post-ex

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.

Michael Marshall: Assessing Small Finds from Roman London

Like any job in archaeology, working with small finds can be a bit of a mixed bag. For every box opened to reveal shiny ‘treasure’ there are countless others containing more prosaic yet interesting finds which are indicative of everyday life and activities in the past. These are really the ‘best bit’ of any assemblage but more numerous still on many urban sites are boxes full of highly fragmentary and corroded ‘dross’. Some of this is completely unidentifiable, fairly undiagnostic (such as fragments of iron sheet or wire) or else tantalisingly close to being a recognisable object leading to some speculative flicking through likely find’s books or trips around the office to bother colleagues.

Today I am working on a post-excavation assessment report for an interesting Roman site in Southwark and it is no exception. This morning I’ll be dealing with some more of the most corroded horrible bulk nails it’s ever been my misfortune to handle (don’t expect any terribly enlightening updates about these) but this afternoon I have some more nice Roman glass to round off the week so stick with me. There are lovely glass vessels in this assemblage and some evidence for glass working – probably the first evidence of this sort from south of the river.

In the meantime I’m off to grab some gloves and some more boxes of nails. At least it’s a bit cooler today. I grew up in Scotland and so anything above about 24°C is a bit on the warm side for me and it was horrible yesterday when it topped 31 degrees in the office. I was wearing gloves and a dust mask and had to close the windows and turn off the fan in my section of the office to stop all the dust, rust and muck from the nails choking the osteologists and finds specialists I share a room with. For the present here’s a photo of my rather generic desk in that room to contrast with all the lovely site photos that I’m seeing appearing on the website already. This is ‘where the magic happens’… or something like that. The sharp-eyed will notice the awesome (free and pretty accurate) BBC prehistory timeline above my computer.

Treasure ahoy!

Where the magic happens