Unfortunately, transmitted light does not always make these watermarks visible to the naked eye or a digital camera when the watermarks are located under a substantial overlying painting or inscription.
The Fraunhofer WKI, together with the Herzog Anton Ulrich Museum and the Institute for Communications Technology at the Technische Universität Braunschweig (IfN), was able to solve this problem with the help of thermography. It enabled us, within an extremely short period of time, to make the watermarks visible in approximately 60 old sketches which had been attributed to Rembrandt and/or those around him.
Thermography makes watermarks visible
In an experimental device (Fig. 1), a thermographic camera produces a picture under favourable thermal conditions which reproduces the differences in the thickness of the paper, whereby the watermark becomes clearly visible. The artwork or texts on the examined material remain thereby to a substantial extent invisible.
Up until now, this optical elimination required elaborate X-ray apparatus which could only be operated in accordance with the appropriate safety precautions. In contrast, the new thermographic method is mobile and simply records and documents watermarks on painted, written or printed sheets and thereby contributes greatly to their (art-) historical investigation. It is a decisive aid in answering the question as to whether, for example, a work previously classified as a Rembrandt was actually produced by one of his pupils or by an imitator in the 18th century (Fig. 2).