## 5 Thermographic Examination of Mosaic Surfaces  
Hagia Sophia in Istanbul is a unique monument with invaluable assorted mosaic categories. The NTUA research team had the opportunity to study Hagia Sophia mosaics in 2000 [31] and in 2010 [32]. In the first case, IRT (AVIO TVS-2000 Mk II LW, 8-12 \(\upmu\)m) was used along with other non-destructive techniques in an attempt to investigate the plastered surfaces of dome mosaics. Interventions in the past have triggered aesthetic problems leading in some cases to severe damage of the mosaics. Such an example is the intervention that occurred in the early 1900s, under the General Directorate of Pious Foundations, mostly known as the \"Vakif intervention\". The plaster of this intervention was covered with an opaque yellow pigment in a glue medium. A pattern of stencil ornaments was applied on this yellow ground [33]. In this work, a plastered mosaic surface (characteristic Vakif intervention surface) in the Hagia Sophia dome was investigated. The surface was situated between the 19 th and the 20 th rib at the northwest part of the dome, 16 m from the centre of the dome. Figure 22 shows the examined surface whilst a thermal image obtained from the investigation of this surface is shown in Figure 23, in order to reveal the mosaic tesserae beneath the plastered surfaces. Due to the different thermal diffusion that each layer renders, IRT was able to detect the different sub-surfaces on the plastered mosaics by presenting the temperature variations on the surfaces. This was accomplished after employing the active thermographic approach, using an external optical heat source in order to uniformly heat the inspected panels. The surface was heated with optical lamps (1000 watts power output from a distance of 50 cm), and the presented thermal image was obtained during the cooling down process. The image illustrates  
Figure 23: Thermal image of the investigated plastered mosaic surface. Figure 22: Photograph of investigated surface in the Hagia Sophia dome. the existence of the coated mosaic, as well as the detached area within it. Darker grey areas, i.e., lower temperature values, represent the mosaic. The detached areas, behaving similar to a sub-surface defect, hold remarkably higher temperature values due to the different thermal diffusion rate (thermal contrast). The higher temperature of the detached area was between 0.8 and 1 \({}^{\circ}\)C.  
On the second visit, IRT (FLIR B200 Western, 7.5-13 \(\upmu\)m) along with other non-destructive was were applied to investigate the mosaics in the galleries. The main aim of this on-site investigation was to evaluate the preservation state of the mosaics and the previous interventions, as well as to detect mosaics in layers below the external plastered surfaces. The results indicated that, with the aid of NDT, it is indeed possible to locate the grid of rendered mosaics. Additionally, we identified the main environmental decay factors (moisture, salt, pollutants), the areas where the mosaic materials (tesserae and bedding mortars) presented decay problems and sub-layers that pose risk of detachment or decay intensification. In Figure 24, we present our investigation of an area of the mosaic of [PERSON] and [PERSON]. This mosaic was rediscovered by the Byzantine Institute in 1934. The lower part of the mosaic is missing. In this case, IRT was applied in order to diagnose the preservation state of the mosaic. This mosaic is located on the interior of an east external masonry, and a past cement intervention is present at its lower part. With the aid of IRT, we investigated areas with different temperatures at the bedding cement mortar, but also at the existing tesseraes. These temperature differences can be attributed to the action of the main deterioration factors, such as the presence of salts and moisture. The air temperature was 23 \({}^{\circ}\)C and the relative humidity was 70%.