Art & Antiquities: Conservation and Preservation

We find out how scientists and art conservationists work together to develop new techniques to preserve our cultural heritage.
02 May 2013
Presented by Chris Smith, Ginny Smith


An aerial view of St Paul's cathedral


The conservation and restoration of great art once relied on only a good eye and talent with a paintbrush. Now though, scientists and art conservationists are working together to develop new techniques to preserve our cultural heritage.

In this episode

A honey bee on a flower

01:10 - Europe to Ban Controversial Pesticides

From 1 December this year, farmers in the EU will be banned from using three insecticides on crops that attract bees...

Europe to Ban Controversial Pesticides

From 1 December this year, farmers in the EU will be banned from using three insecticides on crops that attract bees. The ban comes after some studies implicated the insecticides in the phenomenon known as colony collapse disorder and will last two years.

Since 2006, worrying numbers of worker bees have been disappearing - for example in the US last year, commercial beekeepers lost up to half of their hives. That's not just a problem for those of us who like our honey but is also a huge problem for agriculture. Bees pollinate the crops we eat as well.

Over the last few years evidence has been stacking up suggesting that a relatively new class of pesticides, called neonicitinoids because they are variants of the molecule nicotine, could be at least partly responsible. However, the case is confusing rather than clear cut. This led the European Food Safety Authority to look at the evidence and recommend that three neonicotinoids - clothianidin, imidacloprid and thiamethoxam - pose an unacceptable risk to bees and should only be used on crops that are unattractive to the insects. However, other scientists, including those at the UK's Department for Environment, Farming and Rural Affairs (DEFRA) have been concerned that the studies do not adequately model bees' pesticide exposure to in the field.

While the science is disputed, environmental campaigners have been vocal in calling for neonicotinoids to be banned and in the US they are even suing the Environmental Protection Agency over the continued use of the pesticides. Back in Europe, several countries had taken matters into their own hands with France, Germany, Italy and Slovenia all placing partial bans are in place on some neonicotinoids for specific crops.

And so a vote by European member states this week, an appeal following an earlier vote this year, has resulted in a ban being scheduled from December.

While environmental groups celebrate, however, there are still concerns about the effectiveness of the ban, especially if farmers revert to using the older and more environmentally unfriendly pesticides that neonicotinoids replaced. There are also worries surrounding the loss of productivity that might result from the moratorium on neonicitonoids as neonicotinoid seed treatments have allowed farmers to extend the growing season for many crops, increasing yields.

Europe will, in effect, be a continent wide field trial and monitoring will have to be well planned and executed. The results of the ban could have far reaching consequences.

honeybees on honeycomb

04:20 - Honey essential for bee health

Honey contituents activate detox and anti-bacterial genes in bees, allowing them to break down pesticides and fend off infection.

Honey essential for bee health

Honey contituents activate anti-bacterial and detox-controlling genes in bees, allowing them to break down pesticides and fend off honeybees on honecombinfection. And this may explain bee population drops seen in recent years, because the food substitutes given to farmed bees to replace their honey lack these substances.

Bees are vital to pollinate our food crops, as well as to keep our wild areas diverse and beautiful. But their numbers have been in decline recently. Over the last 5 years, in the US, 30% of managed colonies have been lost each year.

Some people think this could be due to infection by pathogens, or to pesticides affecting the bees' natural behaviour. A new paper, published this week in PNAS, however, suggests it may be due to the food they are eating.

Normally, bees collect pollen and nectar from flowers and use it to produce honey. But when food is scarce, or to save money, bee-keepers often substitute sugar syrup for the honey. Although nutritionally similar, Mao and colleagues from the University of Illinois have shown that it is lacking components that may be vital for the bees' survival.

They extracted four components from honey, 3 of which cause increased activation of a family of genes, called cytochrome p450 oxygenases, that are responsible for detoxifying pesticides and metabolising other plant chemicals in the bee diet including honey flavonoids. The most potent gene-activating chemical was p-coumaric acid, which also up-regulated a gene encoding a protein that has anti-bacterial properties.

But modern bee-keeping methods mean that bees aren't ingesting these compounds in as large a volume. This means they are less able to digest any poisonous compounds they encounter, or defend themselves against bacterial attack, which may be contributing to their decline.


07:08 - Genetic link to migraines

A gene linked to a badly-timed body clock can also cause migraines, new research has shown...

Genetic link to migraines

A genetic cause for migraines has been discovered by scientists in America.

Affecting up to 15% of the Headachepopulation, and more women than men, migraines are a common neurological condition. Patients usually suffer severe headaches, which can be accompanied by an aura or sensory disturbances such as seeing flashing lights, numbness on one side of the body, or hearing strange sounds.

