**2. Materials and Methods**

#### *2.1. X-ray Fluorescence Spectroscopy*

The prints were fully scanned using the NU-ACCESS MA-XRF system described in [23], for which the X-ray tube and polycapillary optic were replaced by a 50 W transmission Rh anode X-ray tube (Varex Imaging, Salt Lake City, UT, USA) and 500 μm collimator, respectively. MA-XRF scans were collected with a dwell time of 0.2 s, at 40 kV voltage and 1.250 A current. An XGLabs Elio X-ray fluorescence spectrometer operating at 40 kV and 600 μA was used to collect spot samples for comparison and confirmation. All XRF data processing to create elemental distribution maps was done in Python with the PyMCA framework [24].
