Archaeological Observation of Firing Defects in Xiangzhou Kiln Porcelain

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Based on the burial environment and morphological characteristics of these artefacts, firing defects in the samples’ body and glaze were observed, recorded and analysed. Eight types of defects were identified in the foetal body, with underfiring defects accounting for the largest proportion. Meanwhile, 12 types of defects were found in the glaze surfaces, with spot defects being the most prevalent. The firing defects indicate that the Xiangzhou kiln possessed a certain degree of originality in raw material preparation, moulding, drying and firing. In addition, it could produce relatively mature, transparent, white-glazed porcelain. Consequently, its porcelain-making process should be at a high level during the same period. Humanities/History Social science/History Physical sciences/Materials science Xiangzhou Kiln Fired defects Fired process Causes of defects Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Ceramics, representing multi-millennia wisdom of Chinese civilisation, epitomise the artistic interplay of aqueous and pyrolytic processes while embodying profound historical, artistic and scientific values. The spiritual connotations manifested through ceramic artefacts authentically reflect ancient Chinese communities’ aspirations for prosperous living and their persistent quest for aesthetic ideals. Throughout ceramic production workflows, from raw material procurement, forming techniques, desiccation protocols, decorative applications, firing regimes to post-firing handling and transportation, any procedural deviation may cause critical defects, rendering ceramic products commercially non-viable [ 1 ]. Given the inherent variability in production contexts across regional kiln complexes, disparities in raw material selection, technical processes and decorative implementations have led to provenance-specific manifestations of ceramic firing defects. Consequently, systematic analysis of these imperfections becomes imperative for the reconstruction of the operational sequences and craft traditions particular to individual historical kiln sites. The Xiangzhou kiln complex is located along the southern bank of the Huan River near Anyang Bridge in present-day Anyang City, Henan Province, mainly distributed across the former Anyang Battery Factory site and adjacent areas. Recognised as the largest celadon production centre in northern China during the Northern and Southern Dynasties (420–589 CE) to the Sui-Tang period (581–907 CE), it also represents one of the earliest northern kiln complexes engaged in white porcelain manufacturing, holding a pivotal status in Chinese ceramic history [ 2 ]. From October 2008 to May 2009, a salvage excavation was performed at the site in response to the construction of Phase I of the Anyang Battery Factory Residential Complex. This marks the third archaeological excavation of the Xiangzhou kiln, yielding 20 systematically arranged excavation units covering a total area of 1,300 m 2 . The investigation recovered 705 diagnostic ceramic artefacts and substantial quantities of kiln furniture (e.g. saggers and spacers), providing critical material evidence for the reconstruction of production-scale parameters [ 3 ]. During the 2009AYTYGJT1 excavation campaign, six ash pits (H1–H6) stratigraphically underlying Layer ③ were identified, alongside two modern backfill pits (K1 and K2). The stratigraphic sequence demonstrates that H4 intrudes into H5 and H6 while being subsequently disturbed by modern backfill pit K1. Although substantial quantities of the Xiangzhou kiln ceramics were recovered from this excavation, features H3, H5 and H6 proved artefact-sterile. This study therefore focuses on the archaeologically productive units H1, H2 and H4, which yielded substantial archaeological assemblages. Feature H1 presents an elliptical planform, centrally positioned within excavation unit T1, conforming to a bell-shaped pit morphology with constricted opening and expanded basal dimensions (Fig. 1 :1). The stratigraphic unit yielded a considerable assemblage of ceramic artefacts, comprising 146 celadon specimens and 9 white porcelain items, alongside subsidiary findings, including ceramic sherds and fired clay lumps. Typological classification demonstrates multiple vessel forms: triple-lug jars, quadruple-lug jars, stemmed plates, vessel lids, bowls, dou stemmed vessels, bottles, cups and bowls ( bo type), exhibiting comprehensive morphological diversity within the ceramic repertoire. Feature H2 exhibits a circular planform located in the southwestern sector of T1, demonstrating an analogous bell-shaped pit morphology with constricted aperture and expanded basal configuration, maintaining sub-horizontal basal inclination (Fig. 1 :2). The assemblage predominantly comprises kiln furniture, including substantial quantities of triangular stilts, annular spacers, kiln wall fragments and fired clay lumps, with subsidiary ceramic sherds. The archaeological recovery yielded 45 celadon specimens typologically classified into bowls, cups, saucers, dou stemmed vessels, vessel lids and bottles, delineating a specialised production waste deposit pattern. Feature H4 exhibits an irregular planform located in the east-central sector of T1, having been intruded by modern backfill pit K1. This context exclusively yielded 54 celadon-glazed porcelain specimens, with vessel forms primarily comprising saucers, plates, bowls, dou stemmed vessels and cups. Based on integrated analysis of the morphological configurations and associated artefact assemblages of ash pits, it is proposed that the three features (H1, H2 and H4) likely functioned as discard deposits associated with ceramic production activities. This type of analogous bell-shaped discard features has been systematically documented at kiln sites, as evidenced by the discovery of circular bell-shaped pits H2 and H17 in Section I north of Xiaohuangye Bridge at the Tang Dynasty (618–907 CE) Gongyi kiln site in Henan Province. These pits exhibited sub-horizontal bases and yielded ceramic artefacts, including ewers, jars and stoves [ 4 ]. At the Song Dynasty (960–1279 CE) Liujiamen Jun kiln site in Yuzhou City, Henan Province, the identified Feature H2 was located in the southeastern sector of the excavation grid, presenting a sub-circular bell-shaped pit morphology. This context yielded a substantial quantity of celadon-glazed porcelain artefacts, with representative specimens including celadon-glazed shallow-bellied plates and Jun-type ewers [ 5 ]. Within the published excavation materials from the Liao Dynasty (907–1125 CE) Chifeng Gangwayao kiln site in Inner Mongolia, three ash pit features (H25, H56 and H18) demonstrated particular typological importance. These contexts contained substantial quantities of discarded ceramic products, commonly interpreted as forming within a relatively short temporal framework [ 6 ]. Feature H8 at the early Yuan Dynasty (1271–1368 CE) Waguantan kiln site in Tianzhu County, Guizhou Province, exhibited a sub-circular planform with slightly constricted upper walls transitioning to a bell-shaped profile. The eastern wall displays an irregular morphology, while the base maintains a sub-horizontal inclination, collectively presenting a relatively crude configuration devoid of discernible anthropogenic modification on its rim, base or walls. The context yielded numerous restorable ceramic vessels, sherds and kiln furniture, predominantly in fragmented or deformed states. The assemblage is typologically dominated by bowls, plates and small cups (zhan), with subsidiary jars and cups. Kiln furniture includes pedestals, spacers and supporting beads. This feature is interpreted as an accumulation deposit resulting from successive infilling of discarded ceramics and kiln implements [ 7 ]. During the ceramic firing process, variations in kiln temperature and the properties of body and glaze materials may induce various types of defects in the finished products. Based on existing literature and relevant studies, the firing defects observed in ceramics can be systematically categorised into several dozen types, as presented in Table 1 [ 8 ]. Table 1 Ancient ceramic manufacturing defects Body Defects Deformation, kiln adhesion, body bloating, underfiring, overfiring, blistering, black core, kiln-induced cracking, hidden cracks, separation of upper and lower sections Glaze Defects Pinholes, glaze holes, speckles, glaze peeling, orange peel texture, glaze deficiency, glaze shrinkage, thin glaze, glaze runs, glaze ripples, glaze contamination (slag inclusion), glaze bubbles, glaze cracking, colour distortion, fire marks, colour contamination, decorative flaws, hidden yellowing, smoke staining, loss of gloss, crystallisation Statistical analysis of the excavated assemblage revealed a total of 256 ceramic artefacts. The collection included 9 white-glazed porcelain specimens, comprising 5 bowls, 3 cups and 1 bottle, accounting for 3.52% of the total assemblage. These specimens represented high-quality products devoid of discernible defects and were therefore excluded from subsequent discussions of firing defects. The remaining 247 celadon-glazed porcelain artefacts constituted 96.48% of the assemblage, with the following typological distribution: 127 bowls, 79 cups, 10 dou stemmed vessels, 9 bo bowls, 2 jars, 3 saucers, 5 bottles, 2 stemmed plates, 1 plate and 9 vessel lids. The body defects identified included underfiring, kiln-induced cracking, kiln adhesion, hidden cracks, delamination, deformation, body bloating and separation of upper and lower sections, totalling 99 artefacts. Among these, kiln adhesion was the most frequently occurring defect, followed by underfiring and deformation. The glaze defects comprised glaze peeling, glaze shrinkage, thin glaze, glaze deficiency, smoke staining, glaze contamination, speckles, pinholes, glaze holes, glaze runs, hidden yellowing and loss of gloss, observed in 69 artefacts. Speckles constituted the predominant glaze defect, followed by hidden yellowing and glaze shrinkage. In addition, 79 artefacts exhibited combined body and glaze defects (Fig. 3 ). 2. Research Objectives Through statistical analysis and investigation of firing defects in ceramics excavated from this kiln site, this study aims to enhance understanding of various aspects of ceramic production technology, decorative techniques and evolutionary changes in firing processes at the Xiangzhou kiln during the Sui-Tang period (581–907 CE). Furthermore, it seeks to provide deeper insights into the organisation and development of ceramic handicraft in the Xiangzhou region. The study focuses on artefacts recovered during the third archaeological excavation campaign, employing systematic observation and documentation of body and glaze defects. This approach facilitates archaeological analysis and investigation of firing-related imperfections in the ceramic assemblage. 3. Materials Selected ceramic samples excavated from the Xiangzhou kiln site were analysed in conjunction with their firing defects [10–13]. The sample characteristics are presented as follows: 3.1 Body 3.1.1 Body Underfiring Specimen H1:22 represents a celadon-glazed cup. The vessel demonstrates complete interior and partial exterior glaze coverage, with the exterior glaze terminating above the foot. The glaze surface exhibits a matte and coarse texture (Fig. 5:1). Among the 247 celadon porcelain artefacts, 66 were underfired, accounting for ~ 27% of the total. This group comprised 26 bowls, 37 cups, 2 bottles and 1 lid. Underfiring represents a notable and typical defect in the porcelain firing process. Its fundamental cause lies in the failure of ceramics to receive sufficient thermal energy input within the kiln mainly manifested as either inadequate peak firing temperature or insufficient soaking time at a high temperature. This prevents the body and glaze from completing the necessary physical and chemical reactions, resulting in an overall ‘immature’ state. Characteristics underfired artefacts include a porous and friable body with high porosity, a dull sound when struck and a rough, lusterless glaze surface due to incomplete melting. The most immediate cause of underfiring is the failure to reach the critical firing temperature, which means that the kiln temperature did not attain the melting point of the glaze. Uneven temperature distribution within the kiln chamber is another contributing factor. In addition, improper arrangement of kiln furniture, as well as insufficient purity in the raw material composition of the body and glaze, can also contribute to the occurrence of underfiring. 