But our understanding of migraines is limited because it's likely that this is not a single condition with a single cause. Instead scientists suspect that migraines may be the end result of a range of different underlying conditions. But by understanding how they occur, it should be possible to gain insights into the underlying mechanisms.

Now, University of California, San Francisco scientist Louis Ptácek and his colleagues, writing in Science Translational Medicine, used genetic sequencing techniques to identify a faulty gene in two families that suffer from migraines.

The families in question also suffered from a rare second type of condition, familial advanced sleep phase syndrome, or FASPS, a sleep disorder in which people are sleepy early in the evening and suffer insomnia in the early morning. But, remarkably, the gene - casein kinase Iδ or CKIδ - that causes FASPS may also cause migraines.

By introducing a copy of the faulty gene found in the families into mice, the scientists discovered that animals were more sensitive to heat and touch, similar to the sensory sensitivity seen in humans during migraine attacks. The mice were also more likely to display a phenomenon called "cortical spreading depression" or CSD, in which the brain cells fire powerful waves of electrical impulses. Moreover, this effect appeared to be linked to increased dilatation of blood vessels in the brain, one of the mechanisms believed to be behind the symptoms of migraines.

But why do changes to the CKIδ gene give migraine-like symptoms? Previous studies have shown that CKIδ interacts with proteins called connexins, found in astrocytes, which are the star-shaped cells that link neurons with blood vessels in the brain. Connexins are structural proteins that electrically couple brain cells together and it is believed that mutating the CKIδ gene reduces its interaction with connexins and, in turn, increasing the spontaneous activity of astrocytes. This change in brain cell signalling leads to increased vasodilatation and thus the debilitating symptoms of migraine.

These findings suggest that, apart from regulating the body clock, these same genes may also play a role in migraine. As the authors acknowledge in their paper, these findings may not represent the causative factor for all migraines, but they do contribute to our understanding of the genetics of migraines and the link between migraines and sleep.

Oil spill

10:01 - Nanosheets Soak Up Spills

A new floating material has been developed which can soak up oil spills, in the ongoing fight against water pollution.

Nanosheets Soak Up Spills

Oil spillA material capable of absorbing up to 33 times its own weight in oils and organic solvents has been developed by scientists in Australia.

The clean-up of oil spills has historically been an expensive and lengthy process, and recent large-scale spillages - such as that from the Deepwater Horizon oil rig in 2010 - highlighted the urgent need for new and reliable water cleaning techniques. Freshwater supplies are also at risk of contamination with organic solvents and dyes discharged by the textile, tannery and paper industries.

But a solution may be in hand. A group from Deakin University in Australia, led by Ying Chen, have developed a porous boron nitride nanopowder, capable of rapidly soaking up over 30 times its own weight in oils, organic solvents and dyes.

To work like this, a material must have a high capacity and large surface area, it must be capable of accommodating high strains (so that it can swell up), and it should be lightweight and easy to separate from water. The material made by Chen and his team ticks all of these boxes.

Their powder consists of individual sheets of boron nitride (BN). At just 1.1 nanometres thick, they're thousands of times thinner than the average human hair. Large pores in each sheet gives the material an incredibly high surface area, allowing it to absorb large quantities of dyes. The BN nanosheets are also hydrophobic, meaning that they repel water and can float on the surface of a spill. Chen also found that when the sheets absorbed engine oil, they could swell by up to 37%, but without causing long-term damage to the structure of the sheet.

1 g of the nanosheets can absorb up to 33 g of impurities; everything from solvents like ethanol and toluene, to heavy-duty engine and pump oils. The process is also very fast; according to Chen, "After just two minutes, all oil has been taken up by the nanosheets".

BN nanosheets are also cost effective - the raw materials are inexpensive, and the production of the final flakes follows a standard industrial process, according to Weiwei Lei, another author of this study. Once the sheets are saturated by the spill, they can be easily collected, cleaned by burning, heating or washing, and recycled, ready to be used several times more.

Other efficient absorption materials for water purification exist, but these are based mostly on carbon. Currently, only these BN nanosheets offer a low- cost, high-capacity, recyclable material which is suitable for use on a wide range of pollutants. Lei is confident that they can scale up their production process, saying "I can't see much technical obstacle from large-scale application", so it shouldn't be too long before we see these nanosheets on the market.

Comet Shoemaker Levy 9 collided with Jupiter in 1993

13:15 - Comet water in Jupiter's atmosphere

Water detected on Jupiter where it shouldn't be has been tracked to a recent comet impact...

Comet water in Jupiter's atmosphere

The crashing of Comet Shoemaker-Levy 9 into Jupiter in July 1994 was a spectacular event seen by thousands around world -- the first time that anyone has watched two solar Comet Shoemaker Levy 9system bodies colliding.

