In 2009, Cologne's historical archives, the most significant such archive north of the Alps, suddenly caved in. Bakery apprentice Kevin, 17, and design student Khalil, 24, lost their lives, and valuable medieval manuscripts and historical photographs were destroyed forever. Mismanagement and faulty construction of a metro tunnel was the cause of the collapse.

This handwritten medieval document shows parts of the 1437 Cologne statutes. More than 1,000 years of regional history were documented in the archive. Among the items are original manuscripts of significant personalities including medieval philosopher and scientist Albertus Magnus, Thomas Aquinas, French leader Napoleon Bonaparte and former German chancellor Konrad Adenauer.

Aid workers hurried to save whatever could still be saved, and time was running out. The first priority was to fix damage that might cause even more damage. Whereas at first it was feared that almost all irreplaceable manuscripts had vanished forever, it then turned out that almost 95 percent of the items could be saved.

Resulting from the building's dramatic collapse, roughly 10 percent of the documents landed down below ground water level. First, these lost items needed to be located and picked up. Wet parts had to be carefully cleaned of dirt and sludge and wrapped into a sheeting before being shock frozen at -22 Celsius to later be defrosted in a special dryer to prevent the formation of moisture.

Countless documents were heavily damaged, deformed or torn apart – and almost worse, tangled-up in total confusion. Thousands of pages needed to be identified, ordered and filed anew if researchers were to be enabled to study them once again. Only a thorough cleansing process could prevent tiny dust particles from slowly eating away at the material.

Even books that did not get soaked needed to be meticulously cleansed before being archived anew. Each single sheet of paper had to be restored by hand. It was particularly important to remove almost invisible traces of mold. Experts wearing gloves and masks took great care to clean the material by hand.

“Von zarter Hand“ ("Of a tender hand") is the fitting title of this 19th century novel by Johannes Richard zur Megede being carefully cleaned. The restoration experts need to take great care, using particular sponges, brushes and paintbrushes. Dust and dirt must be meticulously removed, lest the paper becomes porous so that, over time, it will decompose.

Manuscripts that needed to be frozen must be dried in special machines before dirt and dust may be removed from them. Books are given a new binding. Every single page needs to be polished and burnished before the book can be put in order anew. All materials are also digitized so that researchers can make better use of them.

The technology available today cannot restore all damaged seals and signets. But as such technology may be improved in the future, the single components of the items are being preserved, explained the former head of the restoration team, Nadine Thiel, in 2011.

The experts and aid workers not only examine and restore paper objects, but also film reels. The restoration of the entire inventory of Cologne's historical archive is a very huge task. It's estimated that the works will have to continue for at least 30 years.

A new site for archiving Cologne's treasures is in the works, set to be finished by 2020. It will not only house the manuscripts formerly kept in the old archive, but also a collection of photographs. Meanwhile, the dispute about who is to be held responsible for the 2009 disaster continues, although soon, it will be too late anyway. In March 2018, the statute of limitations will be over.