3.1.2 Kiln Adhesion Specimen H1:7 represents a celadon-glazed bowl. The interior is fully glazed, with three kiln-adhered spur marks at the base centre. These spurs exhibit elliptical contact surfaces measuring 0.8 cm (major axis) × 0.5 cm (minor axis) (Fig. 5:2). Among the 247 celadon porcelain artefacts, kiln sticking affected 89 pieces, representing the most frequent defect, accounting for ~ 36% of the total. This sub-set comprised 64 bowls, 7 contracted-mouth bowls ( bo ), 9 cups, 2 jars, 5 stemmed dishes (dou), 1 bottle and 1 plate. Kiln sticking is a defect that occurs when porcelain fuses to kiln furniture or adjacent vessels during firing. The primary cause involves uncontrolled glaze flow under high-temperature conditions: when kiln temperatures exceed the maximum required threshold of the glaze or when high-temperature soaking time is prolonged, the viscosity of the glaze decreases and fluidity increases, markedly increasing the risk of adhesion. The secondary cause is that the excessive flux content in the glaze composition lowers its melting point, exacerbating flow tendencies. Improper use of kiln furniture and firing atmosphere further contribute to the occurrence of this defect. 3.1.3 Deformation Specimen H1:8 represents a celadon-glazed bowl. The vessel demonstrates deformation in both body and rim sections (Fig. 5:3). Deformation defects were documented in 16 artefacts, 7 comprising bowls, 4 stemmed dishes (dou), 2 contracted-mouth bowls ( bo ), 1 cup, 1 bottle and 1 small dish. Deformation in porcelain production denotes a defect where the body, which is in a softened state during high-temperature firing, loses structural stability under imbalanced mechanical stress. This defect results from factors spanning the entire manufacturing sequence, from raw material preparation to final kiln processes. Root causes for deformation include structural deficiencies in the green body, impractical vessel morphology and improper raw material processing. The contributing factors for this defect encompass uneven shrinkage stress from inadequate drying control, thermal gradients within the kiln chamber and sub-optimal stacking arrangements during kiln loading. 3.1.4 Kiln-induced Cracking Specimen H1:104 represents a celadon-glazed bo bowl. Both the interior and exterior rims exhibit kiln-induced cracks, with the longest measuring 5.5 cm (Fig. 5:4). Kiln-induced cracking defects were identified in five artefacts, comprising 3 contracted-mouth bowls ( bo ), 1 bowl and 1 cup. Crazing represents one of the most prevalent and complex defects in ceramic firing, characterised by the development of surface or internal fissures in porcelain owing to temperature fluctuations or internal stress during the firing process. Rapid thermal shifts are the primary catalyst for such cracking. During the heating or cooling phase, accelerated temperature changes generate differential thermal expansion stress between the body’s interior and surface, subsequently initiating crack formation. Insufficient time allocated for uniform heating or cooling of the ceramic body during firing further predisposes wares to crazing. Additional contributing factors include the structural integrity and morphology of the green body, compositional ratios of raw materials and the prevailing firing atmosphere within the kiln. 3.1.5 Hidden Cracks Specimen H4:1 represents a celadon-glazed dou vessel. Hidden cracks are observed on the interior and exterior surfaces of the bowl, with the longest measuring 3.7 cm (Fig. 5:5). Subsurface micro-cracks were documented in two artefacts: 1 stemmed dish (dou) and 1 contracted-mouth bowl ( bo ). During firing, these hairline fissures develop internally or on ceramic surfaces, often remaining imperceptibly fine. Their formation primarily correlates with thermal fluctuations and uneven thermal expansion during kiln cycles. As temperatures rise, disparate heating of body zones generates differential expansion rates, producing internal stresses that propagate micro-fractures within the ceramic matrix. 3.1.6 Body Bloating Specimen H1:55 represents a celadon-glazed cup. The base centre shows accidental adhesion of black–brown glaze, with adjacent body bloating measuring 0.19 cm in length and 0.39–0.41 cm in width (Fig. 5:6). Body bursting defects were recorded in 13 artefacts, comprising 8 cups and 5 bowls. This phenomenon involves the sudden rupture or explosive fracture of ceramic bodies during firing mainly caused by internal gas expansion or uneven thermal stress. The residual moisture trapped within the green body undergoes rapid vaporisation upon heating, generating substantial vapour pressure. When the exterior clay layer hardens faster than the interior—creating permeability barriers—the pressurised steam cannot escape efficiently, resulting in explosive failure. Contributing factors include firing parameters (temperature and ramp rate), raw material formulation and structural lamination gradients within the green body. 3.1.7 Delamination Specimen H1:124 represents a celadon-glazed bowl. The body cross-section demonstrates delamination measuring 0.36–1.03 cm in length and 0.03–0.16 cm in width (Fig. 5:7). Laminar separation defects were observed in four artefacts: 2 bowls and 2 cups. This defect manifests as planar delamination within the porcelain body post-firing, fundamentally resulting from the complete failure of interlayer bonding forces at elevated temperatures. The formation of delamination is correlated with multiple factors, mainly attributed to thermal inhomogeneity during firing. Differential heating of the ceramic body causes incomplete vitrification in thermally disadvantaged zones, generating structural discontinuities. In addition, excessive residual moisture or heterogeneous raw material distribution predisposes the ware to incomplete sintering, further facilitating interlayer separation. 3.1.8 Separation of Upper and Lower Sections Specimen H4:50 represents a celadon-glazed dou stemmed vessel. The bowl section is completely separated from the stem. Glaze application unevenly extends to the mid-stem area, accompanied by visible glaze runs (Fig. 5:8). Vertical delamination defects were noted in three artefacts: 1 stemmed plates (gaozupan) and 1 stemmed dish (dou). This phenomenon arises during firing when thermal gradients, structural inconsistencies or compositional heterogeneity compromise interfacial bonding between the upper and lower sections of the ceramic body. Disparate heating across vertical zones generates differential thermal expansion between the upper and lower sections, exceeding critical stress thresholds and inducing mechanical separation. Contributing factors include notable temperature differentials between layers during forming stages or inadequate interfacial bonding prior to firing. 3.2 Glaze 3.2.1 Hidden Yellowing Specimen H1:132 represents a celadon-glazed bowl demonstrating yellowish discolouration in the glaze and body cross-section (Fig. 6:1). Among the 247 celadon porcelain artefacts, yellowish discolouration affected 28 pieces, accounting for ~ 11% of the total assemblage. This included 18 cups, 9 bowls and 1 bottle. Yellowish discolouration refers to an uneven, dull-yellowish surface manifestation occurring during ceramic firing. Inappropriate firing temperature constitutes a primary cause: sub-optimal thermal conditions prevent sufficient sintering of the body surface, enabling localised accumulation of iron oxides or other impurities. These compounds undergo incomplete oxidation at high temperatures, resulting in yellowish surfaces. In addition, insufficient oxygen supply during firing contributes substantially to this defect. Metallic oxides, such as iron and copper, may exhibit yellow to deep-yellow hues under reducing conditions, imparting a dull or yellowish appearance to the ceramic surface. 3.2.2 Loss of Gloss Specimen H2:28 represents a celadon-glazed bowl exhibiting bluish glaze tones with complete loss of surface gloss (Fig. 6:2). Gloss impairment was reported in 10 artefacts, comprising 8 bowls and 2 cups. This defect manifests as a lack of surface lustre or matte appearance, resulting in visually unappealing, dull ceramics. The phenomenon typically occurs when the glaze undergoes incomplete vitrification or reactions, often due to inadequately controlled firing parameters that disrupt the development of surface gloss. A primary causative factor is sub-optimal firing temperature or thermal mismanagement. When kiln temperatures fail to reach the glaze’s required melting threshold, the glaze cannot fully fuse into a vitreous layer, yielding matte or non-reflective surfaces. In addition, compromised glaze formulation quality and improper surface preparation of the green body are notable contributing factors. 3.2.3 Speckles Specimen H1:138 represents a celadon-glazed cup. Black–brown speckles are distributed throughout the vessel body (Fig. 6:3). Among the 247 celadon porcelain artefacts, speckling defects were documented in 40 pieces, representing the most prevalent defect, accounting for ~ 16% of the assemblage. The affected forms comprised 24 bowls, 9 cups, 2 contracted-mouth bowls ( bo ), 2 bottles, 1 jar, 1 stemmed dish (dou) and 1 small dish. Speckling manifests as irregular localised discolorations or spot-like blemishes on ceramic surfaces, resulting from inhomogeneous pigmentation owing to glaze composition variations or atmospheric fluctuations during firing. Sub-optimal glaze formulation is a primary causative factor. Metallic oxides (e.g. iron and copper) within the glaze may undergo localised enrichment under conditions of thermal unevenness and oxygen deficiency during firing, generating chromatic aberrations or speckling. In addition, inadequate temperature control markedly contributes to this defect. Further predisposing factors include instability in the kiln atmosphere and compositional heterogeneity of raw materials. 3.2.4 Glaze Contamination (Slag Inclusion) Specimen H2:25 represents a celadon-glazed cup. The interior is fully glazed, demonstrating suspected slag inclusions (glaze contamination) on the lower belly (Fig. 6:4). Surface contamination defects were documented in 18 artefacts, predominantly co-occurring with other defects. The assemblage comprised 7 bowls, 5 lids, 3 stemmed dishes (dou), 2 cups and 1 plate. This defect manifests as irregular accretions of foreign matter on ceramic surfaces during firing, typically appearing as black specks, greyish blotches or chromatic patches. Primary sources include impurities inadvertently introduced into raw materials, glaze compounds or body surfaces—such as iron scale, coal ash particulates or fine mineral grains—that resist complete melting at peak temperatures and persist as embedded contaminants. In addition, airborne kiln pollutants (e.g. soot and smoke particulates) deposit onto ware surfaces under high-temperature conditions, forming adherent residues. Moreover, sub-optimal material formulation or uneven glaze application creates irregular surface topography. Excessive glaze thickness particularly entrains impurities during application, ultimately manifesting as speckling. 3.2.5 Smoke Staining Specimen H1:108 represents a celadon-glazed cup and exhibits distinct reduction staining with smoke-derived ferruginous speckling (Fig. 6:5). Smoke staining defects were documented in 13 artefacts, predominantly co-occurring with other defects. The assemblage comprised 5 bowls, 4 cups, 2 jars and 2 lids. This phenomenon manifests as carbonaceous deposits on ceramic surfaces resulting from exposure to smoke or reducing gases during firing, presenting as black to dark-brown speckles, streaking or irregular tonality. The defect formation is mainly attributable to fuel combustion dynamics and oxygen availability. Insufficient oxygen supply during firing promotes incomplete fuel combustion, generating substantial smoke and carbon monoxide–rich reducing atmospheres. These by-products deposit particulate matter onto ware surfaces, forming carbon-embedded discolorations. At the Xiangzhou kiln, wood fuel usage aggravated this issue: its inherent propensity for incomplete combustion released smoke and fine carbon particulates that readily adhered to ceramics, resulting in pervasive smoke staining. 3.2.6 Glaze Holes (Melting Voids) Specimen H1:34 represents a celadon-glazed bo bowl. The glaze surface displays melting voids, pinholes and a distinct black slag inclusion mark (Fig. 6:6). Glaze pitting defects were documented in 19 artefacts, predominantly co-occurring with other defects. The assemblage comprised 12 bowls, 3 lids, 2 small dishes, 1 contracted-mouth bowl ( bo ) and 1 jar. Pitting arises from localised over-fusing or heterogeneous vitrification of the glaze or body at elevated temperatures, manifesting as surface cavities or depressions. This defect correlates primarily with thermal profiles, glaze formulation and application thickness. Excessively high firing temperatures may induce glaze over-fusion, generating surface voids. Similarly, disproportionate low-melting constituents in glaze recipes or compositional inhomogeneity can induce excessive fluidity during firing, facilitating pit formation. Moreover, thick glaze applications or incomplete degassing of entrapped bubbles during vitrification may generate subsurface voids that erupt as surface pits under thermal stress. 