The scar of that collision was visible on Jupiter for weeks afterward but astronomers wanted to know what the long-term effects of the collision were. In 1997, the European Space Agency's Infrared Space Observatory detected a lot of water in Jupiter's upper atmosphere. There is usually little water so high above Jupiter, but was it from Shoemaker-Levy 9?

There are other possible water sources, including interplanetary dust and one of Jupiter's many satellites. Astronomers couldn't tell. But ESA's Herschel Space Observatory, launched in 2009, has now settled the issue.

A team from the Bordeaux Astrophysics Laboratory in France used Herschel to observe Jupiter's atmosphere.

With its superior resolution it could map the distribution of water and found that the majority was in the southern hemisphere (where the comet hit) and its abundance peaked at 44°S, the same latitude as the comet impact.

The researchers concluded that 95% of the water detected in Jupiter's stratosphere is from Shoemaker-Levy 9.

The results are interesting because they help researchers understand how material was spread around the solar system by comets during its infancy, when there were very many more comets flying around.

Earth, in particular, seems to have much more water than you would expect and some think it was dumped on earth by snowy comets.

Cartoon schematic of an atom

16:49 - Quickfire Science - World's Tiniest Film

IBM have created the world's smallest movie using atoms - here's how they did it!

Quickfire Science - World's Tiniest Film

You might have seen this week that researchers at the computing company IBM have entered the Guinness Book of World Records. They've created the world's smallest movie about a boy befriending and playing with an atom. But the actors in this 90-second clip are carbon monoxide molecules. 

Here's your Quickfire Science about how the film was made from Naked Scientists Elena Teh and Pete Skidmore.

Pete - The movie was made by moving carbon monoxide molecules one at a time across a couple of surface.

Elena - The molecules were moved with a scanning tunnelling microscope which magnified them 100 million times. That's the equivalent of making an orange look the size of the earth.

Pete - Scanning tunnelling microscopes have a needle tip 1 atom wide which can be very delicately controlled to scan the surface of an object.

Elena - If the needle moves close enough to a molecule, the molecule will stick to it because the same force which makes gecko stick to walls, the Van der Waals force.

Pete - The molecule can then be dragged around to any location that the researchers choose.

Elena - During this process, the molecules were kept at minus 260 degrees centigrade to make sure they stayed still and didn't vibrate due to heat.

Pete - Once moved, the molecules stayed in their new position because they form chemical bonds with the copper atoms in the surface underneath.

Elena - The scientists then took an image of the molecules which made up each frame of the film.

Pete - Four scientists worked for 2 weeks to make the 90-second video.

Elena - Researchers hope that in the future, laying out atoms and molecules in different configurations can be used to store data more compactly.

Peter - This will work by using the molecules to replace the 0s and 1s in computer data.

Campylobacter bacteria are the number-one cause of food-related gastrointestinal illness in the UK. This scanning electron microscope image shows the characteristic spiral, or corkscrew, shape and related structures.

18:33 - Killing bacteria with breast milk

Professor Anders Hakansson discusses how a a naturally occurring protein in breast milk is being used to strike a blow against infection...

Killing bacteria with breast milk
with Professor Anders Hakansson, University at Buffalo

In the last month, we've been hearing more and more warnings about the dangers of antibiotic resistance in bacteria. The UK's Chief Medical Officer Dame Sally Davies even warned that within 20 years, people could be dying from routine surgery as a result. But this week, a paper in PLoS ONE revealed how a substance found in human breast milk can make antibiotic resistant strains sensitive to those drugs once again.

To find out more we were joined by Anders Hakansson from the university at Buffalo at the State University of New York.

Chris - So, tell us first of all, what is this protein that you've discovered? What is it doing and where is it?

Anders - Well, so the protein we've discovered is present in human breast milk and it's essentially a complex between alpha lactalbumin which is the most common protein in human milk, and when it attaches itself to a fatty acid which is also prevalent in human milk called oleic acid. A complex is made that has this activity that it can sensitise at the microbial resistant strains of bacteria out to the antibiotics that they are resistant to.

Chris - So, in some way, it renders bugs that can normally grow with impunity in the presence of an antibiotic compound sensitive again. How is it doing that?

Anders - Well, so what we know at this point is that we know that our protein is acting directly on the membrane. It binds to a pump that normally excretes hydrogen ions and all cells - both our cells in the body and bacteria have a tendency to keep different concentrations of ions on either side of the membrane. And by attacking this pump, suddenly, the concentrations cannot be kept up differently on each side. And when that happens, suddenly all the other ion concentrations are kind of messed up, and we get calcium and other types of ions coming into the cell that suddenly destabilises the bacteria to the degree that suddenly now, antibiotics can reach their target much more effectively.

