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…



Iron Age Slag – No Puns Please!

I’m Dawn McLaren and I’m a finds specialist at AOC Archaeology Group based at Loanhead, Scotland. On a day-to-day basis I’m principally involved in the post-excavation analysis of artefacts recovered as the result of developer-led excavations ranging from early prehistoric through to post-medieval in date. To give you an idea of the range of finds that I deal with, in the last couple of weeks I’ve been looking at coarse stone tools and querns from an Iron Age settlement, some pottery from a Bronze Age burial and post-medieval metal finds from an urban site in Edinburgh. It definitely keeps me on my toes!  

Today I’ve been examining some later prehistoric ironworking waste from a multi-phase site at Beechwood, Inverness and I’m really excited about what it is telling us about metalworking on the site.  The site, which was excavated by my colleague Rob Engl and others, revealed several Bronze Age/Iron Age timber roundhouses, palisades and enclosures together with evidence of Neolithic settlement.

Dawn identifying slag from Beechwood

Starting from the beginning, what is ironworking waste?  Basically, it is the non-iron component of ore that is separated out from the iron during smelting and smithing but there is inevitably other associated debris such as bits of ceramic hearth lining and vitrified stone which don’t necessarily need to be connected to metalworking. I’m terribly over simplifying, of course, but I hope this gives you an idea. Visually, this material doesn’t look like much, I admit! It often looks like rusty or glassy shapeless ugly lumps. But I’ve been trying for years to convince people that it’s really interesting and can tell us a lot about metalworking technology.

My first step is always to visually examine (macro and microscopically) the individual pieces looking at the colour, texture, shape and how melted and fused the material is. Another important part of the initial identification is to determine whether the material is magnetic. All of this information helps me to split the assemblage into broad categories: what is ironworking waste and what has been formed as the result of another pyrotechnic process, what is diagnostic of iron smelting and what might be bloom- or blacksmithing debris. Once I’ve identified the individual pieces, I record all the details (e.g. weight, quantity of pieces and measurements) into a spreadsheet so that I can feed in the contextual data later.

Small smithing hearth bottom from Beechwood

I’m pleased to say that the assemblage from Beechwood has a bit of everything!  It’s not a large assemblage but so far I’ve identified several smithing hearth bottoms and fragments of smelting waste so that I can say that both processes were taking place on or around the site.

Smelting slag from Beechwood

Now that my catalogue of the slag is complete I’ve started to look at where the pieces were recovered from. The excavations at Beechwood covered a very large area and I can see from my initial examination that the ironworking debris is focused in two quite disparate parts of the site. One area, which we’ll call A, includes a possible metalworking hearth or furnace associated with smelting slags and the other area, B, which is quite a distance away and must represent a separate focus of activity, has small residual amounts of both smelting and smithing debris. We’ve already had some of the pits and postholes from these areas radiocarbon dated and those associated with the ironworking waste have provided wonderful Iron Age dates.

Looks like my task for tomorrow is to see how the Beechwood evidence fits in to other Iron Age metalworking sites in the area!

For more information on our post-excavation services please check out our website:

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.

Historical metallurgy

Like many archaeologists, outside of my ‘proper’ archaeological job, I seem to find time to get involved with lots of other archaeological activities as a volunteer. One of my many ‘other’ roles is to be the Chairman of the Historical Metallurgy Society. This is occupying quite a bit of my time, so I thought it was worth saying something about it. I guess in many ways it is quite typical of a lot of national and local specialist societies and groups – entirely run by volunteers who are often the world’s leading experts in their particular fields, and producing information at various levels from high-level peer-reviewed academic journals and monographs to informal datasheets for archaeological fieldworkers.

The manufacture and working of metals is one of the most important human activities. Archaeologists encounter metal artefacts and evidence of metalworking in all periods from the Bronze Age through to the post-medieval. However it is often difficult to identify and make sense of. This is why the Historical Metallurgy Society was established almost 50 years ago. The Historical Metallurgy Society is a dynamic and exciting international forum for exchange of information and research in historical metallurgy. This means all aspects of the history and archaeology of metals and associated materials from prehistory to the present day.

Members’ interests range from processes and production through technology and economics to archaeology and conservation. The Society holds several conferences and meetings each year which showcase the latest research, and explore a wide range of metallugical landscapes and locations. The next meeting is in Cardiff in September.

The Society’s datasheets were launched over 15 years ago, and remain a popular resource for archaeological fieldworkers and managers alike. They can be dowloaded free. The archaeology committee is currently updating and enlarging the scope of the datasheets. The datasheets – and indeed Historical Metallurgy Society members – helped to inform the development of the English Heritage guidelines for Archaeometallurgy, and Science in Historic Industries.

A mini-datasheet on ironworking residues has also been produced – aimed specifically at archaeological fieldworkers.

The Society’s archaeology committee has also recently produced a UK-wide research framework for archaeometallurgy called ‘Metals and Metalworking’. This sets out current knowledge and areas where we need to learn more; it can be downloaded from the Historical Metallurgy Society archaeology committee page.

Joining the Society only costs £20! Details can be found here. Members receive a scholarly journal (Historical Metallurgy) twice a year and a newsletter three or four times a year. The Historical Metallurgy Society has an extensive archive of books and reference material, and can put you in touch with expertise all over the world… the Society also provides small grants for research and travel. Please visit the website – or look at our Facebook page!


More post-excavation tasks

After completing this morning’s interim report on the Stirchley Station watching brief, I have now turned my attention to the various odds and ends which have built up during the week and really need to be sorted before the weekend.

First amongst these was finishing the archive for the blast furnaces community archaeology project which we completed in May. Yes, I’m afraid to say that the drawings and finds have been littering the office for the last three months or so – but I am pleased to report that they are now all boxed up and ready to be sent off. Part of the reason for doing this now is that I am preparing a paper on the site for the Historical Metallurgy journal – the site may contain the remains of the first ‘hot blast’ iron smelting furnaces in the world!

Archiving of projects is one of the big issues in commercial archaeology at the moment – with museums finding it hard to take stuff… and even I do sometimes wonder if some of that stuff is really worth keeping.

Something else that has been in the office for a couple of weeks – and this is definitely worth keeping – is this lovely early nineteenth century mixing bowl which has been converted to a flowerpot by someone drilling rather crude holes in the bottom. We found this on another watching brief in Wales.

Welsh mixing bowl converted to a flowerpot

Also today I have been speaking with one of our team who is working on a site in Cheshire, and dealing with a couple of questions from clients. I have hardly had time so far to get down to work on the big excavation write-up, but I think I can have a good crack at it in the next couple of hours.

Here I am at my desk in contemplative mood!

I guess one of the things about being an archaeologist is that it never stops! I am currently the Chairman of the Historical Metallurgy Society, so inevitably bits of the day (and much of the evenings) are taken up in dealing with the activities of our various committees and thinking ahead. It is the society’s 50th anniversary next year, and we have a meeting on Monday to do some intensive planning – so I need to think about that over the weekend.

Tomorrow, just for fun, I am spending the day with a friend (another archaeologist, of course) looking at urban industrial sites and comparing their condition today to how they were when they were officially surveyed 25 years ago. I expect that this will take about 12 hours of our free time!

You do need to be quite mad to do archaeology – but still after 20 years it is very enjoyable!