UCL Primtech 2011 – running the metalworking sessions

At the very beginning of term the Institute of Archaeology, UCL, runs an experimental archaeology course colloquially referred to as ‘PrimTech’. It’s been running since 1982, when I am reliably informed Peter Drewitt was in charge, and takes all of the first year undergraduates away camping for a long weekend at a small scout camp at West Dean College.

This year was a bumper year of 82 students, which in addition to the Society of Archaeology (SAS) Students attending to run evening activities and cook, and all the staff and helpers, took the total number on site to between 120-130 people!

A few staff and demonstrators arrived early in the week, but the bulk of students arrived on two coaches Thursday morning to pitch tents. Food was cooked on site by the extremely able chef Robby Small, helped by SAS members, and sessions were split into half-days. Everything from basketry, to hut building, to flint knapping and spear-making occurred thanks to members of UCL staff and archaeologists and experimenters from across the country.

All of this was run with a very cool head and steady hand by Dr Bill Sillar, who I can only say has the patience of a saint.

This year, owing to my supervisor being rather tied up with his sabbatical and a beautiful new baby girl, I was in charge of running the ‘metal working’ activity. I have to admit this is one of the most fantastic experiences I have had this year. Anyone who has read this blog will know I love teaching , and absolutely love experimental archaeology, so PrimTech was a marriage made in heaven!

As there always is, we had a bit of a tentative start when bits of kit were found to be  a little wanting – out of something like six bag bellows on site, all but one had holes burnt into them! But after a quick trip back home I had the necessary equipment to enact some hasty – and not at all archaeologically accurate! – repairs and by Friday evening we were running very smoothly.

PrimTech furnace and bag bellows. Copyright Charlotte Frearson

In all this I was extremely lucky to have the very experienced hand of Dr Oli Price helping out, and he built an absolutely fantastic and extremely beautiful furnace for the show-smelt on Friday night. Although we supplied the air to this lovely little furnace through the bag bellows, it was covered in holes which would originally have enable the furnace to be wind powered. As a result, every time we pumped the bellows flames roared out of the holes, which looked extremely impressive after dark!

By Saturday everything was going very smoothly, with the sessions running almost perfectly. For the first 30-45mins I sat down with the students and went through the very basics of metal production and casting, and we chatted about the processes and handled metals and ores and objects. Following this I had them mix up some clay with one of a variety of tempers – wood shavings working the best – and build their own little furnaces, which everyone seemed to really enjoy despite it being incredibly messy. At the same time there were chances to swap out and crush malachite ore (though I let them use a hammer rather than a stone!) and charcoal, and get the furnace lit.

Halfway through the session we took a half-hour break in the shade to rehydrate and reapply sunscreen, and then got onto the serious stuff – running the furnaces! This took the form of filling the furnace with charcoal and using the bellows to get it red-hot, before settling a modern graphite crucible in the centre and filling up the edges all around with more charcoal. To keep the fill of the crucible clean, we popped a second one upside down over the top as a lid. After that it was a good 20 minutes hard bellowing, and we were done! In each session we first we used a crucible filled with crushed ore and charcoal to smelt copper, then ran the furnace again with a new crucible filled with various metals – brass, copper, lead etc – and cast into sand moulds.

The casting didn’t go so well – in part because sand moulding is tricky, and in part because doing the pour alone is pretty difficult. Working with crucibles at over 1100C is very hard work – they are rather large objects kicking out terrific quantities of heat, which is intense even when wearing protective kit. Additionally you have to use large tongs to extract the crucible from the furnace and settle it in a sand pit, then swap to different crucibles to lift and pour – this is quite stressful and obviously any wrong move can be dangerous. In one case I couldn’t get the crucible to sit steady in the sand pit and it tipped over – though the sand absorbed all the metal (as it should) and I had cleared a big area so there was no chance of run-off.  This tipping was likely caused by the fact that we had managed to hit such high temperatures that we’d melted the outside of the crucibles and made the undersides rather irregular. As they have only a small base to begin with, this made sitting them safely in the sand rather tricky!

