Imaging by Ultraviolet (UV) fluorescence
Since the UV radiation has a short wavelengths than the visible light, so by using a UV-fluorescence, it is possible to identify multiple paint layers. Previous practical applications showed that the UV imaging is advantageous in detecting any retouching, varnishes, successive over-paintings and old restorations (Rizzutto et al., 2015). In fact, pigments usually show characteristic fluorescent hues when impacted to a UV light. For example, minium (Pb2O5) = orange fluorescence, while azurite = deep blue fluorescence, limonite = yellowbrown fluorescence, and the red ochre shows a deep red fluorescence (see Fig.
13). For detailed information, it is useful to investigate cross-sections under the UV fluorescence microscopy. This microscopic feature is based on coupling a microscope with an ultraviolet radiation instead of visible light. This microscope uses fluorescence and phosphorescence to study the properties of organic or inorganic substances. So, it is able to differentiate compounds and layers in the same cross-sectional sample which works as a key to understand transparent layers (Pelagotti et al.
, 2018). Table 2 summarizes the common abbreviations used for the UV imaging.
Table 2. Common abbreviations used for the UV imaging
VIS Normal image, in visible domain of visible light (380-780 nm),
REF_UV_UV Reflection in the UV domain (360-400);
UVFC Image treatment of REF_UV_UV with VIS to obtain false color lisible image
FLUO_UV_VIS Fluorescence under UV (pic 365 nm) in the visible domain;
REF_NIR_NIR IR reflection under near IR light (780-1000 nm);
IRFC Image treatment of REF_NIR_NIR with VIS to obtain false color lisible image
FLUO_VIS_IR Fluorescence under visible light (pic 690 nm) in near IR domain (780-1000 nm)
Figure 13. An example of a painted surface before and after the exposure to UV light
As good as, the long infrared wavelengths allow better penetrability into the structure of a material. For this, it has been utilized successfully as a photographic technique for underneath paint layers, over-paintings, signatures, etc. (Hain et al., 2003). Well, the degree of penetration depends on the thickness of the paint, the type of paint, and the wavelength of the IR. Many paints appear partially or completely transparent while others, such as black, will absorb the infrared radiation and will appear opaque and dark. An infrared camera captures the radiation reflecting off the surface of the painting, producing a digitized image (an IR reflectogram). The illumination source is a common incandescent lamp radiating enough infrared rays. Modern cameras can record IR-radiation of wavelengths up to 2400 nm. For example, the INOA IR scanner allows a spectral sensitivity up to 1700?nm (Fragasso & Masini, 2011). Abdrabou et al. (2017) have reported the efficiency of technical photographic methods (i.e. UV& IR imaging) for identifying traces of Egyptian blue pigment in a polychrome wooden coffin (GEM 10831).
The optical coherence tomography (OCT) is a light-based imaging technique (Adler et al., 2007). In effect, tomography refers to obtaining 2D data from the internal structure of a 3D object. OCT enables high-contrast non-destructive cross-sectional views and 2D/3D high resolution investigation of semi-transparent objects (less than 20 ?m) (Targowski & Iwanicka, 2012).
Although a significant progress has been undertaken to understanding the chemical nature of ancient painting materials, but insufficiency in testing the mechanical properties is still present. In fact, it is useful to measure the mechanical properties of a paint material to obtain valuable information on the impact of the environmental factors on paint layers. In this way, nanoindentation has become a technical trend to evaluate the mechanical properties of small-size structures. This procedure is successfully applied to study thin films, coatings, paint layers, etc. Mechanical features such as the reduced elastic modulus and hardness are commonly measured by nanoindentation (Salvant et al., 2011). Nanoindentation measures the surface properties of localized small-volume at high spatial resolution, so, it can be used in multi-painted layers to evaluate each paint layer separately (Faisal et al., 2018). Otherwise a few years ago, a nonlinear microscopic tool (femtosecond pump-probe microscopy, FPPM) was improved to provide a nondestructive 3D imaging of stratigraphic painted objects. The near-infrared femtosecond pump-probe optical microscopy has the ability to detect a wide range of molecular structures. Using this ultra-modern technique, capturing images and mapping thick paint layers is obtainable (Villafana et al., 2014).
In summary, this chapter was a trail to provide answers on some inquiries concerning studying ancient Egyptian painted textiles. Based on analytical literature data, the technical approach of painting on textile fabric structure was enough obvious. Painting on textile was a simple technique when compared to other decorating methods used for ancient textiles. Simply, the fiber matrix is coated by a fine gesso layer (gypsum, or calcium carbonate, and animal glue). After the complete drying of the ground layer, it is then painted with tempera technique. In this technique, the pigment powder, which is suspended in an organic medium, is brushed on the gesso layer according to the decoration design, pre-prepared by the artist. Besides in this chapter, the pigments of the ancient Egyptian chromatic palette have been promoted. This chapter pointed out the main analytical methods used to understand the compositional structure of the paint layers. The environmental scanning electron microscopy with energy dispersive X-ray spectroscopy (ESEM-EDX) is usually utilized to record the morphological features, microstructure of the multi-layers and their microchemical analysis. Assorted non-destructive methods are used to perform an elemental scan on the whole surface or single crystals, such as the micro-X-ray fluorescence (XRF) and laser-induced breakdown spectroscopy (LIBS). To characterize the mineralogical content of pigments and the gesso layer, X-ray diffraction analysis is well suited for this purpose. Molecular and Vibrational techniques of Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) and micro-Raman spectroscopy (µ-Raman) are standard methods to identify organic and inorganic compounds. Fundamentally, micro-Raman analysis has been applied with a significant success to mapping the distribution of multi-minerals on the samples surface or in a depth profile. Many published researches showed the possibility of color measurements and visible reflectance spectra as a fingerprint for certain pigments. Finally, imaging techniques (i.e. UV & IR imaging) can be used as a complementary method to permit additional information on the hidden layers, overpaintings, old restorations and the detection of pigment traces. It should be noted that studying composite structures, e.g. painted textiles, still needs further researches to suite compatible analytical methodology and long-term conservation plans.