3.2.7 Pinholes Specimen H1:25 is a celadon-glazed vessel lid. The lid surface contains numerous glaze holes (melting voids) and pinholes (Fig. 6:7). A single pinhole defect was documented on a lid artefact. Pinholes manifest as minute, deep cylindrical voids on ceramic surfaces, resembling needle punctures in their dense, uniform distribution. Their formation is mainly associated with gas entrapment, glaze inhomogeneity, thermal mismanagement and body flaws. During firing, the bubbles trapped within the body or glaze fail to escape efficiently; these may expand under high temperatures, rupturing the surface to form micro-voids. Compositional heterogeneity in the glaze or improper application can create localised thickness variations or undissolved particulates that nucleate pinholes. In addition, sub-optimal firing temperatures or uneven heating prevent complete glaze maturation, inhibiting bubble release and promoting pinhole formation. 3.2.8 Glaze Peeling Specimen H2:6 represents an underfired celadon-glazed bowl. Although originally intended to be fully glazed, the vessel demonstrates extensive glaze peeling, with only residual glaze remaining near the rim area (Fig. 6:8). Glaze peeling defects were documented on two bowl artefacts. This phenomenon manifests as delamination or spalling of the glaze layer due to insufficient adhesive strength between the glaze and the body. Primary causative factors for this defect include inadequate surface preparation of the green body compromising glaze adhesion; sub-optimal glaze formulation featuring compositional heterogeneity or excessively low melting points that inhibit proper bonding; thermal mismanagement during firing, including inappropriate ramp rates, generating differential expansion stresses at the glaze–body interface; and excessive glaze thickness or high absorbency of the bisque body promoting interfacial failure. 3.2.9 Crawling Specimen H2:22 represents a celadon-glazed impressed saucer. The exterior glaze extends to the lower belly, demonstrating glaze runs and shrinkage near the belly area (Fig. 6:9). Among the 247 celadon-glazed ceramic specimens examined, 33 artefacts exhibited crawling defects, accounting for ~ 13% of the assemblage. This phenomenon frequently co-occurred with other production flaws. The affected forms comprised 26 bowls, 3 cups, 1 bo (wide-mouthed vessel), 1 lid, 1 bottle and 1 dou (stemmed vessel). Crawling denotes surface irregularities arising from excessive glaze contraction during the firing process, manifesting as wrinkles, cracks or uneven gloss. Primary causative factors for crawling include thermal expansion coefficient mismatch between the glaze and the ceramic body, excessive glaze thickness, inadequate regulation of firing temperatures and sub-optimal glaze formulation. When substantial disparity exists between the thermal expansion coefficients of the glaze and body, temperature variations induce uneven contraction of the glaze surface, producing an irregular finish. Furthermore, excessively thick glaze layers undergo pronounced shrinkage during the cooling phase, readily forming cracks or wrinkles due to differential stress distribution. 3.2.10 Thin Glaze Specimen H1:82 is a celadon-glazed bo bowl. Thin glaze and uneven slip application were observed (Fig. 6:10). Only one artefact demonstrated thin glaze, identified as a bo (wide-mouthed bowl). Thin glaze refers to an excessively thin glaze layer, typically resulting in insufficient surface gloss, uneven colouration and localised glaze skipping. Primary causes responsible for this include uneven glaze application, over-dilution of glaze slurry, unsuitable glaze formulation and improper firing temperature or cooling control. When the glaze application is uneven, it may fail to uniformly cover the body, resulting in excessively thin glaze layers. If the glaze slurry is over-diluted, the glaze thickness will also be insufficient. In addition, in certain glaze formulations, low-melting components or excessive fluidity may cause the glaze to flow too rapidly, thereby forming thin layers. During firing, excessively high temperatures or overly rapid cooling can also induce excessive glaze flow or contraction, causing glaze thinning. 3.2.11 Glaze Deficiency Specimen H4:24 represents a celadon-glazed bowl. The interior is fully glazed, except for a glaze-deficient base (Fig. 6:11). Two artefacts exhibited glaze skipping, comprising 1 bowl and 1 bo (wide-mouthed bowl). Glaze skipping refers to areas on the ceramic surface that failed to achieve complete glaze coverage, exposing unglazed body material, which compromises aesthetic integrity and functionality. Contributing factors for this include uneven glaze application, problematic glaze slurry viscosity, inappropriate firing temperatures, inadequate body surface preparation and sub-optimal glaze formulation. Uneven glaze application may cause localised glaze skipping, while over-diluted glaze slurry can lead to inconsistent coating. Inadequate or excessive firing temperatures may result in insufficient glaze melting or overly rapid flow, forming glaze-deficient areas. Concurrently, surface contamination of the body or improper glaze formulation can compromise glaze adhesion, thereby causing skipping. Glaze skipping not only diminishes ceramic aesthetics, producing rough surfaces and gloss deficiency, but also compromises functional properties such as water resistance and corrosion resistance. 3.2.12 Glaze Runs Specimen H2:7 represents a celadon-glazed bowl. The exterior glaze extends to the lower belly, exhibiting six distinct glaze run marks appearing as raised black–brown circular droplets, some extending to the foot (Fig. 6:12). Glaze runs were observed on 28 artefacts, predominantly co-occurring with other defects. The affected forms comprised 16 bowls, 6 cups, 2 stemmed cups, 2 dishes, 1 bottle and 1 lid. Glaze runs refer to fine linear striations formed during ceramic firing, typically manifesting as elongated thread-like marks resulting in rough and uneven glaze surfaces with compromised gloss. The formation of glaze runs is attributed to inhomogeneous glaze composition, uneven glaze application, excessively high firing temperatures or overly rapid heating rates, excessively rapid cooling and excessively high glaze viscosity. When the compositional distribution within the glaze is uneven, striations or trails may form during firing. Uneven or excessively thin glaze application causes differential glaze flow, resulting in trail generation. Excessively high temperatures or overly rapid heating induce excessive glaze mobility, forming linear trails. Excessively rapid cooling triggers surface contraction that produces texturing. In addition, glaze slurries with excessive viscosity demonstrate poor flow characteristics during application, potentially resulting in striation formation. 4. Results and Discussion Based on the foregoing analysis, the primary causes of defects in artefacts excavated from the Xiangzhou kiln site may be categorised into two groups: improper firing temperature control and firing atmosphere issues. The majority of firing defects correlate with inadequate temperature regulation, while the kiln atmosphere constitutes a critical contributing factor to defect formation. During the firing process, ceramics undergo a series of chemical and physical transformations, with phase-specific thermal conditions critically determining the final product performance and quality. The firing temperature for the Xiangzhou kiln ceramics is typically > 1100°C [ 14 ]. Archaeological evidence from kiln sites indicates a high prevalence of underfired specimens, likely attributed to considerable thermal discrepancies during firing. In cases of sub-optimal raw material formulations, failure to reach the target firing temperature (1100°C), insufficient holding duration at peak temperatures or excessive thermal gradients, the ceramic body exhibits elevated water absorption rates, dull acoustic responses and rough yellowish surfaces, which are characteristic defects of underfiring. Defects such as body softening and deformation, glaze blistering and vessel overfiring of the vessel may occur if the firing temperature is excessively high with prolonged heat preservation, causing the kiln temperature to exceed the required firing range for the body. During the low-temperature heating stage, an overly rapid temperature rise can lead to body cracking. The fracture surfaces of such cracks appear smooth due to glaze coverage, forming hidden crack defects. During the cooling stage, excessively fast cooling rates in the medium-temperature range—particularly for thick-walled bodies and high-silica products—may induce kiln-induced cracks due to residual quartz phase transformation. The ceramic defects manifest in a sequential spectrum from high to low firing temperatures as follows: smoke staining → normal firing → loss of gloss → hidden yellowing → underfiring (Fig. 6 ). Archaeological excavations of the Xiangzhou kiln site revealed that its kiln structure was a semi-subterranean mantou-type kiln (rounded vault kiln) with a diameter of ~ 1 m [ 15 ]. Characterised by simple construction and limited internal space, this kiln design influences specific atmospheric conditions during firing. When firing ceramics using wood fuel, incomplete oxidisation occurs if combustion is insufficient prior to glaze sealing. Residual carbonaceous and organic materials within the body deposit on surfaces, resulting in smoke staining defects. An excessively strong reducing atmosphere combined with prolonged heat preservation can lead to the entrapment of free carbon particulates within the glaze, resulting in densely distributed dark speckle defects characterised by large diameters. In addition, if the temperature increases too rapidly during firing, causing premature melting of the glaze and inadequate reducing conditions, the transformation of iron ions in the porcelain body will induce hidden yellowing defects. Defects in ceramics occur due to not only operational errors during firing but also various mishaps in the forming stages, including raw material preparation, shaping, body trimming and drying. During kiln firing, pre-existing issues from the forming stages become progressively amplified, ultimately causing ceramic failure. From the perspective of the chaîne opératoire, sequential progression from ceramic formation through firing enables the classification of defect causes into the following five types: Category I: Operational errors occurring during the raw material preparation stage. The porcelain bodies of the Xiangzhou kiln ceramics were crafted from kaolin clay sourced from the low mountainous and hilly areas surrounding present-day Shanying Town in southwestern Anyang [ 16 ]. The production process involved sequential steps of material selection, crushing, levigation and sedimentation, with the processed clay then wedged into brick-shaped blocks before being shaped on the potter’s wheel. Due to the mineral composition of the raw materials, the Xiangzhou kiln’s clay contained relatively high concentrations of iron and titanium elements. Compared with porcelain clays from southern China, the Xiangzhou kiln’s clay body exhibits relatively coarser texture, greater thickness and inferior chromatic quality, typically appearing in bluish-grey tones with darker colouration. Consequently, during the refining process, the porcelain body of the Xiangzhou kiln underwent insufficient grinding and sieving. Combined with excessively strong reducing atmosphere in the kiln, iron impurities in the clay material developed colouration, resulting in irregularly sized spots in pale yellow, brownish and black hues on the ceramic surfaces. Non-uniform clay preparation leads to air pockets within the body, which can easily cause delamination defects at this stage. Improper body formulation or inadequate processing, causing uneven sintering and shrinkage, may also cause deformation defects. Insufficient sieving during the preparation of body and glaze materials enables fusible impurities to mix into the body. During firing, the combustion and melting of these materials result in glaze hole defects. The presence of impurities and hard particles (e.g. pebbles and quartz) within the body can lead to body bloating defects due to cracking during firing. Similarly, a considerable mismatch in the thermal expansion coefficients between the body and glaze may cause this defect. To compensate for the drawbacks of the porcelain clay, ceramics from the Xiangzhou kiln often applied a layer of white slip onto the body after forming. This process concealed the darker grey or greyish-brown body, creating a smoother and more refined surface. The application of slip enhanced glaze colouration and improved the aesthetic quality of the ceramic surfaces. The Xiangzhou kiln ceramics predominantly feature partial glaze coverage on their surfaces, with the