Max Frei theory revisitation: does really strokes depth change along time?

A serious problem in questioned document examination is to establish the age of written lines. With respect to paper dating, in the past, Max Frei theory (based only on microscope analysis) claimed that strokes are time-dependent. Therefore, according to this theory, it has been asserted that from the analysis of the strokes depth changes it is possible to try to date the handwritten document (to find out that the document is older than ...). In the present work, we investigate the strokes depth change by a laser profilometer considering not only the time but also microclimatic variations. First, we analyze the stability of stroke characteristics along the time. In particular, we demonstrate that if the document is preserved without change of temperature and humidity, the depth of the strokes has not appreciable changes. In this way, we have the purpose to verify the real possibility of documents dating by means of Max Frei theory. Subsequently, we test how the 3D profile of strokes changes in connection with the microclimatic variations. In particular, we test humidity variations. With this experiment, we show that humidity variations reduce the strokes depth. Moreover, this effect shows a non-linear trend, leaving a hysteresis on the depth. Finally, we show that the analysis of 3D stroke profile is unable to determine the age of documents.