Roman Glass Research Project

It’s a long time since I wrote an entry for this blog  – I have been taking a rest from the Basinghall glass project  while other work has taken over – lots of it ….   Meanwhile John Shepherd  and I,  with  the MOLA photographer Andy Chopping  and the rest of the publication team  have been putting the finishing touches to our little book about the glass workers of Roman London.  This takes the evidence from Basinghall Street as a starting point, but reviews all the current evidence for the history of glass working in the  city.  We describe all the various stages of glass production  illustrated by brilliant action photographs  taken at the workshops of the Roman Glassmakers, Mark Taylor and David Hill.  The book is topical, very, very colourful  and available from MoLA – take a look …..

www.museumoflondonarchaeology.org.uk/english/publications/pubDetails.htm?pid=111

I have now recorded over 1500 nearly complete moils and fragments of moil and I am beginning to count moils instead of sheep at night! John has sorted and recorded a similar number, so the end is in sight and we hope to finish this stage of the project by the end of February. Then we will start on the challenging task of analysing the data and seeing what it all means.

I have had a few questions about Roman glass this week. Tom wanted to know how it survives in the ground for so long without breaking. Well of course most of the glass we deal with is broken, – it is usually rubbish, vessels which have been broken and thrown away, or put into ‘bottle banks’ for recycling, but the glass itself survives so well because it was very stable.

The main ingredient of Roman glass was silica, in the form of sand. Soda was added to this as a flux, to reduce the melting temperature of the sand and calcium (lime) was added to stabilise the mixture. The whole lot was heated in a furnace, in crucibles, or in a large tank, until it melted and fused together to form raw glass, which in its natural form has a greenish blue colour, due to impurities (iron oxide) in the sand. Glass was made first in the eastern Mediterranean region, where all the raw ingredients were in plentiful supply, and was exported in large chunks to be reworked into vessels.

  Glass was used for many purposes, and while it was probably always quite expensive, there is evidence throughout the empire for a massive increase in the use of glass vessels in everyday life after the invention of glass blowing. We can certainly see this in London, where glass was used at table for serving vessels and for drinking cups. Bottles and flasks were used for transporting all sorts of liquids, – wine, oil and water and many different foods. Some of our finest glass vessels come from burials, where they were placed intact, containing food, drink or perfumes for the deceased, with no intention of recovery. These bottles never found their way into the recycling collections.

Glass was also used for windows, to make tesserae for the most elaborate floor and wall mosaics, for stirring rods, used with cosmetics and unguents, and for gaming counters.

Another long gap between postings, due to being entirely swamped with work. At the beginning of February I took time off to do some lecturing on Roman glass to students from the University of London – a chance to talk about the history of glass working and to show pictures of some very fine vessels. They also had a chance to handle some of the more mundane fragments.

Since then John and I have settled down in earnest to the problem of recording the moils (those little cylinders of glass left on the blowing iron when a vessel was removed). Considering that every one of these little cylinders of glass represents a blown vessel, it is important to work out how many complete moils there may have been. The differences in size and shape might also indicate different techniques, perhaps the idiosyncrasies of individual glass blowers. For example, some moils are very long and slender, while others, cut off very close to the blowing iron, are extremely short. Now this might be because they are from different types of vessel, but it might also be the way in which particular craftsman worked.

On our best examples it is possible to see a mark left by the edge of the blowing iron, which gives us its diameter and we have decided to divide the moils into groups based on the probable diameter of the iron. We have then subdivided these groups according to particular features of the moil – length, thickness, shape (cylindrical or tapering) and so on. All these basic measurements are being recorded on a database, together with the weight of the fragment. We are also recording how much of a complete moil each fragment represents, using an adaptation of a method originally devised for pottery studies (a complete moil is 100%).

All these measurements take some time, but we have to ensure that we are recording as much as possible, as we will not get another chance! Of course many of the fragments, although identifiable as moils, are so incomplete that we cannot take any measurements, but I reckon, that we still have several thousand moils to classify and record individually. I have probably done about 750 so far ……

If anyone (is there anyone out there?) has any burning questions about the project – or anything else about Roman glass, do post a comment, and I shall do my best to answer it – in between moils of course!

Back in the office after a rather protracted Christmas break followed by a long weekend away with friends and in theory revived, with batteries recharged and able to tackle all the problems of the new year. In fact I have been so busy in the last three weeks that I have completely failed to write this diary, so it is time to catch up.

Everyone was very enthusiastic about the finds displays at the Museum before Christmas and this is a photo of the glass table – we really do have an astonishing quantity of glass working waste. It is not easy to take photographs in these rather dark conditions, but the shot gives some idea of what we were able to display.

Since my return, I have finished going through the entire assemblage, separating it into the different categories of waste, all now neatly boxed, describing every identifiable vessel, and weighing every accession. We now have over 47k of raw material, tank metal from the furnace.

This includes some massive individual chunks, one weighing 30k, but it is still a huge quantity. The picture shows the huge lump of glass being excavated. It is irregular in shape, filling a small pit and it may have flowed as molten glass from the tank furnace.

