# Terahertz Inspection of Buildings and Architectural Art
## 2 Terahertz Uses in Architectural Art
### Wall Paintings  
Wall paintings are the oldest known form of figurative artwork dating back to the prehistoric period [21]. The conservation of wall paintings inevitably concerns not only the paintings themselves, but also the supporting architectures and building materials [22]. Detailed knowledge of the internal structure supporting a wall painting may be considered fundamental for conservation purposes, allowing for the localization of cracks and layers detachments, as well as for identification of regions where filling or consolidation is needed. In historical Europe, wall paintings were typically executed on wet (fresco) or dry (secco) plaster painted with pigments dissolved in limewater (fresco) and/or with organic binders (secco). Several plaster or mortar layers typically cover the wall structure before the paint application, normally characterized by increasing thicknesses and coarseness starting from the supporting structure. Aggregates within the mortar or plaster may vary and include stone, sand, straw, fibers etc. It has been established that THz radiation is generally able to provide information about the interim region of a wall painting up to approximately one centimeter into a plaster wall surface (for \(n=2.2\)) [23]. However, this number needs to be adjusted, depending on the refractive index of the constituent material of the wall and the extent of the scattering effect caused by the granular surface and internal structure on the radiation. To the best of our knowledge, a terahertz time-domain system (Picometrix T-ray 4000) was used to study wall painting replicas for the first time by [PERSON] et al. in 2008 [24]. They imaged graphite preliminary sketches (underdrawing) covered by layers of paint and a top thick layer of gypsum plaster; they, thus, demonstrated the feasibility of using the terahertz technique for inspecting wall paintings, specifically for imaging underdrawing and paint layers covered by a plaster topping layer. The first report on a real wall painting scanned with a THz-TDS system was a fragment about 25 cm in the width of a wall painting from Dazhao Monastery (Mongolia). The remain, representing a bust of a woman, was scanned in 2010 by [PERSON] et al. [25]. The distribution of different materials on the painting surface was mapped and the painting stratigraphy was reconstructed from the terahertz measurements. The surface depiction was clearly imaged in the reflection THz image thanks to the contrast given by pigments with relatively high reflectance (minimum, cinnabar, and lead white) as compared to the other ones (small, orpiment, malachite, and red ochre) present in the panting. In addition, the layers composing the multilayered painting support, i.e., the fine preparation layer, coarse preparation layer and clay wall, were identified in the non-invasive THz cross-sections (B-scans). When compared to other painting typologies, such as paintings on canvas and wooden panel, scanning wall paintings with a reflection THz time-domain system is complicated by the fact that walls are typically rough and uneven, thus deviating the THz beam reflected at the surface from the detector and causing beam defocusing in the optical axis direction. In addition, the typical grain-like structure of wall painting surfaces causes scattering of THz waves, strongly influencing the output of the measurements. In fact, the spatial scale of refractive index fluctuations due to variations in surface roughness can be, in many cases, comparable to the wavelength of the THz signal. [PERSON] et al. [26] have defined a correction procedure for adjusting the THz data acquired by scanning the uneven wall paintings of the dwellings at the Neolithic Catalhoyuk archeological site (Turkey). THz scans were performed in a \"pitch-catch\" configuration. They first created a (x,y,z) three-dimensional surface profile based on the time delay difference of reflected signals from a zero plane defined by the time shift of the signal which gave the largest amplitude.