Smashing malachite (copper ore). Copyright Charlotte Frearson

Despite this problem lots of students got to go home with lumps of metal they had made themselves, and they were all very positive about the sessions. A couple of times we failed to get the furnace to temperature, but this was largely down to the extreme weather – it was almost 30C during the day and we were working with a very hot furnace without any shade. Unsurprisingly not everyone could face bellowing hard in those conditions! I gave very thorough safety briefings at the beginning of each session, everyone had to wear suitable safety kit, and I was the one handling the crucibles, so I think the safety side was well covered. The one thing that was harder to deal with was the weather – some of the students didn’t tolerate the heat well and had to lie down in the shade for parts of the sessions despite the fact that I made sure we took regular breaks to hydrate. I assume past practitioners would have had similar issues, and that  it might be another contributing factor to the location of smelting sites adjacent to rivers.

The weekend was an absolutely fantastic experience, and I can only hope I will get the chance to run an activity at PrimTech 2012.Whilst the sessions were run primarily in order to give students an understanding of the processes which generate the archaeological material they will study, and the issues around spotting and interpreting these, I know that I learnt quite a lot and improved my own understanding of the processes. I had some absolutely fantastic students who were very engaged, very interested and really motivated. I hope that they had as much fun as I did, and that I get to work with people as great as them next time!

Sampling Blooms from Austria

Today I sampled some bloom pieces that make up part of a small set of debris sent to me by Brigitte Cech, an independant archaeological researcher working on the Ferrum Noricum sites.

Brigitte was pretty sure this was likely to be a Roman site, so she had a look using a metal detector and a little bit of sampling excavation, and pulled out some ore, three blooms, slag fragments and furnace lining (amongst other stuff!). I’ve mentioned this stuff before, as I was pretty excited about working on it.

Today I cut off some small pieces from the three blooms. Blooms are the accumulated iron that is produced in the furance – they’re called blooms due to their ‘spongy’ appearance and the way they build up over time. Essentially they look like nasty pieces of corroded rock. They’re heavier than rock though, but that’s a bit hard to spot if they’re not too big and not too small. However if you cut them open, you get to see something a lot more exciting. Basically the whole nasty lump is made of shining bright iron!

To put that in perspective, that iron was smelted sixteen hundred to two thousand years ago. It’s lain in the ground for all that time, and look how little rust there is! I’m told that it’s pretty rare for an archaeological bloom to look like that – most are just massive lumps of rust with a core of metal surviving. My friendly lab technician asked me if it was material from a modern experimental smelt! But Brigitte tells me that they have pottery and brooches from the site that date it securely to the Roman period, so the lack of corrossion is very notable.

It makes me think that there’s something else in the iron here that’s inhibiting the corrosion. Unfortunately the analyses are going to have to wait until the mid-end of May, but I’m sure I read something somewhere about something special in the Ferrum Noricum ores. I’m currently waiting for Brigitte’s book (see below)- it’s still being catalogued by the library – but I suspect there’ll be something like that going on.

Reference:

Die Produktion von Ferrum Noricum am Hüttenberger Erzberg : die Ergebnisse der interdisziplinären Forschungen auf der Fundstelle Semlach/Eisner in den Jahren 2003-2005 = The production of Ferrum Noricum at the Hüttenberger Erzberg : the results of interdisciplinary research at Semlach/Eisner between 2003-2005
Brigitte Cech (Hrsg.) Wien : Österreichische Gesellschaft für Archäologie, 2008.

Iron ore reduction in a bloomery furnace – part 3

Well it looks like I’ve wrapped up the first draft of my iron smelting chemistry and processes piece and sent it off to my supervisors. It’s not the literature review they hoped for, but I figure this way I won’t have to explain anything when I talk about why this paper is ground-breaking or that paper is facile. I also didn’t want to assume my secondary supervisor, who isn’t a technologist, knew much about iron smelting.

During the writing process I went looking for a schematic diagram of a bloomery furnace, just for illustration, but for some reason I couldn’t find one – not even using Google image search. As a result I knocked one up on Word and thought I’d post it here in case anyone needs one. It’s not particularly artistic or complex, but it is handy.

Simple schematic diagram of Romano-British slag-tapping bloomery furnace

In addition I also wrote up a quick simple flow chart for Romano-British iron smelting. Again it’s not complicated, but I like the way that ‘observation’ sits at the centre of the smelting operation. By this I mean observation of the level of the charge as it sat in the furnace chimney, observation of any fire or smoke at the top of the furnace, and observation of the slag levels, either through listeing to the furnace (slag covering the tuyure gives rise to a ‘booming’ sound during the smelt) or through opening a slag-tapping hole.

