Inside art

A CT scanner can penetrate art treasures without causing damage

You don’t simply cut up or saw through an artwork to study it. Historians have recently started using a clever 3D X-ray scanner for that. But can these devices handle a carpet too?

Text: Pepijn van der Gulden, image: Johannes Abeling

Have you ever crept into a carpet? Sophia Coban is a mathematician, and today she’s going to try to penetrate the fibres of a weave pattern. Not with a pair of scissors and a lot of fiddling, but with an ultra-modern CT scanner. This experimental machine is located at CWI, the Dutch national research institute for mathematics and computer science. It’s meant to be used for medical research, but also for studying art objects three-dimensionally.

Historians are closely observing Coban to see whether she will manage to unravel the secrets of an ancient carpet. Never before has a carpet been observed through a scanner. What kind of  history will the carpet reveal? And what’s the use of this kind of research?

The internal structure of the carpet becomes visible on the screen. (credits: Johannes Abeling)The internal structure of the carpet becomes visible on the screen. (credits: Johannes Abeling)

Losing the thread

Using an insanely expensive scanner to capture  a carpet three-dimensionally isn’t something that happens every day. You can hold the carpet and examine it from any angle without being afraid that it will fall apart. So studying something like that must be a piece of cake, a lot easier than a mummy, for example. What’s more, carpets have a familiar structure: the threads have been woven crosswise on a loom.

Yet we’re not able to see as much with the naked eye as the experts would like, says Rijksmuseum historian Suzan Meijer, who has brought the piece in with her. The patterns in carpets are made by knotting coloured threads into the fabric. These knots can be fixed in different ways. But usually it’s almost impossible to see how, because the knots happen to hide the fabric behind them. ‘Because a carpet is pressed so hard, you can hardly see the rest. The knots are completely covered. You would actually need to prise the knots out to see it.’ And that’s something you would rather not do with a carpet that’s centuries old.

Maarten van Bommel, professor of Conservation Science at the University of Amsterdam, hopes that the scanner will tell us more. ‘By discovering how a carpet is made, you can attribute it or date it.’ The production method is a kind of signature of the maker. That’s why it’s interesting to know how the carpet is structured.

Density matters

carpet fragments in the scanner

The CT scanner should provide assistance. The large grey cabinet with a black sliding door wouldn’t attract much attention if it weren’t for the intimidating yellow sticker on its front: Caution, radioactive! ‘The scanner uses X-rays and the effect is similar to X-ray machines in hospitals,’ Coban explains. X-rays are projected towards a sensor on the other side of the machine. ‘If you place something in between, then the X-rays lose some of their energy.’ If a dense materials is placed there, such as bone, then it will block more radiation than something soft like muscle tissue or cells. ‘The scanner records these differences in density.’ This method can also be used on other materials. A carpet is just as easy to scan as a broken leg. However, the radiation level does have to be adjusted , because a carpet will block less radiation than a piece of bone.

Spotting differences

For now it’s still unclear whether it will be possible to look inside a carpet. Anything can be scanned, as long as there are differences in density, Coban says. That’s precisely what makes a carpet a difficult sample . ‘It’s going to be tricky, because it consists largely of the same material.’ The weave is made out of one kind of cotton, the threads of which are literally woven as well. There’s only one way of discovering whether it will really work: by actually trying it out. Coban puts the small carpet in the scanner, after having put it on a stand (see box entitled ‘Dynamic DIY’ on next page). As if the yellow warning sticker isn’t intimidating enough, a red light starts to flash now too. Luckily the cabinet is protected by a layer of lead. So we have nothing to fear. A grey haze appears on the screen. It seems as if the X-rays don’t really know what to do initially with the uniform density of the carpet, which makes it look like something that’s been fused into a lump.

3D takes time

It’s now that Coban’s expertise comes into play. She can improve the result. ‘It’s very similar to photography. I adjust the shutter speed and the diaphragm to increase the amount of information.’ The X-ray has to connect with the density of the carpet. That’s why she changes the power and the duration of the X-ray. And sure enough, it generates a sharper scan. The X-ray scanner doesn’t recognise colour, but Coban can make the image crisper by giving each type of material a different colour.

For the time being, the machine has only made a two-dimensional X-ray. The trick is to convert this into a three-dimensional image. As soon as the carpet is in focus on the picture, the CT scanner sets to work. A turntable with the carpet installation on it starts to slowly rotate inside the machine. This makes it possible to calculate the three-dimensional image (to find out how that works, see the box ‘Modelling with X-rays’, on the right).

