Scientific Analysis Reveals the Archaeological Connotation of the Earliest Bronze Round-sculpted Horses in China: a Comprehensive Technical Investigation

doi:10.21203/rs.3.rs-4648742/v1

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Scientific Analysis Reveals the Archaeological Connotation of the Earliest Bronze Round-sculpted Horses in China: a Comprehensive Technical Investigation

https://doi.org/10.21203/rs.3.rs-4648742/v1

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The two bronze horses unearthed from the tomb during the late Shang period on the Loess Plateau represent the earliest known examples of independent round-sculpted horse artworks in China, but their archaeological connotation has not yet been effectively recognized. Based on the preliminary observation of surface craft traces, we conducted a comprehensive technical investigation on the bronze horses, including the technology of casting, alloying, production of clay casting cores as well as the potential casting place, utilizing optical microscopes (OM), a scanning electron microscope-energy dispersive spectrometer (SEM-EDS), an X-ray diffractometer (XRD) and a synchronous thermal analyzer. The comparative study with related materials indicates that the two bronze horses are a pair of drinking vessels imitating the animal shape, with a style that combines local and Central Plains elements. They were cast in the Loess Plateau, while it is likely that the metal raw materials and production technology were either directly or indirectly imported from the Central Plains. In this study, we conducted technical characterization of the clay casting materials utilized in Lijiaya culture’s bronze artifacts for the first time, which is of enlightening significance for comprehensively revealing the picture of the bronze handicraft industry in the Loess Plateau during the late Shang period.

Yanjiagou tomb

Lijiaya culture

The late Shang bronze

Bronze horse

Casting clay core

Casting technology

Origin of domesticated horse

Horses played an important role in the early stages of human civilization. Serving as representatives of strategic resources and productivity, they have significantly altered the patterns and pace of human interactions across the pan-Eurasian continent. In 2005, two realistically shaped bronze horses were unearthed from a destroyed Late Shang tomb on the Loess Plateau in China (Yanjiagou Village, Ganquan County, Shaanxi Province), representing the earliest independent round-sculpted horse artworks found in the country and the sole example of its kind in Shang Dynasty bronze artifacts (Fig.1, 2a, 2b). Hiroki Kikuchi, from the perspective of zooarchaeology, highlighted that these two bronze horses are among the rare archaeological findings discovered outside Anyang, the capital of the late Shang Dynasty, which depict human-horse relationships beyond mere horse bones[^1].

Nearly 70 artifacts, predominantly bronze items such as food containers, drinking vessels, and weapons (Fig. 2c), were excavated from the tomb, along with a small assortment of gold foil decorations, turquoise ornaments, bone arrowheads, and animal teeth. The tomb was initially dated to the late stage of the second phase through the fourth phase of the Yin Ruins period (approximately 1191-1046 BCE) and is associated with the Lijiaya culture[^2], which is commonly linked to the powerful state "Guifang" on the northwest of the Shang Dynasty[^3]. The Yanjiagou tomb, with the highest number of unearthed artifacts and the most diverse contents among similar tombs in the Loess Plateau during the Late Shang period, has garnered widespread interest from scholars since its discovery. Archaeologists have engaged in thorough discussions regarding significant issues such as the dating of the tomb, typological features of artifacts, the assemblage of vessels, as well as the cultural elements and regional interactions they reflect, ultimately reaching a certain consensus[^4,5,6,7,8].

At the close of 2020, Northwest University commenced the scientific conservation and restoration of the bronze artifacts unearthed from Yanjiagou tomb, presenting an opportunity to conduct a preliminary analysis of the alloy composition and metallographic structure of various vessels, including Ding, Gui, Yan, You, and Gu[^9]. Subsequently, Yangxinghe Liu systematically analyzed the alloy composition, metallographic structure, trace elements, and lead isotope ratios of these bronzes. Within the framework of the "Oxford Research System"[^10], she conducted multidimensional comparisons with materials from surrounding areas to explore issues such as metal flowing patterns and raw material sources[^11].

However, there is currently insufficient research on the two bronze horses, and our understanding of them remains limited. The existing knowledge primarily includes: (1) The objects are solid in form with no internal cavities;^[²^,12] (2)They depict the appearance of Equus przewalskii;^[²^,¹²^,13] (3)They depict the appearance of saddles^[²^,¹²^]; (4)It is possible that they were used for burial or sacrificial purposes^[²^,⁴^,¹²^]. In Yangxinghe Liu’s work mentioned above, only in-situ elemental composition data and rust samples’ lead isotope data were acquired^[¹¹^]. The former exhibits exceptionally high lead content due to the influence of surface corrosion products, posing challenges in accurately reflecting the material information of the objects. Its reference value is limited, making it difficult to delve into a comprehensive discussion.

In the field of metallurgical archaeology, clay molds and cores have always been highly valued. Scholars typically conduct analyses from the perspectives of major and trace element composition, mineral composition, and petrographic structure of the samples, comparing them with other molds, cores, pottery sherds, soil, etc. to explore the production technology and potential casting places of bronze artifacts[^{14,15,16,17,18,19,20,21,22,23,24,25,26]}. In this study, a comprehensive sampling and analysis were conducted on the residual casting cores of the bronze horses, alongside an examination of the alloying and casting technologies employed in creating the artifacts. Based on this, we combine existing archaeological materials to conduct a discussion on the technical features, functions, and sources of the bronze horses, to reveal more valuable information about these precious artifacts.

Overview of objects and samples

The two bronze horses share a similar form and are depicted in a standing posture. Apart from the parallel lines that represent the mane and tail hair, there are no other additional decorative elements. The surface is completely covered with red and green corrosion products and hardened soil. The bronze horse GQ744-1 exhibits a slightly greater height and is characterized by the absence of its left ear and front legs. similarly, the GQ744-2 bronze horse displays missing front legs and the right hind leg. Residual casting clay cores were found inside all of their extant hooves. Except for the right front leg of GQ744-2, which is suspected to be connected to the horse's abdomen, all other broken legs are in a closed state (Fig. 2a, 2b).

In this study, we mainly sampled and analyzed the residual casting clay cores, the filling soil, as well as the inner surface hardened substance of the bronze horses. Additionally, we also sampled the bronze horses’ metal body, buried soil, and other bronze artifacts’ casting clay cores for comparative analysis. The sampling procedure adheres to the principle of minimal intervention, aiming to minimize any impact on the value of the artifacts and their subsequent restoration work. Detailed sampling information is presented in Table 1.