The most astonishing figure is probably that for the moils, which you will remember are the little cylinders of glass left on the blowing iron after a vessel has been removed. Every complete moil therefore represents the blowing of one vessel. We have a total weight of over 6.5k of moils and an estimated count of over 5000 fragments. Of course most of these are incomplete and many are extremely small fragments, so John and I are devising ways in which to record them in order to establish how many vessels they represent. We are also looking at the best ways in which to classify and measure them. I’ll keep you posted, but I can definitely say that the recording is going to take some time …..

 Although I made good progress last week, and have now gone through more than half  the glass, various other activities  (mostly singing) took up all my spare time  and I failed to write this diary.     

 This week  started very well and by Tuesday I only had about another 600 ‘accessions’ to sort and record, but I was then struck down with a nasty flu-like bug which is doing the rounds. Two days in bed, unable even to watch daytime TV!  And delirious dreams about monstrous glass moils are not to be recommended!    

At least I have got over the worst of it now, but I was sorry to miss a reception held at the Museum for our clients and sponsors,  for which I had prepared a display of the glass from Basinghall Street – and it  looks very impressive presented all together in this way.  My colleagues very kindly arranged it all for me and stood in for me on the night – I am hoping that someone took a photograph.

Back at my desk now, and trying to catch up a little before the holiday –back on January 2nd  ….    A very happy Christmas and New Year ….

Although at times these days it seems to dominate my existence, I do have a life beyond glass …. and last Monday I spent the morning doing something completely different, talking about the archaeology of Roman London to a group of A level students at the Museum. It was a chance to think about the history of recorded discovery in London, which goes back to the 16th century and to look at some of the amazing discoveries of the 20th century, all illustrated by pictures of excavations and some of my favourite finds photographs. Excavations have revealed the physical remains of buildings but the objects found within these buildings, in rubbish dumps, in ditches and streams can help us to interpret how the people of Roman London lived, what they wore, how their houses were furnished, what they cooked with … and so on. The fruits of all this research can be seen in the displays at the Museum and in our publications.

This is one of the most interesting an unusual oil lamps found in recent years, on a site in Southwark. It is made of pottery and is moulded in the shape of a human foot, wearing an elaborate sandal. The area of soot around the big toe shows that is was not only decorative, but functional.

On Tuesday I returned to the glass project and I have now worked through about a quarter of the total number of boxes, weighing every fragment and separating out the runnels, droplets, burnt vessels, moils and all the other categories of production waste so that we can look at them in more detail later. There is quite a large amount of window glass among the cullet, collected for recycling. Window glass was a luxury, used only in the more substantial buildings and bath houses – in the early days of the city at least the windows of most private houses would have been shuttered and rather dark.

One piece of window glass from Basinghall Street is of interest as it still has mortar attached to it – evidence that it had already been used in a building, and was to be melted down with the rest of the cullet.

I have made quite good progress during the remainder of the week, and am just about on target to finish this stage by Christmas.

I have now started on the huge task of going through the entire collection to record the details of the different types of waste on our database. I am sorting the different categories into separate boxes as I go, so that we can go back and look at the various groups in more detail later on. Meanwhile I am weighing everything and recording the dates and descriptions of the more complete, identifiable glass vessel fragments, generally those with rims or bases. There are over 2000 separate accessions so it may take some time!

Some of the bags are ‘bulk accessions’, containing many fragments. The largest of these are collections of vessel glass which was used as cullet, melted in the furnace to be used as raw material for blowing new vessels.

On one large piece you can see individual vessel fragments which have fused together in the heat. We have already sorted the glass into different colours and I am quickly going through the bags again to look for any rim fragments which we may have missed first time around. Most of the glass is in various shades of blue-green, the natural colour caused by the presence of iron oxides in the sand from which it was made originally, but quite a lot is better quality colourless glass. I am weighing all this glass, but life is too short to count every fragment!

I have been very lucky this week to have had some help with some final work on all those sieved glass residues. Monica, one of my colleagues, has done a brilliant and painstaking job of sorting out the smallest fragments into their various colours for which I am very grateful indeed.

John and I are writing a short booklet about the glass project. We have completed the first draft and are now sorting out the illustrations – lots of them. We hope that it will be published in the spring – something to look forward to for the New Year …

One of the important questions which we will try to answer is the scale of the glass production at Basinghall and this means that we will have to weigh the entire collection, and for some types of production waste, count every fragment.

I have started this week by looking at the tank metal, the raw molten glass from which vessels were blown. There is no evidence from London that glass was ever made from the raw materials. Instead, it was either imported from the Mediterranean in large blocks, or alternatively broken vessel and window glass, known as cullet, was collected for recycling. This was common practice throughout the Roman world where there appears to have been a trade in broken glass, rather like modern bottle banks, and the evidence suggests that it was the prime source of the basic material for the London glassblowers.

The glass was melted in a furnace, in rectangular tanks, hence the term ‘tank metal’ and at Basinghall some large fragments have straight edges, where they lapped the side of the tank. It looks as if the glass has been allowed to cool in the tank as some of it shows distinctive crystallisation, and this might mean that it was unused – perhaps thrown away when something went wrong with the furnace, or the workshop went out of use – there are so many possibilities and questions!

