The characterisation of pigments used in X-ray rock art at Dalakngalarr 1, central-western Arnhem Land

Thermally-induced color transformation of hematite: insight into the prehistoric natural pigment preparation

Since the prehistoric era, hematite has been known as a reddish color pigment on rock art, body paint, and decorating substances for objects discovered almost worldwide. Recently, studies about purple hematite used in prehistoric pigment have been done vigorously to investigate the origin of the purple pigment itself. These previous studies indicate that the differentiation of crystallinity, crystal size, morphology, and electronic structure can cause the color shift, resulting in purple hematite. In this study, we conducted a detailed study of the sintering temperature effects on the formation of hematite minerals. This study aims to reveal the structural, crystallography, and electronic transformation in hematite due to heating treatment at various temperatures. The hematite was synthesized using precipitation to imitate the primary method of hematite formation in nature. The sintering process was carried out with temperature variations from 600 °C to 1100 °C and then characterized by crystallographic and structural properties (XRD, Raman Spectroscopy, FTIR), particle size (TEM), as well as electronic properties (DRS, XANES). The crystallinity and particle size of hematite tend to increase along with higher sintering temperatures. Moreover, we noted that the octahedral distortion underwent an intensification with the increase in sintering temperature, which affected the electronic structure of hematite. Specifically, the 1s → 3d transition exhibited lower energy for hematite produced at a higher temperature. This induced a shift in the absorbed energy of the polychromatic light that led to a color shift within hematite, from red to purple. Our finding emphasizes the importance of electronic structure in explaining hematite pigment’s color change rather than relying on simple reasons, such as particle size and crystallinity. In addition, this might strengthen the hypothesis that the prehistoric human created a purple hematite pigment through heating.

Analysis of Iron Anchor Diseases Unearthed from Gudu Ruins in Xianyang City, Shaanxi Province, China

Iron cultural relics are easily affected by environmental factors and can completely rust away. As early as the Qin Dynasty in ancient China, Xianyang Gudu was part of the most important transportation route to the West from ancient Chang’an; research into Xianyang Gudu has provided important information for understanding the historical changes in ancient China, East–West trade, and ancient boating technology. In this research, we use the iron anchors unearthed from the Gudu ruins in Xianyang City, Shaanxi Province, China as the research object; then, we used a scanning electron microscope–energy dispersive spectrometer (SEM-EDS), a high-resolution X-ray diffractometer (XRD), ion chromatography, and other methods to detect the corroded products of the iron anchors, and analyzed the iron anchor diseases in different preservation environments to explore the relationship between iron anchor disease and the preservation environment. This research found that the corroded products of the iron anchors contained the harmful tetragonal lepidocrocite (β-FeOOH) and that a high concentration of salt ions in the river channel accelerated the corrosion of the anchors; this analysis, based on the disease results, can provide a basis for the subsequent scientific restoration of iron anchors.

Investigation of Deterioration for Large Outdoor Iron Statues Relics: A Case Research of Chairman MAO Iron Statue in Qinghai, China

In this study, the Chairman MAO iron statue in Qinghai, China was analyzed via multi-analytical methodology, including polarizing microscope, SEM-EDS, metallographic analysis, high-resolution X-ray diffractometer, ion chromatographic analysis, silver nitrate titration detection in rust samples. The iron cultural relics are affected by factors such as their chemical composition, metallographic structure and surface characteristics, casting process, and natural storage environment. Although the corrosion rate is different, the corrosion is inevitable. With the corroborative evidence derived from the above analyses, it could be determined that the long-term preservation of cast iron statues in outdoor natural environments is prone to oxidation and corrosion, which due to the poor air circulation inside the statue of Chairman Mao Zedong, the humidity is significantly higher than that of the outside. In addition, due to the large temperature difference between day and night in this area, the condensed water is easy to form, causing the inner cavity of the statue and the welding parts to be rusted particularly seriously by the electrochemical corrosion and chemical corrosion. Compared with single chemical corrosion, electrochemical reaction can greatly accelerate the corrosion of iron. This result provided important scientific basis onto the production crafts of the precious casting, and the correlation between environment and deterioration for large outdoor iron relics, contributing to the conservators to make informed decisions on restoration.

The colours of Rome in the walls of Cástulo (Linares, Spain)

Wall paintings have become one of the most relevant, complex and challenging research subjects in Archaeometry. Minimally- or non-invasive, accurate and multidisciplinary methods are needed to successfully address the problems posed by their physical and chemical properties and by their analysis techniques. Specifically, the analytical method implemented for the study of this type of samples must enable a precise separation of the chemical information from backgrounds and scenes, allowing the identification of pigment’s components in overlapping layers, the detection of minority components and the elucidation of pigment mixtures. Thus, this paper puts forward a multidisciplinary approach towards these goals by means of the combined use of micro Energy Dispersive X-ray Fluorescence (µEDXRF) surface mapping and single-spot micro-Raman spectroscopy and µEDXRF analysis. The samples under research come from the site of Cástulo (Linares, Spain), one of the most important Roman cities in the Iberian Peninsula. It must be emphasized the uniqueness of the walls of Cástulo, their optimal conservation state and the richness and variety of the colour’s palette used in their decoration, which make them an excellent and representative example of Roman wall paintings.

Hunter-Gatherers Harvested and Heated Microbial Biogenic Iron Oxides to Produce Rock Art Pigment

Red mineral pigment use is recognized as a fundamental component of a series of traits associated with human evolutionary development, social interaction, and behavioral complexity. Iron-enriched mineral deposits have been collected and prepared as pigment for use in rock art, personal adornment, and mortuary practices for millennia, yet little is known about early developments in mineral processing techniques in North America. Microanalysis of rock art pigments from the North American Pacific Northwest reveals a sophisticated use of iron oxide produced by the biomineralizing bacterium Leptothrix ochracea; a keystone species of chemolithotroph recognized in recent advances in the development of thermostable, colorfast biomaterial pigments. Here we show evidence for human engagement with this bacterium, including nanostructural and magnetic properties evident of thermal enhancement, indicating that controlled use of pyrotechnology was a key feature of how biogenic iron oxides were prepared into paint. Our results demonstrate that hunter-gatherers in this area of study prepared pigments by harvesting aquatic microbial iron mats dominated by iron-oxidizing bacteria, which were subsequently heated in large open hearths at a controlled range of 750 °C to 850 °C. This technical gesture was performed to enhance color properties, and increase colorfastness and resistance to degradation. This skilled production of highly thermostable and long-lasting rock art paint represents a specialized technological innovation. Our results contribute to a growing body of knowledge on historical-ecological resource use practices in the Pacific Northwest during the Late Holocene.

Figshare link to figures: https://figshare.com/s/9392a0081632c20e9484.