Documents in archives are mostly marked with a traditional metal or self-inking stamp with the name of the respective institution. However, this simple and widespread procedure has its drawbacks. It is visible, which reduces the aesthetic value of the document. Moreover, commonly used stamp inks can be easily removed.

"Especially in the 1990s, many significant memory institutions were affected by theft. Among the most famous cases were the thefts of old maps from the in . But our archives and libraries were not spared either, with items such as letters and manuscripts by the elder or the sheet music of Beethoven's disappearing from them. The value of these documents ranged in the hundreds of millions of crowns. Over the years, the stolen documents reappeared at foreign auctions or in antique shops. However, it was not always easy to identify the owner. Traditional archival marks can be dissolved or erased, leaving nothing on the document to prove ownership," explains the main project coordinator, Dean of , Michal Veselý.

With the spread of digital technologies, systems based on barcodes or QR codes have also become popular. However, their disadvantage is their visibility and ease of copying. Thus, they do not serve any protective role and are only used for document identification.

Experts from the therefore began working on a new invention – a hidden identification element that is completely invisible to the naked eye and cannot be revealed even by the traditional UV method.

It can be printed using common techniques, such as screen printing or pad printing. "The mark can take the shape of a ring or an irregular blot. It carries a chemical code created by the ratio of the contained metal oxides – exotic lanthanoids. Only an X-ray fluorescence spectrometer can reveal it by recognizing the exact elemental composition and reading the numerical signal," adds Veselý.

The patented technology allows for the use of as many codes as there are oxides in the printing mixture and in what ratio. "We can compose a mixture containing three, four, or five components represented in five different concentrations. In total, we are capable of creating up to 5⁵ identification marks, which is on the order of thousands. This is certainly more than we currently have archives or similar suitable institutions in ," continues the head of the development team.

In addition to the chemical mark itself, experts have added a locator for its position, which can be detected somewhat more easily – with a simple infrared laser. "It has the advantage of penetrating through glass. Thus, we can scan from a distance through an antique shop window to see if the document contains this mark. If so, thanks to the special nature of the compounds used, the spot will glow green. It then becomes clear that our identifier is hidden inside, and its specific code can then be decoded by the spectrometer," explains Veselý.

Marking a document, artwork, or photograph with an invisible mark is simple and inexpensive. A custom printing varnish will be created for those interested, in which metal oxides will be mixed in various ratios to create a unique numeric code. This special mixture can then be printed using pad printing by any printing press.

"To complete the process, a specific code administrator would also be needed – ideally a state institution such as the or , equipped with an X-ray fluorescence spectrometer, that would assign a secret code to applicants and instruct the manufacturer on the composition of the respective printing varnish. This institution would then be able to identify the owner or custodian of found archival items, books, or other art artifacts," he adds.

Private sector companies and state institutions have already expressed interest in the invention. Besides marking paper documents, the mark can be used to verify artwork. "Art prints, which can be endlessly printed with one particular serial number, are easily forged. Photographers complain that their work is far too easy to copy," Veselý concludes, highlighting further applications.

The team from Faculty of Chemistry BUT collaborated on the development of the invisible identification mark with the Centre of Organic Chemistry, the National Archives, the Institute of Chemical Processes of the CAS, the University of Chemical Technology in Prague and the Institute of Chemical Technology for the Restoration of Monuments. All institutions contributed equally to the patent.

(mar)