During the eighteenth century, Giovanni Domenico Tiepolo (son of the more famous Giovanni Battista Tiepolo) was commissioned by the prince-bishop of Würzburg to depict the biblical story of the flight of the Holy Family from King Herod. But did he start the series of 24 etchings, called The Flight into Egypt, in his hometown of Venice, Italy, or only after he arrived in Würzburg, Germany, at around 1750?

Art historians in Freiburg, Germany, have now acquired the technology to help them solve this long-standing mystery. The method allows watermarks to be seen in situ without damaging the paper in which they are embedded. It was developed by scientists at the Technical University Braunschweig and the Fraunhofer Institute for Wood Research, also in Braunschweig, and was first tested last year on Rembrandt sketches.

Paper mills have characteristic watermarks that are visible when the paper is held to the light, although not when they are obscured by ink. X-ray analysis can be used to identify obscured watermarks, but it needs special conditions to protect the paper from radiation. Delicate materials therefore usually need to be transported, and museum curators are often reluctant to do this.


In the new method, a plate warmed to 35–40 °C is placed behind the paper for one second, during which an infrared camera captures the heat passing through it. The outline of the watermark is revealed because it lets more heat through than does the rest of the paper. The picture here illustrates an early etching for the series, together with its watermark (inset), as exposed by the thermography technique. It is part of the exhibition Giandomenico Tiepolo: The Flight into Egypt, which opens this week and runs until 16 September at the Augustiner Museum in Freiburg. All the watermarks stem from a paper mill not far from Würzburg, so Domenico almost certainly started work on the series after he arrived in Germany.