Recyclable materials

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 Part 2

So the many, many boxes of nails, thank god, are now a distant memory. At the assessment stage we only do a fairly coarse quantification in order to determine the potential of the material for further work. This sometimes just involves weighing and counting the fragments but when preservation is good enough some other data can be collected such as number of complete nails from each context (divided into broad size categories), minimum number of nails and comments on particularly distinctive styles or features. The point of this isn’t to write a definitive account of the use of nails on the site but to assess their potential for further analysis, decide what role they will play in the final publication and how they can help us to address research questions.

Unfortunately, it’s probably not worth doing much more work on the Southwark nails as they are in terrible condition. Most are completely encrusted or incomplete and the assemblage is quite small with a maximum of c. 15 fragments from any given context making any inferences of limited value.

Much more exciting this afternoon is the Roman glass and glass working waste which will definitely feature in the final publication. As mentioned briefly in my previous post, this seems to be the first Roman glass-working evidence from this side of the river. The types of waste include ‘moils’ (glass from the end of the blowing iron left behind when you crack the vessel off) as well as a variety of melted, fused and runny lumps.  Threads, pulls and trails etc derive from more detailed manipulation of glass during decoration or the addition of handles etc.

Glass working waste from Basinghall, London: Threads and Nails( (c) Andy Chopping, MOLA)

The assemblage is relatively small so far with only about 25 moils worth of fragments accounted for, each of which equates to a vessel manufactured onsite. This estimate is based on EME (estimated moil equivalent) a technique lifted from pottery studies (EVEs) which is calculated by measuring the proportion of the moil diameter present in each fragment. Of course many more vessels could have been made and the moils recycled or not recovered. Vessels were being made from both naturally coloured blue-green glass and amber coloured glass.

Glass working waste from Basinghall, London: Moils ( (c) Andy Chopping, MOLA)

The general range of waste types is not dissimilar to those found at the much larger glass-working dumps at Guildhall Yard and Basinghall Street across the river in Londinium (see pictures below) and, like those dumps, the waste was found alongside lots of broken vessel and window glass intended for recycling. Raw Roman glass was brought all the way from the Mediterranean so recycling this ‘cullet’ made good economic sense. Identifiable fragments of bottles, beakers, jugs and jars from amongst the smashed up vessels suggest a probable date in the early to mid 2nd century AD for the glass working.

If glass working interests you check out this website and a great little book called Glass workers of Roman London by John Shepherd and my colleague Angela Wardle, which provides an interim popular account of their work on the Basinghall assemblage and the techniques of glass making.  Their work on the final monograph is nearing completion, but luckily the new evidence from Southwark should still just about make it into the gazetteer of glass-working sites included in the text, and contribute to their discussion of the organisation of the industry.

That’s enough from me. I don’t have time to tell you about the lamps, finger rings, combs, figurines, crucibles, hairpins, querns, toilet instruments, tools or the large and interesting assemblage of glass vessels I have already recorded from the site. Unfortunately, I can’t even tell you about the cosmetic mortar or the blue blobbed glass beaker, probably an import from the Rhineland, which I recorded yesterday. The whole point of the assessment stage is so we can get our head around what we’ve got, and how best to study and publish it, so if you want to know more you’ll need to hang about. This is certainly shaping up to be an interesting site and I’ve already spent too long waffling here and not enough time doing my glass data entry.

Now if you’ll excuse me, I need to get back and finish this context before the end of the day, so I can get to the pub on time.