Study on Decay Patterns of the Stone Reliefs of the Tāq-e Bostān Historical-Cultural Site

The historical-cultural site of the Tāq-e Bostān is located in the Porav-Biston Mountain range. This area is attributed to the Sassanid era, which includes three main parts of stone reliefs so-called "the Great Arch", "the Small Arch" and "the Relief of Ardeshir II". Since its creation, this site has been constantly exposed to environmental factors and has rarely been studied in terms of decay. Therefore, this study was conducted to investigate the decay patterns of stone reliefs by field studies and macroscopic surveys. Studies have shown that the crusts (mainly in the Great Arch) have covered the surface of the reliefs in orange to brown and even gray tones. In particular, on the western side of the Great Arch, these crusts have distorted most of the reliefs. The small arch and the relief of Ardeshir II are less affected by surficial crust. The reason of crust formation on the reliefs of the Great Arch is mainly the outflow of water from the natural channels inside the rock, so that by sprinkling among the reliefs, it has caused transfer and then deposition of dissolved minerals. Some microorganisms and bacterial residues are observed in the Relief of the Tree of Life, the motifs of the angel, the walls on the east and west sides of the small arch, and the Reliefs of Ardeshir II. Due to lichen activity in these areas, the pitting pattern has caused distortion and roughness of the surface. Also, the restoration interventions such as cement used to fill the rock veins have caused the formation of effloresces and microcracks in the surrounding areas. Therefore, from the point of decay patterns, it became clear that the water flowing and the dissolution occurred by this phenomenon, along with the activity of microorganisms and previous incorrect interventions, are the most important factors of deterioration of the surfaces of reliefs of the Tāq-e Bostān.