The effect of case hardening on the natural preservation of stone monuments, Fraktin monument, Türkiye

Abstract Case-hardening processes that develop in monuments reduce the porosity of rocks while increasing surface hardness. This situation may contribute to the transfer of cultural stone heritage markers to future generations by limiting the penetration into the rock structure of water, which is the most destructive agent in atmospheric weathering processes. This study focused on the Fraktin relief monument, which was created by the Hittites in the 13th century BCE by carving into the rock surface. The site was investigated to reveal the case-hardening mechanism and its effects on the physical properties of the rock. The geochemical formation mechanism of the surface-hardening process, which contributed to the survival of the Fraktin relief monument, was examined by use of scanning electron microscope–energy dispersive spectrometry (SEM-EDS) and analysis by an inductively coupled plasma–mass spectrometer (ICP-MS). In addition, the effect of the hardening on the physical properties of the rock was investigated with nondestructive tests applied both in situ and in a laboratory. The SEM-EDS and ICP-MS analyses revealed that amorphous silica filled the pores of the rock and created surface hardening. This situation has increased the geotechnical properties of the unit in which the monument was carved, making it more resistant to atmospheric processes.