Table 1. Sampling information
Sample number	The objects sampled	Sampling position	Sample type	Sample features
C001	The GQ744-1 horse	Bottom of the right hind hoof	Casting clay core	Grayish-yellow powder and pieces
C002	The GQ744-1 horse	Bottom of the left hind hoof	Casting clay core	Dark-brown powder and pieces
C003	The GQ744-2 horse	The root of the tail	Casting clay core	Red-brown powder
C004	The GQ744-2 horse	Bottom of the left hind hoof	Casting clay core	Grayish-yellow powder and pieces
C005	The GQ743-1 Ding	Inside a broken leg	Casting clay core	Hard black pieces
C006	The GQ743-3 Ding	Inside the leg 3-1	Casting clay core	Hard grayish-black pieces
C007	The GQ743-3 Ding	Inside the leg 3-2	Casting clay core	Hard grayish-black pieces
C008	The GQ743-3 Ding	Inside the leg 3-3	Casting clay core	Hard grayish-black pieces
C009	The GQ743-4 Ding	Inside the leg 4-1	Casting clay core	Yellowish-brown powder and pieces
C010	The GQ743-4 Ding	Inside the leg 4-2	Casting clay core	Yellowish-brown powder and pieces
C011	The GQ743-4 Ding	Inside the leg 4-3	Casting clay core	Red-brown powder and pieces
S001	The GQ744-1 horse	Inside the mouth	Filling soil	Yellowish-brown powder and pieces
S002	The GQ744-1 horse	The outer surface of the neck	Buried soil	Yellowish-brown powder
S003	The GQ744-2 horse	Inside the mouth	Filling soil	Yellowish-brown powder and pieces
S004	The GQ744-2 horse	Inside the abdomen	Filling soil	Yellowish-brown powder and pieces
S005	The GQ743-1 Ding	Inner surface	Buried soil	Yellowish-brown powder
I001	The GQ744-2 horse	Inner bottom surface of the mouth	Hardened substance	Red-brown pieces
I002	The GQ744-2 horse	The broken right hind leg	Hardened substance	Dark-red and yellowish-brown layered pieces
B001	The GQ744-2 horse	The broken right hind leg	Metal body	A dark-yellow piece

Analysis methods

Investigation of the casting technology

Ancient bronzes were predominantly produced through casting, and whether employing the piece-molds or lost-wax method, a well-designed gating system is essential for achieving the intended shapes, patterns, and hollow structures[^27]. By examining the morphological characteristics and surface technical traces (such as parting lines, core supports, and repair blocks) of the object, it is possible to partially reconstruct its technical details[^28,29,30]. According to this, we conducted an initial investigation into the casting technology employed in creating the bronze horses during the process of sampling and restoration.

Analysis with optical microscopes (OM)

We conducted petrographic observations on the casting clay cores, filling soil, and hardened substance to visually compare the grain’s size, psephicity, color, and distribution among these different samples. Due to the samples’ diminutive size, rendering them unsuitable for petrographic thin section preparation, the block-shaped ones were embedded in epoxy resin, followed by meticulous polishing, and subsequently subjected to direct observation under an optical microscope. The utilized equipment is the HIROX RX-100 three-dimensional digital video microscope system, equipped with the AC-2016VD lens capable of magnification ranging from 20 to 160 times.

Furthermore, the metal body was also embedded in epoxy resin, meticulously polished, and subsequently subjected to etching using an ethanol solution containing ferric chloride and hydrochloric acid[^31]. The resulting microstructure was then examined under a metallographic microscope. The utilized equipment is the XD30M series metallographic microscope manufactured by Sunny Optical Technology Co., Ltd., equipped with a 10x eyepiece lens as well as 5x, 10x, 20x, and 50x objective lenses.

Analysis with a scanning electron microscope-energy dispersive spectrometer (SEM-EDS)

Embedded and polished samples were subjected to carbon coating, followed by observation of their microstructure utilizing a scanning electron microscope (TESCAN VEGA-3XMU). The elemental composition of different regions was measured utilizing an energy dispersive spectrometer (OXFORD INCA x-act). The scanning electron microscope is equipped with a tungsten lamp electron gun, operating at an excitation voltage of 20 kV. Elemental analysis was conducted in backscattering mode, with a working distance of 15 mm. Each region was measured five times in parallel, and the mean value was calculated. Subsequently, the results were normalized and presented in terms of weight percentage (wt%).

Furthermore, the relative volume of grains(≥10μm), clay matrix(<10μm), and pores in the casting cores, filling soil, and hardened substances were quantified using a point counting method[^32] applied to SEM images. The equivalent projected area diameter of each counted grain was calculated, with the formula as follows:

In this formula, “d” represents the equivalent projected area diameter, “S” represents the projected area of the grain, and the proportionality constant “” is taken as 3.14. The counting interval is set to be greater than or equal to the maximum grain diameter in the sample to avoid duplicate counts, and it is ensured that each sample has a minimum of 200 counting points. Point counting and grain size calculation were conducted separately using Fiji and Auto CAD software.

Analysis with an X-ray diffractometer (XRD)

We conducted X-ray diffraction analysis on the casting clay cores, hardened substances, and various soil samples to qualitatively determine their mineral composition. The samples were finely ground in an agate mortar and then sieved through a 200-mesh sieve before undergoing instrumental testing. The X-ray diffractometer used is the Rigaku Ultima IV from Japan, operating with CuKα radiation at 40kV and 40mA. The scanning parameters included a speed of 1°/min and a range of 5-70°. The obtained XRD patterns were cross-referenced with the ICDD (International Centre for Diffraction Data) database using Jade 6 software to ascertain the mineral composition.

Analysis of thermogravimetry-differential scanning calorimetry (TG-DSC)

Thermal analysis can partially reveal the heating history of clay materials, and provide qualitative and semi-quantitative assessments of specific inclusions, such as calcite[^33], thereby aiding in the determination of the cores’ firing temperature and the attribution of the filling soil and hardened substance. The analytical instrument utilized is a NETZSCH STA 449 F5 synchronous thermal analyzer manufactured in Germany. For each test, the sample was weighed to 8±0.05 mg and then placed in an aluminum crucible. Subsequently, it was heated from 30℃ to 1000℃ at a rate of 10K/min in a nitrogen atmosphere, resulting in the acquisition of the thermal flow/mass loss synchronous curve.