Basic flowchart of Romano-British slag-tapping iron smelting

The flow chart is meant to be a simple introduction – as I said before I’ve tried to make a more complex one containing everything, but it’s just so tricky! The actual chemistry of reducing iron ore to iron is really simple, but the practicalities of the process are much more complex.

For earlier posts, see:

Iron ore reduction in a bloomery furnace – part 2
Iron ore reduction in a bloomery furnace – part 1

Why do I study iron?

I’ve been doing some reading in an attempt to wrap up my piece on iron smelting chemistry and processes, and I was thinking about why iron is such an interesting subject to study.

I think it probably comes down to the juxtaposition between incredible utility and practicality of the metal, and the sheer difficulty and technical sophistication needed to produce the metal and really develop the best of its properties.It’s simultaneously extremely functional and actually quite expensive and in some cases status bearing.

Iron itself is a strong, hard, flexible and above all workable metal that can be given an extremely sharp edge. As a material it had no comparison during the pre-medieval period. In general it hits western Europe by 1000 BC at its earliest, often becoming practically common place for objects such as swords by around 400 BC and then extremely common for use in a dizzying array of uses by the Roman period (around 100 BC into AD 100).

Without iron, there would be no large buildings, either. Cathedrals and castles are all held together using iron clamps, so were many of the ancient buildings like the Parthenon and the Pantheon. Even smaller buildings and wooden fortifications would hardly have been possible without iron nails. As soon as we could, we used iron to make almost any conceivable thing, because it was harder and sharper and just plain better.

But one of the most intriguing things is that all the complex processes necessary to produce iron, select or control for additives like carbon or phosphorous, and then work the iron at various extremes of heat and cold to change and alter the internal crystal structures and hence achieve the desired properties, all of these were done by practitioners with no actual knowledge of these unseen things.

They didn’t know how much carbon or phosphorous was in the iron – they knew what ores and processes they were using and how these effected the working properties of the iron. They couldn’t see the crystal structures of the metal bar and note how they were changing them during smithing, they could only judge by colour and deformation, practice and experience.

Ancient practitioners had to rely on empirical evidence, rather than any theoretical structures. They couldn’t explain why things changed they way they did, but they had to understand them through physical experience. Some of the processes they undertook had very narrow limits within which they had to keep, or the process would fail. So narrow that I wonder how they could have judged them at all sometimes. How amazing is that?

Another interesting fact about iron, is that the ores are available almost anywhere. No one could control iron production, once knowledge of how to do it was readily available. The knowledge of copper smelting appears to have been reasonably widespread, and the basic processes are the same.

Although most iron was first introduced as an import, once people got the knowledge of how to smelt it, it spreads all over the place. It’s really a rather democratic activity in western Europe, something that doesn’t seem to have been controlled in most countries.

I can find slag from production or smithing on almost any Roman site in Britain. How cool is that? People were making or working this incredibly useful metal all over the place, in their homes, in villas, in cities and towns and legionary camps. I suspect that all kinds of people from all over the world and in all kinds of contexts had the knowledge to either produce or work iron, and I think that makes it a really exciting thing to study, particularly in the Roman period.

New projects and anticipation!

Not my image, but very awesome!

So here at Finds and Features life’s been pretty hectic. Not only do I have a bunch of deadlines, but I’ve just moved house! On the up side, I now have a large room in a lovely flat with lovely people, and even some shelves of my own. On the down side, all my photocopies are now in seemingly random boxes and I’ve no idea where half my stuff is.

However today has been particularly exciting as I’ve suddenly got two more projects in the pipeline. Unfortunately it looks like both will have to wait until May before I can actually give them the time they deserve, but that’ll come round soon enough I suspect!

For the first project I’ve received a small but perfectly formed package of smelting debris from a site associated with the Hüttenberger Erzberg in Kärnten, Austria. That’s where the famous Roman ferrum Noricum steel/iron is thought to have been produced. From what I understand the site is a smaller satellite furnace and slag heap situated very close to the mines, Roman in period and probably in use from the first century BC through to the fourth century AD.