Chris - Because if you take penicillin for example, a lot of bacteria that are resistant to that make an enzyme that binds the penicillin molecule and stops it working. So, how would this process re-sensitise bugs to penicillin? Campylobacter

Anders - That is a good question. So, what we have done at this point is, we have looked at methicillin-resistant and also actually, penicillin-resistant bacteria, but most of the bacteria we've looked at are more penicillin-resistant based on their ability to change their penicillin-binding protein from the outside rather than the enzymatic activity. But the bacteria that do have the enzyme, what seems to be happening when we add our hamlet protein is that messing around with the ion concentrations renders the bacteria, enables to make energy. And most of these enzymes requires energy to act properly. And the other thing that seems to be happening is that the cell wall structures are changing to the degree that antibiotics have an easier way to get access to them even though there are enzymes around that can cleave the penicillins.

Chris - So, would these effects be active against a broad class of bacteria or will it only work against some bacteria?

Anders -  So, at this point, we have quite good evidence that it will work against a number of different species of bacteria. So, the paper that has just published was talking mostly about methicillin-resistant staphylococcus aureus. With this, we wanted to make a case with the bacterium that is a big clinical problem. But we've also tested E. coli and a number of both gram positive and gram negative bacterium that are kind of common in clinical situations and where antibiotic resistance is a big problem, and it seems to be working on all of those.

Chris - Which is very good news. So, how would you see this actually being deployed? It's a protein, so you couldn't just swallow this as a pill.

Anders - No, that's right. So, if we were to swallow it, it would probably be - it's not really degraded very well, but it's not absorbed very well from the gut. So, the way I foresee it at this point is that - what we're pursuing at the moment is mostly topical applications. So, with Staphylococci, we have wound infections and we have pneumonia where we can do inhalation rather than eating a pill. We could go into vancomycin-resistant enterococci or clostridia which also are gastrointestinal infections and in that case, you can actually eat the protein together with the antibiotics and give an effect on the mucosal surface because this is not inside the body in the bloodstream.

Chris - Is it safe and well-tolerated?

Anders - Yes, as a human breast milk protein that infants are drinking it daily and we have tested this at very high concentrations, both in animal models as well as in humans actually, and we don't see any side effects whatsoever.

Chris - So, is there not a way of making the human body make its own hamlet protein so that you don't have to give it? You could just turn on the ability of the body to make it itself.

Anders - Well, the only problem is that this is only produced during lactation.  So, it's only produced in human milk. So, there's no other cells that are actually producing it. So, you have to - the other option will be to drink breast milk rather than making synthetic protein. And that might actually work well as well.

An aerial view of St Paul's cathedral

24:32 - Conserving a Cathedral

Oliver Caroe of St Paul's Cathedral discusses how he looks after one of London's most famous landmarks.

Conserving a Cathedral
with Oliver Caroe, St Paul's Cathedral

The conservation and restoration of great art once relied only on a good eye and talent with a paint brush.  Now though, scientists and art conservationists are working together to develop new techniques to preserve our cultural heritage.

In 2011, one of the most iconic buildings in the UK - that's St. Paul's Cathedral completed a 15-year 40 million-pound restoration project to remove centuries of London grime from its walls. State-of-the-art techniques were required to restore that building to its former glory and to find out more, we were joined by Oliver Caroe who's the current surveyor of the fabric at St. Paul's. 

Ginny - Now Oliver, buildings like St. Paul's seem very permanent, but what actually happens to them over time? An aerial view of St Paul

Oliver - Well, they're made of stuff. They're made of stone and wood, and metal, and all of this stuff in itself intrinsically has a life span to it. But it's in an environment and the environment can intrude on how it behaves rather aggressively and of course, there are many enemies to old buildings. But water is our main enemy, dirt and other. And then there is the sort of the chemical soup of the environment generally in the atmosphere. But of course, we mustn't forget the people are a great enemy of buildings and they get up to mischief, they damage. But they also damage with good intentions and we have to watch out for that too.

Ginny - So, what's St. Paul's actually made of?

Oliver - Primarily, Portland stone, but much else besides and that's a long and complicated story and many books have been written about it.

Chris - What actually is Portland stone?

Oliver - Calcium carbonate, it's sea shells settled on the seabed, compressed and made into a lovely homogenous matrix of a very enduring and very durable stone.

Chris - But susceptible to acid attack.

Oliver - Very much susceptible to acid and a great irony of course that St. Paul's Cathedral was built and funded by the attacks on coal coming in to London. And that coal actually, not only creates acid rain, but soils the cathedral. And Christopher Wren in his own day had the cathedral cleaned because he recognised the damage caused by coal.

Chris - How quickly does grime build up and how thick is the stuff that gets deposited?