Most of the fragments are a natural blue-green in colour, but some of the large fragments are a much darker green. There is actually quite a lot of variation in colour, but some is due to the size of the fragments, so I have had to record their general size as well as the weight of the tank metal from each context (archaeological layer in the ground). Later in the project we will be looking at the chemical composition of some of the chunks of glass to see if they may have come from different sources.

After a day or so, I have recorded over 18.5k of tank metal from 125 contexts – that is a lot of glass!

This week I have finished sorting the residues from a single huge deposit of glass waste, selecting ever smaller fragments. The largest pile is naturally coloured blue-green vessel glass, which was collected for recycling, but I have found many more tiny fragments of glass thread, some less than 1mm in diameter. These were produced at various stages in the glass-working process, sometimes when testing the viscosity of the molten glass.

Strands and threads were also formed when a lump of hot glass, the gather, was removed from the furnace on the end of a blowing iron before it was inflated. Minute fragments fell on to the workshop floor and these are extremely fragile, which suggests that our dump of waste was from the working area itself.

When sorting out some colourless glass fragments I was very pleased to spot some distinctive glass with a fine crackle or crazing on the surface. This might be a faulty vessel, made in the workshop and thrown away, but it is too early to say yet – we shall have to look out for some more.

John and I have been looking at photos of modern glassblowers working in the Roman style with an experimental furnace, to see what sort of waste is produced by the various activities. This is a fascinating exercise and I am learning a lot. We hope to be able to classify our fragments by the production process to gain further insights into Roman glass-working techniques. Next week I shall begin the mammoth task of recording it all in greater detail.

First steps
So my first job is to complete the sorting of the glass residues which have all been sieved to retrieve the tiniest fragments. I have about 3kg of these, which were roughly sorted at assessment, but I am now going through them again, so that we can retrieve the maximum amount of information. The unsorted pile looks quite a jumble, but gradually this falls into more ordered heaps. You can see the different glass colours quite easily.

Roman glass was made by heating silica (sand), soda, which was used as a flux to reduce the melting temperature and calcium (lime) which acted as a stabiliser. Iron oxide, a naturally-occurring impurity in the sand produces a the characteristic blue-green shades of much Roman glass, but various minerals were added to produce coloured glass and also to make the clear colourless glass which became fashionable in the later 1st century AD. Most of the glass at Basinghall is blue-green, but much is colourless and there are a few fragments in dark blue and amber.

Sometimes, sorting these fragments into ever smaller heaps begins to feel endless, and I need a good light. Not a job for these dark evenings – its time to go home …..

Photos: a heap of unsorted Roman glass; Roman glass sorted into the different types of waste

The site

The site at 35 Basinghall Street lies on the western edge of the Walbrook

valley, on the fringes of a marginal area, away from the main focus of residential settlement in Londinium. Before the start of the excavation we already knew that the Walbrook valley was an area of industrial activity, with the site of a major pottery workshop on the eastern side of modern Moorgate. There was also evidence for leather and bone-working and for a 2nd century glass furnace on the edge of a canalised Walbrook tributary, suggesting that this part of the town was occupied by small workshops.

Discovery
When almost 70kg of glass waste was found in the south-west corner of the site at Basinghall we were faced with a colossal task. Usually, at MoLAS, every fragment of Roman glass is accessioned (given a unique number) and recorded on our database, but with tens of thousands of fragments, this was clearly impossible.

After initial recording the first stage in the post-excavation process is always an assessment of the material, looking at its significance in relation to the site, to Roman London as a whole and, as in this case, to the wider Roman world. Clearly we had to establish rapidly exactly what we were dealing with and how much of it there was in order to devise a programme of study and research which would lead to publication.

The scale of the problem
Most of the waste came from a single context, the fill of a pit, so we sorted this, by glass colour, into the different types of waste; raw materials in the form of molten glass from the tank, which was part of the furnace and cullet, broken vessel glass which was collected for recycling and production waste.

This took various forms which I shall discuss in later postings, but the most easily recognisable fragments were molten fragments and runnels from the furnace, which had fallen on to the workshop floor, and moils, little cylinders of glass which were left on the blowing iron after the vessel was removed. Each moil therefore represents a single glass vessel and normally these fragments would have gone back into the melting pot to be blown again.

At this first stage, we accessioned only the best examples of the moils and other types of waste, and all the vessels which could be recognised by form, bagging up most of the moils as ‘bulk’ accessions. Even so, we made out over 2000 record cards. Most glass-working sites in London have produced only about a dozen moils, but at Basinghall we estimate that we have over 3,000.

The questions
There are some fundamental research questions which we are going to try to answer during our work on this project and some of these can only be answered by analysis of the glass composition, a very expensive process. We hope to work out, for example, where the raw materials came from and how they were prepared, what the glass workers were making and the techniques which they used, when the workshop operated and for how long. We shall explore these and many other detailed aspects of the glass-working industry in London over the next few months.

Having devised and costed our programme of research, we presented it to the very supportive developers of the site, Stanhope plc, who have generously agreed to fund it. We are now ready to start the programme and hope that our initial estimates will prove to be accurate!