application of slip constituting a distinctive technical feature. Among the celadon-glazed ceramics of the Xiangzhou kiln, uneven slip application is observed on exterior surfaces. However, no slip traces are detected on ceramics with fine-textured bodies and whitish colouration. The majority of these ceramics are light-coloured or white-glazed porcelain. Owing to the relatively pure and white body material, slip application was seldom adopted for surface refinement. This demonstrates that the Xiangzhou kiln selectively utilises slip to modify ceramic surfaces according to the specific quality of the body material. Category II: Operational errors occurring during the shaping stage. In the production of the Xiangzhou kiln ceramics, the wheel-throwing technique is predominantly employed for vessels such as bowls and plates. Appendages including spouts, handles and lugs were typically formed through moulding or hand-modelling. Bowls and cups were generally created via single-stage wheel throwing, while stemmed plates and other forms were created using a composite construction method: the dish and stem were separately wheel thrown and then joined using a slip mixture comprising porcelain clay and glaze materials. Inadequate adhesion between components can easily lead to separation defects in the upper and lower sections of the vessel. In addition, deformation defects may arise from either excessive or insufficient forming pressure during the shaping process. Category III: Operational errors occurring during the drying stage. During the drying process of ceramic artefacts, body deformation and cracking can be induced by a soft body, improper control of the drying regime or uneven shrinkage caused during drying. Owing to the notable variation in the thickness of the body in different parts of the artefact, the shrinkage rate and stress conditions during high-temperature firing are likely to be uneven. Improper drying can lead to the generation of internal stresses within the body, causing deformation during firing. When the body is still pliable, excessive stress can distort and deform the body. Once the surface of the body hardens, if the magnitude of the stress exceeds the strength of the body, it will result in cracking. Category IV: Operational errors occurring during the glazing stage. The Xiangzhou kiln employed dipping glazing as its primary method, supplemented by auxiliary techniques such as brush and swirling glazing. The process involved manually grasping the base of the vessel and inverting it into a glaze slurry bath to apply the coating. For common vessels such as bowls and cups, the interiors were typically fully glazed, demonstrating noticeable glaze accumulation within the cavities. Externally, glazing was often partial, terminating below the rim or extending only to the abdominal region, while the foot rims remained unglazed, exposing the raw clay body. The glaze surfaces typically exhibited fine crackling patterns, with predominant hues ranging from celadon, celadon-grey, to celadon-yellow, characterised by a transparent vitreous quality that revealed the underlying ceramic body. Thicker glaze layers exhibited deeper olive-green to dark-green shades, while thinner areas manifested as pale celadon or celadon-grey tones. If the relative density of the glaze slurry is too low, the resulting glaze layer during application exhibits insufficient thickness, leading to coarse surface textures, poor gloss retention and ultimately thin-glaze defects. Conversely, excessively high slurry density may cause poor adhesion at angular intersections of the ceramic body, thereby promoting crack propagation during the drying and firing stages. This condition frequently manifests as glaze cracking (crazing) and thickened glaze accumulation on the finished surface. In addition, inadequate bonding between the glaze and body prior to or during firing can result in glaze spalling, characterised by partial or complete detachment of the vitreous layer. An excessively high moisture content in the ceramic body prior to kiln entry or excessive steam generation during firing can compromise the bonding integrity between the body and glaze, causing glaze detachment and pinhole defects. Inadequate dispersion of glaze slurry during application may cause localised thickening of the glaze layer. Upon melting, this thickened glaze migrates to form linear ridges or circular cord-like structures, which is termed glaze veining. Prolonged glazing duration leads to excessive absorption of slurry by the body, creating trapped air bubbles within the glaze matrix. Such conditions further promote the formation of closed pores within thickened glaze layers, ultimately manifesting as pinhole defects. Category V: Operational errors occurring during the kiln loading and firing stage. As saggar firing was not yet adopted during this period, open firing remained the predominant method. Kiln workers adapted their approach based on the kiln’s internal structure and spatial configuration as well as the types of vessels being produced. They applied appropriate glaze slurries to various greenware pieces, which were then dried and loaded into the kiln for single high-temperature firing. In the production process, a combined system of kiln posts with various supporting implements (including spurs, spacer beads, setters and other kiln furniture) was used, and these tools were repeatedly used to enhance firing efficiency and output (Fig. 7 ). Some artefacts exhibit sand adhesion on their bases likely resulting from direct contact with the kiln floor. In addition, a stacking method was implemented, where multiple vessels were fired in direct contact without intermediate supports, leaving residual fragments from stacked pieces within the vessels [ 17 ]. In open firing without saggars, irregular loading practices during greenware arrangement and kiln stacking caused the body surfaces to readily adhere to debris such as grog particles, sand grains, glaze slag and kiln ash, manifesting as surface contamination defects. Furthermore, due to the open firing method, improper kiln stacking disrupts the balanced spacing between greenware bodies, causing displacement of the greenware pieces and resulting in direct contact between adjacent bodies. An excessively dense arrangement of products with reduced or eliminated inter-vessel spacing inevitably leads to mutual adhesion between the rim exteriors or belly sections of adjacent vessels, resulting in kiln adhesion defects. The absence of saggars during firing enables the backflow of flue gases within the kiln to directly erode the glaze surfaces, resulting in blackened and greyish discolouration characterised by smoke staining. 5. Conclusions Through examining defective specimens from the Xiangzhou kiln, it is evident that improper control of glaze and body materials by kiln workers during production stages—including raw material preparation, drying, body trimming, decoration, glazing and firing—readily leads to ceramic defects such as delamination, deformation, edge slumping, underfiring and glaze shrinkage. These defects resulted in low firing success rates and substantial quantities of sub-standard products, which were ultimately deemed unmarketable as quality commodities and systematically discarded in concentrated deposits near the kiln site. Archaeological research has shown that these ceramics with firing defects enable the reconstruction of historical loading and firing techniques. As one of the northern folk kilns produced celadon during its early phases, the Xiangzhou kiln exhibited certain rudimentary aspects in its ceramic technology. The prevalence of various defects in its ceramic products logically aligns with this technological context. Simultaneously, the Xiangzhou kiln was capable of producing mature white porcelain characterised by pure white and dense bodies without slip application, featuring lustrous and immaculate glazes. Therefore, the ceramic technology of the Xiangzhou kiln occupied an advanced level among contemporaneous kilns. Based on the Xiangzhou kiln ceramics discovered during the 2009 third excavation campaign—primarily unearthed from bell-shaped ash pits at the site and demonstrating pronounced firing defects—these pits are inferred to be discard deposits for storing rejected ceramics. This phenomenon shows the emergence of specialised production in porcelain manufacturing during this period. As one of the early celadon kiln sites in northern China, the Xiangzhou kiln provides crucial insights into ceramic production processes through the analysis of firing defects observed in its sub-standard products. By categorising and statistically examining these defects, researchers can reconstruct the production stages of the Xiangzhou kiln’s ceramic industry and deduce the specific techniques and technologies used in its historical manufacturing practices. The Xiangzhou kiln provides substantial physical evidence for research on the typology of early ceramics, technological evolution in firing techniques and development of decorative craftsmanship in the Xiangzhou region. This evidence markedly enhances our understanding of the distribution patterns of ceramic kiln sites in the area, particularly regarding the origins of celadon and white-glazed porcelain production. These findings hold crucial importance for investigating the production systems, spatial distribution of Sui Dynasty (581–618 CE) kilns as well as the genesis and development of early northern celadon and white porcelain. Declarations Funding 2025 General Project for Humanities and Social Sciences Research in Henan Provincial Higher Education Institutions (Project Approval Number: 2025-ZDJH-418) Postgraduate Education Reform and Quality Improvement Project of Henan Province (Project Approval Number: YJS2025XQLH40) Author Contribution M.J. and X. wrote the main manuscript text and J.K. prepared figures 1-8. All authors reviewed the manuscript. References Lu, X. R. Ceramic Technology. Changsha: Hunan University Press, 2005, p. 187. Kong, D. M. Research on the Xiangzhou Kiln in Anyang and Related Issues. Yindu Academic Journal 1 (2014) 34–38. Anyang Municipal Institute of Cultural Relics and Archaeology. Kneading Earth into Gold: New Archaeological Discoveries on the Xiangzhou Kiln and Its Celadon Wares. Zhengzhou: Zhongzhou Ancient Books Publishing House, 2018, pp. 4–9. Li, H. 2016. Ancient Lime Kilns and Various Porcelain Relics Discovered at the Gongyi Kiln Site in Henan. China Cultural Relics News August 26, 2016, p. 8. Peking University Center for Chinese Archaeology Research, Henan Provincial Institute of Cultural Relics and Archaeology. Excavation Brief Report on the Liujiamen Jun Kiln Site in Shanghou Town, Yuzhou City, Henan Province. Cultural Relics 11 (2003) 26–52. Guo, Z. Z, Su, D. Highlights of Liao and Jin Porcelains Excavated from the Chifeng Gangwayao Kiln Site. In Chinese Ceramic Studies (Vol. 11). Beijing: Forbidden City Press. 2005. Department of Archaeology, Sichuan University; Guizhou Provincial Institute of Cultural Relics and Archaeology; Tianzhu County Cultural Relics Management Office.Excavation Brief Report on the Yuan Dynasty Kiln Site at Waguantan in Tianzhu County, Guizhou Province. Archaeology 3 (2016) 45–67. Yu, H, Yang, Z. Z. Fundamentals of Ancient Ceramic Restoration. Shanghai: Fudan University Press, 2014, pp. 25–29. Shi, Q, Guo, Z. M, Hao, J. J. Research and Application Considerations on Defects in Traditional Ceramics. Chinese Ceramics 6 (2007) 36–38. Feng, X. M. Chinese Ceramics Dictionary. Beijing: Cultural Relics Publishing House, 1998, pp. 391–395. Li, J. J. Ceramic Technology. Beijing: China Light Industry Press, 2006, pp. 495–510. Xu, S. Y, Xu, K. Chinese Ceramics Dictionary. Beijing: China History and Culture Press, 2013, p. 443. Terminology of Defects in Daily-Use Ceramics, pp. 6–10. Fang, K. K. Application of Modern Experimental Techniques in Xiangzhou Kiln Ceramics [D]. Zhengzhou: Zhengzhou University. 2017. Anyang Municipal Institute of Cultural Relics and Archaeology. Kneading Earth into Gold: New Archaeological Discoveries on the Xiangzhou Kiln and Its Celadon Wares. Zhengzhou: Zhongzhou Ancient Books Publishing House, 2018, p. 53. Anyang Municipal Institute of Cultural Relics and Archaeology. Kneading Earth into Gold: New Archaeological Discoveries on the Xiangzhou Kiln and Its Celadon Wares. Zhengzhou: Zhongzhou Ancient Books Publishing House, 2018, p. 52. Ming, Z. F, Kong, D. M, Jiao, P. Discussion on Issues Related to Xiangzhou Kiln Celadon Wares in Anyang: A Case Study Based on the 2009AYTYGJT1 Dataset. Yellow River, Loess, and Yellow People, 14 (2018) 31–40. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 05 Sep, 2025 Editor assigned by journal 05 Sep, 2025 Submission checks completed at journal 14 Aug, 2025 First submitted to journal 06 Aug, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7310003","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":500498547,"identity":"3b609baa-653c-4207-b83f-2e2ed7c5dd47","order_by":0,"name":"MING Chao-fang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAq0lEQVRIiWNgGAWjYDACZhBhY8PDz95Akpa0NBnJngMkWZV22MbghgORinXbeQ+/+JFwnofhBgPjh485RGgxO8yXZtmTcJuHcXYDs+TMbURp4TEzZvxxm4dZ5gAbMy/RWhgSzvGwSSQQr8X4MUPCAR4eUrSYMfYkJPNI8BxsJtIv588Yf/iRYGdvf7z54IePxGgBAjYJCM3YQJx6IGD+QLTSUTAKRsEoGJkAAJoyMgUN3lPmAAAAAElFTkSuQmCC","orcid":"","institution":"Anyang Normal University","correspondingAuthor":true,"prefix":"","firstName":"MING","middleName":"","lastName":"Chao-fang","suffix":""},{"id":500498548,"identity":"ca166fe4-3358-4b07-8da7-0b3bab059d30","order_by":1,"name":"JIANG Meng-xue","email":"","orcid":"","institution":"Anyang Normal University","correspondingAuthor":false,"prefix":"","firstName":"JIANG","middleName":"","lastName":"Meng-xue","suffix":""},{"id":500498549,"identity":"89bf272c-5e8d-47ae-8095-cf3165b291e8","order_by":2,"name":"XIE Ling-li","email":"","orcid":"","institution":"Anyang Normal University","correspondingAuthor":false,"prefix":"","firstName":"XIE","middleName":"","lastName":"Ling-li","suffix":""},{"id":500498550,"identity":"d43d9708-a4c1-44b2-b8e6-69dc8d7c89d2","order_by":3,"name":"De-ming KONG","email":"","orcid":"","institution":"Cao Gao Mausoleum Museum","correspondingAuthor":false,"prefix":"","firstName":"De-ming","middleName":"","lastName":"KONG","suffix":""}],"badges":[],"createdAt":"2025-08-06 12:53:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7310003/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7310003/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":89280724,"identity":"e1640e1e-e57f-4fad-b0a9-9261e15bae3a","added_by":"auto","created_at":"2025-08-18 10:28:40","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":104480,"visible":true,"origin":"","legend":"\u003cp\u003eLocation of the Xiangzhou kiln site.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7310003/v1/158f824a0916675b2410120c.jpg"},{"id":89282484,"identity":"34bb8b93-3602-47c7-a59c-0d24fc676952","added_by":"auto","created_at":"2025-08-18 10:44:40","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":29971,"visible":true,"origin":"","legend":"\u003cp\u003eAsh pit profile in plan: (1) H1 profile in plan and (2) H2 profile in plan.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7310003/v1/2c3768672efcae7d157f07a1.jpg"},{"id":89281112,"identity":"ae79ac25-c66c-4ad4-91a6-98d8299cbaf2","added_by":"auto","created_at":"2025-08-18 10:36:40","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":24416,"visible":true,"origin":"","legend":"\u003cp\u003eWhite porcelain bowl of the Xiangzhou kiln: (1) H1:114; (2) H1:122; and (3) H1:123.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7310003/v1/9c2ff547a6e428f1272f385d.jpg"},{"id":89280727,"identity":"8ef06afb-9cdc-421d-827d-95b458a79efe","added_by":"auto","created_at":"2025-08-18 10:28:40","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":31027,"visible":true,"origin":"","legend":"\u003cp\u003eStatistical diagram of firing defects in each abandoned pit of the Xiangzhou kiln\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7310003/v1/5b6cb83c852ab5b418181de0.jpg"},{"id":89280734,"identity":"225ec625-f2af-4afe-a4c2-3a0b5baf23c2","added_by":"auto","created_at":"2025-08-18 10:28:40","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":101826,"visible":true,"origin":"","legend":"\u003cp\u003eDefects in the firing of the body of green porcelain in the Xiangzhou kiln:\u003c/p\u003e\n\u003cp\u003eBody underfiring, H1:22; (2) kiln adhesion, H1:7; (3) deformation, H1:8; (4) kiln-induced cracking, H1:104; (5) hidden cracks, H4:1; (6) body bloating, H1:55; (7) delamination, H1:124; and (8) structural separation H4:50.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7310003/v1/cf9be02cc4bf11b778519902.jpg"},{"id":89281118,"identity":"43540631-8995-4eb6-b7ff-f3ed9c97ae60","added_by":"auto","created_at":"2025-08-18 10:36:40","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":96007,"visible":true,"origin":"","legend":"\u003cp\u003eDefects in the firing of the glaze of green porcelain in the Xiangzhou kiln:\u003c/p\u003e\n\u003cp\u003e(1) Hidden yellowing, H1:132; (2) loss of gloss, H2:28; (3) speckles, H1:138; (4) glaze contamination, H2:25; (5) smoke staining, H1:108; (6) glaze holes, H1:34; (7) pinholes, H1:25; (8) glaze peeling, H2:6; (9) glaze shrinkage, H2:22; (10) thin glaze, H1:82; (11) glaze deficiency, H4:24; and (12) glaze runs, H2:7.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7310003/v1/00b4c0e3aec9382b03c3b8ce.jpg"},{"id":89280732,"identity":"e50ad9c2-29be-4c1d-87ed-65dfd5016787","added_by":"auto","created_at":"2025-08-18 10:28:40","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":52852,"visible":true,"origin":"","legend":"\u003cp\u003eCeladon fired in the Xiangzhou kiln:\u003c/p\u003e\n\u003cp\u003e(1 and 2) smoke staining, H1:25 and H1:81, respectively; (3) normal firing, H1:32; (4) loss of gloss, H2:28; (5) hidden yellowing, H1:132; and (6 and 7) underfiring, H1:108 and H1:22, respectively.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7310003/v1/8415da58a09049775f45bf08.jpg"},{"id":89283100,"identity":"3d647a59-2b0b-4e11-b9c6-c72e374523c4","added_by":"auto","created_at":"2025-08-18 10:52:40","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1111928,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7310003/v1/f08a57f4-2cf6-4490-839a-ba6f104f7730.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Archaeological Observation of Firing Defects in Xiangzhou Kiln Porcelain","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eCeramics, representing multi-millennia wisdom of Chinese civilisation, epitomise the artistic interplay of aqueous and pyrolytic processes while embodying profound historical, artistic and scientific values. The spiritual connotations manifested through ceramic artefacts authentically reflect ancient Chinese communities\u0026rsquo; aspirations for prosperous living and their persistent quest for aesthetic ideals. Throughout ceramic production workflows, from raw material procurement, forming techniques, desiccation protocols, decorative applications, firing regimes to post-firing handling and transportation, any procedural deviation may cause critical defects, rendering ceramic products commercially non-viable [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Given the inherent variability in production contexts across regional kiln complexes, disparities in raw material selection, technical processes and decorative implementations have led to provenance-specific manifestations of ceramic firing defects. Consequently, systematic analysis of these imperfections becomes imperative for the reconstruction of the operational sequences and craft traditions particular to individual historical kiln sites.\u003c/p\u003e\u003cp\u003eThe Xiangzhou kiln complex is located along the southern bank of the Huan River near Anyang Bridge in present-day Anyang City, Henan Province, mainly distributed across the former Anyang Battery Factory site and adjacent areas. Recognised as the largest celadon production centre in northern China during the Northern and Southern Dynasties (420\u0026ndash;589 CE) to the Sui-Tang period (581\u0026ndash;907 CE), it also represents one of the earliest northern kiln complexes engaged in white porcelain manufacturing, holding a pivotal status in Chinese ceramic history [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFrom October 2008 to May 2009, a salvage excavation was performed at the site in response to the construction of Phase I of the Anyang Battery Factory Residential Complex. This marks the third archaeological excavation of the Xiangzhou kiln, yielding 20 systematically arranged excavation units covering a total area of 1,300 m\u003csup\u003e2\u003c/sup\u003e. The investigation recovered 705 diagnostic ceramic artefacts and substantial quantities of kiln furniture (e.g. saggers and spacers), providing critical material evidence for the reconstruction of production-scale parameters [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eDuring the 2009AYTYGJT1 excavation campaign, six ash pits (H1\u0026ndash;H6) stratigraphically underlying Layer ③ were identified, alongside two modern backfill pits (K1 and K2). The stratigraphic sequence demonstrates that H4 intrudes into H5 and H6 while being subsequently disturbed by modern backfill pit K1. Although substantial quantities of the Xiangzhou kiln ceramics were recovered from this excavation, features H3, H5 and H6 proved artefact-sterile. This study therefore focuses on the archaeologically productive units H1, H2 and H4, which yielded substantial archaeological assemblages.\u003c/p\u003e\u003cp\u003eFeature H1 presents an elliptical planform, centrally positioned within excavation unit T1, conforming to a bell-shaped pit morphology with constricted opening and expanded basal dimensions (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e:1). The stratigraphic unit yielded a considerable assemblage of ceramic artefacts, comprising 146 celadon specimens and 9 white porcelain items, alongside subsidiary findings, including ceramic sherds and fired clay lumps. Typological classification demonstrates multiple vessel forms: triple-lug jars, quadruple-lug jars, stemmed plates, vessel lids, bowls, dou stemmed vessels, bottles, cups and bowls (\u003cem\u003ebo\u003c/em\u003e type), exhibiting comprehensive morphological diversity within the ceramic repertoire.\u003c/p\u003e\u003cp\u003eFeature H2 exhibits a circular planform located in the southwestern sector of T1, demonstrating an analogous bell-shaped pit morphology with constricted aperture and expanded basal configuration, maintaining sub-horizontal basal inclination (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e:2). The assemblage predominantly comprises kiln furniture, including substantial quantities of triangular stilts, annular spacers, kiln wall fragments and fired clay lumps, with subsidiary ceramic sherds. The archaeological recovery yielded 45 celadon specimens typologically classified into bowls, cups, saucers, dou stemmed vessels, vessel lids and bottles, delineating a specialised production waste deposit pattern.\u003c/p\u003e\u003cp\u003eFeature H4 exhibits an irregular planform located in the east-central sector of T1, having been intruded by modern backfill pit K1. This context exclusively yielded 54 celadon-glazed porcelain specimens, with vessel forms primarily comprising saucers, plates, bowls, dou stemmed vessels and cups.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBased on integrated analysis of the morphological configurations and associated artefact assemblages of ash pits, it is proposed that the three features (H1, H2 and H4) likely functioned as discard deposits associated with ceramic production activities. This type of analogous bell-shaped discard features has been systematically documented at kiln sites, as evidenced by the discovery of circular bell-shaped pits H2 and H17 in Section I north of Xiaohuangye Bridge at the Tang Dynasty (618\u0026ndash;907 CE) Gongyi kiln site in Henan Province. These pits exhibited sub-horizontal bases and yielded ceramic artefacts, including ewers, jars and stoves [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. At the Song Dynasty (960\u0026ndash;1279 CE) Liujiamen Jun kiln site in Yuzhou City, Henan Province, the identified Feature H2 was located in the southeastern sector of the excavation grid, presenting a sub-circular bell-shaped pit morphology. This context yielded a substantial quantity of celadon-glazed porcelain artefacts, with representative specimens including celadon-glazed shallow-bellied plates and Jun-type ewers [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Within the published excavation materials from the Liao Dynasty (907\u0026ndash;1125 CE) Chifeng Gangwayao kiln site in Inner Mongolia, three ash pit features (H25, H56 and H18) demonstrated particular typological importance. These contexts contained substantial quantities of discarded ceramic products, commonly interpreted as forming within a relatively short temporal framework [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Feature H8 at the early Yuan Dynasty (1271\u0026ndash;1368 CE) Waguantan kiln site in Tianzhu County, Guizhou Province, exhibited a sub-circular planform with slightly constricted upper walls transitioning to a bell-shaped profile. The eastern wall displays an irregular morphology, while the base maintains a sub-horizontal inclination, collectively presenting a relatively crude configuration devoid of discernible anthropogenic modification on its rim, base or walls. The context yielded numerous restorable ceramic vessels, sherds and kiln furniture, predominantly in fragmented or deformed states. The assemblage is typologically dominated by bowls, plates and small cups (zhan), with subsidiary jars and cups. Kiln furniture includes pedestals, spacers and supporting beads. This feature is interpreted as an accumulation deposit resulting from successive