Analysis results

Casting technology of the bronze horses

The two bronze horses underwent post-production surface refinement, during which some technical traces were removed. Nevertheless, the remaining parting lines still indicate that they were cast with the piece-molds method. Taking one of them as an example, the body mold was divided into two halves along the mane and backbone, with each half featuring an ear-shaped cavity on its inner surface. The rear section of the mane is integrated with the “saddle” structure, while being separated from the front section, resulting in discontinuous hair patterns between the two sections (Fig. 3a). Additionally, there is a gap between the “saddle” and the horse’s body, suggesting that the “saddle” was individually cast before being combined with the body. A bottom mold should be positioned between the four legs, and between this mold and body molds, parting lines were formed at the junction of the body side and belly, chest and belly, as well as each leg’s front and rear edges (Fig. 3b). The continuous parting lines extending from the hind legs’ rear edge to the tail’s sides (Fig. 3c) suggests the presence of a tail mold, which was linked to both the body and bottom molds. The tail root is thickened on the inner side into a wedge shape, which is believed to have been intentionally created by carving a groove into the bottom mold surface to reinforce the strength of the tail root and prevent breakage or detachment (Fig. 3d). The facial features were also captured in a mold, including the eyes (Fig. 3f, 3g). The section from the chest to the lower jaw may be molded separately or as part of the body molding, and the residual parting line at the lower jaw suggests that the mold of this section was also divided into two halves (Fig. 3h). The hollow nature of the body and hooves suggests the presence of clay cores during the manufacturing process, while the solid metal legs indicate that the two kinds of clay cores were not interconnected. There should be clay supports attached to the body core, which are the same thickness as the casting cavity, thus forming a hollowed-out structure for the nose and mouth (Fig. 3i).

Furthermore, it is essential to incorporate metal spacers between the body core and the body mold as well as the bottom mold to ensure the stability of the casting cavity structure. Nevertheless, due to extensive corrosion, no discernible traces of these spacers were evident on the horses’ surface. We have also captured X-ray images of the horses, but no significant findings were obtained. Further examination and confirmation are required using more advanced CT (computed tomography) technology.

Analysis results of the metal body sample

The metallographic structure

The metallographic analysis results of the bronze horse exhibit characteristic casting microstructure. In Figure 4, the dendritic segregation of α solid solution is evident, with refined dendrite growth indicating favorable cooling conditions during casting^[³¹^]. A substantial quantity of (α+δ) eutectoid structure occupies the interdendritic spaces, while lead is distributed within the matrix in the form of spherical shapes and fine particles. In addition, the microstructure contains numerous spherical free copper phases and cuprous oxide phases, which result from the deposition of copper in various valence states during the corrosion and mineralization processes within the pores formed by casting shrinkage or lead loss[^34,35].

The alloy composition and micro-region composition

Figure 5 depicts the backscattered electron images at different magnifications and the EDS test regions of the sample, with corresponding test data presented in Table 2. The analysis indicates that the bronze horse is a Cu-Sn-Pb ternary alloy, with an average tin content of 15.10% and lead content of 6.41%, respectively. It can be classified as a lead-tin bronze with a relatively high tin content[^36]. Considering the minor mineralization present in the sample, which has resulted in the partial loss of copper and lead elements, the actual alloy composition may exhibit higher levels of copper and lead. The presence of this kind of composition is more prevalent in the bronze ritual vessels excavated from the tombs of high-ranking nobles at Anyang during the Yin Ruins Phase Ⅱ^[³⁶^,37,38], and can also be identified in certain Yin Ruins style bronze vessels unearthed in the Loess Plateau area[^39]. However, it exhibits distinct differences from the overall characteristics of lead-rich, arsenic-rich, and low-tin content found in other Yanjiagou tomb’s bronze vessels^[⁹^,¹¹^]. Moreover, the presence of impurity elements such as As, Fe, Sb, Ni, Ag, and Zn in the sample is at remarkably low levels, suggesting a high degree of refinement in the raw materials utilized for casting the bronze horse.

The micro-region composition analysis further confirms the results of the metallographic observation. In Figure 5b, SEM images revealed grayscale disparities between the inner and outer regions of the α solid solution dendrites, indicating the presence of intragranular segregation, with the dendrite centers predominantly composed of pure Cu and Sn elements and minimal mineralization. Conversely, the (α+δ) eutectoid structure exhibits higher oxygen content, suggesting preferential corrosion of the tin-rich phase. Additionally, lead particles contain a certain amount of sulfur, potentially originating from ore or sulfur-containing substances in the burial environment.

Table 2. EDS analysis data of the metal body sample
Test region	Elemental composition（wt%）
Test region	Cu	Sn	Pb	As	Fe	Sb	Ni	Ag	Zn	Co	Bi	Au	O	S	Cl
Fig. 5a-region 1	66.67	15.65	7.16	0.07	0.07	0.41	—	—	—	—	0.18	0.23	7.31	0.21	2.04
Fig. 5a-region 2	62.81	15.17	9.50	0.15	0.03	0.40	—	0.19	—	—	—	0.13	8.70	0.46	2.47
Fig. 5a-region 3	69.78	13.08	4.57	0.17	0.13	0.16	0.08	0.16	—	—	0.04	0.14	7.04	0.17	4.49
Fig. 5a-region 4	67.27	16.05	5.73	0.12	0.24	0.42	0.06	0.08	—	0.05	0.18	0.47	6.39	0.17	2.77
Fig. 5a-region 5	69.84	15.55	5.08	0.09	0.24	0.16	0.03	0.22	—	—	—	—	6.54	0.02	2.24
Fig. 5a-mean	67.27	15.10	6.41	0.12	0.14	0.31	0.03	0.13	—	0.01	0.08	0.19	7.20	0.21	2.80
Fig. 5b-point A	2.43	0.15	66.56	0.05	—	—	—	0.04	0.05	—	—	0.96	20.89	8.87	—
Fig.5b-point B	90.30	7.34	0.08	0.20	0.24	0.08	0.04	0.02	—	—	0.45	0.29	0.74	0.15	0.05
Fig. 5b-point C	21.92	38.80	14.45	0.10	—	0.96	0.03	—	—	—	0.19	0.33	21.43	—	1.78
Fig. 5b-point D	85.21	0.06	0.26	0.08	—	—	0.12	0.04	—	—	—	—	14.11	—	0.12

Analysis results of the casting clay core and soil samples

The petrographic characteristics

The optical micrographs in Figure 6 depict the casting core, filling soil, and hardened substance. Among these, the morphological characteristics of the three filling soil samples from the bronze horses exhibit a fundamental similarity, with a matrix displaying a grayish-brown hue and only sporadic well-sorted fine grains (Fig. 6g-6i). The morphological characteristics of the hardened substance from the GQ744-2 bronze horse’s mouth (Fig. 6j, 6k) are similar to those observed in the casting core from its left hind hoof (Fig. 6c). Their matrix exhibits a reddish-brown hue, interspersed with a sparse distribution of large grains, predominantly manifesting angular and subangular shapes. The casting core matrix of the GQ744-1 bronze horse exhibits a slightly darker hue compared to that of the GQ744-2 bronze horse. While the size of the large grains is similar in both, there is a slightly higher roundness observed in the grains of the GQ744-1 bronze horse. Additionally, differences are also noted in the C001 and C002 clay cores for the GQ744-1 bronze horse’s hooves, with the latter being looser in texture and containing fewer large grains. The casting cores of the three Ding exhibit distinct variations in matrix hue, grain size, and grain morphology (Fig. 6d-6f), all differing from those of the bronze horses’ casting core. Notably, the GQ743-3 Ding’s casting core is predominantly composed of clay matrix, with few large grains. Some pores display a banded structure, likely resulting from the combustion of plant fibers.