The material I have was collected as part of a brief sample scrape of the slag heap. However due to the skill of the excavator I’ve got slags of varying type, ore samples, furnace lining, and even fragments of iron bloom. That was a real surprise! It’s pretty rare to get bloom pieces, as that’s what the smelters were interested in producing, after all. But I’ve got a bit of everything! An analysts dream!

This of course is great, as it actually syncs with my PhD topic and will be great comparative data. However the other project I’m hoping to take on is looking at some more Roman brooches (I know, my pet subject!).  I’m particularly interested in a wasp brooch, which has weird metal bits seemingly embedded in it, and a slightly creepy looking fly with massive weird eyes that looks both copper alloy and silver. Oh, and there’s also  the potential of looking at a votive deposit which includes a religious headdress, votive leaves, and what look like possible mini eagle standard-things! I’ll post pictures as soon as I get the go-ahead.

In other news, tomorrow I’m off to the 2010 Roman Archaeology Conference/Theoretical Roman Archaeology Conference (RAC/TRAC) in Oxford. Really looking forwards to see what’s going on in Roman archaeology at the moment. It’s rather easy to feel a bit isolated at my university, what with there being more Mayan archaeologists than there are Romanists and Anglo-Saxon scholars combined!

Although I’m only attending Friday I’ll post my thoughts online when I get a chance – though that may not be until Monday as I’m helping out and note-taking at a European Early Iron Workshop on Saturday and Sunday. If I don’t pass-out with exhaustion somewhere along the line!

Iron ore reduction in a bloomery furnace – part 2

I’ve spent a lot of hours over the last couple of days trying to express every variable of bloomery iron smelting and their complex and dependent relations. At the moment, I can’t seem to do it in any way that doesn’t look like a spider covered in multi-coloured ink had a seizure on my page.

The best I can do is below – and although I think it looks pretty it only covers the general shape of smelting, it isn’t as awesome as I was planning. Anyway, I thought I’d share the first draft with you for two reasons. I hope that if anyone has any better ideas, comments, corrections or other reading they will comment or get in touch, and I may be able to improve the diagram. I also hope that anyone trawling the web for information on this topic will find the diagram to be of some use.

Iron smelting is a much more complicated process than copper smelting, despite that fact that both could be conducted in essentially the same structure and using much the same techniques and materials.

During copper smelting liquid copper can be produced, because copper has a melting point of 1084 C and furnaces can conceivably get up to or above that temperature. However iron melts at around 1536C, a temperature which is incredibly difficult to reach without modern machinery, and at which the ceramics of pre-modern furnaces are likely to melt and collapse.

As a result, iron is very rarely liquid during the smelting process. When it is produced, it is generally thought to remains solid. Luckily when iron oxide and silica are present they can melt together, and this forms the slag that is the most common archaeological evidence for smelting. I like to think of the slag as having a similar beneficent effect to  ’primordial soup’ – it allows the transport of the tiny fragments of iron metal so that they can conglomerate as a solid mass, and it covers it from the strong oxidising effect of the air being pumped into the furnace.

For good slag formation we need enough carbon monoxide to be circulating inside the furnace system, and high enough temperatures, so all three of these components effect the outcome of the smelt and are inextricably linked. But of course, you can’t just throw in more CO if its not working. What you can do is fiddle with the proportions of fuel to ore you put in, the size of these pieces, the amount of air you force into the furnace, and you can patch the furnace so that the CO doesn’t escape through the furnace walls.

The diagram below is a summary of this. Oh, except for the fact that you can also add ‘flux’ to ease the formation of slag, and I haven’t put that in. But you can imagine it in the bottom pile of variables if you like. Enjoy!

The full reference is:

Tylecote, R.F., Austin, J.N. & Wraith, A.E., 1971. The mechanism of the bloomery process in shaft furnaces. Journal of the Iron and Steel Institute, 209, 342-363.

It’s a rather full-on and large report of a campaign of experimental work undertaken by Tylecote and his colleagues, but it’s essentially invaluable. Most of what we know of how the various parts of the process contribute to the final product (the bloom) and how they interact comes from experimental work, of which Tylecote is the dominant contributor.

