X Ray Digital Radiography (RX) and Computed Tomography (TC)
Digital radiography can be very useful especially for paintings, both on canvas and on wood. It is possible to obtain information about the conservation status, the characteristics of the background, the manufacturing techniques and the underlying paint layers. Tomography can be successfully used for all the artworks in which the third dimension is also important, such as figurines, vases, ornaments or other artefacts. It allows seeing the inside of the artefact in a non-invasive way, getting preliminary information about its preservation state and manufacturing technique. (See the example beside: the three-dimensional reconstruction of a Diana head showing a very small thickness defect). With our innovative instruments it is possible to analyse objects of different sizes and types, changing in real time the acquisition conditions in order to get the best results depending on the studied materials. Much of our analysis systems are transportable and therefore allow to perform analysis directly in museums or in restoration studios, after the approval of safety qualified expert.
X-Ray Fluorescence (XRF)
X-Ray Fluorescence (XRF) is an analytical technique that can be applied to most inorganic materials directly in situ. It allows quickly performing multielemental non-invasive / non-destructive analysis and therefore it has been one of the earliest techniques applied in the cultural heritage field. The analysis of the “secondary” X-ray spectrum emitted by the sample after the excitation due to a “primary” X-ray beam allows for the identification of the chemical elements constituting the sample. The emitted energies are indeed related to the atomic species present in the material and thus allow identifying the atomic composition of the analysed object in a short time (typically of the order of a few minutes). In order to obtain molecular information, it is necessary to perform stoichiometric calculations or to employ complementary techniques (eg. Raman and IR spectroscopy). Common applications of XRF are: the identification of the different components of a metal alloy, of the pigments used in glass or in a paint. From the pigment identification it is possible to know the artist’s palette and identify fake parts.
Some of the available XRF instrumentation allows for obtaining images in two or three dimensions:
XRF confocal scanning
X Ray Diffraction (XRD)
X Ray Diffraction (XRD) is used to determine the mineral phases that characterize the studied materials. The technique is particularly useful in the study of cultural heritage and of different types of archaeological materials. It is especially efficient in order to determinate the nature of the pigments in paints, frescoes, mural paints, parchments and manuscripts. Other applications are related to the determination of corrosion or degradation patinas in ancient metals and in materials used in historical buildings. In addition, XRD is also used for the individuation of the nature of degradation processes and to identify appropriate conservation protocols. Recently, the technique has also been used in forensics for attribution and authentication of historical and artistic materials.
Depending on the instrumentation, the technique may require sampling or may be completely non-invasive, non-destructive.
Alfa Particle Induced X-ray Emission (alpha-PIXE)
The portable PIXE-alpha technique is particularly appropriate for the compositional analysis of surfaces. The chemical information is obtained from the analysis of the X-ray spectrum emitted by the sample after the irradiation with an alpha particle beam. The analytical range of the particles used as primary radiation is limited to 5-10 micron. For this reason, the system is especially appropriate for the elemental analysis of the most superficial layer without the interference of the components belonging to the innermost layers. Compared to X-ray fluorescence, this technique also allows obtaining quantitative information on the concentrations of the elements present on the analyzed surfaces. Useful information can be obtained on the technologies used and on the origin of the raw materials. In general, alpha-PIXE is widely used in the analysis of pigments (ceramics, paintings, frescoes), patinas and alteration of metals.
Raman spectroscopy is the most powerful non-invasive and non-destructive technique for molecular analysis among those currently available for the analysis of cultural heritage. It is based on the inelastic scattering of light by the sample and it can provide information on the molecular composition, the bonds, the chemical environment, the phase and the crystalline structure of the test sample. It is appropriate for the analysis of materials in most forms: gas, liquid and amorphous or crystalline solid. The technique is used for the physical and chemical characterization of paint pigments, but also for the identification of the type of stones or gems. Raman spectroscopy has become one of the most used techniques for the study of art works, since it allows for material identification through a comparison with standard Raman spectra. This technique indeed provides essential information about the nature, the state of degradation and, sometimes, on the origin of the used material, significantly contributing to restoration and conservation projects. The portable instrumentation allows for the in-situ analysis of art works of any size. Through a microscope (Raman microspectroscopy) it is possible to characterize pigment grains down to dimensions of the order of the micron, thus allowing, for example, to separately obtain the spectra of the pigments mixed by the painter to achieve a specific colour.
Multispectral imaging is a reflectance technique consisting in the observation of an image produced by:
– the radiation reflected by the object in the visible region (Trichromatic RGB Imaging, Macro photography, Macro photography with grazing light);
– the radiation reflected by the object in the near infrared region (reflected infrared photography);
– the interpolation of the infrared image with the three separate Red, Green, Blue images (false color photography);
– the fluorescence radiation emitted by the object after the exposure to ultraviolet rays (ultraviolet fluorescence photography).
These techniques allow for the observation of different morphological qualities of the object, such as:
– the paint thickness (through the Trichromatic RGB Imaging)
– the executive traces, such as the “ductus” of the brush stroke, the engravings and the visual interferences due to the supports (with Macro photography)
– the preparatory drawing (with Infrared photography)
– the restorations (with Ultraviolet Fluorescence photography)
The colour is not a physical quantity but a perception that, in addition to subjective characteristics, depends on the light source used. The fruition and the conservation of polychrome works of art depend both on the emission spectrum and the illuminance level of the light source. The spectrum influence the appearance of the colour while the brightness is related to any damage to the work. The colorimetry is a technique used to specify with objective data a colour by three numerical coordinates.
Infrared thermography is widely used in the archaeometric field, because of its non invasiveness-non destructiveness and because of the possibility to perform analyses in situ. This technique is based on the detection of a temporal sequence of thermograms (images of the surface temperature of the analysed object) after a thermal perturbation caused by the absorption of a light pulse. By the comparison between the different thermograms, hidden elements with physical or structural properties different from those of the background may be pointed out. In genral, in the thermographic sequence it is possible to see elements, defects and inhomogeneities belonging to different layers of the analysed object.