Identifying the Glaze Structure of Historical Clay Bricks: A Case Study of Persepolis Clay Bricks

Abstract Glazes include a wide range of organic and inorganic compounds. During the Achaemenid period, one of the decoration methods of buildings and palaces was the use of colored glazed clay bricks. This experimental-laboratory research was carried out regarding valid scientific documents. Purpose: to identify the constituent elements of glazes in Achaemenid clay bricks. Research Question: What are the constituent elements and the main coloring factors in gray and azure blue glazes of Achaemenid clay bricks in Persepolis? This research aims to identify the structure of the glaze of Persepolis clay bricks. Among the different devices that are used to identify minerals, X-Ray Diffraction and Scanning Electron Microscope laboratory methods were used. The XRD device was used to detect the type of material and determine the type of constituent phases and the SEM device was used to determine the amount of constituent elements of the material. The glazes of Persepolis bricks are white, pale green, dark green, dark brown, gray, fawn yellow, and azure blue. In this article, the structure of gray and azure blue glazes was identified and studied. The study samples were physically sampled as a tiny flake-shaped layer of glaze from the building body on the eastern wall of the exclusive women's palace during the time of Xerxes. In total, three samples of gray glaze and three samples of azure blue glaze were selected. According to the results, the existence of silica and porosity of the body and also the existence of iron and magnesium in the composition of the glazes were confirmed, which were probably present in the original composition (base glaze), showing that besides the color effects, they were also present in them as impurities. Based on the structure of gray and azure blue glazes, both glazes are alkaline. The scientific and academic application of this research is for the protection and restoration of brick glazes and also for the use of the Ministry of Cultural Heritage, Handicrafts, and Tourism, as well as students in restoration of cultural and historical artefacts in Art universities.