Making Sense of Analytical Data

After the inital excitement of the arrival of new material in the lab, curiosity had to be curbed and the main task of the day tackled. This task was to process and interpret anayltical data acquired last week during many days work on the SEM. I use many different analytical techniques to investigate the more important archaeometallurgical residues passing through the lab – and the analytical SEM is one of the most useful.

BSEM Roman smelting slag

Backscattered electron image of a tapped Roman iron smelting slag. The field of view is 2.5mm.


The backscattered electron images reveal compositional contrasts through their grey scale. In this image the dominant phase, appearing pale grey, is fayalite (an olivine mineral, approximately Fe2SiO4).

Across the centre of the image is a discontinuity, produced by the chilling of the surface of an individual lobe of slag as it flowed from the surface and cooled in the air.

The crystals are large, suggesting the slag cooled slowly, and the lobe margin is not marked by the development of much iron oxide, so this example probably cooled right in the mouth of the furnace.

As well as producing these images, the analytical SEM also permits chemical microanalyses from tiny spots or areas of the sample.

The second backscattered electron image shows a tiny detail of the first image, with the location of microanalyses.

Detail of Roman iron smelting slag

Detail of Roman tapped iron-smelting slag. Field of view is approximately 0.17mm.


The instrument provides the chemical analyses, but they then have to be recast as mineral formulae – and that was today’s task. With many hundreds to do that was a substantial task in front of the spreadsheet. Gradually a picture emerges of the overall composition of the slag and of its constituent minerals.In this instance, the slag proved to be typical of residues produced during the smelting of iron ores from the Forest of Dean. That is a useful result in itself, allowing one aspect of the economy of this Roman settlement to be understood. As other samples from the same site are interpreted further details will emerge – permitting reconstruction of the yield and efficiency of the furnace as well as aspects of the technology itself.

Spreadsheet of chemical data

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


Archaeometallurgical residues provide a very direct link back to a particular occasion in the past, when an artisan did a particular job in a particular way. The waste material provides key evidence for that moment in time. Although studying the waste, rather than the product, might seem perverse, there is often a richer set of evidence about hte nature of the process to be gleaned from the residues than from the artefact. Crucially, the residues also typically remain close to the site of the activity, whereas the products were dispersed after production and may not be able to be linked back to their point of origin.

Careful investigation of such archaeometallurgical residues may allow us to come as close as we ever could do to looking over the shoulder of the Roman smith at his work.

Interpreting Ancient Metalworking

The Day of Archaeology is a pretty busy one in the office – not just the usual need to get specimens analysed and reports out of the door, but also with the added urgency of being almost the last day in the office before holidays.

As an archaeometallurgical specialist, I examine assemblages of metalworking residues (mainly slag…) on behalf of field archaeologists, both in academia and in the commercial world. My particular interest is in iron – so although I undertake projects dealing with all sorts of materials, it is with iron that there is the greatest synergy between my commercial work and my research interests. You might have thought we already know all there is to to know about iron making and iron working – but nothing could be further from the truth. This is a dynamic and rapidly advancing branch of archaeometallurgy and experimental work on various techniques is a key aspect of what I do – at least when the opportunity arises.

The reports I’m completing today include two for assemblages from a pair of adjacent Early Medieval sites in central Ireland. Intepreting such material entails bringing together various strands of data:

– there is the overall make-up of the assemblage, the types of slag, their proportions and distribution within the site. Much of that information is produced during the assessment stage of the project.

– there are detailed observations to be made about the form of individual pieces of slag. Often they can be identified to a general process or technology at this stage.

– there are bulk chemical analytical data. I use information generated by XRF (X-Ray Fluoresence Spectrometry) for the major elements and by ICP-MS (Inductively-coupled plasma – mass spectrometry) for the trace elements – thats over 50 elements altogether.

– and there are also the microstructural and microanalytical data that can be obtained by examining polished blocks of material under the SEM (scanning electron microscope). This gives information on the individual minerals within the slag: what they are, how they formed and sometimes what reactions were taking place in the slag before it solidified.

That, then, are the various sorts of data, but the challenge (and the fun) is in the synthesis of that information into an intepretation. That interpretation needs to be both scientifically rigorous and archaeologically useful. It needs to reflect the place of the metalworking activity in the lives, culture and economy of real people. Its not just a case of what was happening, chemically, within a hearth or furnace – but what that means in a human context.

So where is the synthesis of today’s data going? Well, one of the key observations on the material I’m writing up today is that the morphology of the slag tells me it comes from iron working (rather than primary smelting), but it contains a high proportion of material (particularly the elements manganese and barium) that must have been derived from the original smelting of the iron ore. This means that these slags were generated during the refining of the raw iron bloom to produce a useable material.

Slag under the SEM

A manganese- and barium-rich slag under the SEM

One of the great debates in early ironworking studies at the moment is whether such slags were generated during a bloomsmithing operation (thats to say the smith alternately heated the raw iron and forged it with a hammer to drive out the slag impurities) or by a remelting process (in which the smith completely melted the raw iron to allow the escape of the trapped slag). In the past it has been assumed that all bloom refining was by bloom smithing – now it seems remelting may have been much more important than we thought.

It is to debates such as this that experimental work can make a great contribution.

remelting hearth in operation

An experimental approach to studying bloom refining - a bloom remelting experiment run with friends in Virginia

Today’s  report writing was, at one level, supplying data and interpretation to a developer-funded project – and relates to the interpretation of life in 7th century Ireland. At another level it was another piece of the jigsaw in trying to understand a key early technology used in many parts of Europe. It will be a while before that all comes together as a comprehensive understanding of the technique – but when it does, that information can then be fed back again into the understanding of people’s lives 1400 years ago.

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.