How do you open an old locked chest when you don’t have a key? Breaking the chest would work, of course, but that isn’t an option if the chest is a valued historic artefact. So when curators at a Czech museum were faced with the challenge of opening a locked guild chest, they found a different solution. With help from researchers they were able to reveal the secret lock mechanism, and after a series of puzzles reminiscent of an elaborate heist they finally opened the chest.

The chest they tried to open was a large steel guild chest, estimated to be from the early 19th century. At 60 kilograms it weighed as much as a person, and getting into the locked chest without a key would be a challenge. Guild chests were used to store important documents or valuables for groups of merchants or other professionals, and often served a ceremonial function as well. The locks on these chests were often elaborately designed to keep people out and two hundred later they were still doing their job.

But to be able to properly restore the chest, the museum needed to open it without damaging it. They had help from Jozef Kaiser and colleagues of Brno University of Technology, who reported in the journal PLOS One how they used a technique called X-Ray computed tomography (CT) to create a series of X-Ray images of the lock mechanism in the lid.

Or rather, locks. One of the first things they noticed was that there wasn’t just one lock, but three locking systems, connected by a series of keys, bolts and secret buttons.

From the CT images, the researchers were able to recreate a 3-D representation of the locks and see how the different systems interacted. The first mechanism needed the missing key to move a plate. The second mechanism involved a button hidden in the decoration of the frame of the lock. Finally, another secret button would pop up the third mechanism.

At least, that’s how it would have worked if they had a key and if the lock was fully functional. But one of the surprises from the 3-D model was that the chest was technically already partially unlocked. The lock was just stuck, and missing a spring.

Even though the lock was stuck, they now had enough information to figure out how to pick it. First, they created a copy of the key, based on the shape of the keyhole. Then they pushed a long iron rod through a hole in the bottom of the chest, to maneuver the mechanism that the spring would have controlled. And finally, after all this time, the chest was open.

Inside the chest, the lockpicking research team found the missing spring that had fallen from the mechanism. And that was it. There was nothing else inside the chest. No centuries-old documents or surprises.

But the chest itself is a historic artefact, and now that it was open, the lock mechanism could be fully restored. The museum has left the chest unlocked, though. Just in case the lock breaks again and requires another scientific puzzle quest to open.

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