Restoring the Masters

Sally Woodcock talks about the science behind preserving great works of art
02 May 2013

Interview 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.


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