’m investigating are all brick and mortar masonry, with concrete filling up the center wall core. This is what Vitruvius called opus caementicium. To be honest, I’m most interested in the people who made it: the builders who developed this wonderful, magical material that is still performing successfully more than 2000 years after it was first installed. Where did they get their materials? Why were certain materials preferred over others? How were the materials processed and mixed together? How did builders’ choices affect the concrete and its performance? Were the same mix types used for both public and private structures? Why is this stuff still standing? These are the questions driving my research, and I am looking to answer them by investigating the material itself.

To give a quick overview, the mortar and concrete I am analyzing was made of lime, volcanic sand aggregate, and water. Sounds rather simple, however, the combination of materials they were using produced complex chemical reactions, known to modern concrete scientists as pozzolanic reactions, which resulted in a sophisticated, high quality material. My sample collection was collected from a series of structures in Ostia from the 2nd century CE, by which time – at least in Rome – concrete was well-developed and had been employed in large-scale Imperial building projects. My task now is to analyze the Ostian structures to determine how well-developed their concrete industry had become by that time. The benefit of a site like Ostia is that the ancient city is left largely in tact without modern development. This means that unlike in Rome, where centuries of modern development has destroyed all but the most protected monumental structures, it will be possible to evaluate the buildings within their original cultural context.

The analytical techniques employed for my research are borrowed from geology and concrete science, which makes this a truly interdisciplinary project. My samples are essentially synthetic composites of natural materials that can be investigated with traditional petrography. I’m using light microscopy of thin sections to identify and quantify the aggregate, to describe the cementitious matrix, and to identify any  obvious degradation features or alteration products. Today I’m working on point counting one of the samples, which is pretty straight forward. I move across the sample in 1 mm steps, and at each location I record what I see in the cross hairs of the eyepiece. Besides the obvious benefit of quantifying each of the different components, I’m also getting to the know the sample really well. As I go, I’m recording information about the state of degradation or alteration, the shape and fillings of any cracks or holes, particle size and shape, and any other details that may give me a clue about what the builders were doing when they made the concrete.

I am also using scanning electron microscopy (SEM) to collect high-resolution, high-magnification backscatter images of the samples. At this scale I can get a better look at the binder-aggregate interface to see how well-bonded these components are. It is also possible to see any microscopic cements that have formed in pores, cracks, and the vesicles of aggregate clasts that would otherwise not be visible. The SEM also detects the atomic weights of everything in the sample, which show up as differences in the greyscale colour of the image. It  also can calculate the chemical composition of the different components, so using a combination of chemical data and backscatter images, I can determine what types of cements have formed (strengthening) and how much leaching has occurred across the matrix (degradation). The ratio of calcium to silica is key in both cases.

X-ray diffraction is also on the menu, assuming I can find the funding to pay for it. This technique is incredibly useful for identifying the mineral assemblage in rocks and materials. In this case, I will use it to confirm the original petrographic identification of minerals in the aggregate and to find any other alteration minerals that could not be seen in thin section. The presence of certain minerals like gypsum or ettringite usually indicate alteration of the mortar itself, but minerals such as stratlingite and calcium-aluminum-silicate-hydrates suggest the mortar was rather well-formed in the first place.

So today, I’ll be giving an account of what it’s like for me in the lab. I realize that being stuck in the lab sounds like a death sentence to some people, but for me, it’s where the magic happens.