# Rock Emissivity Measurement for Infrared Thermography Engineering Geological Applications
## 7 Conclusions  
In this paper, the experimental estimation of rock emissivity was carried out to provide new scientific data on the use of infrared thermography (IRT) for engineering geological purposes. The development of new survey remote methodologies has opened the way to the application of IRT in laboratory and field activities involving rocks, through both qualitative and quantitative approaches. In the second case, the surface temperature of a rock framed by a thermal camera can be properly estimated by knowing its characteristic emissivity. In this paper, the emissivity was estimated herein on 27 oven-heated specimens belonging to 15 different lithologies among sedimentary, igneous and metamorphic ones by employing a thermal camera operating in the 7.5-13 \(\upmu\)m spectral range. The achieved results show that tested rocks are characterized by high emissivity values, generally greater than 0.89, with the exception of polished rock granite surfaces, which show a slightly lower value due to the reflection effect brought by the polishing treatment (Figure 7). Moreover, each rock type is not characterized by a univocal emissivity value, but rather by narrow ranges of variability arising from the heterogeneity of the rock surface. In fact, it must be taken into account that rocks are natural aggregates of mineral affected by different textural features playing a key role on their emissivity. The first effect observed herein is played by pores, whose presence leads to the basic physical property known as porosity. In this case, the key controlling element is the presence of macroscopic pores, whose IRT attitude in oven-heated specimens is a high surface temperature at the pore location. Therefore, it is self-evident that the emissivity of a rock surface is conditioned by pores, which contribute to enhance its value (Figure 7). A further controlling feature is the mineral content, which is particularly evident for holocrystalline rocks, where minerals are macroscopically recognizable according to their different thermal behavior. It was experienced herein that the IRT output of dark minerals, such as micas in granites, is represented by positive anomalies (high temperatures) and high emissivity, while lighter minerals, such as quartz, are affected by lower temperature and emissivity (Figure 7). Therefore, both pore and mineral effects play a controlling role on the rock emissivity, although with different weights depending upon the rock type (the emissivity of holocrystalline rocks with no macroscopic pores is more affected by the mineral effect, while sedimentary porous rocks mainly respond to the pore effect). The combination of such two effects is so relevant that the rock color, at the hand scale specimen, seems not to produce relevant emissivity variations. Based on such considerations and for IRT applications requiring a general framing of the rock surface, it is reasonable to overcome these effects by considering an average emissivity value for each rock type. Nevertheless, cases of same rock types but with different porosity must be distinguished (e.g., massive and vesicular basalts). The results achieved herein surely represent a scientific step forward in the employment of IRT to rocks and pave the way to further studies aiming not only at increasing the scientific casuistry, but also at shedding light on new potential use of the IRT technology on rocks. Conceptualization, S.M. ; methodology, S.M. and G.P. ; software, S.M. ; validation, S.M. and G.P. ; formal analysis, S.M. ; investigation, S.M. ; data curation, S.M. ; writing--review and editing, S.M. ; visualization, G.P. ; supervision, S.M. and G.P. ; project administration, G.P. ; funding acquisition, G.P. All authors have read and agreed to the published version of the manuscript. This paper was financially supported by the Department of Biological, Geological and Environmental Sciences, University of Catania, Project: \"CH2V--Cultural Heritage Hazard and Vulnerability\", granted to [PERSON].