Additionally, a particularly unique sample is the hardened substance on the inner surface of the GQ744-2 bronze horse’s broken right front leg (Fig. 6l). Its matrix consists of red and yellowish-brown layers, and the grain size is slightly larger than that of the filling soils. The red layer exhibits greater density, while the yellowish-brown layer contains more pores, with local inclusions of green corrosion products. An orange band is present at the interface between the two kinds of matrix, potentially indicating differing chemical compositions.

We then conducted point counting analysis (see Sup. 1) on the high-quality SEM images of the casting cores and soil samples and subsequently generated a stacked bar chart illustrating the percentage bulk composition (Fig. 7) as well as a boxplot depicting grain size distribution (Fig. 8) for each sample. Upon examination of the figures, it is evident that the bulk composition of the casting cores C002, C004, and the hardened substance I001 exhibit similarities, with a higher content of grain observed in the casting core C001. Furthermore, there is a relatively lower presence of large grains in the casting core C002 compared to consistent distribution patterns of grain sizes among C001, C004, and I001, aligning with microscopic observations.

About the bronze Ding, the bulk composition of the casting cores for the same vessel exhibits a high degree of similarity, while there are significant distinctions between different vessels. However, in terms of grain size, there is a substantial disparity between C007 and C008, which are from the same vessel, potentially attributable to the stochastic errors arising from the limited number of grain statistics.

The filling soil S003 exhibits higher porosity and lower grain content compared to the aforementioned samples and is devoid of grains larger than 50μm in diameter. Considering its loose and fragile characteristics, it is likely that the sample represents soil that infiltrated the bronze horse during burial, rather than being part of the casting core.

According to relevant studies, the casting molds, cores, and models unearthed at Yin Ruins have a low content of clay matrix and a high content of silt, achieved through the process of washing loess raw materials^[¹⁶^,40,41,42]. In comparison, the proportion of clay matrix of Ganquan bronze artifacts appears to be marginally higher. Additionally, the coarse sand content in the outer layer of the double-layer mold and the core of the Yin Ruins is significantly higher compared to that in the inner layer and the model, suggesting a potential sand-adding process during the former’s production^[¹⁶^,⁴⁰^,⁴¹^]. The situation with the Ganquan bronze artifacts’ casting cores is analogous. Apart from C005, the grain sizes of the other clay cores do not conform to a normal distribution. The primary grain size is concentrated within the silt range (10-60μm). There is a lower content of coarse sand (>60μm), yet it exhibits good sorting, suggesting a potential anthropogenic addition of the coarse sand.

The major elements

We have chosen ten soil elements (Na, Mg, Al, Si, P, K, Ca, Ti, Mn, Fe) and three alloy elements (Cu, Sn, Pb) as the subjects of analysis. The results show that the red matrix of hardened substance I002 is predominantly composed of Cu element, with a minor presence of Pb, accounting for mass ratios of 84.27% and 11.95%, respectively; while the yellowish-brown matrix is primarily constituted by Pb, representing a mass ratio as high as 98.70%. Consequently, it can be preliminarily inferred that I002 does not originate from the residual casting core but rather represents a corrosion layer formed through outward migration and deposition of alloy elements during burial. As for other samples, since the elements of Cu, Sn, and Pb are very minimal, we only present the soil elements in the form of oxides and list the normalized average element content in Table 3.

Table 3. Chemical composition of the casting core, filling soil, and hardened substance
Sample number	The objects sampled	Average content of major elements（wt%）
Sample number	The objects sampled	Na₂O	MgO	Al₂O₃	SiO₂	P₂O₅	K₂O	CaO	TiO₂	MnO	Fe₂O₃
C001	The GQ744-1 horse	2.75	2.37	12.26	55.70	1.58	4.24	15.23	0.69	0.08	5.11
C002	The GQ744-1 horse	2.19	1.81	11.39	69.50	2.26	3.58	3.79	0.54	0.05	4.89
C004	The GQ744-2 horse	2.51	2.74	12.02	56.95	2.03	4.01	13.96	0.60	0.05	5.13
C005	The GQ743-1 Ding	4.92	1.35	16.41	67.48	0.13	4.25	1.69	0.51	0.06	3.21
C006	The GQ743-3 Ding	1.62	1.86	22.54	59.64	0.41	4.69	1.91	1.32	0.01	6.00
C007	The GQ743-3 Ding	1.90	1.88	22.74	58.91	0.26	5.14	1.51	1.31	0.03	6.32
C008	The GQ743-3 Ding	1.59	1.64	21.88	60.29	0.60	4.77	1.64	1.45	0.06	6.09
C009	The GQ743-4 Ding	1.23	1.66	11.09	69.57	0.40	4.15	6.42	0.53	0.05	4.91
C010	The GQ743-4 Ding	2.61	1.41	10.48	71.63	0.27	3.64	5.43	0.54	0.04	3.95
C011	The GQ743-4 Ding	2.61	1.53	10.21	71.98	0.21	3.23	5.71	0.61	0.08	3.84
S001	The GQ744-1 horse	2.84	2.63	13.51	59.27	0.26	4.31	10.44	0.72	0.10	5.93
S004	The GQ744-2 horse	2.79	2.57	12.74	59.66	0.25	4.13	11.74	0.61	0.10	5.41
I001	The GQ744-2 horse	3.19	2.12	12.37	63.92	0.34	3.54	8.73	0.65	0.06	5.07

The data in the table reveals that SiO₂ and Al₂O₃ are the predominant constituents of the samples, with content ranges spanning from 55.70% to 71.98% and 10.21% to 22.74%, respectively, indicating substantial variability among the samples. The primary source of SiO₂ is attributed to quartz grains within the samples, and its relative content may be influenced by processes such as the washing of loess and the addition of sand during manufacturing. The presence of higher levels of P₂O₅ in samples C001, C002, and C004 suggests the possible addition of plant ash to the raw materials. Additionally, it is important to note the presence of CaO, which has been identified as a significant tracer element in previous research on the potential places of bronze casting. Owing to variations in leaching processes, the concentrations of Ca and Mg in the loess deposits within the Yellow River basin are notably higher than those found in the red soil deposits of the southern region, with a 4% disparity serving as an approximate threshold for differentiation^[¹⁴^,¹⁵^,²⁰^]. The molds, cores, and models utilized in the casting of ancient bronzes were often crafted from locally sourced materials, and the firing process has minimal impact on the concentrations of most major and trace elements, thereby allowing for the preservation of the compositional characteristics of the indigenous raw materials to a significant extent within the finished product. In comparison, the Ca content in the clay cores and filling soil of the two bronze horses is notably higher, with some reaching levels exceeding 15%; the Ca content in the clay cores of GQ743-4 Ding is marginally lower, approximately 6%; while the Ca content in the clay cores of GQ743-1 and GQ743-3 Ding is exceedingly low, not surpassing 2%. These findings suggest that the unearthed bronze artifacts from the Yanjiagou tomb may have been sourced from diverse casting workplaces, with indications that the bronze horses were likely cast in a northern region and certain bronze Ding could potentially be linked to casting workshops in southern regions. Particularly noteworthy is GQ743-3 Ding, as its clay cores also exhibit lower levels of Na and Mg but higher levels of Al, Ti, and Fe—indicative of soil composition characteristics specific to southern regions.

