A MOBILE XRF SCANNER FOR A REAL-TIME ELEMENTAL IMAGING OF PAINTED ARTWORKS

Painted artworks are complex multi-layered materials studied over the time with several sophisticated imaging techniques. Scanning MA-XRF technique provides information on pigment materials and their distribution over the painted surface in non-invasive manner. Further, it provides information on conservation state of artworks and also their authenticity can be approached. The penetrating nature of X-rays allows visualizing overpainted hidden pictorial compositions, not visible to naked eye. In the present PhD activity an advanced XRF scanning system for a real time imaging of painted artworks has been developed at the LANDIS laboratory of LNS-INFN and IBAM-CNR in Catania (Italy).The MA-XRF scanner presents a modular set-up to be assembled/disassembled in a short time (30-60 min) and easily moved in museums for analyses in situ. Spectrometric head is moved by three linear stages covering an area up to 110 × 70 × 20 cm3 (XYZ) . All motion sensors and their synchronization with the acquisition system are fully controlled by a custom developed and programmed Control Unit (CU) and all instrument operations can be in real-time monitored by a graphical user interface (GUI). The CU controls also the Z-axis movement, along the focal distance, via the communication with a laser triangulation sensor maintaining dynamically constant the painting-spectrometer distance in case of non-flat sample surface. Further, the CU interrupts immediately the scan if the safety distance is overcame. SDD detector coupled to a Digital X-ray Processors (DXP) that works in a time-list event mode (TLIST) allows to perform scans in real time (or in continuous mode). Ultra-fast scans can be performed up to 100 mm/sec covering the total area in 4.2 hours. The MA-XRF device is aimed to the macroscopic analysis of large size paintings. However, the use of a polycapillary lens equipping the X-ray source, allows to combine imaging at both the macro and micro scale of length with a simple adjustment of the sample position along the focal distance obtaining a lateral resolution up to 35 µm. A long range optical microscope is used for the accurate positioning of the painting. To date, performances of this device in terms of lateral resolution, scanning speed and dimensions of the scanning area, makes it as one of the most (or even the most) efficient portable MA-XRF systems available in the scientific community for studying paintings. X-ray spectra are processed in real-time during the scanning by the non-linear least-squares fitting procedure developed in PyMCa and integrated in the in-house programmed analysis software with a maximum processing speed of 5000 fitted spectra per second. Further a number of editing and mathematical tools for processing the images in live-mode are available. During the present research activity, the scanner was validated by studying three painted artworks on different support (canvas, wood and paper) and different size, for which the instrument has been moved in situ. MA-XRF provided valuable information through the determination of the pigment palette on painting techniques and artist connections with other workshops, the degradation state of the paintings, the reconstruction of the creative process by revealing overpainted figure and hidden inscriptions as well by the identification of changes introduced during the creation of the work. Analytical results obtained by MA-XRF analysis of the Paston Treasure , a mid-1670s oil on canvas painting at the Norwich Castle Museum (UK); painted wooden coffers belonging to the funerary collection of the tomb of Kha and Merit (1549-1292 BC) at the Museo Egizio in Torino (Italy); and an Italian illuminated manuscripts manufactured in the 14th century at the Fitzwilliam Museums (Cambridge) are presented and discussed.