Rot stopped - the science of preservation

Rot stopped - the science of preservation

  • Artefacts from the Mary Rose
    Artefacts from the Mary Rose

Artefacts from the Mary Rose at the Mary Rose Museum www.maryrose.org

Archaeological artefacts often begin to decay as soon as they are removed from the ground or water. Halting the process takes a combination of art, expertise and a big dose of chemistry. Words by Lucy Jolin.

Gold remains perfect; fragments of bone can survive cremation. Bronze tarnishes and iron can corrode to nothing. Insects feast on palm-leaf manuscripts. Old glass takes on a beautiful, iridescent patina of decay. One moment can survive centuries of burial if the soil is right, while whole civilisations are lost through an accident of climate. And some long-gone objects still stubbornly cling to life: wood can leave an echo in grains of sand, while a pattern on medieval linen can imprint itself on bronze. Preservation is a complex, patchy story, and yet whole historical arcs have been built on little but the tenacity of certain metals.

Preserving the Mary Rose

Sometimes preservation is an exact science. On 19 July 1545, just two kilometres from the entrance to Portsmouth Harbour, the Mary Rose – a warship built by Henry VIII and named after his sister – met a fleet of French galleys and sank with the loss of around 500 crew members and 19,000 artefacts. At least, that is how many have been recovered from the wreck site so far – the salvage operation began in 1971 and continues today, even though the ship itself was raised in 1982.

There are ceramic jugs and bronze bells, leaded brass chainmail, ivory and wooden nit combs, and wooden surgeon’s canisters containing traces of a grey powder of beeswax and poppy oil, used to treat inflammations and as a painkiller. There are pine tool holders, rat bones, and an oak, bark and yew board for tables (an ancestor of backgammon). But although they have been successfully salvaged, the process of preserving these objects must continue – and creates ongoing challenges, particularly when it comes to waterlogged wooden objects.

“The brief was quite vague: help preserve the Mary Rose,” says chemist Dr Zarah Walsh, who took on the job of finding a new way to preserve waterlogged wooden objects while undertaking postdoctoral research in the Chemistry Department’s Melville Laboratory of Polymer Synthesis. (Professor Mark Jones, head of conservation at the Mary Rose Trust, and Dr Oren Scherman, lecturer in Chemistry, were principal investigators on the project.)

A natural solution is better

Keeping intact an object that’s been under the sea for 500 years is not as simple as just removing it from the water. Waterlogged wood can lose up to 70 per cent of its volume when it dries. It warps and cracks. Traditionally, wooden objects are preserved using polyethylene glycol (PEG). Large pieces are sprayed; small pieces are submerged in the substance. It works by liquid exchange. The PEG pushes the water out and replaces it, stabilising it and bulking it up.

“But it’s not a perfect solution,” explains Walsh. “PEG can degrade down to formic acid, which is bad for the cellulose in the wood. In seawater, iron degrades down to nothing, so the wood can be saturated with iron ions. Add that to the presence of sulphur in the water from organic deposits – dead fish – and you get sulphuric acid, which is also damaging to cellulose. And because PEG is very good at moving ions from one place to another, it allows those iron ions to move through the wood. So instead of getting a local problem, you get a widespread problem.”

Dr Walsh and research co-author Emma-Rose Janeĉek came up with a simple and elegant solution to tackle these problems: a new treatment made from non-synthetic substances – principally chitosan, a natural antibacterial which gives structure to shrimp shells, and guar, a bean commonly used as a food thickener. The new treatment is about to be tested on larger pieces of wood. “It was good to use a substance from the sea,” says Walsh. “It felt great to think about the circular nature of it: it was all about things coming from the water.” (Those shrimp shells preserve lives, too: chitosan is the key ingredient in the HemCon bandage introduced by the US Army in 2002, as its binding properties can help stem uncontrollable bleeding on the battlefield.)

Climate can help preserve manuscripts

Sometimes preservation is a glorious marriage of ancient and totally modern. In the University Library sit more than 1000 tiny manuscripts from south Asia, written in a variety of languages including Sanskrit, Nepalese and Tamil. They have been created not just on paper but also on pieces of palm leaf and birch bark – many of which only survived because they happened to be written or preserved in Nepal and therefore in the right climate.

“Paper was introduced to India relatively late – not before the 12th century, and it didn’t really catch on until the 15th century,” says Dr Vincenzo Vergiani, senior lecturer in Sanskrit, who has recently completed a project to catalogue the collection. “Now, in India itself – and by that I refer to more or less what it is today, the modern state of India, Nepal, Bangladesh and Pakistan – the climate is such that manuscripts don’t survive long. Palm leaf is very sturdy material. It’s very good for a writing support but in the heat and humidity, bugs feast on it. So the further south you go, the shorter the life of the manuscript. In south India they don’t survive for more than 100 years, 200 years at the most, which means they need to be recovered regularly. In the north, it is a bit less hot and humid. But Nepal, on the other hand, has the perfect climate. It’s more temperate – cold winters, less humid in the summer. So there you have manuscripts going back to the first millennium AD.”

The palm-leaf manuscripts are surprisingly hardy, says Vergiani: you don’t even need to wear gloves when you handle them, and they are, of course, kept in strictly temperature-controlled conditions. But the collection has also been digitised, meaning that researchers don’t even need to be in the same room as the manuscripts they are working on. In many cases, examining an object digitally can be even better than doing so in real life: you can magnify it and get up close without the risk of damaging it for future generations.