Unravelling the knots

Twenty minutes pass while the scanner meticulously does its work. As soon as the carpet has rotated 360 degrees, the carpet appears on the screen as a magnificent 3D model. Up close the threads, which normally seem so perfectly arranged, look like hairy cables crawling over and through each other. The scanner can distinguish differences in thickness the size of a human hair, so the thin threads actually resemble the mooring lines on an ocean liner.

Now the scanner’s added value becomes clear. Coban can virtually turn, stretch or dissect the carpet. What would it look like if you cut straight through the middle of it with a razor-sharp knife? Or what would remain if you unravelled all of the knots on top with tweezers? Thanks to the scanner, these kinds of operations can be performed with the click of a mouse. The historians watch enthusiastically as the inside of the carpet is exposed.

As they had hoped, it’s possible to see which knotting techniques were used. The lines have been laid along the mooring sites in different ways, the threads of the weave. Some of the embellishments are secured symmetrically, while others show through the background pattern completely asymmetrically. That should yield enough information to determine the origin of this carpet. Is it from a Persian house or an Ottoman sultan’s palace?

Fabric for the future

Neither as it turns out. This carpet is from 1991. A.D. of course. Though the maker isn’t completely certain anymore. In fact, she’s sitting behind the computer following the proceedings, as her creation is being digitally dissected. Historian Suzan Meijer wove the small carpet once upon a time as a test, to get more of a feel for the craft. ‘After all these years I can finally see where I have made mistakes.’

Nonetheless, there is reason for the historians to be enthusiastic today. Because the scan results prove that CT scanners are definitely something that can be used with fabric. ‘This was the very first test to see what we can do with this system,’ Van Bommel says. ‘We intend to only work on historical objects when it really serves a purpose.’ The scanner has already proven its merit with other art objects (see the box entitled ‘Something to practice on’). Now that the method appears to work with carpets, the scan project can move on to the next stage. An entire Persian carpet would be hard to fit into the contraption, but it should be quicker and easier to determine the production method from now on based on smaller pieces of fabric. And Coban intends to regularly crawl into fabric in the future.

  

Something to practice on

It’s extremely delicate work, but it’s well worth the effort: a Chinese puzzle ball.

In order to explore the scanner’s possibilities, a great variety of objects were scrutinised. Chinese puzzle balls from the Rijksmuseum, for example. These are skilfully carved balls made of ivory. The first ball has a second sphere carved inside it and so forth, with as many as seven layers. That makes these objects difficult to examine. The CT scanner makes it possible to look at the carvings of the innermost sphere as well, without having to break open the delicate object. The bones of the Java Man, an early hominid, were scanned here as well. The bones are from the Naturalis collection and tightly  fit into the scanner. Scanning the skeleton in parts made it possible to digitise the full bone.

Modelling with X-rays

Does the structure of the fabric differ enough to allow it to be scanned?

A CT scanner is actually an overenthusiastic X-ray machine. It doesn’t make one photo of a given object but thousands, and each of these from a different angle. A computer combines these flat images into a spatial representation. Hospitals have been using it for some time now to localise tumours, for example. Increasingly, people are exploring its potential uses beyond hospitals. And that really is a completely different ball game, according to professor of virtual reality Robert van Liere at CWI, the Dutch national research institute for mathematics and computer science. ‘We’re doing much more than simply laying a carpet in a hospital scanner. Museum objects are completely different than people.’ A person consists of tissue, bone and water, whereas a museum object is made up of different, sometimes unknown materials – and rarely of water. That means adapting the reconstruction methods. ‘Our role is to improve the technique by gaining a better understanding of the methods.’ The scanner that was used for the carpets, among other objects, is not an ordinary CT scanner incidentally. This new machine makes it possible for the first time to look inside objects in real time. When that needs to be done, you can immediately zoom in or adjust the scanner. That saves a lot of time and exposure via harmful X-rays.

Dynamic DIY

Sophia Coban and piece of carpet inside the scanner

To operate a 3D scanner you need to be more than just well-versed in mathematics. Before Sophia Coban put the carpet into the CT scanner, she built a stand with wooden plates, styrofoam, needles and double-sided tape to keep the carpet upright. ‘I had to ensure that it didn’t move, otherwise the image would be out of focus. I studied for years, but these kinds of DIY challenges are also an essential part of my job.’ Coban shows us a cupboard full of tape, styrofoam and cheap abrasive sponges. ‘Really practical material for this kind of work.’ Luckily, the DIY materials don’t need to be subtly concealed: the X-rays go right through them. And because abrasive sponges have a different density, Coban can easily remove the accessories on the computer.