In this work, we demonstrate the usefulness of multivariate techniques for the analysis of two-dimensional (2D) spectral images obtained by proton beam ionization (PIXE mode) and combined proton beam and photoionization with an X-ray tube (PIXE + XRF mode). Two different multivariate analysis approaches were used: (i) principal component analysis (PCA) for dimensionality reduction followed by k-means clustering for the identification of different sample regions and (ii) t-distributed stochastic neighbour embedding technique (t-SNE). In PCA + k-means clustering, similar pixels were grouped into different clusters where a direct connection between individual clusters and elements identified from cluster spectra resulted in the fast image segmentation and identification of different sample regions. t-SNE was used for dimensionality reduction and simple 2D visualization of high dimensional data. Three different cases were investigated: (i) qualitative analysis of 2D spectral maps having pixel spectra with a high number of counts per pixel in the full range of measured X-ray energies collected in PIXE + XRF mode excitation, (ii) qualitative and semi-quantitative analysis of 2D spectral maps having pixel spectra with medium to low counts per pixel collected in PIXE mode, and (iii) qualitative and quantitative analysis of 2D spectral maps having medium to low statistics per pixel obtained in PIXE mode. In the actual case studies, we identified all the pigments in artificial and real samples, namely, illumination from a historical book, and quantitatively characterized the identified gold layer and Niello decoration on an archaeological plate of Roman origin. In the last example, we were able to identify the sample regions with similar layer thicknesses and obtain the layer thickness and elemental concentrations. We demonstrated that high statistics spectra that would contain enough information for qualitative and/or quantitative analysis of major, minor and even trace elements can be deduced using multivariate analysis methods even from low-statistics individual pixel spectra collected during 2D scanning of objects under investigation. This could be of particular importance for sensitive samples that could be damaged during long irradiation.

Stone monuments and buildings are susceptible to weathering. Carbonate-based stones are especially vulnerable in acidic environments, whereas magmatic acidic stones are more susceptible to chemical weathering in basic environments. To slow down surface corrosion of limestone and marble artworks/buildings, protective coatings which inhibit calcite dissolution have been proposed. In this work, samples from two stone types with different porosity were treated with ammonium oxalate (AmOx) to create a protective layer of calcium oxalate (CaOx) using the previously developed brushing method. Two different synchrotron microscopy experiments were performed to determine its protective capability. X-ray powder diffraction (SR-μ-XRPD) in transmission geometry allowed visualization of the distributions of calcium carbonate and oxalates along the sample depths. In a second step, X-ray fluorescence (SR-μ-XRF) was used to check the efficiency/integrity of the protective surface coating layer. This was done by measuring the sulfur distribution on the stone surface after exposing the protected stones to sulfuric acid. XRPD showed the formation of a protective oxalate layer with a thickness of 5–15 µm on the less porous stone, while a 20–30 µm thick layer formed on the more porous stone. The XRF study showed that the optimal treatment time depends on the stone porosity. Increasing the treatment time from 1 to 3 h resulted in a decreased efficiency of the protective layer for the low porosity stone. We assume that this is due to the formation of vertical channels (cracks) in the protective layer.

Textile metal threads were used to decorate historical Croatian textiles. There are three basic types of metal threads usually used on historical textiles in Croatia. These are narrow stripes, wires, and combined metal textile yarn called “srma”, made of metal thread spirally wrapped around the nonmetal textile yarn. Textile yarns were made of silk, linen, wool, or cotton. Metal threads were primarily made of gold, silver, and copper, and different alloys of these metals or threads are layered in the structure. Analysis of metal threads with three different methods was made and the most adequate method for the analysis of metal threads from historical textiles was established. Metal thread analysis was performed with scanning electron microscopy with an energy-dispersive X-ray detector (SEM-EDX), which was determined to be the most suitable for the analysis of historical textiles if cross-section analysis of metal threads is also performed. Textile threads from combined metal textile threads were analysed with a light microscope. This information of the metal threads’ content and structure as well as the composition of textile thread can lead to an understanding of the technology of production threads and also temporal and spatial dating of textile objects which is helpful to conservators and restorers of valuable historical textiles.