What can scientific investigation of glass from wrecks tell us?
For British Science Week (6-15 March 2020) I am delighted to welcome my colleague Dr Sarah Paynter, Materials Scientist at Historic England, as our guest blogger. She describes some of her recent work involving analysis of glass from wreck sites and what these finds can tell us about wrecks. Some of the key finds recently analysed include those from designated wreck sites, such as the wreck of the London, lost off Southend-on-Sea 355 years ago this week on 7 March 1665, or the ‘Wheel Wreck’ off the Isles of Scilly.
Over to Sarah:
The Historic England laboratories at Fort Cumberland specialise in the conservation and analysis of all kinds of ancient and historical materials. We have worked on archaeological glass for many years but when our remit expanded to include wreck sites, we had the opportunity to work on glass artefacts recovered from the sea. It has been an eye-opening experience . . .
Cargo vessels and warships alike contain a surprising amount of glass, some as part of the vessel structure, such as windows at the stern, but there are also glass components in the instruments (sand glasses and sundial/compasses) and personal belongings (spectacles and mirrors), as well as fine goblets for officers’ use, and often a great many glass bottles and beads amongst the cargo.
On a wreck site this glass can be found scattered in a debris field and the preservation can often be remarkable – despite terrible explosions, navigational errors, violent storms and loss of life, glass bottles can still occasionally be found intact and unopened with the stopper in place.
Ironically, a damaged object generally holds more promise for us than an intact one, because we can usually take a small, fairly unobtrusive sample from a previous break. We use pincers or glass cutters to clip the broken edge, giving us a sample a few millimetres across, which exposes fresh glass. We need this fresh surface to obtain a good chemical analysis because even seemingly well-preserved objects are altered by their time in the depths.
The surface is usually covered in a fragile skin of iridescent, flaky, weathered glass, as well as concretions, marine organisms and sandy mud, all of which limit the usefulness of surface analyses. We can identify old breaks because these also have a matt, altered surface, whereas any breaks that have occurred during recovery and post-excavation handling are shiny and smooth.
The chemical make-up of the glass, and the environment that it has lain in, both have a huge impact on its condition when it is recovered centuries later. English medieval glass made before the mid-16th century tends to degrade very quickly, whereas later glass can be miraculously preserved because it is chemically more resistant to weathering (Historic England 2018).
Scientific analysis of glass:
We use several different analysis techniques in our work, depending on our research questions, the size and condition of the object, and whether we can take a sample. The scanning electron microscope (SEM) can show us the structure on a microscopic scale, as well as chemical composition, even if the sample is only a few millimetres wide.
The benchtop XRF (X-ray fluorescence) spectrometer provides similar chemical information and we can sometimes fit intact objects into the machine, which is ideal if we cannot take a sample from them. We also have a portable XRF machine, which gives less complete results, but which can be used on any object, even those that are still wet or very large. It can also be taken out on site as it is about the size of a hairdryer and not much heavier.
One of the main advantages of working on wreck material is that we often know to the day, when the ship was lost, and perhaps the ports of origin and destination. This means wreck sites can provide precisely dated material for the archaeologist. The objects might be found in a case packed for transport, in a chest of personal belongings or on the deck where they were being used. Accounts of the time may even provide us with details of ship architecture, provisions, armaments, cargo and crew, and the life of the vessel from the shipyard through to eyewitness accounts of its final journey. It is very rare in land-based archaeology to have so much information around the context of an object. Samuel Pepys, the famous diarist, gives a detailed account of the devastating loss of the London, in an entry on March the 8th 1665:
“This morning is brought me to the office the sad newes of “The London,” in which Sir J[ohn] Lawson’s men were all bringing her from Chatham to the Hope, and thence he was to go to sea in her; but a little a’this side the buoy of the Nower, [the Nore] she suddenly blew up. About 24 [men] and a woman that were in the round-house and coach saved; the rest, being above 300, drowned: the ship breaking all in pieces, with 80 pieces of brass ordnance. She lies sunk, with her round- house above water. Sir J[ohn] Lawson hath a great loss in this of so many good chosen men, and many relations among them. I went to the ‘Change, where the news taken very much to heart.”
Bottles, instruments and windows: tracing technological changes:
The reasons for analysing archaeological and historical glass are varied, but we are always aiming to answer a question, either to identify something, or to work out how old it is or where it is from. Wreck material also serves another purpose because we can often date it so precisely, so we are using our analyses of glass objects from precisely dated wrecks to add to a kind of ‘calibration curve’ for how the composition of windows, bottles, beads and vessels change over time in England. On the occasions when we are presented with glass from an unidentified wreck, we have used our ‘calibration curve’ to estimate the date of the wreck from the composition of glass bottles or trade beads. For example, bottles were recovered from the protected wreck site of an unidentified vessel with a cargo of probable mining machinery in the Isles of Scilly, known as the ‘Wheel Wreck’. Analysis of the bottles added to growing evidence that the vessel might be older than previously thought.
This ‘calibration curve’ works especially well for post-medieval glass because technological developments appeared thick and fast from the 16th century onwards, and glass compositions changed quite rapidly (whereas in earlier periods, the technology used to make glass remained fairly constant for centuries at a time).