To further trace the potential source of the clay cores used in casting the bronze horses, we gathered chemical composition data for some soils, molds, and cores from the Loess Plateau and its surrounding areas for comparative analysis (see Sup. 2) ^[¹⁹^,²¹^,²²^,²³^,²⁴^,²⁵^,²⁶^{,43,44,45,46,47,48,49,50]}. It should be noted that some of the bronze casting sites where molds and cores have been unearthed have a different dating from the Yanjiagou tomb, but the geochemical changes over such a time scale can be virtually ignored. The considerations of local craftsmen in selecting raw materials should be similar at different periods, thus these samples retain their comparative significance. We utilized the SPSS software to perform factor analysis on the seven elements Na, Mg, Al, K, Ca, Ti, and Fe. The Kaiser-Meyer-Olkin (KMO) test statistic yielded a value of 0.623 (>0.5). Additionally, Bartlett's sphericity test produced a probability value of P=0 (<0.05), leading to the rejection of the null hypothesis. The first two factors extracted through principal component analysis account for a cumulative variance contribution rate of 65.18%. The factor scatter plot (Fig. 9) reveals that Al₂O₃, K₂O, and TiO₂ exhibit higher loadings on factor 1, whereas CaO demonstrates elevated loadings on factor 2. Although most of the reference samples come from loess deposit areas in northern China, the clay cores of the bronze horses from Yanjiagou tomb exhibit a distinct difference in distribution range when compared to samples from casting workshops at Yin Ruins, Lijia, Guanzhuang, etc. It also demonstrates a certain range overlap with samples from Houma, while sharing a similar distribution center with soil samples from Yanjiagou, Heimugou, and Peijiamao. These findings suggest a higher likelihood that the bronze horses were locally cast on the Loess Plateau. The clay cores of the GQ743-1 and GQ743-3 Ding are situated at a significant distance from the aforementioned northern sample group, suggesting that these two artifacts may have been cast in a specific southern region before being transported to the Loess Plateau.

The mineral composition

The XRD patterns and mineral identification results for the casting core, soil sample, and hardened substance are presented in Figure 10 and Figure 11. Notably, the mineral composition of the three filling soil samples (S001, S003, and S004) within the bronze horses are entirely consistent, primarily comprising quartz, albite, adularia, microcline, dolomite, calcite, muscovite, hornblende, and chlorite. The common mineral types and proportions of the buried soil (S002 and S005) closely resemble those of the filling soil, but they also contain additional minerals such as cerussite, hydrocerussite, and malachite, which are rich in lead and copper. These minerals are likely to have resulted from the migration of metal elements from the bronze’s body during corrosion and subsequent deposition in the burial environment.

The mineral composition of the hardened substance on the inner bottom surface of the horse’s mouth (I001) is in accordance with those of the casting cores from its hooves. Combining the results of XRD, OM, and SEM-EDS, it can be inferred that this sample is also indicative of a residual casting clay core. In comparison to the filling and buried soils, chlorite was not detected in the bronze horses’ casting cores, possibly due to the washing process of raw materials leading to a reduction in clay mineral content. Furthermore, it is noteworthy that the absence of calcite in C002 differs from other casting cores of the horses and directly contributes to the lower Ca content in this sample. Some researchers have observed that the clay cores utilized in the casting of large bronze artifacts may exhibit notable variations contingent upon the location of sampling, with segmented casting being identified as a contributing factor to this diversity[^51]. Although the main body of the Yanjiagou bronze horses was cast in a single operation, it is plausible that non-uniform materials were employed in fashioning the ceramic molds and clay cores during this process.

Owing to the limited sample size, only the yellowish-brown layer’s mineral composition was analyzed in I002. The results indicate that the high lead content is primarily attributed to cerussite and phosgenite, with minor amounts of quartz, microcline, albite, and muscovite. Combining observations from optical microscopy leads to the conclusion that this sample is not a residual casting core but rather a combination of surface corrosion products and soil mineral grains. Additionally, the mineral composition of the casting cores for the three bronze Ding differs significantly from that of the two bronze horses, aligning with the findings of petrographic and chemical composition analyses. Firstly, the clay cores of the three bronze Ding are devoid of dolomite, furthermore, the clay cores of GQ743-1 and GQ743-3 Ding lack calcite. The substantial disparities in mineral composition further underscore the distinctions in the casting provenance of the bronze horses and the Ding.

The thermal response of the samples

The TG-DSC curve provides a comprehensive depiction of the thermal and mass changes in the target sample across varying temperatures, thereby elucidating the reaction type and chemical composition. Research indicates that ceramic products derived from clay undergo different physical and chemical transformations at varying temperatures during the firing process, with the irreversible changes serving as reliable indicators of the maximum effective heating temperature of the sample[^52,53,54,55]. Taking calcareous clay as an example, the interlayer water and adsorbed water are first removed at approximately 200℃. This process is endothermic, reversible, and constitutes a physical alteration. The dehydroxylation of layered silicate minerals (such as clay minerals and micas) occurs within the temperature range of approximately 450℃ to 900℃, leading to the removal of structural water. This process is characterized by its endothermic nature and irreversibility. Certain clay minerals may undergo dehydroxylation at lower temperatures, contingent upon the specific mineralogical composition and crystalline lattice structure^[⁵⁴^]. Within the temperature range of 600℃ to 900℃, calcite undergoes endothermic decomposition, yielding CaO and liberating CO₂ gas. At temperatures exceeding 950℃, the primary layered silicate minerals are completely decomposed and react with CaO to generate novel secondary minerals.