infilling of discarded ceramics and kiln implements [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eDuring the ceramic firing process, variations in kiln temperature and the properties of body and glaze materials may induce various types of defects in the finished products. Based on existing literature and relevant studies, the firing defects observed in ceramics can be systematically categorised into several dozen types, as presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eAncient ceramic manufacturing defects\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBody Defects\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDeformation, kiln adhesion, body bloating, underfiring, overfiring, blistering, black core, kiln-induced cracking, hidden cracks, separation of upper and lower sections\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eGlaze Defects\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePinholes, glaze holes, speckles, glaze peeling, orange peel texture, glaze deficiency, glaze shrinkage, thin glaze, glaze runs, glaze ripples, glaze contamination (slag inclusion), glaze bubbles, glaze cracking, colour distortion, fire marks, colour contamination, decorative flaws, hidden yellowing, smoke staining, loss of gloss, crystallisation\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eStatistical analysis of the excavated assemblage revealed a total of 256 ceramic artefacts. The collection included 9 white-glazed porcelain specimens, comprising 5 bowls, 3 cups and 1 bottle, accounting for 3.52% of the total assemblage. These specimens represented high-quality products devoid of discernible defects and were therefore excluded from subsequent discussions of firing defects. The remaining 247 celadon-glazed porcelain artefacts constituted 96.48% of the assemblage, with the following typological distribution: 127 bowls, 79 cups, 10 dou stemmed vessels, 9 \u003cem\u003ebo\u003c/em\u003e bowls, 2 jars, 3 saucers, 5 bottles, 2 stemmed plates, 1 plate and 9 vessel lids.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe body defects identified included underfiring, kiln-induced cracking, kiln adhesion, hidden cracks, delamination, deformation, body bloating and separation of upper and lower sections, totalling 99 artefacts. Among these, kiln adhesion was the most frequently occurring defect, followed by underfiring and deformation. The glaze defects comprised glaze peeling, glaze shrinkage, thin glaze, glaze deficiency, smoke staining, glaze contamination, speckles, pinholes, glaze holes, glaze runs, hidden yellowing and loss of gloss, observed in 69 artefacts. Speckles constituted the predominant glaze defect, followed by hidden yellowing and glaze shrinkage. In addition, 79 artefacts exhibited combined body and glaze defects (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2. Research Objectives","content":"\u003cp\u003eThrough statistical analysis and investigation of firing defects in ceramics excavated from this kiln site, this study aims to enhance understanding of various aspects of ceramic production technology, decorative techniques and evolutionary changes in firing processes at the Xiangzhou kiln during the Sui-Tang period (581\u0026ndash;907 CE). Furthermore, it seeks to provide deeper insights into the organisation and development of ceramic handicraft in the Xiangzhou region. The study focuses on artefacts recovered during the third archaeological excavation campaign, employing systematic observation and documentation of body and glaze defects. This approach facilitates archaeological analysis and investigation of firing-related imperfections in the ceramic assemblage.\u003c/p\u003e"},{"header":"3. Materials","content":"\u003cp\u003eSelected ceramic samples excavated from the Xiangzhou kiln site were analysed in conjunction with their firing defects [10\u0026ndash;13]. The sample characteristics are presented as follows:\u003c/p\u003e\n\u003cdiv id=\"Sec4\"\u003e\n \u003ch2\u003e3.1 Body\u003c/h2\u003e\n \u003cdiv id=\"Sec5\"\u003e\n \u003ch2\u003e3.1.1 Body Underfiring\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:22 represents a celadon-glazed cup. The vessel demonstrates complete interior and partial exterior glaze coverage, with the exterior glaze terminating above the foot. The glaze surface exhibits a matte and coarse texture (Fig. 5:1).\u003c/p\u003e\n \u003cp\u003eAmong the 247 celadon porcelain artefacts, 66 were underfired, accounting for ~\u0026thinsp;27% of the total. This group comprised 26 bowls, 37 cups, 2 bottles and 1 lid. Underfiring represents a notable and typical defect in the porcelain firing process. Its fundamental cause lies in the failure of ceramics to receive sufficient thermal energy input within the kiln mainly manifested as either inadequate peak firing temperature or insufficient soaking time at a high temperature. This prevents the body and glaze from completing the necessary physical and chemical reactions, resulting in an overall \u0026lsquo;immature\u0026rsquo; state. Characteristics underfired artefacts include a porous and friable body with high porosity, a dull sound when struck and a rough, lusterless glaze surface due to incomplete melting. The most immediate cause of underfiring is the failure to reach the critical firing temperature, which means that the kiln temperature did not attain the melting point of the glaze. Uneven temperature distribution within the kiln chamber is another contributing factor. In addition, improper arrangement of kiln furniture, as well as insufficient purity in the raw material composition of the body and glaze, can also contribute to the occurrence of underfiring.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec6\"\u003e\n \u003ch2\u003e3.1.2 Kiln Adhesion\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:7 represents a celadon-glazed bowl. The interior is fully glazed, with three kiln-adhered spur marks at the base centre. These spurs exhibit elliptical contact surfaces measuring 0.8 cm (major axis) \u0026times; 0.5 cm (minor axis) (Fig. 5:2).\u003c/p\u003e\n \u003cp\u003eAmong the 247 celadon porcelain artefacts, kiln sticking affected 89 pieces, representing the most frequent defect, accounting for ~\u0026thinsp;36% of the total. This sub-set comprised 64 bowls, 7 contracted-mouth bowls (\u003cem\u003ebo\u003c/em\u003e), 9 cups, 2 jars, 5 stemmed dishes (dou), 1 bottle and 1 plate. Kiln sticking is a defect that occurs when porcelain fuses to kiln furniture or adjacent vessels during firing. The primary cause involves uncontrolled glaze flow under high-temperature conditions: when kiln temperatures exceed the maximum required threshold of the glaze or when high-temperature soaking time is prolonged, the viscosity of the glaze decreases and fluidity increases, markedly increasing the risk of adhesion. The secondary cause is that the excessive flux content in the glaze composition lowers its melting point, exacerbating flow tendencies. Improper use of kiln furniture and firing atmosphere further contribute to the occurrence of this defect.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec7\"\u003e\n \u003ch2\u003e3.1.3 Deformation\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:8 represents a celadon-glazed bowl. The vessel demonstrates deformation in both body and rim sections (Fig. 5:3).\u003c/p\u003e\n \u003cp\u003eDeformation defects were documented in 16 artefacts, 7 comprising bowls, 4 stemmed dishes (dou), 2 contracted-mouth bowls (\u003cem\u003ebo\u003c/em\u003e), 1 cup, 1 bottle and 1 small dish. Deformation in porcelain production denotes a defect where the body, which is in a softened state during high-temperature firing, loses structural stability under imbalanced mechanical stress. This defect results from factors spanning the entire manufacturing sequence, from raw material preparation to final kiln processes. Root causes for deformation include structural deficiencies in the green body, impractical vessel morphology and improper raw material processing. The contributing factors for this defect encompass uneven shrinkage stress from inadequate drying control, thermal gradients within the kiln chamber and sub-optimal stacking arrangements during kiln loading.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec8\"\u003e\n \u003ch2\u003e3.1.4 Kiln-induced Cracking\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:104 represents a celadon-glazed \u003cem\u003ebo\u003c/em\u003e bowl. Both the interior and exterior rims exhibit kiln-induced cracks, with the longest measuring 5.5 cm (Fig. 5:4).\u003c/p\u003e\n \u003cp\u003eKiln-induced cracking defects were identified in five artefacts, comprising 3 contracted-mouth bowls (\u003cem\u003ebo\u003c/em\u003e), 1 bowl and 1 cup. Crazing represents one of the most prevalent and complex defects in ceramic firing, characterised by the development of surface or internal fissures in porcelain owing to temperature fluctuations or internal stress during the firing process. Rapid thermal shifts are the primary catalyst for such cracking. During the heating or cooling phase, accelerated temperature changes generate differential thermal expansion stress between the body\u0026rsquo;s interior and surface, subsequently initiating crack formation. Insufficient time allocated for uniform heating or cooling of the ceramic body during firing further predisposes wares to crazing. Additional contributing factors include the structural integrity and morphology of the green body, compositional ratios of raw materials and the prevailing firing atmosphere within the kiln.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec9\"\u003e\n \u003ch2\u003e3.1.5 Hidden Cracks\u003c/h2\u003e\n \u003cp\u003eSpecimen H4:1 represents a celadon-glazed dou vessel. Hidden cracks are observed on the interior and exterior surfaces of the bowl, with the longest measuring 3.7 cm (Fig. 5:5).\u003c/p\u003e\n \u003cp\u003eSubsurface micro-cracks were documented in two artefacts: 1 stemmed dish (dou) and 1 contracted-mouth bowl (\u003cem\u003ebo\u003c/em\u003e). During firing, these hairline fissures develop internally or on ceramic surfaces, often remaining imperceptibly fine. Their formation primarily correlates with thermal fluctuations and uneven thermal expansion during kiln cycles. As temperatures rise, disparate heating of body zones generates differential expansion rates, producing internal stresses that propagate micro-fractures within the ceramic matrix.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec10\"\u003e\n \u003ch2\u003e3.1.6 Body Bloating\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:55 represents a celadon-glazed cup. The base centre shows accidental adhesion of black\u0026ndash;brown glaze, with adjacent body bloating measuring 0.19 cm in length and 0.39\u0026ndash;0.41 cm in width (Fig. 5:6).\u003c/p\u003e\n \u003cp\u003eBody bursting defects were recorded in 13 artefacts, comprising 8 cups and 5 bowls. This phenomenon involves the sudden rupture or explosive fracture of ceramic bodies during firing mainly caused by internal gas expansion or uneven thermal stress. The residual moisture trapped within the green body undergoes rapid vaporisation upon heating, generating substantial vapour pressure. When the exterior clay layer hardens faster than the interior\u0026mdash;creating permeability barriers\u0026mdash;the pressurised steam cannot escape efficiently, resulting in explosive failure. Contributing factors include firing parameters (temperature and ramp rate), raw material formulation and structural lamination gradients within the green body.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec11\"\u003e\n \u003ch2\u003e3.1.7 Delamination\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:124 represents a celadon-glazed bowl. The body cross-section demonstrates delamination measuring 0.36\u0026ndash;1.03 cm in length and 0.03\u0026ndash;0.16 cm in width (Fig. 5:7).\u003c/p\u003e\n \u003cp\u003eLaminar separation defects were observed in four artefacts: 2 bowls and 2 cups. This defect manifests as planar delamination within the porcelain body post-firing, fundamentally resulting from the complete failure of interlayer bonding forces at elevated temperatures. The formation of delamination is correlated with multiple factors, mainly attributed to thermal inhomogeneity during firing. Differential heating of the ceramic body causes incomplete vitrification in thermally disadvantaged zones, generating structural discontinuities. In addition, excessive residual moisture or heterogeneous raw material distribution predisposes the ware to incomplete sintering, further facilitating interlayer separation.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec12\"\u003e\n \u003ch2\u003e3.1.8 Separation of Upper and Lower Sections\u003c/h2\u003e\n \u003cp\u003eSpecimen H4:50 represents a celadon-glazed \u003cem\u003edou\u003c/em\u003e stemmed vessel. The bowl section is completely separated from the stem. Glaze application unevenly extends to the mid-stem area, accompanied by visible glaze runs (Fig. 5:8).