In archives, documents are usually marked with traditional metal or self-inking stamps bearing the name of the relevant institution. While this is a simple and widespread method, it has several drawbacks. It’s visible and thus diminishes the aesthetic value of printed materials. Moreover, stamp ink can be easily removed.

“During the 1990s, many prominent memory institutions were affected by thefts. Among the most notorious cases were the thefts of old maps from the Lobkowicz collection in Nelahozeves. But Czech archives and libraries weren’t spared either – letters and manuscripts by Alexandre Dumas Sr. or Beethoven’s Moonlight Sonata disappeared. These documents were worth hundreds of millions of Czech crowns. Years later, some resurfaced at foreign auctions or in antiquarian bookstores. But it wasn’t always easy to determine ownership. Traditional archive stamps can be dissolved or wiped off, leaving nothing to prove provenance,” explains the project’s principal investigator, Dean of FCH BUT, Michal Veselý.

With the rise of digital technologies, barcode and QR code systems have become popular. However, their downside is visibility and easy replicability. They serve only to identify a document – not to protect it.

Experts from the Institute of Physical and Applied Chemistry began developing a new invention: a hidden identification feature invisible to the human eye and undetectable by UV light.

The mark can be printed using common techniques such as screen or pad printing. “The mark can take the form of a ring or an irregular blot. It carries a chemical code determined by the ratio of contained metal oxides – specifically exotic lanthanides. Only an X-ray fluorescence spectrometer can detect them by identifying the elemental composition and reading the numeric signal,” adds Veselý.

The patented technology allows for as many codes as there are combinations of oxides used and their ratios. “We can mix a composition with three, four, or five components in five different concentrations. That gives us up to 5⁵ identification marks – thousands of unique codes. That’s certainly more than there are archives or relevant institutions in the Czech Republic,” the project lead continues.

In addition to the chemical code, researchers added a locator to help find the mark’s position more easily – using an ordinary infrared laser. “The advantage is that it penetrates glass. You can scan a document through an antique shop window and check for the mark. If it’s there, the spot glows green thanks to the special compounds used. That indicates our invisible identifier is present, and its specific code can then be read using the spectrometer,” Veselý explains.

Marking a document, artwork, or photograph with the invisible code is simple and affordable. A custom printing lacquer is created for each client, containing metal oxides in specific ratios that form a unique code. The special mixture can be applied using common pad printing techniques.

The mark contains a chemical code defined by the ratio of metal oxides | Author: Jan Prokopius

“To fully implement the process, we’d ideally need a dedicated code administrator – preferably a state institution like the National Archives or National Library. This body would have the necessary spectrometer and would assign secret codes to applicants and provide manufacturers with the formula for the printing lacquer. That way, the institution could verify the ownership of recovered artifacts, books, or other artworks,” Veselý adds.

The invention has already drawn interest from private companies and public institutions. Beyond marking paper documents, the technology can help verify artwork authenticity. “Art prints, for instance, are easy to fake – you can keep printing the same edition number endlessly. Photographers often complain that their work is too easy to copy,” concludes Veselý, outlining further potential applications.

The development of the invisible identification mark was a joint effort of the Faculty of Chemistry BUT, the Centre for Organic Chemistry, the National Archives, the Institute of Chemical Process Fundamentals of the Czech Academy of Sciences, the University of Chemistry and Technology in Prague, and the Institute of Chemical Technology for the Restoration of Cultural Heritage. All institutions contributed equally to the patent.