Preservation is not about an imagined mythical state

Preservation isn’t just about objects. Environment is when it gets really tricky. “When you’re excavating, it’s the structures and the things all around you that contribute to your thoughts,” says Dr Helen Geake, Visiting Scholar at the McDonald Institute for Archaeological Research. “With a grave, you’re more likely to stand around discussing what that funny brown stain is than you are to talk about the objects, because that brown stain is much more fleeting. If you don’t talk about it then, if you don’t take a photograph of it then, you are now going to dig it away because you have to find out what’s underneath.”

It’s an ongoing story – sometimes, a messy one. “It’s tempting to be critical of things people have done in the past, but if they hadn’t done them, we wouldn’t have that object now,” says Julie Dawson, senior conservator at the Fitzwilliam Museum. When we imagine a preserved object, we tend to imagine that it is in its original form, “an imagined mythical state”, she says.

But conservators no longer attempt to achieve this. These days, past attempts to preserve have become part of the object’s story: the 19th-century iron nails hammered through an ancient Egyptian coffin in order to hold it together, the corroded Roman bronzes chemically stripped and repatinated by an early collector in order to give them a more acceptable aesthetic.

Dawson points to Object GR.2.1885, a statuette of Apollo. Originally it was just an anonymous torso carved in ancient times. In 1793, British artist John Flaxman and the Italian sculptor Antonio D’Este added arms, legs, a head and a quiver of arrows. “It’s become something else. Now, it tells us also about what somebody who’s restoring a piece of classical sculpture in the 18th century thought and felt about it, and what was informing that practice at that time.”

Inferring existence from artefacts

It’s also about what isn’t there: inferring the existence of things that you don’t have, based on what you do. So if you find woodworking tools, then you know people were working wood. Or sometimes, you can use a proxy: one thing that represents something else. “For example, Ben Cartwright, a recent PhD student of mine, was looking at textile working and how it might have changed through time with the arrival of the Vikings,” says Dr James Barrett, reader in medieval archaeology at the McDonald Institute for Archaeological Research. “The textiles themselves are rarely preserved, but it’s possible to find their impressions in metalwork, in graves.

“Or take the Scar Viking boat burial on the Scottish island of Sanday, which had very poor preservation of wood. Archaeologists there were able to actually take blocks of the sand where the wood had been, impregnate them with resin and then look at them in detail. In doing that, you can see where the planks and the caulking between them were. And in this instance, there were even some grains of a different sand in the caulking that allowed them to infer that the boat had not been made in Scotland but imported from somewhere else.”

When Geake, a regular on Channel 4’s Time Team, participated in a BBC television programme on early-medieval seafaring, the production company received a furious letter concerning the discussion on the role of women within Viking society. “This is a lie!” it read. “The Vikings were all men!” It was funny, she says, but it illustrated a truth about preservation: we build our stories of the past simply on the accident of what’s been left behind. We can’t adjust for that, or attempt to produce a full understanding of the past, she says. All we can do is attempt to understand our understanding.

This article first appeared in CAM - the Cambridge Alumni Magazine, edition 74. Find out how to receive CAM.

Beagle specimens

Preservation

Keeping intact an object that’s been under the sea for 500 years is not as simple as just removing it from the water. Waterlogged wood can lose up to 70 per cent of its volume when it dries. It warps and cracks. Scientists are working on a new treatment made from non-synthetic substances – principally chitosan, a natural antibacterial which gives structure to shrimp shells, and guar, a bean commonly used as a food thickener. 

Sarcophagus

Analysis

Certain materials fluoresce when exposed to ultraviolet light – a useful property when a conservator is trying to decode a degraded or restored surface. On an Egyptian coffin, traces of natural resin varnish applied as part of the original decoration emit a pale greenish glow. Modern glues or paints, applied during early conservation treatments and which can be difficult to detect with the naked eye, stand out by fluorescing in contrasting colours. 

Carolina parakeet specimen

Maintenance

The Museum of Zoology is currently undergoing a major three-year redevelopment. As part of this project, more than 2000 mounted, stuffed birds are being repaired and cleaned. Many of the birds are frozen (to eradicate insects) and then dry cleaned with a brush and a museum vacuum. In some cases, this is followed by wet solvent cleaning. To repair and stabilise feathers, a Preservation Pencil is used to fire a fine jet of moisture at the specimen, allowing the feathers to be manipulated back into their original shape. Each bird can take anywhere from 20 minutes to two days to clean and repair. 

Velvetbean caterpillar moth

Extending life

Insect pests like moths and carpet beetles can destroy textile and fur collections. Traditional fumigation methods are toxic and prohibited by law, so at the Scott Polar Research Institute (SPRI) freezing is used to kill pests instead. Textiles are sealed in polythene bags with as much air squeezed out of them as possible in order to prevent surface condensation during the freeze-thaw process. The packages are frozen at -30 degrees for a week, which kills all life stages of the pest. However, some pest eggs are very tough and can withstand extreme cold for a long time. The SPRI operates a strict quarantine programme, freezing all new material as well as anything suspected of infestation. 

Inset images: Beagle specimens, University Museum of Zoology, Cambridge; Sarcophagus via Creative Commons, Andrew Moore; Carolina Parakeet specimen, University Museum of Zoology, Cambridge; Velvetbean caterpillar moth via Creative Commons, USGS Bee Inventory and Monitoring Lab