Figure 12 depicts the TG-DSC curves for selected samples. Following baseline correction with blank tests, all the samples exhibit an upward curvature, attributed to changes in specific heat during the heating process. Despite this, combined with the results of XRD, certain endothermic peaks remain discernible. The endothermic peaks observed in the bronze horses’ casting core and filling soil samples at temperatures around 600-700℃ are accompanied by a significant decrease in mass, indicative of the decomposition of calcite and dolomite. Furthermore, the endothermic peak and conspicuous weight-loss step exhibited by the hardened substance I002 at 300-400℃ correspond to the decomposition of cerussite. Additionally, the minor endothermic peak at 573℃ corresponds to the phase transition from α-quartz to β-quartz. Notably, the endothermic peaks associated with dehydration and dehydroxylation processes are relatively inconspicuous and can only be inferred from TG curve analysis.

Comparatively, the TG curves of the casting cores and filling soil for the bronze horses both exhibit significant dehydroxylation weight-loss steps below 300℃, with a more pronounced weight loss; however, the dehydroxylation weight-loss step for the casting cores of the bronze Ding is less apparent, with a smaller weight loss. This implies that the casting cores’ firing temperature of the bronze horses may be lower than that of the three bronze Ding. Given that these artifacts have been buried for several thousand years, rehydroxylation has occurred in the clay cores due to environmental conditions[^56]. Therefore, a conservative estimate suggests that the actual highest firing temperature of the bronze horses’ casting cores is slightly lower than the onset temperature of calcite decomposition, approximately around 500℃. With relatively lower firing temperatures, the cores exhibit limited hardening, ensuring their collapsibility and facilitating easy removal to achieve the hollow structure of the casting objects. Additionally, it retains a degree of deformability in the cores, thereby reducing the risk of objects’ cracking during casting.

Discussion of relevant questions

The technological characteristics of the bronze horses

Upon analysis of the aforementioned results, it is evident that the bronze horses discovered in the Yanjiagou tomb exemplify the fundamental characteristics of piece-molds casting technology prevalent during the Shang and Zhou dynasties. The initial stage of the bronze production process involves creating a model capable of producing various components of the mold or core box. The striking resemblance between the two bronze horses strongly implies their potential origin from a shared model. Yue conducted a comparative analysis of the bronze vessels unearthed at Yin Ruins, positing that vessels with identical shapes were cast from the same model, while the differences in local patterns resulted from unclear mold imprints necessitating subsequent refinement. The craftsmen in the Late Shang Dynasty had already acquired the expertise to replicate multiple items using either a clay model or an existing bronze vessel[^57]. Nevertheless, certain scholars have a more stringent criterion for “casting with the same model”, contending that definitive evidence of one model producing multiple vessels did not emerge until around 500 BCE[^58].

In this research, the absence of intricate patterns on the surfaces of the two bronze horses poses challenges in determining whether they were cast from a common model. However, compared to vessels such as Ding, Gui, and Gu with regular cross-sectional geometric shapes, producing two bronze horses separately while ensuring a high degree of similarity in their body shapes presents considerable difficulty. Skilled craftsmen who are well-versed in the principles of piece-molds casting typically do not intentionally seek complex methods. Upon further comparison, it is observed that the two bronze horses exhibited more pronounced disparities in facial features and mane details than in their bodily forms, implying a respective treatment of these aspects. While conclusive evidence is lacking to ascertain that the two bronze horses were cast from an identical model, it can be confirmed that their casting processes are interconnected. It is plausible that they originated from the same model or master model, or one may have served as the model for the other, which can be easily accomplished provided that no patterns are imprinted. Subsequently, through the utilization of local molding and direct engraving of patterns onto the mold’s inner surface, distinctive details of the facial features as well as the mane and tail hair can be achieved. The presence of raised patterns on the mane and tail indicates the utilization of mold engraving, a traditional decorative technique that had existed in the Central Plains of China since the middle Shang Dynasty[^59].

Additionally, the craftsmen employed separate casting technology to fabricate the two bronze horses, to seamlessly integrate the tails with their bodies. Upon observation, it has been noted that the parting lines on both sides of the tail are superimposed onto the tail’s pre-existing surface but lack a distinct boundary with the body. This implies that the tail was first cast and subsequently placed within an overall mold, with the body part being cast later and naturally integrated with the tail. The wedge-shaped protrusion located at the tail root’s inner side is also part of the body’s casting. Existing evidence suggests that the separate casting technology may have originated during the transition from the early Shang Dynasty to the middle Shang Dynasty. The earliest method involved a “casting-on” approach, where the object’s body was cast first, followed by casting the attachments onto it. This is exemplified by the discovery of bronze Jia and Gui from the Panlongcheng site in Huangpi, Hubei Province[^60]. Furthermore, the "wrapping-type precasting method" used in Yanjiagou’s bronze horses appeared in the late stage of the middle Shang Dynasty[^61]. During the Yin Ruins Phase II, a variety of separate casting techniques were undergoing refinement and widespread adoption^[³⁶^,⁶¹^]. Concurrently, bronze artifacts with a Central Plains style, which were produced using these techniques, also emerged in the Loess Plateau region^[²⁸^]. The casting techniques demonstrated in the bronze horses are akin to those prevalent in Anyang during the same period, suggesting a probable influence from the Central Plains regime.

In terms of quality, the two bronze horses are meticulously crafted and exhibit no apparent casting shrinkage or signs of repair casting, setting them apart from the vessels such as Ding and Gui unearthed together. The clay core material is characterized by a low clay matrix and high silt content, combined with a lower firing temperature to ensure minimal shrinkage, excellent thermal shock resistance, and effective collapsibility. The technology of creating clay casting materials has its origins in pottery making, but their respective applications have led to distinct technical characteristics. The craftsman responsible for crafting the bronze horses possessed a profound understanding of this and applied it to product quality control.

As for the alloy composition, the bronze horse contains approximately 15% tin, falling within the normal range for tin usage in bronzes but significantly higher than that of all the weapons found together. The ritual vessels from the Yanjiagou tomb are primarily composed of red copper, lead-arsenic bronze, and arsenic bronze, with lower alloy element content and no tin^[⁹^,¹¹^]. Conversely, the alloy composition of the bronze horse demonstrates significant conformity with the bronze ritual vessels unearthed from high-ranking noble tombs at Anyang during Yin Ruins Phase Ⅱ, as well as certain imported Anyang bronze ritual vessels unearthed on the Loess Plateau^[³⁶^,³⁷^,³⁸^,³⁹^]. The scarcity of tin resources has led to a stratification in the tin content of bronze artifacts, reflecting social hierarchies. The elite nobility have had greater access to these resources, even resulting in instances of excessive tin usage, notably during the Yin Ruins Phase Ⅱ at Anyang^[³⁶^,³⁸^]. In contrast, the population in the Loess Plateau during the same period faced a scarcity of metal resources, resulting in low alloy element content as a characteristic of most artifacts^[³⁹^]. The significance of the bronze horse, with its technological quality approaching that of Anyang, to the inhabitants of the Loess Plateau cannot be overstated. It may serve as a localized manifestation of Anyang's technological dissemination, a point which will be further explored in subsequent sections.

