Chemistry

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.

Anglo-Saxon CSI: Sittingbourne (Conservation Science Investigations)

CSI Volunteer Richard Senior's raw gold and garnets

Investigative conservation of Anglo-Saxon grave goods

The X-raydiograph shows copper, iron and bone - decorations sewn onto a tunic perhaps?

Conservation volunteer Pat at the microscope

Today I have been supervising some of my volunteers and speaking to visitors at our shopping mall conservation lab. We have been running for nearly two years and have just reached 5,000 volunteer hours for investigative conservation of several hundred artefacts from 65 graves. We are on the last grave for this project – but there is still the finds from the other half of the cemetery to be worked on. Tomorrow we close our doors for fundraising for that project. fingers crossed that we’ll be open again soon! For general info on our community conservation project see a great video made on our opening day – http://digital.kent.gov.uk/2800. and/or visit our website – you can also ‘befriend’ us if you like as we just set up a facebook page too. Volunteer Pat Horne says: ” Today I am working on an object that is really perplexing. It is a ‘blocklifted’ assemblage of finds from a woman’s grave. I am trying to discern the different materials it is made from (we have found mineral preserved bone and textile, possibly leather iron and copper alloy). It has become very fragile, so I am repackaging it to make it more secure before continuing to work on it. this artefact has to be looked at along with others in this grave. There are several with the same ‘figure 8’ copper alloy shapes. so imagination is working overtime trying to puzzle it out – great stuff!” .

Janice Monday is also working on a find from a woman’s grave: “I am working on a small object which, from the X-ray, appears to be minute thin pieces of wire bundled through a loop possibly of bone. there are three more baffling pieces associated with the main part.”

Both Pat and Janice have been volunteering at CSI: Sittingbourne since we began in Oct. 2009 (2 and 1 days per week). We have recently begun training a new group of volunteers (there were 80 on our waiting list!) – one of our new recruits has just returned from panning for gold in Northern Scotland… he popped in to show me some of the gold and garnets he came back with. I didn’t know that garnets were sometimes found alongside gold, when panning – we decided we should look out what is known about the sources of gold and garnets in the Anglo-Saxon period and I encouraged Richard to join the Historical Metallurgy Society to find out more about those iron age camps located at his ‘gold hot spots’ that he was wondering about. We also discussed him posting up his photos to our facebook page and staying in contact while we are closed for fundraising.

– Another day draws to a close at CSI, now on to other tasks, like writing a reference for a past conservation student intern and submitting a paper for publication in the proceedings from PARIS4, Copenhagen… that’s about my conservation work on an early Christian monastery on Sir Bani Yas Island, Abu Dhabi, but that’s another story…


Freezing on a Summer’s day

It’s a beautiful sunny morning here in Portsmouth and quite warm already. We’re in the English Heritage Archaeological Conservation laboratory. Just to cool down a bit we opened up our freeze drier to weigh some wood. It’s -30°C in the chamber! In there we’ve got barrel staves from the Stirling Castle shipwreck, a Medieval set of stocks from Barking Abbey and Roman bowl fragments from Birdoswald fort.

All the wood was found wet and has been treated with Polyethylene Glycol before it was frozen and placed inside the vacuum freeze drier. The freeze drier removes the water in the wood via sublimation (where the frozen water is transformed to the gas state without going through the liquid state). This water is collected in the small chamber as ice . We take the wood out every 2nd day and weigh it. The wood is dry when it stops losing weight. Once it’s dry it can be handled, studied or displayed.

Weighing one of the Stirling Castle barrel staves

The small condenser chamber with all ice removed from the wood.

You can find out more about conserving wet wood via:

www.english-heritage.org.uk/publications/waterlogged-wood/

Karla Graham and Angela Karsten

Radicarbon dating samples

Was able to escape the thrills of writing about pigs diets and isotopes, as I was just dragged off to advise on, and select samples for radiocarbon dating from the Banjo enclosure at North Down, Dorset. This is a large Bournemouth University dig, which had a team of up to 160 excavators and specialists. The 2011 season finished only a couple of weeks ago and post-ex is now in full swing. As for the samples, there wasn’t a lot to choose from. It’s crucial to pick a bone (or whatever else) that you can be confident has not been kicking about for long periods prior to ending up in the deposit you want to date. You don’t want to waste your money dating something that was dug out of a much earlier pit before finding its way into your deposit. One bone, a cattle radius, was about three quarters complete and although it had evidence of weathering showing it could have been on the surface for some time I’m confident it will provide a good date for the ditch from which it came.