Iron-Rich Black Spots and Rust-Brown Glaze Transitions on a Yuan-Dynasty Qingbai Double-Gourd Ewer: Portable X-Ray Fluorescence Analysis and Optical Documentation

Nona Dronova Independent Researcher Research Article — Archaeometry and Chinese Ceramic Technology 2026 Abstract This study presents an archaeometric investigation of a Yuan-dynasty (1279–1368) Chinese Qingbai porcelain ewer in the form of a double gourd (hulu, 葫芦), decorated with distinctive iron-rich black spots and rust-brown glaze transitions. Three areas of the vessel were examined using portable X-ray fluorescence spectroscopy (pXRF): the pale Qingbai glaze, a black decorative spot, and a rust-brown region. The measurements revealed substantial differences in elemental iron concentrations: 0.913 wt% Fe in the pale glaze, 5.722 wt% Fe in the black spot, and 1.161 wt% Fe in the rust-brown area. The black decorative spot contains approximately 6.27 times more iron than the pale glaze. This pronounced enrichment establishes the iron-rich chemical character of the dark decoration. Four photographic images document the distribution of black spots, ochre-colored glaze regions, pale glazed surfaces, and the unglazed foot. Particular attention is given to the irregular boundaries between colored regions and to clusters of dark inclusions occurring near these transitions. The analytical results are discussed in relation to published archaeometric studies of iron-rich decoration on Chinese Qingbai porcelain, including research on the microstructure and coloration mechanisms of iron spots from Jingdezhen. The study contributes original elemental data and optical documentation to the investigation of iron-bearing glaze decoration on Yuan-dynasty Chinese porcelain. Keywords: Yuan dynasty; Qingbai porcelain; double-gourd ewer; hulu; iron-rich black spots; rust-brown glaze; Jingdezhen; portable X-ray fluorescence; pXRF; Chinese ceramic technology; archaeometry. 1. Introduction Qingbai porcelain represents one of the major achievements of Chinese high-fired ceramic technology during the Song and Yuan dynasties. Its characteristic pale, translucent glaze was developed through the interaction of silica-rich ceramic materials, feldspathic fluxes, and carefully controlled firing conditions. Although Qingbai porcelain is commonly associated with pale bluish-white or ivory-colored surfaces, certain historical vessels exhibit additional decorative effects, including dark iron-rich spots and brown or ochre-colored glaze regions. Such features are particularly relevant to the study of medieval Chinese kiln technologies because their appearance reflects variations in the distribution, concentration, and chemical behavior of iron-bearing materials during high-temperature firing. The present investigation concerns a Yuan-dynasty Qingbai porcelain ewer shaped as a double gourd, or hulu (葫芦). The vessel exhibits a pale glaze with scattered black decorative spots, broad rust-brown and ochre-colored regions, and local transitions between these contrasting surface effects. The purpose of this study is to characterize the elemental differences between three visually distinct areas of the vessel and to examine their relationship to published research on iron-rich coloration mechanisms in Chinese ceramics. 2. Historical and Art-Historical Context 2.1. The Double-Gourd Form The double-gourd form has a long history in Chinese material culture. Known as hulu, it is associated with longevity, abundance, protection, and auspicious symbolism. During the Yuan dynasty (1279–1368), double-gourd forms appeared in a variety of ceramic vessels, including ewers with curved spouts and handles. The investigated vessel combines two rounded sections in a vertically arranged double-gourd silhouette. Its form includes a curved spout, an arched handle, and a small attachment loop. The vessel therefore belongs to a recognizable historical category of Chinese pouring vessels. 2.2. Qingbai Glaze and Iron-Rich Decoration Qingbai porcelain is characterized by a high-fired silicate glaze whose appearance may vary from pale blue-white to ivory or slightly greenish-white. Iron, even when present in relatively small quantities, can influence the optical properties of these glazes. Local concentrations of iron-bearing material may produce brown, reddish-brown, or nearly black decorative effects. The technological study of such decoration is particularly important because the same ceramic surface can exhibit substantial variation in color and elemental composition. The investigated Yuan-dynasty ewer provides an example of this phenomenon. 