This is how X-rays reveal the hidden secrets of the Prado Museum | Technology

A sign warning of possible radiation in the area welcomes you to an unknown enclave of the Prado Museum. Located in a restricted area of ​​the basement, particularly safe for working with radiation, is a laboratory unlike what one would expect to find in an art museum. In front of the entrance door, a light box which occupies practically the entire wall is covered with dozens of fragments of x-ray film which allow us to deduce what is done in this place. These are images of different fragments from a multitude of paintings in the museum: the face of a king, the support of a sculpture, the detail of a shoulder or even the corner of the support of a canvas . In this place, art is x-rayed.

Although it is still unknown, x-rays began to be used in the art world shortly after the technique was discovered. Its first application was in medicine. Later, they made the leap to anthropology and the study of mummies, eventually arriving at the analysis of paintings. At the Prado Museum, the first x-rayed painting was Lowering, by Van Der Weyden, in the mid-1970s. Half a century later, the use of radiation to unravel the past from paintings and sculptures is part of everyday life not only in this museum, but in galleries around the world. Laura Alba Carcelén, superior technician of the Department of Technical Documentation and Laboratory of the National Museum of the Prado, is the encargada of the radiography, a herramienta that considers especially useful in the art: “It deals with an essay that is not invasive and that does not require Sampling. The object will not be damaged and this can be done throughout the work. There is no application limit. Infrared reflectography, for example, we use it on paint, but not on objects because it does not penetrate them,” he explains in this laboratory. Thanks to infrared reflectography, we discovered, for example, that in painting Christ between the Virgin Mary and Saint John the Baptistby Jan Gossaert, the heads were drawn independently on sheets of paper which were then glued to the support and which, in addition, were traced from the original (Polyptych of the Mystical Lambby Jan and Hubert van Eyck) or an intermediate model.

Laura Alba moves an image taken by reflectography of “Christ between the Virgin Mary and Saint John the Baptist”. On the right, the large X-ray viewer which is located in front of the entrance.
Alvaro Garcia

However, the data must be interpreted. “Some training is necessary because all the information is superimposed on the same image and you have to know how to discern in which layer the elements you see are located. Sometimes it’s very obvious and other times it’s more difficult. Something can confuse you and lead you to mistakes,” says the specialist.

In order to unlock all the secrets hidden, for example, in a large-format painting, it is necessary for a team of four people specialized in handling works of art to move it to that location. Then it is placed on the wall which achieves the greatest possible distance to reduce geometric deformation and distortion. The amount of film that will be needed must also be calculated and cut and assembled with the edges overlapping so as not to waste centimeters.

The images are taken in one go and developed plate by plate. In order to be able to view them in detail on the computer and store them digitally, in addition to the physical archives, X-ray films are scanned and negatives are scanned, which have a high resolution and allow the work to be analyzed in maximum detail. By digitally processing the images, we can, for example, remove a frame to better observe the face, as in Children playing dice. In this case, thanks to the x-ray, it was discovered that beneath these children, there was another painting: a full-length portrait of a knight of the Order of Malta. To study a case like this in detail, the digital image also allows you to remove those elements that prevent others from being appreciated, although this must be done with the greatest care so as not to lose information that could be relevant to the investigation.

Laura Alba points out the man behind “Children play dice”.Alvaro Garcia

All digital images are archived, but are also stored on specially designed plans. To accommodate the largest x-rays, some drawers must measure three or four meters. The art world must adapt to elements designed for the industry. Large x-ray films, for example, are often used in aeronautics, for welding planes or oil pipelines. A museum must adapt materials developed for other fields to its needs. “Reflectography was developed specifically for its application to the study of painting and art, but the challenge with radiography is that there is no equipment for us because the world of x-rays is very powerful outside of heritage. We have to study all the materials and equipment that come to the market to see which ones we buy, from industry or medicine, but generally from industry,” explains Alba Carcelén.

Once everything is ready to take the X-ray, the laboratory technician closes the room where the table and the X-ray machine are located and prepares to enter the necessary parameters so that the image is captured correctly: we establish the kW of voltage, mAh of intensity and minutes. With these three variables and certain mathematical formulas you play to obtain a satisfactory result. However, this is not simple and several tests must be performed. Laura Alba explains: “In general, you want to see everything, so what you are looking for are conditions that allow you to observe the area of ​​greatest and least absorption; hence the tests. To do a test, you look for an area of ​​maximum and minimum absorption. Voltage would be the quality of the radiation, the energetic power it has, and intensity, the quantity of radiation. This will normally define much of the grayscale, detail…. The greater the intensity, the more detail and a greater range of grays you will have. In Children playing dice You can see that the bottom part is much more absorbent than the top part. The top one has a layer of preparation that absorbs less and the bottom one is painted on another board.

When working with ionizing radiation as is done in industry, those who use this technique must obtain a radioactive installation supervisor authorization from the Nuclear Safety Council (CSN), according to the specialist. In addition, as these are exposed personnel, the workers who carry out their work in these installations must wear an indirect reading dosimeter, which records the radiation received and which is reviewed monthly by a company approved by the CSN. There is also an area radiometer at this location which is currently used to check radiation. The CSN also carries out inspections of installations, checks on the success of medical examinations, etc.

In a place like this, where art history mixes with mathematical formulas, one might wonder what academic training is necessary. According to Laura Alba, one can come from different fields of training, but, in her case, after scientific studies in high school, she obtained a diploma in Fine Arts with a specialty in Restoration. The colleague most specialized in reflectography is an art historian and, according to Laura Alba Carcelén, the one who takes the x-rays at the British Museum is an archaeologist and the one at the Louvre is a physicist. “The important thing is not so much the taking of the image, but rather the interpretation. A person of science and literature can learn it; the important thing is experience,” he says.

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