Oliver - That's an answer that science has to tell us. Old age is beautiful and I think one mustn't sort of dodge away from that, we love things to look old. But there are encrustations in St. Paul's Cathedral which are literally inches thick and they are quite beautiful. One mustn't ignore that and that they are like frozen black waterfalls of grime even like exquisite folded gossamer fabric. It is exquisite, but...

Chris - Who would've thought that the dirt could be regarded as kind of an added value.

Oliver - Well, it doesn't sort of play on radio, but it is something that you have to see.

Ginny - So, if water is bad for limestone, then how did you go about cleaning it?

Oliver - Well, there are lots of methods and in the past, water has been used aggressively and literally flooding the surface of the cathedral. Back in the 1970s and '80s, that was done extensively and you get some rather nasty unintended consequences. What we do now on the outside of the cathedral and in the last programme that my predecessor Martin Stancliffe led heroically really for 15 to 25 years in fact, they used microabrasives and very, very carefully, just slightly wet abrasives are being used. And then there are lots of other techniques besides.

Chris - But you said that water was bad. Why was just washing it bad? How about every time it rains?

Oliver - Stone has a skin just like you and I. If you sit in the bath and scrub yourself in water - actually, it's not very comfortable if you do that for too long. So, exfoliation has its demerits, but stone doesn't regrow its skin like we do. So, you can actually drive water through the surface of the stone, it loses its hardness and that then leaves sort of soft material behind. But also, what can happen is you get water into the joints of the stone which goes into the core of the wall, and the middle of the wall is lots of rubble held together with lime mortar, and you don't know where it's going then. So, you have to be very, very careful.

Ginny - So, what kind of techniques can you use that don't involve water?

Oliver - Well, on the inside of the cathedral, we used a piece of chemistry - a really quite exotic piece of chemistry and a material called Arte Mundit which is actually rubber, slightly alkaline rubber that was sprayed onto the cathedral and at a very special St. Paul's formulation. And that again, it's very beautiful, the way you do it and it stinks, but you apply it on like a poultice. You leave it for just under 24 hours and then you can peal it off and you can literally see the dirt peel off.

Chris - This is just what you do in a beauty parlour. I mean, not that I'm speaking from experience. Maybe you Ginny, I don't know, but I've heard that people apply these things to their skin and then sort of peel them off and all the kind of gunge comes with it. It's sort of similar there.

Oliver - It's exactly like that and that there is a lovely analogy between sort of makeup and sort of health treatments, and this process, but I think...

Chris - How do you know it would do no harm then? I mean, that's the key thing, isn't it? I mean, you've said here, "It's critical to do no harm." So, how do you know this isn't harming the stone?

Oliver - Well, just a stretch in the analogy with beauty treatments, you wouldn't do some things yourself if you knew it might do you harm.

Chris - Well people have breast implants all the time, I mean, to make a point. I mean, that people do do cosmetic things and actually, they do turn out to have a sting in the tail.

Oliver - And they're not reversible at the end of the day. We have to enter into this world, I think firstly with a philosophical framework and that in a sense is a branch of history of art. But I think most people will have heard of William Morris and the sort of basic intention that you don't go and muck around with all buildings because you lose something every time you do that. And then in the 20th and 21st century, we have lots of international charters - the Burra Charter, the Icomos Charter - these are perhaps a bit obscure, but they sort of try and establish a philosophical framework of why we do things to old buildings.

Chris - But when you buy say, a touch-up kit for your car, it always says, "Try this on a small area first." Do you have a small area of St. Paul's that's your sort of test area when you come along with your latex mask, and think, "I'll just try this here" and if it does make a bit of a mess, it's minimised the damage rather than comprehensively doing a 40 million quid project all at once?

Oliver - Well, I think some misadventures of the past, we strenuously tried to avoid in this latest campaign by doing endless trials and there about 10 sets of trials, all carefully reported and examined by many experts and the people who give permission for these works to establish that actually, we do no harm. I think that has to be our first priority, but any intervention has some sort of long term ramifications, so we keep an eye on things. We also think it's very important to keep records of what we did because there's nothing worse than going back and saying, "Well, we think something happened here, but we don't know." And then you can't track the impact of that.

Chris -  So, is this sort of an on-going project now with the latex cleaning. Is this something that you will then, a bit like the Forth Bridge - you're basically starting as soon as you finish - so you just keep on going doing these peels to keep the surface clean or is this just something you're trying out?

Oliver -  No. This is now being completed and actually, one of those very bold things at the cathedral. It is under Martin Stancliffe is they said, "If we're going to do this clean, we've got to do it all. We can't leave the cathedral half clean." That would've been awful.