\u003c/p\u003e\n \u003cp\u003eVertical delamination defects were noted in three artefacts: 1 stemmed plates (gaozupan) and 1 stemmed dish (dou). This phenomenon arises during firing when thermal gradients, structural inconsistencies or compositional heterogeneity compromise interfacial bonding between the upper and lower sections of the ceramic body. Disparate heating across vertical zones generates differential thermal expansion between the upper and lower sections, exceeding critical stress thresholds and inducing mechanical separation. Contributing factors include notable temperature differentials between layers during forming stages or inadequate interfacial bonding prior to firing.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\"\u003e\n \u003ch2\u003e3.2 Glaze\u003c/h2\u003e\n \u003cdiv id=\"Sec14\"\u003e\n \u003ch2\u003e3.2.1 Hidden Yellowing\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:132 represents a celadon-glazed bowl demonstrating yellowish discolouration in the glaze and body cross-section (Fig. 6:1).\u003c/p\u003e\n \u003cp\u003eAmong the 247 celadon porcelain artefacts, yellowish discolouration affected 28 pieces, accounting for ~\u0026thinsp;11% of the total assemblage. This included 18 cups, 9 bowls and 1 bottle. Yellowish discolouration refers to an uneven, dull-yellowish surface manifestation occurring during ceramic firing. Inappropriate firing temperature constitutes a primary cause: sub-optimal thermal conditions prevent sufficient sintering of the body surface, enabling localised accumulation of iron oxides or other impurities. These compounds undergo incomplete oxidation at high temperatures, resulting in yellowish surfaces. In addition, insufficient oxygen supply during firing contributes substantially to this defect. Metallic oxides, such as iron and copper, may exhibit yellow to deep-yellow hues under reducing conditions, imparting a dull or yellowish appearance to the ceramic surface.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec15\"\u003e\n \u003ch2\u003e3.2.2 Loss of Gloss\u003c/h2\u003e\n \u003cp\u003eSpecimen H2:28 represents a celadon-glazed bowl exhibiting bluish glaze tones with complete loss of surface gloss (Fig. 6:2).\u003c/p\u003e\n \u003cp\u003eGloss impairment was reported in 10 artefacts, comprising 8 bowls and 2 cups. This defect manifests as a lack of surface lustre or matte appearance, resulting in visually unappealing, dull ceramics. The phenomenon typically occurs when the glaze undergoes incomplete vitrification or reactions, often due to inadequately controlled firing parameters that disrupt the development of surface gloss. A primary causative factor is sub-optimal firing temperature or thermal mismanagement. When kiln temperatures fail to reach the glaze\u0026rsquo;s required melting threshold, the glaze cannot fully fuse into a vitreous layer, yielding matte or non-reflective surfaces. In addition, compromised glaze formulation quality and improper surface preparation of the green body are notable contributing factors.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec16\"\u003e\n \u003ch2\u003e3.2.3 Speckles\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:138 represents a celadon-glazed cup. Black\u0026ndash;brown speckles are distributed throughout the vessel body (Fig. 6:3).\u003c/p\u003e\n \u003cp\u003eAmong the 247 celadon porcelain artefacts, speckling defects were documented in 40 pieces, representing the most prevalent defect, accounting for ~\u0026thinsp;16% of the assemblage. The affected forms comprised 24 bowls, 9 cups, 2 contracted-mouth bowls (\u003cem\u003ebo\u003c/em\u003e), 2 bottles, 1 jar, 1 stemmed dish (dou) and 1 small dish. Speckling manifests as irregular localised discolorations or spot-like blemishes on ceramic surfaces, resulting from inhomogeneous pigmentation owing to glaze composition variations or atmospheric fluctuations during firing. Sub-optimal glaze formulation is a primary causative factor. Metallic oxides (e.g. iron and copper) within the glaze may undergo localised enrichment under conditions of thermal unevenness and oxygen deficiency during firing, generating chromatic aberrations or speckling. In addition, inadequate temperature control markedly contributes to this defect. Further predisposing factors include instability in the kiln atmosphere and compositional heterogeneity of raw materials.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec17\"\u003e\n \u003ch2\u003e3.2.4 Glaze Contamination (Slag Inclusion)\u003c/h2\u003e\n \u003cp\u003eSpecimen H2:25 represents a celadon-glazed cup. The interior is fully glazed, demonstrating suspected slag inclusions (glaze contamination) on the lower belly (Fig. 6:4).\u003c/p\u003e\n \u003cp\u003eSurface contamination defects were documented in 18 artefacts, predominantly co-occurring with other defects. The assemblage comprised 7 bowls, 5 lids, 3 stemmed dishes (dou), 2 cups and 1 plate. This defect manifests as irregular accretions of foreign matter on ceramic surfaces during firing, typically appearing as black specks, greyish blotches or chromatic patches.\u003c/p\u003e\n \u003cp\u003ePrimary sources include impurities inadvertently introduced into raw materials, glaze compounds or body surfaces\u0026mdash;such as iron scale, coal ash particulates or fine mineral grains\u0026mdash;that resist complete melting at peak temperatures and persist as embedded contaminants. In addition, airborne kiln pollutants (e.g. soot and smoke particulates) deposit onto ware surfaces under high-temperature conditions, forming adherent residues. Moreover, sub-optimal material formulation or uneven glaze application creates irregular surface topography. Excessive glaze thickness particularly entrains impurities during application, ultimately manifesting as speckling.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec18\"\u003e\n \u003ch2\u003e3.2.5 Smoke Staining\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:108 represents a celadon-glazed cup and exhibits distinct reduction staining with smoke-derived ferruginous speckling (Fig. 6:5).\u003c/p\u003e\n \u003cp\u003eSmoke staining defects were documented in 13 artefacts, predominantly co-occurring with other defects. The assemblage comprised 5 bowls, 4 cups, 2 jars and 2 lids. This phenomenon manifests as carbonaceous deposits on ceramic surfaces resulting from exposure to smoke or reducing gases during firing, presenting as black to dark-brown speckles, streaking or irregular tonality. The defect formation is mainly attributable to fuel combustion dynamics and oxygen availability. Insufficient oxygen supply during firing promotes incomplete fuel combustion, generating substantial smoke and carbon monoxide\u0026ndash;rich reducing atmospheres. These by-products deposit particulate matter onto ware surfaces, forming carbon-embedded discolorations. At the Xiangzhou kiln, wood fuel usage aggravated this issue: its inherent propensity for incomplete combustion released smoke and fine carbon particulates that readily adhered to ceramics, resulting in pervasive smoke staining.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec19\"\u003e\n \u003ch2\u003e3.2.6 Glaze Holes (Melting Voids)\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:34 represents a celadon-glazed \u003cem\u003ebo\u003c/em\u003e bowl. The glaze surface displays melting voids, pinholes and a distinct black slag inclusion mark (Fig. 6:6).\u003c/p\u003e\n \u003cp\u003eGlaze pitting defects were documented in 19 artefacts, predominantly co-occurring with other defects. The assemblage comprised 12 bowls, 3 lids, 2 small dishes, 1 contracted-mouth bowl (\u003cem\u003ebo\u003c/em\u003e) and 1 jar. Pitting arises from localised over-fusing or heterogeneous vitrification of the glaze or body at elevated temperatures, manifesting as surface cavities or depressions. This defect correlates primarily with thermal profiles, glaze formulation and application thickness. Excessively high firing temperatures may induce glaze over-fusion, generating surface voids. Similarly, disproportionate low-melting constituents in glaze recipes or compositional inhomogeneity can induce excessive fluidity during firing, facilitating pit formation. Moreover, thick glaze applications or incomplete degassing of entrapped bubbles during vitrification may generate subsurface voids that erupt as surface pits under thermal stress.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec20\"\u003e\n \u003ch2\u003e3.2.7 Pinholes\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:25 is a celadon-glazed vessel lid. The lid surface contains numerous glaze holes (melting voids) and pinholes (Fig. 6:7).\u003c/p\u003e\n \u003cp\u003eA single pinhole defect was documented on a lid artefact. Pinholes manifest as minute, deep cylindrical voids on ceramic surfaces, resembling needle punctures in their dense, uniform distribution. Their formation is mainly associated with gas entrapment, glaze inhomogeneity, thermal mismanagement and body flaws. During firing, the bubbles trapped within the body or glaze fail to escape efficiently; these may expand under high temperatures, rupturing the surface to form micro-voids. Compositional heterogeneity in the glaze or improper application can create localised thickness variations or undissolved particulates that nucleate pinholes. In addition, sub-optimal firing temperatures or uneven heating prevent complete glaze maturation, inhibiting bubble release and promoting pinhole formation.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec21\"\u003e\n \u003ch2\u003e3.2.8 Glaze Peeling\u003c/h2\u003e\n \u003cp\u003eSpecimen H2:6 represents an underfired celadon-glazed bowl. Although originally intended to be fully glazed, the vessel demonstrates extensive glaze peeling, with only residual glaze remaining near the rim area (Fig. 6:8).\u003c/p\u003e\n \u003cp\u003eGlaze peeling defects were documented on two bowl artefacts. This phenomenon manifests as delamination or spalling of the glaze layer due to insufficient adhesive strength between the glaze and the body. Primary causative factors for this defect include inadequate surface preparation of the green body compromising glaze adhesion; sub-optimal glaze formulation featuring compositional heterogeneity or excessively low melting points that inhibit proper bonding; thermal mismanagement during firing, including inappropriate ramp rates, generating differential expansion stresses at the glaze\u0026ndash;body interface; and excessive glaze thickness or high absorbency of the bisque body promoting interfacial failure.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec22\"\u003e\n \u003ch2\u003e3.2.9 Crawling\u003c/h2\u003e\n \u003cp\u003eSpecimen H2:22 represents a celadon-glazed impressed saucer. The exterior glaze extends to the lower belly, demonstrating glaze runs and shrinkage near the belly area (Fig. 6:9).\u003c/p\u003e\n \u003cp\u003eAmong the 247 celadon-glazed ceramic specimens examined, 33 artefacts exhibited crawling defects, accounting for ~\u0026thinsp;13% of the assemblage. This phenomenon frequently co-occurred with other production flaws. The affected forms comprised 26 bowls, 3 cups, 1 \u003cem\u003ebo\u003c/em\u003e (wide-mouthed vessel), 1 lid, 1 bottle and 1 dou (stemmed vessel).\u003c/p\u003e\n \u003cp\u003eCrawling denotes surface irregularities arising from excessive glaze contraction during the firing process, manifesting as wrinkles, cracks or uneven gloss. Primary causative factors for crawling include thermal expansion coefficient mismatch between the glaze and the ceramic body, excessive glaze thickness, inadequate regulation of firing temperatures and sub-optimal glaze formulation. When substantial disparity exists between the thermal expansion coefficients of the glaze and body, temperature variations induce uneven contraction of the glaze surface, producing an irregular finish. Furthermore, excessively thick glaze layers undergo pronounced shrinkage during the cooling phase, readily forming cracks or wrinkles due to differential stress distribution.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec23\"\u003e\n \u003ch2\u003e3.2.10 Thin Glaze\u003c/h2\u003e\n \u003cp\u003eSpecimen H1:82 is a celadon-glazed \u003cem\u003ebo\u003c/em\u003e bowl. Thin glaze and uneven slip application were observed (Fig. 6:10).\u003c/p\u003e\n \u003cp\u003eOnly one artefact demonstrated thin glaze, identified as a \u003cem\u003ebo\u003c/em\u003e (wide-mouthed bowl). Thin glaze refers to an excessively thin glaze layer, typically resulting in insufficient surface gloss, uneven colouration and localised glaze skipping. Primary causes responsible for this include uneven glaze application, over-dilution of glaze slurry, unsuitable glaze formulation and improper firing temperature or cooling control. When the glaze application is uneven, it may fail to uniformly cover the body, resulting in excessively thin glaze layers. If the glaze slurry is over-diluted, the glaze thickness will also be insufficient. In addition, in certain glaze formulations, low-melting components or excessive fluidity may cause the glaze to flow too rapidly, thereby forming thin layers. During firing, excessively high temperatures or overly rapid cooling can also induce excessive glaze flow or contraction, causing glaze thinning.