Mineralogical Phase Transition on Sandwich-like Structure of Clinky Pottery from Parthian Period, Iran

摘要

Parthian period (250 B.C. - 224 A.C.) in the west of Iran is specified by appearance of Clinky pottery. In this study, Clinky pottery was investigated to evaluate the impact of raw materials and firing conditions (temperature and kiln atmosphere) due to the sandwich structure on this type of pottery. Samples were mineralogically and chemically characterized with petrography, semi-quantitative X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) analysis. Hematite and magnetite in the matrix of these samples indicate that firing process carried out under oxidation- reduction condition. High temperature products such as gehlenite, secondary anorthite, pyroxenes and pseudomullite, denote on firing temperature approximately between 950 to 1000 degrees C. This study shows that kiln atmosphere has changed to reduction in a temperature of about 850 to 950 degrees C. In reduction conditions, vitrification occurs approximately in 50 degrees C less than oxidation condition. Consequently, amorphous phase develops in the matrix. Formation of this phase helps to internal reducing and poor diffusion of oxygen to the matrix and therefore, cause the appearance of magnetite in dark-gray core. In the final stage of firing (ca. 1000 degrees C), fast but controlled blowing of oxygen was happened. In this condition, iron ions in the vicinity of oxygen is changed to hematite and the color of surface became orange-brown. Accordingly, it seems that the vitrification and formation of Si-O-Si polymeric structure in final stage of firing and furthermore formation of secondary SiO2, is the reason for formation of sandwich structure by preventing of oxygen penetration into the body. Due to this reason, iron ions near the surface were oxidized and magnetite remained in the core due to the lack of oxygen.