The Last of England, by Ford Madox Brown

33:04 - Restoring the Masters

Sally Woodcock talks about the science behind preserving great works of art

Restoring the Masters
with Sally Woodcock, Fitzwilliam Museum

This week Ginny Smith visited the Fitzwilliam Museum in Cambridge to meet Sally Woodcock who is currently completing her PhD at the Hamilton Kerr Institute to find out how oil paintings age.

Ginny - Today, we've come to Fitzwilliam Museum to have a chat about some of their paintings. Obviously, we've got all these beautiful paintings from hundreds of years ago that we want to keep and preserve so that our children and our children's children can still look at them. But what happens to paint as it gets older?

Sally - Well, I think you can see here in the Fitzwilliam Museum, many of the paintings will tell you that story because paint, as it dries, it changes, but then there's also changes in relation to its environment, so it will respond to change. And so, you've got paintings here in the museum that show overall patterns of ageing cracks and then specific areas of drying cracks, and then I think we're looking at an Edward Lear at the moment here, the Temple of Apollo, which has fabulous concentric, spider's webs of cracks where at some point in the distant past, someone has bashed it, thought nothing of it and then 20 or 30 years later, this spectacular crack pattern propagates in the paint film. And those are permanent. They are part of the history of painting.The Last of England, by Ford Madox Brown

Ginny - So, what I can see here, it's a beautiful huge painting with trees and rocks, and most of the time, it looks perfectly fine. But if you get it so that the light hitting it at a certain angle, there does seem to be a sort of ring of circles so you think that's where someone actually bashed into the painting.

Sally - Yes, it's an impact crack and it's likely to be where somebody bashed the back or the front, and it just allows the tension that's in the paint film to release and you just get this crack pattern. When you look at a painting at an angle, you normally talk about looking at it in raking light and it's one of the methods of examination we use just to be able to look at the surface. And it gives you an idea of the surface topography.

Ginny - What's the difference between drying cracks and ageing cracks?

Sally - Superficially, you can see a difference because drying cracks occur when the paint film is very brittle and therefore, they're rather sharp and they normally form an overall network, often over the whole painting. Sometimes they're more prominent in certain passages of paint, but they've got sharp angular crack patterns and they differ from drying cracks which occurred at much earlier stage in the painting's ageing when the painting is drying and is still a quite flexible film. And they flow slightly, so they're often rather curved at the edges. Quite often the actual aperture of the crack is rather wider and often, the surface look at worst like a crocodile handbag. And they again can't be treated because they are part of the original materials of the painting even if it wasn't the artist's intention for it to look like that.

Ginny - So, why do some of these paintings get lots of drying cracks, others get more ageing cracks, and some look perfectly normal even after hundreds of years?

Sally - There are an awful lot of variants. One thing is the environment in which they're kept. If you keep a painting in a very stable environment, it will reach equilibrium with even on paper quite poor conditions. The reason for the drying cracks is really the materials they used and it's often the proportion of dryers and drying oil and also, the layer structure.

So, the traditional way of painting is fat over lean. You want your lower layers to dry before you put your upper layers on. What you find is if you do it the other way around, the lower layers are still drying while the upper layer forms a film and then you'll often find one pulls the other apart. So, a very common reason why British 19th century and 18th century paintings cracked was the addition of asphalt or bitumen which doesn't dry. It was so common in British paintings of that period, it's known as 'craquelure anglais' by the French rather rudely, but actually, their painting aren't quite as bad as ours and you'll find it in British and American pictures of this period.

Ginny - Is that the same stuff that they use for pavements?

Sally - Not quite, but it is a refined tar product and it's also in Egyptian mummies.  They used asphalt and things from the mummification process and a pigment was made from mummified corpses in the 19th century known as mummy brown - a very odd use of archaeological artefacts. And when Edward Burne-Jones, the Victorian painter, found what his mummy contained, he insisted on giving it a burial in the garden, because he was so horrified to have body parts in it. So, that's an odd use of a non-drying component.

Ginny - Can you tell about what components go in to making paint?

Sally - Well, in its simplest form, it's a mixture of a medium and a pigment, and the medium will usually be with oil paintings, a drying oil, most commonly linseed, but then people started doing things to their linseed oil to try and change its handling properties and drying properties so they added manganese or lead driers to speed up the drying because it's quite a slow drying film. Or they would do things like heat body it, stand it out in the sun, or boil it to pre-polymerise it. All change the thickness of it and to change the drying components.

Ginny - So, polymerisation is when you have a load of small molecules and then they combine to make one long molecule. So that often makes things more viscous and in this case, you mentioned that they did this to help them have a nice texture to paint with. So, I guess causing them to polymerise would make it a bit thicker and hopefully make it dry more quickly and make it nicer to work with. Other than cracking, what else can happen to a painting over time?