World of Iron Conference 2009

Wow, wasn’t that cool!  Last week was spent Tuesday-Thursday afternoons at the World of Iron Conference, which was held at the Natural History Museum in the Flett Theatre.

Wikimedia Commons image by David Illif

The conference itself ran Monday pm through to Friday am, but unfortunately due to financial and time constraints (ie I had to take most of the time as personal leave and pay for the conference myself)  I could only make the afternoon sessions. However, I have to say that I really enjoyed them and found most of the papers to be very interesting. The sessions I attended were:

Invention, Innovation and Inspiration Chaired by Gill Julef (Exeter University)
Theoretical Approaches to Technology Chaired by Pierre Lemonnier and
Scientific Approaches to Technology Chaired by Vincent Serneels (Fribourg University)

Hearing Pierre Lemonnier chair was awesome, as he is one of the greats of ethnographic and anthropological study of technology (chain operatoire etc), and Gill Julef is a good speaker and comes across as sharp as a button and the kind of person you could have a really good conversation with. I have to admit to not knowing exactly who Vincent Serneels was… but he did chair his session well, and handled the extended discussion afterwards.

The sessions themselves were good, though the Theoretical Approaches was perhaps a little dry – conversation during the post-conference drinks indicated that most of the hard-core archaeometrists/archaeological scientists felt that very little of the ethnography/anthropology discussed (excepting Louise Iles, UCL) was applicable to them  - which was rather not the point of the session.

In addition, there did seem to be rather a lot of shouting about menstruation, blood, furnaces being women, furnaces having breasts, furnaces giving birth, impregnating the furnace, nakedness (or not) near furnaces, semen and other bodily fluids, which made all of us British and north-western Europeans a little uncomfortable. To be frank, it was all a little bit weird! I have to admit to having problems trying to relate much of this work to my view of British prehistoric practices, but maybe that just shows my own weaknesses – still, I rather imagine the Iron Age Briton to be too busy being grumpy about the terrible weather to think about dancing round his furnace and splattering it with blood!

However any weakness that afternoon was made up for by the fantastic papers given in the Scientific Approaches session. Wow! I mean, I am a self-confessed geek, but it was really great to see so many people using statistical multivariate techniques to interrogate their analytically-derived datasets.

My favourite paper was probably Michael Charlton’s Measuring variation in iron production slags. He used a few different techniques, including Kernal Density Estimation (which was new to me) to look at his data, and he seemed to have some really promising ways of  classifying and grouping his results in a meaningful manner. Also of interest was a paper (unfortunately not fully given as the author was taken ill) on fluid dynamics and modelling furnace interiors from Exeter University, which suggested promising things.

Attendance to the conference was reasonable, considering the niche subject – I think they had around 100 people every day, and a lot of people were only popping in for one or two days.  The theatre could hold 250, so looked a little empty, but there were enough people there to make it seem full enough.

I didn’t think much of the timing though – it was clearly set up during university reading week  so that the organisers weren’t teaching etc, however it was also half-term so the museum was packed – to the extent that queues were going all the way out and into the access tunnel from the tube station. Whilst we were given passes to the conference, it took a bit of persuasion to get the guards to let me in ahead of the queues, and the press of people inside and the queues for the toilets were very tiresome.

However the Museum itself was a great venue, and having the conference buffet and drinks  in the Treasures Gallery was really lovely, as it’s full of beautiful objects and looks lovely at night. The buffet was pretty poor if you’re an aspiring vegetarian like me (I think there were two or three things I could eat), but although the variety was limited the food was good quality and the red wine certainly seemed to get the conversation flowing!

Of course, the venue and the food were all down to sponsorship deals with Tata Steel and Rio Tinto that had been struck as little as a month before the recession began to bite. The conference itself was largely the product of Xander Veldhuijzen, a post doctorate at the Institute of Archaeology ULC, and  Jane Humphris, a PhD student also at the Institute, with Thilo Rehren, Professor in Archaeological Materials and Technologies at the Institute.

As a result it looks like it’ll probably be a one-off, but it was well worth the effort and the organisers did very well bringing it all together. I certainly met some really interesting people and left the event with my head buzzing with ideas every evening. Fingers-crossed that Jane Humphris finds a post after completing her PhD – she’d certainly make a good addition to any faculty if her conference-organisation skills are anything to go by!