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Infrared photos offer 'behind the scenes' look at works of art

June 08, 2011

What if you suddenly find that you've inherited a European castle and all the furnishings and artworks inside?

Sell the castle and list the furnishings on eBay. But what about the paintings? What if there's a masterpiece worth millions hanging on the ancient walls?

How do you know that the artworks are what the signature says they're supposed to be? That someone didn't just paint a copy with modern materials and make it look old?

Call in the Art Detectives, or rather Corine Schleif, professor of art history at ASU, and Volker Schier, an affiliate of the Arizona Center for Medieval and Renaissance Studies.

Since about 2002, Schleif and Schier have done "undercover" work examining paintings with infrared photography – utilizing the invisible rays just beyond the red end of the visible spectrum – to learn about the original preliminary drawings for the painting, layers of paint, changes in the artist's focus and more.

Schleif, who spends her summers in Germany, often is asked to analyze paintings in the many churches in Nuremberg, most of which date back to the 1400s, and in fact, she already has studied a great many of them.

When Schleif and Schier are called in, Schier said, "The pieces of art are usually up for restoration, and the restorers need to know more about the paintings before touching them. I actually like to come when the surface of a painting has been cleaned by the restorers. This makes it easier for me to penetrate paint layers, since otherwise there might a fairly large amount of candle soot and other particles on the surface that effectively shield the lower layers from the 'gaze' of my camera."

For example, Schleif and Schier recently were asked to photograph and analyze the Glockengiesser epitaph in St. Lorenz Church in Nuremberg, which was painted shortly after the death of its honoree, Agnes Glockengiesser, in 1433.

"The epitaph, or memorial painting, depicts the 'Dormition of the Virgin,' or the death of Mary," Schier said. "In this case, infrared photographs were important since the restorers had determined that large parts of the floor but also some of the mantles and tunics had been painted over in the 19th century. During the restorations these added layers were removed. In order to prepare the removal of paint layers it is essential to have information about the layers below."

Sometimes, whole paintings disappear under new artwork – for example, when Lutheran scenes were painted over Catholic ones after the Reformation. 

"In the 17th century several paintings from the 15th century in Nuremberg churches were completely painted over with new motifs," Schier said. "Restorers had noticed this and they had removed small patches of the surface paint layers at some less obtrusive parts of the paintings. This provided the proof that there were different paintings underneath but of course these approximately 1-by-2-inch patches do not provide any information about the iconography."

With infrared photography, Schier and Schleif could determine what was under the more modern paint, and then let the restorers decide what to do. In the case of these particular artworks, the restorers are have not yet made a decision about their strategy.

Infrared images also can give hints into the working of the artist's mind. In the Glockengiesser epitaph, for example, the artist's initial design for the sleeve of Christ's tunic was different from the later execution, Schier said. "The left sleeve was initially drawn much lower. The artist then corrected this and lifted the arm of Christ, and he also made the sleeve much longer.

"If you look at the right hand of Christ you can also see the underdrawing for the fingers. Traces of the underdrawing appear all over the infrared photogram."

"Infrared allows you to look through the painting process. You see the different steps involved in it. You can see the underdrawings," Schleif said. "You can ask the question of who did the underdrawing? Sometimes the master did the underdrawings and apprentices completed the works. You can study the styles of underdrawings, and attribute them."

The approximate age of a painting can also be revealed through infrared photography, particularly when certain pigment colors are examined.

For example, if the camera can "see" through the blue, it is almost certainly a paint made of lapis lazuli, not azurite. Lapis was a luxury product since it had to be imported from Afghanistan, while azurite was mined in several locations in Europe. "If I can't get through," Schier said, "it's azurite.

"I think the basic question about lapis and azurite was how much a donor wanted to spend on a painting. If lapis was used, the donor had a lot of money."

Possible misunderstandings between patrons and artists also can be detected, as can places in the painting where the artist changed his mind.

In the Imhoff epitaph in the Church of Saint Sebald in Nuremberg, for example, which is a memorial to Ursula Imhoff, Schleif and Schier discovered the that painting included depictions of both of Ursula Imhoff's husbands - a very unusual departure from the custom of the time.

"The Imhoff epitaph poses many questions," said Schleif. "Why was it commissioned so many years after Ursula's death in 1504? Why did those ordering the work determine that both husbands should be included? Were changes made while the work was in progress? Is it possible that initially only one man was depicted?"

After finding only one man under the top layers of paint, Schier speculated, "Perhaps the daughters didn't like having their father left out.”

When Schleif and Schier began attempting to take infrared photos, spurred by their interest in the Imhoff Epitaph, they had to work at night to avoid visible light, and put the cameras under big black cloths to further block light and reflection. They used film with special filters, and had to develop the film themselves.

Their first roll of film was a disappointment, Schier said. "I had calculated an approximate exposure time based on the speed of the film and we started taking images with one of our medium format cameras and flashlight illumination.

"We ran a couple of rolls of film and in the evening I started to develop them in my darkroom. When I removed the first roll of film from the developing tank I was shocked: The film was almost completely blank. Only a very faint image was detectable under close inspection. With the next rolls I drastically extended the developing time and could obtain printable images."

Once the Sony DSC-F828 digital camera based on a CCD chip was released in 2004 Schier immediately purchased one. "We were able to push the upper limit of infrared wavelength to approximately 1100nm, which is quite a jump," Schier said. "We now could use 'denser' filters and generally look much deeper into the paint layers."

The infrared examination of paintings is still a rare art, said Schier and Schleif, and it's still more rare for students to have access to them. They gladly invite ASU students to work with them during summers, using their personal equipment, and Schleif includes information about infrared tools in her art-history courses.

"Usually only major museums have an infrared camera," Schier said. "You have to learn to use the equipment on your own. Infrared techniques are not written up, and there are hardly any books because technology is changing so rapidly. 'Infrared Imaging for Dummies' hasn't been written yet."

Getting the images is one thing, but reading them is another. "You need a solid art background to read them, and our approach is to get those two things together," Schleif said.

"Only when working with a professor who is regularity using this technology – such as Corine – students can learn about this important technology that should be part of their art historical 'toolkit.'" Schier said.

"ASU is very special, since – as far as I am aware – students at only one other university in the United States use this technology."