Sally - Well, it can actually change in quite a few different ways. The pigments themselves can change. Sometimes they darken, more commonly they lose colour. So you'll see here at the Fitzwilliam Museum, they have a very celebrated collection of flower paintings. Quite a few of them have strangely blue lookinh leaves because the yellow lake that was added to blue to make it green has disappeared. It's completely fugitive. And so, what you're left is with just the blue components, so the tonal balance in the painting has completely changed, and that's normally as a result of light.

That's why the museum controls light below certain levels - both UV and lux to try and make sure that actually, that's not going to happen anymore than it's happened already. You also find drying oils can become increasingly transparent. So, some of the reason why sometimes you see things underneath that you weren't meant to see like under drawing, is the refractive index over time changes and you start to see things which once were covered. So, you might see what are called 'pentimenti' or the changes of mind. The artists tried a different position for a hand or a different position for a vase or something, and you start to see them because of this change in refractive index.

Ginny - And refractive index is how the light is bent when it hits the different medium. So, if it's bent a lot, you'll see something different to if it's only bent a little bit. And you say that can change over time as the oils get older. Is that right?

Sally - It's part of the ageing process in paintings. There's increasing transparency. So, you've got various things going on, so you've got increasing transparency, but then you've got a darkening of the actual oil medium as well. So, it tends to become slightly more yellow and some artists try to compensate from this when they knew this was happening by painting paler, hoping that over time, they would darken down.

Plastic and fabric buttons

41:47 - Preserving Plastics

Plastic objects are older than we think, and need conserving, says Dr Anita Quye.

Preserving Plastics
with Dr Anita Quye, University of Glasgow

It probably comes as no real surprise that 100-year-old oil paintings like those in the Fitzwilliam Museum might need conserving, but conservators are also finding that more modern materials like plastic are also degrading. We spoke to Anita Quye who's a lecturer in conversation science from the University of Glasgow to find out whether this is just a modern problem.

Anita - Well, it's quite interesting. It's been going on for about 30 or so years.  In the late '80s, people were starting to notice that some plastics maybe weren't surviving quite as long as we might have expected them to. They were starting to do odd things. They were starting to crack up, they were starting to weep, which sounds a bit extreme, but they were giving out liquids. And so, there's quite a concerted effort from that period on and it's escalating now. More and more people internationally are collaborating to look into the chemical reasons to why these materials are starting to degrade.Plastic buttons

Chris - Because of course, we're all brought up and told, plastic is really bad because if it goes into a landfill, it's going to last a million years before it breaks down and it soils the environment, and these things last forever. But actually, that isn't true and also, by the sound of it - what you're saying - there is a lot of it about that is in need of preserving because it became so ubiquitous and people don't regard it as very important.

Anita - Absolutely. Plastics, for their absolutely fascinating and perhaps quite a remarkably long history behind them, I think most people maybe think of plastics being things like PVC which you might associate to be like the 1940s and post-World War 2 chemical efforts from that. But it actually goes back a lot earlier. We're talking about the 1860s.

We're talking about the period when arts and science really were coming together to create these new man-made materials. And the ironic thing behind is that a lot of these early synthetic plastics which were appearing in the Victorian period and becoming quite novel materials were based on natural polymers, things like cellulose and paper, and cotton, were being chemically modified to create some of these early ones. People might have heard of celluloid for example, which is cellulose nitrate. And then by the early 20th century, scientists were starting to take these plastics that you could mould, that you could make into films and starting to make fibres from them.

So, the very first synthetic fibre was something called rayon which people might be familiar with and that's where you take that natural cellulose polymer and you break the cotton polymer down and then you regenerate it again, and you can control the length of the polymer chains, and you can make a much better fibre from it. So, we're talking about collections that people might not associate with plastics and the materials have gone from being this novel material first of all. By the 1940s, becoming a very utilitarian and very much a futuristic look at what we can do - this is the future guy's - kind of material. So then, by the time it gets the 1960s and they start to get a slightly different kind of imagery behind them - they're disposable, they're not long lasting. So, it's gone through this wave of being a novelty to being something that's disposable and I think that's lasted with us today and now, like you say, the environmental issues that are behind them. But they are misbehaving and they're starting to degrade and...

Chris - But chemically, what underlies that degradation because most people regard plastics as relatively inert or stable? So, what's going on inside the plastic to make it do that?