3. Description of the Investigated Object The investigated object is a Yuan-dynasty Qingbai porcelain ewer in the form of a double gourd. Its principal characteristics are: Double-gourd body composed of two rounded sections. Curved pouring spout. Arched handle with a small attachment loop. Pale ivory-white Qingbai glaze. Irregular black and dark-brown decorative spots. Extensive ochre and rust-brown glaze transitions. Unglazed circular foot with visible firing-related surface characteristics. The dark decorative spots vary in size and distribution. Some occur as isolated, relatively large formations, while others appear as small scattered inclusions or clusters. The rust-brown coloration forms broader areas that merge irregularly with the pale glaze. The resulting surface demonstrates a complex relationship between the ceramic glaze and iron-bearing decorative material. 4. Materials and Analytical Method The elemental composition of the vessel was examined using portable X-ray fluorescence spectroscopy. The measurements were recorded on 28 October 2026 using instrument serial number 940487. Analytical conditions Parameter Description Analytical technique Portable X-ray fluorescence spectroscopy (pXRF) Instrument serial number 940487 Analytical mode Geochem (3-Beam) Extra Acquisition time 45 seconds per measurement Sample ID 10 Measurement No. 16 Pale Qingbai glaze Measurement No. 17 Black decorative spot Measurement No. 18 Rust-brown glaze region Procedure Non-destructive, in situ analysis The three measurements were performed on visually distinct regions of the same ceramic object. The resulting elemental concentrations were compared to characterize the chemical heterogeneity of the glaze and decorative areas. 5. Experimental Results 5.1. Comparative Elemental Composition Table 1. Portable X-ray fluorescence measurements of the Yuan-dynasty Qingbai double-gourd ewer (Sample ID 10) Element Pale glaze, No. 16 Black spot, No. 17 Rust-brown region, No. 18 Mg (ppm) ND, <4200 4000 ND, <4500 Al (%) 6.96 5.57 5.89 Si (%) 39.00 28.45 33.52 P (ppm) 198 291 342 S (ppm) 3400 2840 4170 K (%) 3.769 2.435 3.322 Ca (%) 2.864 2.442 2.926 Mn (ppm) 997 1208 799 Fe (%) 0.913 5.722 1.161 Ni (ppm) 36 44 30 Cu (ppm) 26 35 28 Zn (ppm) 36 16 29 Rb (ppm) 664 561 621 Sr (ppm) 78 85 87 Y (ppm) 26 28 27 Zr (ppm) 52 46 50 Nb (ppm) 27 22 24 Ba (ppm) 133 136 147 Hf (ppm) 14 8 13 W (ppm) 16 15 29 Pb (ppm) 8 9 9 U (ppm) 9 7 8 ND indicates that the element was not detected above the instrument's reported detection threshold. Values reproduce the elemental concentrations reported by the pXRF instrument. The measurements were acquired from different surface regions and represent their respective X-ray fluorescence responses. 5.2. Iron Enrichment in the Black Decorative Spot The principal analytical result is the pronounced enrichment of iron in the black decorative region. The measured iron concentrations are: Pale Qingbai glaze: 0.913 wt% Fe. Black decorative spot: 5.722 wt% Fe. Rust-brown region: 1.161 wt% Fe. The iron enrichment factor between the black spot and the pale glaze is: 5.722 / 0.913 = 6.27. The black decorative spot therefore contains approximately 6.27 times more elemental iron than the pale glaze. The iron enrichment factor between the black spot and the rust-brown region is: 5.722 / 1.161 = 4.93. These results demonstrate that the black decorative spot represents a highly iron-enriched region of the ceramic surface. 5.3. Iron-Oxide Equivalent For comparison with ceramic literature reporting iron as Fe₂O₃, the measured elemental iron values can be expressed as theoretical Fe₂O₃ equivalents using the conversion factor 1.4297. Table 2. Elemental iron and theoretical Fe₂O₃ equivalents Surface region Fe (wt%) Fe₂O₃ equivalent (wt%) Pale Qingbai glaze 0.913 1.305 Black decorative spot 5.722 8.181 Rust-brown region 1.161 1.660 Fe₂O₃ equivalent is a conventional calculation based on elemental iron content. It does not constitute mineralogical identification of hematite or any other specific iron-bearing phase. The results provide a useful quantitative basis for comparison with published studies of iron-bearing Chinese ceramic glazes. 5.4. Manganese and Other Elements Manganese is present in all three investigated regions. Its concentration reaches 1208 ppm in the black spot, compared with 997 ppm in the pale glaze and 799 ppm in the rust-brown region. The increase in manganese accompanies the pronounced enrichment in iron. However, the magnitude of the iron difference is substantially greater, identifying iron as the principal measured element associated with the black coloration. The black region also exhibits lower measured silicon and potassium concentrations than the pale glaze. These differences indicate compositional variation between the analyzed surface regions. 6. Optical and Photographic Observations Four photographic images were examined to characterize the surface morphology and distribution of decorative colors. 6.1. Photograph 1: Lower Glazed Surface The first image documents an irregular combination of pale glaze, ochre-colored areas, and numerous small dark inclusions. The dark particles occur individually and in localized clusters. Their distribution is particularly noticeable near the transitions between pale and rust-brown regions. The surf