Hematite as unprecedented black rock art pigment in Jufri Cave, East Kalimantan, Indonesia: the microscopy, spectroscopy, and synchrotron X-ray-based investigation

Recent research on black pigment rock art reported on various prehistoric image sites globally consisting of carbon compounds and manganese oxides. In this paper, we report the unprecedented use of hematite as a black pigment material in rock art. Combination of scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS), X-ray fluorescence (XRF), synchrotron radiation X-ray diffraction (SR-XRD), X-ray absorption near-edge spectroscopy (XANES), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy was able to provide detailed information on the physicochemical characteristics of red, purple and black pigment materials from Jufri Cave, Sangkulirang, East Kalimantan. The results show that the mineral hematite is the main component of all pigment materials. However, the hematite crystals in each pigment possess different properties of crystallinity and octahedral symmetry of the Fe3+ ion. The characteristic differences in hematite compounds are possibly due to the high-temperature heating process and could be caused by natural geological or anthropological factors. Besides, the discrepancy of crystallinity and distortion of octahedral symmetry in the hematite structures are presumably responsible for the color variety among hematite crystals in the pigment material. The information on the physicochemical properties obtained from this research certainly adds new insights into the characteristics of pigments in the diversity of rock art in Indonesia.

Automated summary

This paper reports the unprecedented use of hematite as a black pigment material in rock art and analyzes the physicochemical characteristics of red, purple, and black pigment materials from Jufri Cave, East Kalimantan, using various techniques. The results reveal that hematite is the main component of all pigment materials, but the properties of hematite crystals differ among different pigments, possibly due to high-temperature heating or natural geological and anthropological factors. The variation in crystallinity and octahedral symmetry distortion of hematite structures may account for the color diversity observed in the pigment materials.