Anita - Well, when a plastic is formulated - so, these are very early ones, the cellulose nitrates, the cellulose acetates, we found through some research that we've been doing is that the actual way that you make the plastic is actually unfortunately a misfortune. So, when - like I say, you take the cellulose and you treat it with acids for example, and part of the process, whether you're making cellulose nitrate or cellulose acetate, is you create a cellulose sulphate as an intermediate and then you can control this nitration or acetylation process much more in a controlled fashion. And we found that certain levels, about 5mg of sulphate remaining in the plastic - per gram of plastic - is enough to indicate that that particular material is either degrading or has a potential to degrade. Then there are things like the PVCs for example, PVC, we might recognise it in our homes, in our window frames for example, you can get U-PVC window frames and that's a very hard material. But manufacturers can add in plasticisers to make that into a PVC plastic bag for example which you might be familiar with, as a carrier bag. So, you can get these formulations where you're introducing chemicals into them and over decades, over centuries, you're now starting to get these materials moving within the plastic. So PVCs for example nowadays, some of the old ones are degrading is because the plasticiser is moving through the plastic and appearing as a sticky liquid on the surface.

Chris - Can we reverse these changes?

Anita - Good question. I think if we ask a polymer scientist, the first thing that they normally respond to is, "well, why don't you just coat everything? Why don't you try and re-introduce these plasticisers again?" But I think Oliver mentioned actually about reversibility - something that when it comes to preservation of collections, we're trying to be as hands off as possible. So, we're trying to control environments, trying to control humidity and temperature which has an impact on how these materials behave. But we're actually a little bit cautious nowadays about coating things or trying to change the material back. Certainly, from a chemical point of view, once these degradation processes happen, you can't chemically change them back. The amount of energy that you need is very difficult to re-introduce. And from a conservation point of view, to start interfering with the materials, starting to put another material onto it can have devastating effects. And unfortunately, through textile conservation, we found that there was a great love of using soluble nylon as an adhesive for adhering support fabrics onto things like tapestries in the 1960s and '70s and the nylon material at that time was very reversible and you could take it back off with a solvent. But nowadays, it's polymerised and it's become hardened, and it's almost irreversible to remove it. So, there's a great deal of wariness about doing that kind of interventive treatment at the moment.

Chris - So, is it curtains, Anita, for some things that we have in our museums, are we going to see them disappear because we just do not have the ability to preserve these plastics because they will inexorably degrade?

Anita - Yes. We're getting to that point now I think where we're just faced with more and more things coming to our collections that we're having to make decisions and it's a big wide issue. Again, a mountain will be eroding but very slowly. Some materials in our collections nowadays are doing similar changes, but they're happening over a much faster period than we can imagine. So, we're going to have to start making some decisions about them and unfortunately, we don't think we'll be able to keep everything. Museums are constrained economically and have to think carefully about their collections too, but to conservation, we're doing our absolute best that we can in collaboration with scientists around the world.

Chinese ritual wine server (guang) from 1100 BCE

How do we date ancient artefacts?

Hannah - So, how best to date ancient artefacts? For the answer, we turn to our very own resident expert and Naked Archaeologist Diana O'Carroll...

Diana - Well, the answer is a bit of both. Sometimes archaeologists make use of the dates recorded by the civilisations, but most of the time, it's better to get a carbon dated check that these timeframes are accurate. When you have a king proclaiming that his ancestor ruled for 400 years for example, (it does happen) you'll want to check this out with other evidence. Now archaeologists use a number of tools to put dates on things they find and they tend to fall into two categories - absolute and relative. With relative dating, dig finds are placed in order and that can be done using stratigraphy. Stratigraphy refers to the layers of Earth or mineral deposits in which remains are found and the deeper you go, the older the layer will be. Another relative dating technique uses typology. Humans tend to go through fashions in the many things they make, whether it's pottery, road building, house construction or metal forging, we can usually identify what period the objects we find belong to. But this is only because we already have a back catalogue with which to compare them. And it's why you'll often hear an archaeologist pull a broken unimpressive bit of pottery out of the ground and exclaimed with excitement. It's usually quite useful for dating that stratigraphic layer with.

Hannah - But surely, sometimes these layers get mixed? Old bits of pottery might be used for generations or what happens when someone goes against the fashion trend? In this case, an absolute dating technique is required. Back to Diana.

Diana - Carbon dating is probably the best known chemical technique although there are others. When organisms such as people, animals, crops and trees are alive, they're continuously exchanging carbon with their environment. There are two main types of carbon out there. The one we're interested in is carbon 14 because it's unstable. When an organism dies, that carbon 14 starts to decay at a measurable rate. All you need to do to work out when that organism died is to see how much carbon 14 is left in proportion to the other forms of carbon. So, to sum up, the Mayan civilisations did have a calendar of their own, the one that was post in last year and sometimes they were kind enough to leave dates from carvings and encryptions about their rulers. They didn't however leave dates for the minutiae of daily life. And that's when we need absolute dating.

Hannah - Thanks Diana O'Carroll for a trip through the dating world and, as Evan_au points on the forum, there are other clever archaeological tricks like optical dating which examines when buried minerals were last exposed to daylight.


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