When you hold an old print or drawing in front of a light source, you can often discern an image or pattern within the paper itself – a watermark. They play an important role in identifying fakes and dating old master prints. Sometimes the writing or drawing is so dense that it renders it difficult to identify the watermark under normal light. Now a scientific team in Germany has developed a new method for identifying even poorly discernible watermarks.
The physicist Peter Meinlschmidt at the Fraunhofer-Institute for Wood Research in Germany developed a quite simple, but effective thermographic method to make watermarks clearly visible. He uses the fact that printed ink is transparent under thermal radiation. A thermal source is placed behind the drawing or print; a digital camera which is sensitive to thermal radiation shows the watermark density differences in the paper. As an example, under “normal”, visible light, the watermark in the drawing by Jan Lieven is barely visible (left). A thermal infrared picture of the same drawing shows clearly a crown watermark (right).
Pictures from http://www.wki.fraunhofer.de/projekte/wki-pmt-2.html
Original oder Fälschung? (in German)