Dean

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.

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…

 

 

Boreal CRM Archaeology in Northern Canada

I work for a small consulting firm which conducts cultural resource field surveys of proposed oil and gas developments in the fields, forests, mountains and boreal muskeg of northeastern British Columbia. Each working day, we load up our big 4×4 trucks with ATV’s or skidoos and head out to play with the moose and bears in Canada’s great backyard. We are assigned specific project areas to inspect, such as pipelines and oil or gas leases, which take anywhere from a few hours to a few days to fully investigate. We are generally looking for areas which might have been favourable camping locations, such as creek banks or elevated terrain features with a bit of a view. Once we find one of these areas within our projects, we check surface exposure – if any – for cultural material (generally lithics in this neck of the woods – pre-contact ceramic technology didn’t catch on this far north and the acidic soils quickly deteriorate unburnt bone and wood) and then we conduct a little subsurface inspection through shovel testing. If we are able to find any flakes or tools, we then try to establish the boundaries of the cultural area through more shovel testing and then return with this information to our clients, oil and gas exploration firms. In the vast majority of cases, they will choose to alter their development to avoid the terrain feature upon which the site rests; if not, we will be asked to conduct an excavation to fully investigate the site area and record all associated features and remove all the cultural material. Today, July 29, 2011, was in the middle of just such an excavation for us.

Earlier this week our team of four drove two hours further north along the Alaska Highway and signed ourselves in to the remote camp north of the Sikanni Chief River which would be our base for the next two weeks. Then we drove another hour along a very rough dirt road to where we unloaded our ATV’s and then ‘quadded’ (as we say) a further 11 kilometres into the forest along steep and very challenging seismic line trails that are used by everyone up here as accesses into the bush. We arrived at the site, established a 10 m by 15 m grid (had to cut down a few spruce trees for this) over the cluster of shovel tests that contained cultural material, took some photos, drew up a plan and began to dig.

Peter and Dean excavate the site

 

So far this week, we’ve excavated about half of the site area and found several suspected hearth-features, about 150 flakes and half a dozen stone tools. Projectile points are the real glory finds for any pre-contact excavation but everyone – myself included – loves to hold the scrapers. It’s always a thrill to find anything that has been hidden for so long and not only are they lovely to look at but they fit so comfortably in the hand, as the tool-maker has often thoughtfully included some kind of groove for the thumb – unlike arrows and spearheads, these are objects that were meant to be held, and they convey a quiet, homely domestic atmosphere to the site.

Black chert scraper

We will continue the dig for another week or so, until we are satisfied that we have removed all the cultural material from in front of the bulldozers which will be coming in after us, and then we’ll de-camp back to our home base to finish up our reports and any cataloging. The artifacts will eventually wind up in the local museum system and we will head back out into the wild to check new locations.