Instrumentation and method

Fourier transform-infrared spectra were acquired in attenuated total reflectance modality (ATR FT-IR) on each single material (few grains of pigment/ dye and small amounts of resin/ binder), using a Bruker Vertex 70 FT-IR spectrophotometer along the 400 - 4000 cm-1 spectral range, with a 4 cm-1 resolution, and a Harrick MVP-2 Star single angle ATR unit.

Panels were analyzed with the following multispectral imaging techniques.

An XRITE Classic ColorChecker® of 24 colours and a 99% white ceramic tab were used as references.

Two infrared reflectography (IRR) were adoted: the IRR 950 nm was acquired with an X-Nite Nikon D810 IRUV camera equipped with a R72 Hoya IR filter, the IRR 1150 nm was acquired with an Art Innovation Artist Camera, illuminating in both cases with Ianiro Varibeam Halogen 800W lamps.

The ultraviolet fluorescence images (UVF) were recorded with an X-Nite Nikon D810 camera equipped with a Hoya IRUV-cut filter. The ultraviolet reflectography (UVR) was conducted with an X-Nite Nikon D810 IRUV camera equipped with an X-Nite BP1 filter (i.e. 330 – 630 nm and 930 – 1400 nm bandpass filter) coupled to a Peca 900 filter (i.e. 250 – 400 nm and 675 – 775 nm bandpass filter), illuminating with Labino® UV FLOODLIGHT lamps (emission peak at around 365 nm).

False colour images (FC) were elaborated using Adobe Lightroom and Adobe Photoshop softwares. The IRR-FC images were obtained by combining the IRR and the R and G channels of the RGB photograph, assigning the R to the G channel, the G to the B channel and the IR image to the R (empty) channel of the false colour image. The UVR-FC images were obtained by inserting the G and B channels of the picture and the UVR component respectively into the R, G and B channels of the false colour image.

X-rays fluorescence spectra (XRF) were acquired on the panel: here one spectrum per pigment (or dye) is reported. Analyses were conducted with a portable Micro-EDXRF Bruker Artax 200 spectrometer with a fine focus X-rays source and a Molybdenum anode, 4096 channel-ADC, 1.5 mm spot size, anodic voltage 30 kV and anodic current 1300 mA, purging helium gas in order to detect low atomic weight elements.

Fiber optics reflectance spectra (FORS) were acquired on the panel: here one spectrum per pigment (or dye) is reported. Analyses were conducted with a compact portable system based on two spectroanalysers (a MCS 601 UV and MCS 611 NIR 2.2 Zeiss Multi Channel Spectrometer system), one CLH 600 tungsten halogen lamp and optical fibres of 1 mm in core diameter. Measures were acquired in 0°/2x45° geometry along the 350 – 2200 nm wavelength range, with a spectral resolution of about 2.5 nm in the Vis range and 18 nm in the NIR range.