CT scanner brings Rembrandt paintings closer together

The centre is one of Europe’s best X-ray research facilities, and has a specially designed CT scanner which may be able to solve a dilemma that has hampered efforts to unravel the Rembrandt mystery: A fixed wooden frame around the portrait of the woman prevents researchers from being able to see the growth rings in the original wooden panel. However, if they remove the frame, they risk damaging the painting.

“The CT scanner allows us to see the growth rings without removing anything,” says Carsten Gundlach, Senior Executive Research Officer at DTU Physics. He adds:

“We place the painting on a rotating table in the CT scanner and then take 360-degree X-rays, just like in a hospital. The X-rays allow us to see through the painting, and what it looks like at depth. This means that we will be able to see the growth rings,” he says.

The growth rings are key, as they can be used to fix the age of the wooden panel and the tree’s place of origin. If this data matches the wooden panel used for the portrait of the man, it increases the likelihood that the man and woman were painted as a couple.

Tree fingerprint

“Hey, this is perfect!” This was the reaction of dendrochronologist Aoife Daly to the processed scan images Carsten Gundlach and Jakob Sauer Jørgensen, Senior Researcher at DTU Compute, sent to her.

A dendrochronologist is a kind of tree detective who can determine the age and origin of a tree by measuring its growth rings. Before getting started, Aoife Daly was unsure whether this task would even be possible.

“I crossed my fingers that we would get some cross-sections that were detailed enough for me to work with. The painting was much larger than the CT scans of objects I usually work with, where about 30 centimetres in diameter is usually about the limit. Larger size often means low resolution and blurry images,” she explains.

Fortunately, the resolution and sharpness of DTU’s images were ‘much better than expected’, and Aoife Daly could begin measurements on the wood without difficulty.

“I measure from the innermost to the outermost growth ring right through the wood. This results in a sequence of measurements. They can be plotted as a graph, where you can see the variation. If growth has been high for a year, the growth ring will be wide and this will be a high point on the curve, while low growth will mean a narrow growth ring that is a low point in the curve,” explains Aoife Daly.

The complete growth curve for the tree forms a unique pattern that can be seen as its fingerprint. Aoife Daly can use this to determine how old the tree is, using correlation statistics. She can then use a tree register—a collection of growth ring datasets from all over Europe - to consider where the tree has grown.

In her work on the Rembrandt painting this method led her to a clear result, which she could compare with a similar analysis of the portrait of the man undertaken by The Metropolitan Museum of Art in New York. Together, these gave her the final conclusion.

Reunited

This autumn, the two Rembrandt paintings hung side by side at the Nivaagaard Malerisamling art museum in the ‘Rembrandt Reunited’ exhibition.

They are oblivious to the years of research collaboration behind the scenes that has sought to illuminate their past connection, and which has brought art historians around the world closer to an answer.

“There is every reason to believe that these two paintings are some of the first oval portraits Rembrandt painted in 1632, and while some questions remain open, we believe they may be a pair,” note Jørgen Wadum and Angela Jager, an art historian from the RKD Institute for Art History in The Hague.

Aoife Daly’s analysis shows that the trees used to make the oval panels on which the two portraits are painted were cut down in the 1620s. This is consistent with Rembrandt painting the portraits in 1632.

The analysis also shows that the wood used in the panels for both paintings originates from a specific area in the Baltic States, known today as Lithuania. Two of the boards used—one from each panel—have such a strong corelation that they probably derive from the same forest.

“Taken together, these observations makes it appear likely that the timber was felled in the same region of Lithuania, during the same period. The cut wood was shipped to a port city and then on to Amsterdam, sorted and purchased by a panel maker, who then made the panels used by Rembrandt,” explains Aoife Daly.

New collaborations on past mysteries

However, much of the wood used to make panels in Amsterdam in the 1600s originates from Lithuania. It is therefore impossible to say whether the panels were purchased by Rembrandt together, or separately.

What is clear is that the scans are considered ‘enormously valuable’ by the parties involved in the collaboration, and at DTU, the researchers hope that this will lead to more collaborations on past mysteries.

“We seek to benefit society, and we think this is an example of how we can contribute to performing a different type of research to what is typically associated with DTU. We hope there will be more of this in the future,” says Carsten Gundlach.