We can see how quickly glass technology changed in later periods by comparing the glass objects from two Navy warships: the London, which exploded off Southend on the 7th of March 1665, and the Stirling Castle, which was wrecked alongside three other warships a few decades later in the Great Storm of the 26th of November 1703. The shape and composition of green glass bottles has already changed subtly even in this brief period of less than 50 years, to the extent that they began during the 18th century to resemble modern bottles more closely. (Burton 2014)
We can see some differences in the images below:
At this time, bottles were made by gathering hot glass on the end of a blowing iron, inflating a bubble in the glass to form the body of the bottle, and lengthening the neck. The sides and base of the bottle body could be shaped using a mould or a flat surface, even the floor. The end of the bubble was pushed in to create a ‘push-up’ at the bottle base, so that it would stand on a flat surface.
To finish the bottle, the pontil iron was attached to the bottle base so that the neck of the bottle could be broken free of the blowing iron, and then the rim shaped by further working or by applying more glass. Finally the bottle was detached from the pontil, leaving a pontil scar.
Finds specialists can use the characteristics of bottle bases, including the type of pontil mark and the shape of the push-up, to work out the date of a bottle or where it was made. We can also look at the composition of the bottle glass to see if it matches our analyses of other English-made bottles. With cargo vessels, analysis is a useful tool to investigate the movement of goods around the world. We can use analysis to work out where goods were made and where they were going to.
Glassworkers could also make different types of glass, depending on what the glass was going to be used for. For a lens in a pair of spectacles or an instrument, like the pocket sundial/compass from the London, they used special, purified, more expensive ingredients to obtain a colourless glass (instead of the common green). Colourless glass was also used to make mirrors and the best goblets. The closely guarded industrial secrets for making colourless glass were originally brought to England in the 16th century, along with other technology, by glassworkers from Continental Europe, and their expertise led ultimately to a revival of the English glass industry.
Glass windows on ships are a particularly striking feature and appear to develop in parallel with those on buildings. Glass had become more widespread in the windows of ordinary homes in Britain by the early 17th century, and ships dating to the 17th century, like the London, also had windows incorporated into the elaborately decorated stern, where the captain’s cabin would be situated.
In the past, glassblowers had several techniques for making flat glass for windows.
One technique involved making rectangular sheets of glass (broad or cylinder glass) by elongating a blown bubble of glass into a more cylindrical shape using gravity, which involved swinging the blowing iron back and forth whilst standing on a platform, or over a pit. The cylinder was cut along its length, then unfolded and flattened.
Alternatively, a round sheet of glass, known as a crown, was made by blowing and shaping a bubble, which was then transferred to a pontil iron rod so that the other end of the bubble could be opened up. When the glass was spun, it opened into a disc shape, or ‘crown’. When the pontil was removed it left a ‘bull’s eye’ in the middle with a pontil mark. Diamond-shaped quarries of glass, for glazing windows, were cut from the thinner glass around the edges.
In earlier windows, the glass quarries were joined together using bendy lead strips known as cames, and the glass tends to have a greenish colour, similar to contemporary bottles, or the green glass component from the London sundial shown above.
Later in the 17th century wooden glazing bars were adopted to hold the glass panes in place. As time went on, the technology for making large sheets of flat glass improved, so window panes could be made larger, and the ingredients used to make the glass were improved so that the glass became increasingly colourless (Dungworth 2012).
By contrast, the tiniest glass objects we have encountered so far on wreck sites are glass beads, which were made and traded on a vast scale in the past, either small and plain beads, or elaborately multicoloured examples. The better-known European manufacturers were based in Venice, Amsterdam and Bohemia, where huge numbers were made, and there were also established bead-makers in the Indo-Pacific region and Africa.
European beads were widely transported by sea, with a commensurately widespread distribution in archaeological contexts, reaching the American and African continents. Plain, monochrome beads can be superficially difficult to tell apart just by looking at them, but examining their composition will usually give us enough clues to work out when and where they were made. So glass beads from wrecks can also help to answer archaeologists’ questions at wreck sites around the world, as a means of dating contexts and investigating trade.
At Fort Cumberland, the work on all kinds of finds from wreck sites around the coast of England continues to aid in our understanding and management of wreck sites. There can be few more appropriate locations to investigate the remarkable finds from historic ships than in Portsmouth, a port city and also home to the Mary Rose Museum and the National Museum of the Royal Navy.
Discover our online dive trails: visit wreck sites without getting wet!
With particular thanks to colleagues at Historic England (Angela Middleton, Serena Cant), the HMS London licensees, Cotswold Archaeology, Michael Walsh, Jörn Schuster, Kevin Camidge, David Dungworth, Florian Strӧbele, Fred Hocker, Niklas Eriksson, ‘The Glassmakers’ Mark Taylor and David Hill, Alastair Miles at the Mary Rose Trust, Diana Davis at the National Museum of the Royal Navy, the Nautical Archaeology Society, and all those collaborating on the wreck sites described here.
Burton D, 2014 Antique Sealed bottles 1640-1900 and the families that owned them. Antique Collectors Club Ltd.
Dumbrell R 1992 Understanding antique wine bottles. Antique Collectors Club Ltd.
Dungworth D 2012 ‘Historic window glass. The use of chemical analysis to date manufacture’ Journal of Architectural Conservation 18, 7-25.
Gillespie CC (ed) 1959 A Diderot Pictorial Encyclopedia of Trades and Industry. New York: Dover.