# Finding the Hugo de Groot book chest

In the television documentary _Historisch Bewijs_ [Historical Evidence] that aired in March, researchers of the Rijksmuseum in Amsterdam were pursuing a most remarkable question: in which chest of books did Hugo de Groot escape from castle Loevestein in 1621? At CWI, Joost Batenburg and Francien Bossema of the Computational Imaging group provided supporting evidence by the use of a tailored X-ray imaging technique.

To refresh your memory: Hugo de Groot, born in Delft 1583, was a Dutch humanist and jurist, well-known for his _De iure belli ac pacis_ (On the Law of War and Peace). In 1618 Hugo was imprisoned in castle Loevestein, for collusion in a religious conflict with the state. His unconventional escape out of the castle, by hiding in a book chest for several hours - breathing through the keyhole, entered Dutch history books. After that the chest was kept in the family, but it was somehow lost in the century that followed. In the centuries afterwards the chest surfaced multiple times, but as of today it is still a mystery which chest -- if any -- is the original one. In the documentary researchers examine three candidates: one at castle Loevestein, one at Museum Prinsenhof in Delft, and one at the Rijksmuseum.

When researchers of _Historisch Bewijs_ wanted to look at tree rings in the chest's planks, to find out if any of the chests was "too young", they found out that they couldn't do it alone. The rings on the exterior of the chests were not visible enough, and as such they needed to look inside of the wood. They contacted CWI and asked for Francien's expertise on X-ray CT, a scanning technique used in hospitals that can also be used to investigate objects. The technique rotates an object around in an X-ray beam and then calculates a 3D image out of that. Unfortunately it turned out that the chests were too heavy for the set-up at Rijksmuseum, and too big for the scanner at CWI, and for a moment it looked like they were in dire straits.

Now comes what television likes to call "a true Dutch innovation". By moving the chest sideways through the view of the X-ray camera, Francien and Joost manage to squeeze a bit of 3D information out of the plank, just enough to recognize the ring structures. "For me, this result was quite unexpected," Francien explains, "as we were not sure this would give us enough information to visualize the rings." Although CT from limited angles is nothing new to X-ray scientists, Francien and Joost were lucky. “Only elongated structures such as stripes can really be recovered using this method, luckily tree rings are basically that,” Francien says. In a follow-up project she hopes to further explore limited-angle scanning for big objects, like furniture.

Unfortunately, the chests that were investigated with the X-ray technique were found to be from the 16th century, and thus no candidate was eliminated. Nevertheless, other evidence was put forward, and you are invited to watch _Historisch Bewijs_ [edu.nl/rt9fm] (Dutch only) to find out which chest made it. Additionally, Francien and Joost hosted the camera crew team at CWI, and demonstrated CT scanning in the 15-minute supplementary _Historisch Bewijs Extra - CT scans_ [edu.nl/e8quf], featuring CWI's FleX-Ray Laboratory and a mini-book chest with LEGO Hugo de Groot.