The morphology and functions of the bronze horses

The author of the Yanjiagou tomb excavation report conducted a preliminary investigation into the shape characteristics of the bronze horse, and his conclusions were generally acknowledged by fellow researchers. Nevertheless, there is a need for further discussion on two aspects: (1) Whether the bronze horse is solid or hollow? (2) Whether the elliptical structure on the back of the bronze horse represents a "saddle"?

The first question involves the judgment of the filling material’s nature within the bronze horse. In ancient bronze castings, the utilization of clay cores not only saved metal raw materials but also mitigated defects arising from entrapped gases during the pouring process[^62]. The clay core is typically removed after casting while the blind core completely encased in casting liquid remains within the final object. Analysis results indicate that the filling soil inside the bronze horses exhibits similarities in petrographical structure, elemental composition, and mineral component to the buried soil enveloping the objects’ surface. however, it differs notably from the residual cores in the hooves, suggesting its nature is soil that has been deposited during the burial process. The original clay core within the horse’s body had been removed, although some residue remained on the inner surface which is difficult to clean. Nevertheless, the exposed hoof clay core, which was easier to clean, was preserved completely, suggesting a deliberate removal process aimed at achieving a functional hollow structure within the horse’s body. In essence, both bronze horses function as containers.

For the second question, we contend that the elliptical structure on the back of the bronze horse does not correspond to a saddle depiction. This assertion is predicated on the positioning of the mane above said elliptical structure, which still exhibits hair patterns, indicating that it traverses through the "saddle", thereby deviating from the overall realistic logic embodied by the bronze horse. Furthermore, the elliptical structure is separate from the horse's body, with a gap connecting to the horse's abdominal cavity. Consequently, it is plausible that this structure serves as a potential vessel lid, while the mane of the horse adeptly functions as a lid handle.

Archaeological materials indicate that bronze vessels depicting realistic animal images first emerged during the late Shang Dynasty and gained popularity in the periods of Western and Eastern Zhou. Primarily utilized as wine containers, these bronze vessels collectively known as “Niaoshouzun” (bird and beast-shaped Zun) [^63] encompass three distinct types: Zun, Gong, and You. The imitated animals include owl, duck, buffalo, rhino, goat, elephant, pig, horse, tiger, rabbit, fish, etc. (Fig. 13) Niaoshouzun was typically possessed by elite nobles and played a significant role in rituals and commemorations[^64]. The Yanjiagou bronze horses’ symmetrical upright shape and the back lid designed to match the body are reminiscent of features found in other quadruped animal vessels, suggesting they should be classified within the Niaoshouzun system of the Shang and Zhou dynasties and named “horse Zun”. Previously, the discovery of “horse Zun” was limited to a single example from the middle Western Zhou period, namely the “Liju” Zun also from Shaanxi province. This artifact shares a similar style with the Yanjiagou horse Zun and may indicate a specific relationship of transmission and influence between them^[¹³^,65]. During the late Shang Dynasty, the distribution of Niaoshouzun was concentrated in Anyang and the Xiangjiang River basin[^66]. Some scholars suggest that the abundance of distinctive bronze artifacts found in the latter area indicates a developed bronze civilization and independent bronze handicraft industry, which had close technological exchanges with the Central Plains. The realistic animal-shaped bronze vessels discovered in Anyang are thought to reflect southern traditions[^67,68,69]. Additionally, it has been proposed that the bronzes discovered in the Xiangjiang River basin are linked to the southward migration of Shang’s remnant people and local powers in the Jianghan Plain during the transition from the Shang to the Zhou Dynasty, and the probability of locally casting animal-shaped vessels in the Xiangjiang River basin is minimal[^70]. The earliest origins of the Niaoshouzun remain uncertain, but it is evident that such vessels were not prevalent in the Loess Plateau region. The pair of horse-shaped Zun, part of a drinking vessel set, likely emerged under the influence of neighboring cultural circles, reflecting the acceptance and observance of “Shang rites” by the inhabitants of the Loess Plateau[^71].

The sources of the bronze horses

In recent decades, a large number of late Shang Dynasty bronze artifacts have been accidentally discovered in the Loess Plateau during production and construction activities. Through the progression of proactive archaeological excavations, these bronze artifacts have been gradually ascribed to the Lijiaya culture, a local state culture. In terms of morphological and decorative styles, the bronze group from the Loess Plateau demonstrates a diverse source, drawing influences from the Central Plains, northern grasslands, and Guanzhong region. Researchers have categorized them into three distinct styles: local style, Shang style, and mixed style. It is postulated that the local-style and mixed-style artifacts were locally produced in the Loess Plateau[^79,80]. Previously, the Yanjiagou bronze horses were considered typical local-style artifacts^[⁷^,¹¹^]. These artifacts are deeply influenced by the northern grassland bronze culture, with a tradition of depicting round-sculpted animals. However, they were primarily utilized for local decorations, and independent round-sculpted artifacts were very rare. Following the preceding discussion, the bronze horses are believed to exhibit influences of Shang culture in terms of their shapes and technological details, yet their completely realistic and unadorned features distinguish them from typical Shang-style artifacts. Consequently, they should be hybrid artifacts that combine styles of the Central Plains and the local area. Cao highlighted the sporadic discovery of the mixed-style artifacts across the Loess Plateau, suggesting that they were unlikely to be bespoke items from Anyang, but rather products made by craftsmen who had mastered Anyang's techniques within their indigenous workshops on the Loess Plateau^[⁷^]. In recent years, the largest architectural remains of the late Shang Dynasty outside of Yin Ruins were found in Qingjian County, the core area of Lijiaya culture. Within these remains, three houses yielded Lijiaya culture’s ceramic molds, models, and clay cores for the first time, which were used to cast a variety of items such as containers, weapons, and chariot equipment[^81]. This discovery suggests that the population on the Loess Plateau had developed a certain scale and level of metallurgical production, and there were technical capabilities for the bronze horses’ local casting. Furthermore, the analysis of the casting cores from Yanjiagou tomb indicates that the bronze artifacts unearthed were sourced from diverse casting workshops, and probably that the two bronze horses were locally cast in the Loess Plateau region, aligning with discussions on styles.

