# Terahertz Inspection of Buildings and Architectural Art
## 4 Monitoring of the Conservation Process of a Mural
The sample removal resulted in severe damage to the reinforcing material; therefore, THz-TDS was used for the first time in 2016 in order to prevent further damage and weakening of the protective plaster. Other methods, such as ultrasonic examination, were not considered because the depth resolution is inappropriate for the small cavities formed. So far, the evolution of the cyclodotdecane/air layer has never been non-destructively probed on a real sacrificial render system. The aim of our measurements with THz-TDS was to determine the size and dimension of the cyclodotdecanelayer and the airgap that forms after its sublimation process between the original surface and the covering sacrificial render system. In this section we present the results of non-destructive measurements performed by terahertz time-domain spectroscopy (THz-TDS) both on mockups and in-situ on the conservation intervened mural of the Konstantinbasilika. Figure 6 shows the area that we used for monitoring. The images show the mural before the conservation intervention, and the result after the intervention. Aluminum foils can be seen on the surface of the protective plaster layer that were used to generate nearly-perfect terahertz reflections, to be used in the data processing as reference. The stratigraphic measurements were taken in the areas delimited by the aluminium foil squares. In parallel with the conservation intervention, mockups were fabricated for laboratory measurements. Woodenolds of 14.5 cm \(\times\) 12 cm were prepared, a base of Roman cement was placed at the bottom; subsequently, cyclododecane was applied with a brush to the surfaces by double coating reaching a thickness of approximately 0.6 mm, over it a reinforcing mesh made of para-aramid fibres (KEVLAR) was placed. Finally a plaster layer of \(\sim\)7 mm up to the edge of the mold was applied. The thickness of the upper protective plaster and the layer of cyclododecane corresponded approximately to the real conservation measure at the Konstantinbasilika. While the mock-ups were kept in a laboratory with a controlled temperature of \(\sim\)20 \({}^{\circ}\)C, the mural was exposed to the uncontrolled outdoor conditions that prevailed in Trier during the year that passed between measurements. All of the measurements were performed with a fiber-coupled terahertz time-domain spectrometer using a \"pitch-catch\" terahertz path like the one shown in Figure 1b but using high density polyethylene lenses instead of off-axis parabolic mirrors. The spectrometer is based on a mode-locked ultrafast Er:fiber laser which is used to gate both, an emitting and a receiving, photoconductive antennas. The transceiver ensemble was mounted on an \"x-y\" motorized stage that allowed automated multiple point measurements. Various measurements over the course of 300 days were taken on the mockups. A typical terahertz waveform from one of the mockups is shown in Figure 7a, the first pulse, at \(t=0\), corresponds to the air-plaster interface, while the second pulse corresponds to a superposition of two echoes from the gap initially generated by the cyclododecane layer. By using sparse deconvolution [48] on the signal, we obtain the impulse response function shown in Figure 7b, where it is clear that the second pulse is actually the superposition of two echoes, which are shown in more detail in Figure 7c. The separation between these two echoes depends on the physical separation between the plasters as well as the refractive index of the material in between. Therefore if the cyclododecane, whose refractive index is \(n\sim 1.53\pm 0.02\) with  
Figure 6: Photographs from the Konstantinbasilika identifying the area where the conservation intervention was performed. The right-hand-side bottom panel shows the area after conservation with the aluminium foil references around the positions studied. negligible dispersion, sublimates as expected over the course of several months, the separation between these two echoes is expected to decrease.