From a geological perspective, the lack of metal mineral resources in the Loess Plateau necessitates the reliance on raw material input from other regions for local bronze casting. Additionally, it is also important to consider the practice of recycling complete artifacts and remelting them. The alloy composition of the bronze horse suggests a deliberate focus on quality control, with little likelihood of using recycled materials. Possibly, precious input metal ingots may be utilized following Anyang's technical standards. Based on previous research on lead isotopes and trace elements, it is evident that during the Late Shang period, the Loess Plateau region shared similar ore sources of bronzes with the Central Plains^[¹¹^,³⁹^]. The high radioactive origin lead was found in the raw materials used for casting the two bronze horses, prevalent during Yin Ruins Phase Ⅰ to Phase Ⅱ. While this type of lead material was also widely utilized in contemporaneous regional bronze civilizations around the Central Plains and its precise source remains unclear, it provides evidence that the Loess Plateau region played a role in the late Shang dynasty's metal resource distribution network. Local powers probably acquired the necessary raw materials, technology, and even skilled craftsmen for casting the bronze horses from direct or indirect interactions with the Shang dynasty through means such as warfare, rewards, economic exchanges, and population migration.

The final topic for discussion pertains to the sources of the bronze horses’ imagery. Presently, the prevailing perspective posits that equine domestication initially occurred in the middle and western Eurasian steppe around the 4th to 3rd millennium BCE before disseminating globally[^82,83,84]. While sporadic evidence of human-horse interactions exists in northern China during the Late Paleolithic era[^85], a definitive timeline for equine origins within China remains elusive. It was not until the late Shang period that horse bones, chariots, as well as associated horse artistic depictions and written records began to proliferate across archaeological sites throughout the expansive Central Plains region typified by Yin Ruins. Nonetheless, the representation of horses in the flourishing bronze art of this period remained relatively scarce, with all known instances being concentrated in the northern region. The Yanjiagou bronze horses stand as the sole tridimensional depiction of its kind. Furthermore, the regions flanking the Longshan Mountain, western Guanzhong Plain, and the Loess Plateau exhibit the most abundant evidence of horse utilization beyond the direct governance of the Shang Dynasty. Geographically contiguous and positioned within the “crescent-shaped” cultural diffusion region[^86], they function as conduits for interaction between the Central Plains and regions to the west and north. It is plausible that these areas served as continuous suppliers of horse resources to the dynasties in the Central Plains. Recently, proactive archaeological excavations in the Loess Plateau region have yielded significant findings, including the discovery of the late Shang period burials of high-ranking nobles accompanied by chariots and horses, with bronze chariot equipment exhibiting typical Central Plains styles[^87]. Analysis of the horse bones indicates that during the late Shang period, inhabitants of the Loess Plateau had access to high-quality horse resources[^88], which played a crucial role in interest exchange with the Central Plains. Furthermore, their innovative incorporation of horse imagery into ritual objects reflects a deep reverence for this animal, potentially elevating horses to a spiritual totem akin to the owl depicted on the Niaoshouzun of the Shang people.

Through a comprehensive technical examination and archaeological contextual analysis of the two bronze horses unearthed at Yanjiagou tomb, we can derive the following key conclusions:

The two bronze horses are not solid sculptures as previously believed, and the depiction of a "saddle" on their backs is inaccurate. They function as a pair of drinking vessels, specifically known as “Niaoshouzun” (bird and beast-shaped Zun), which were prevalent during the Shang and Zhou dynasties. They reflect the Lijiaya culture’s acceptance and observance of “Shang rites”.

The bronze horses exemplify the advanced techniques of mold engraving decoration, same model casting, separate casting, alloying, and clay core making, which reflect the technical characteristics and standards of the Shang Dynasty. It is plausible that they were created by craftsmen from Anyang or local craftsmen who had acquired knowledge of Anyang's techniques.

The bronze horses were most likely cast in the local area, with minimal likelihood of being cast in Anyang and subsequently imported to the Loess Plateau. The incorporation of tridimensional horse imagery into bronze ritual vessels also demonstrates a distinct local originality.

In summary, the Yanjiagou bronze horses are likely ritual vessels crafted by the inhabitants of the Loess Plateau, influenced by the Shang culture and incorporating their own cultural characteristics and preferences. They utilized imported technical expertise and metal resources from the Shang Dynasty to locally produce these objects. The bronze horses serve as a reflection of the intimate connection between Lijiaya culture and the Shang Dynasty, offering a glimpse into the interactions between the Central Plains regime and its neighboring states. The current research is actively engaged in exploring the late Shang state culture in the Loess Plateau region. This study represents the first comprehensive technical examination of typical bronze artifacts in this area, to provide a scholarly reference and insight for fully elucidating the local population's bronze handicraft industry in future studies.

OM: Optical Microscope;

SEM-EDS: Scanning Electron Microscope-Energy Dispersive Spectrometer;

XRD: X-Ray Diffractometer;

ICDD: International Centre for Diffraction Data;

TG-DSC: Thermogravimetry-Differential Scanning Calorimetry;

CT: Computed Tomography;

KMO: Kaiser-Meyer-Olkin;

Availability of data and materials

All data generated or analyzed during this study are included in this published article and its supplementary information files.

Competing interests

The authors declare that they have no competing interests.

Funding

This research is funded by the “Conservation and Restoration Project of Bronze Cultural Relics in Ganquan Museum” (ZC2020-69) and China’s National Key Project “Physicochemical Characteristics of Bronze Corrosion and Typical Bronze Disease Mechanism” (2020YFC1522001).

Authors' contributions

YH analyzed the petrographic characteristics, major elements, mineral composition, and thermal response of casting cores, and was a major contributor in writing the manuscript. XNQ investigated the casting technology of the bronze horses. YXHL conducted the metallographic analysis and alloy composition analysis. YLL collected and organized relevant archaeological materials. CL presided over the whole research work and designed the overall framework of the manuscript. All authors read and approved the final manuscript.

Acknowledgments

We would like to thank Ms. Jing Wang from Ganquan Museum, Mr. Hailong Guo from Luoyang City Cultural Relics and Archaeology Research Institute, and two graduate students (Haoyue Du and Ziming Tang) from the School of Cultural Heritage at Northwest University for providing valuable assistance during the sampling process. We also extend our sincere gratitude to Mr. Yonggang Wang, author of the Yanjiagou tomb’s excavation report, and Dr. Ruiliang Liu of the British Museum for their insightful suggestions in this study. Last, A special appreciation to Miss Qi Liang for her invaluable moral support throughout the writing process of this manuscript.

Authors' information

YH is a Ph.D. candidate majoring in archaeology at the School of Cultural Heritage, Northwest University, his main research direction is scientific analysis and conservation of bronzes and murals.

XNQ is employed at the Institute of Cultural Heritage, Northwest Nonferrous Engineering Co., Ltd., specializing in the authentication of ancient bronze artifacts.

YXHL graduated from the School of Cultural Heritage, Northwest University with a master's degree in cultural heritage conservation, she is now working in the Beijing Institute of Archaeology.

YLL is the director of the Ganquan Museum.

CL is a professor at the School of Cultural Heritage, Northwest University, his main research direction is scientific analysis and conservation of bronzes and murals.

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No competing interests reported.