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec24\"\u003e\n \u003ch2\u003e3.2.11 Glaze Deficiency\u003c/h2\u003e\n \u003cp\u003eSpecimen H4:24 represents a celadon-glazed bowl. The interior is fully glazed, except for a glaze-deficient base (Fig. 6:11).\u003c/p\u003e\n \u003cp\u003eTwo artefacts exhibited glaze skipping, comprising 1 bowl and 1 \u003cem\u003ebo\u003c/em\u003e (wide-mouthed bowl). Glaze skipping refers to areas on the ceramic surface that failed to achieve complete glaze coverage, exposing unglazed body material, which compromises aesthetic integrity and functionality. Contributing factors for this include uneven glaze application, problematic glaze slurry viscosity, inappropriate firing temperatures, inadequate body surface preparation and sub-optimal glaze formulation. Uneven glaze application may cause localised glaze skipping, while over-diluted glaze slurry can lead to inconsistent coating. Inadequate or excessive firing temperatures may result in insufficient glaze melting or overly rapid flow, forming glaze-deficient areas. Concurrently, surface contamination of the body or improper glaze formulation can compromise glaze adhesion, thereby causing skipping. Glaze skipping not only diminishes ceramic aesthetics, producing rough surfaces and gloss deficiency, but also compromises functional properties such as water resistance and corrosion resistance.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec25\"\u003e\n \u003ch2\u003e3.2.12 Glaze Runs\u003c/h2\u003e\n \u003cp\u003eSpecimen H2:7 represents a celadon-glazed bowl. The exterior glaze extends to the lower belly, exhibiting six distinct glaze run marks appearing as raised black\u0026ndash;brown circular droplets, some extending to the foot (Fig. 6:12).\u003c/p\u003e\n \u003cp\u003eGlaze runs were observed on 28 artefacts, predominantly co-occurring with other defects. The affected forms comprised 16 bowls, 6 cups, 2 stemmed cups, 2 dishes, 1 bottle and 1 lid. Glaze runs refer to fine linear striations formed during ceramic firing, typically manifesting as elongated thread-like marks resulting in rough and uneven glaze surfaces with compromised gloss. The formation of glaze runs is attributed to inhomogeneous glaze composition, uneven glaze application, excessively high firing temperatures or overly rapid heating rates, excessively rapid cooling and excessively high glaze viscosity. When the compositional distribution within the glaze is uneven, striations or trails may form during firing. Uneven or excessively thin glaze application causes differential glaze flow, resulting in trail generation. Excessively high temperatures or overly rapid heating induce excessive glaze mobility, forming linear trails. Excessively rapid cooling triggers surface contraction that produces texturing. In addition, glaze slurries with excessive viscosity demonstrate poor flow characteristics during application, potentially resulting in striation formation.\u003c/p\u003e"},{"header":"4. Results and Discussion","content":"\u003cp\u003eBased on the foregoing analysis, the primary causes of defects in artefacts excavated from the Xiangzhou kiln site may be categorised into two groups: improper firing temperature control and firing atmosphere issues. The majority of firing defects correlate with inadequate temperature regulation, while the kiln atmosphere constitutes a critical contributing factor to defect formation.\u003c/p\u003e\n\u003cp\u003eDuring the firing process, ceramics undergo a series of chemical and physical transformations, with phase-specific thermal conditions critically determining the final product performance and quality. The firing temperature for the Xiangzhou kiln ceramics is typically\u0026thinsp;\u0026gt;\u0026thinsp;1100\u0026deg;C [\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]. Archaeological evidence from kiln sites indicates a high prevalence of underfired specimens, likely attributed to considerable thermal discrepancies during firing. In cases of sub-optimal raw material formulations, failure to reach the target firing temperature (1100\u0026deg;C), insufficient holding duration at peak temperatures or excessive thermal gradients, the ceramic body exhibits elevated water absorption rates, dull acoustic responses and rough yellowish surfaces, which are characteristic defects of underfiring. Defects such as body softening and deformation, glaze blistering and vessel overfiring of the vessel may occur if the firing temperature is excessively high with prolonged heat preservation, causing the kiln temperature to exceed the required firing range for the body. During the low-temperature heating stage, an overly rapid temperature rise can lead to body cracking. The fracture surfaces of such cracks appear smooth due to glaze coverage, forming hidden crack defects. During the cooling stage, excessively fast cooling rates in the medium-temperature range\u0026mdash;particularly for thick-walled bodies and high-silica products\u0026mdash;may induce kiln-induced cracks due to residual quartz phase transformation.\u003c/p\u003e\n\u003cp\u003eThe ceramic defects manifest in a sequential spectrum from high to low firing temperatures as follows: smoke staining \u0026rarr; normal firing \u0026rarr; loss of gloss \u0026rarr; hidden yellowing \u0026rarr; underfiring (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eArchaeological excavations of the Xiangzhou kiln site revealed that its kiln structure was a semi-subterranean mantou-type kiln (rounded vault kiln) with a diameter of ~\u0026thinsp;1 m [\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e]. Characterised by simple construction and limited internal space, this kiln design influences specific atmospheric conditions during firing. When firing ceramics using wood fuel, incomplete oxidisation occurs if combustion is insufficient prior to glaze sealing. Residual carbonaceous and organic materials within the body deposit on surfaces, resulting in smoke staining defects. An excessively strong reducing atmosphere combined with prolonged heat preservation can lead to the entrapment of free carbon particulates within the glaze, resulting in densely distributed dark speckle defects characterised by large diameters. In addition, if the temperature increases too rapidly during firing, causing premature melting of the glaze and inadequate reducing conditions, the transformation of iron ions in the porcelain body will induce hidden yellowing defects.\u003c/p\u003e\n\u003cp\u003eDefects in ceramics occur due to not only operational errors during firing but also various mishaps in the forming stages, including raw material preparation, shaping, body trimming and drying. During kiln firing, pre-existing issues from the forming stages become progressively amplified, ultimately causing ceramic failure. From the perspective of the cha\u0026icirc;ne op\u0026eacute;ratoire, sequential progression from ceramic formation through firing enables the classification of defect causes into the following five types:\u003c/p\u003e\n\u003cp\u003eCategory I: Operational errors occurring during the raw material preparation stage. The porcelain bodies of the Xiangzhou kiln ceramics were crafted from kaolin clay sourced from the low mountainous and hilly areas surrounding present-day Shanying Town in southwestern Anyang [\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]. The production process involved sequential steps of material selection, crushing, levigation and sedimentation, with the processed clay then wedged into brick-shaped blocks before being shaped on the potter\u0026rsquo;s wheel. Due to the mineral composition of the raw materials, the Xiangzhou kiln\u0026rsquo;s clay contained relatively high concentrations of iron and titanium elements. Compared with porcelain clays from southern China, the Xiangzhou kiln\u0026rsquo;s clay body exhibits relatively coarser texture, greater thickness and inferior chromatic quality, typically appearing in bluish-grey tones with darker colouration. Consequently, during the refining process, the porcelain body of the Xiangzhou kiln underwent insufficient grinding and sieving. Combined with excessively strong reducing atmosphere in the kiln, iron impurities in the clay material developed colouration, resulting in irregularly sized spots in pale yellow, brownish and black hues on the ceramic surfaces. Non-uniform clay preparation leads to air pockets within the body, which can easily cause delamination defects at this stage. Improper body formulation or inadequate processing, causing uneven sintering and shrinkage, may also cause deformation defects. Insufficient sieving during the preparation of body and glaze materials enables fusible impurities to mix into the body. During firing, the combustion and melting of these materials result in glaze hole defects. The presence of impurities and hard particles (e.g. pebbles and quartz) within the body can lead to body bloating defects due to cracking during firing. Similarly, a considerable mismatch in the thermal expansion coefficients between the body and glaze may cause this defect.\u003c/p\u003e\n\u003cp\u003eTo compensate for the drawbacks of the porcelain clay, ceramics from the Xiangzhou kiln often applied a layer of white slip onto the body after forming. This process concealed the darker grey or greyish-brown body, creating a smoother and more refined surface. The application of slip enhanced glaze colouration and improved the aesthetic quality of the ceramic surfaces. The Xiangzhou kiln ceramics predominantly feature partial glaze coverage on their surfaces, with the application of slip constituting a distinctive technical feature. Among the celadon-glazed ceramics of the Xiangzhou kiln, uneven slip application is observed on exterior surfaces. However, no slip traces are detected on ceramics with fine-textured bodies and whitish colouration. The majority of these ceramics are light-coloured or white-glazed porcelain. Owing to the relatively pure and white body material, slip application was seldom adopted for surface refinement. This demonstrates that the Xiangzhou kiln selectively utilises slip to modify ceramic surfaces according to the specific quality of the body material.\u003c/p\u003e\n\u003cp\u003eCategory II: Operational errors occurring during the shaping stage. In the production of the Xiangzhou kiln ceramics, the wheel-throwing technique is predominantly employed for vessels such as bowls and plates. Appendages including spouts, handles and lugs were typically formed through moulding or hand-modelling. Bowls and cups were generally created via single-stage wheel throwing, while stemmed plates and other forms were created using a composite construction method: the dish and stem were separately wheel thrown and then joined using a slip mixture comprising porcelain clay and glaze materials. Inadequate adhesion between components can easily lead to separation defects in the upper and lower sections of the vessel. In addition, deformation defects may arise from either excessive or insufficient forming pressure during the shaping process.\u003c/p\u003e\n\u003cp\u003eCategory III: Operational errors occurring during the drying stage. During the drying process of ceramic artefacts, body deformation and cracking can be induced by a soft body, improper control of the drying regime or uneven shrinkage caused during drying. Owing to the notable variation in the thickness of the body in different parts of the artefact, the shrinkage rate and stress conditions during high-temperature firing are likely to be uneven. Improper drying can lead to the generation of internal stresses within the body, causing deformation during firing. When the body is still pliable, excessive stress can distort and deform the body. Once the surface of the body hardens, if the magnitude of the stress exceeds the strength of the body, it will result in cracking.\u003c/p\u003e\n\u003cp\u003eCategory IV: Operational errors occurring during the glazing stage. The Xiangzhou kiln employed dipping glazing as its primary method, supplemented by auxiliary techniques such as brush and swirling glazing. The process involved manually grasping the base of the vessel and inverting it into a glaze slurry bath to apply the coating. For common vessels such as bowls and cups, the interiors were typically fully glazed, demonstrating noticeable glaze accumulation within the cavities. Externally, glazing was often partial, terminating below the rim or extending only to the abdominal region, while the foot rims remained unglazed, exposing the raw clay body. The glaze surfaces typically exhibited fine crackling patterns, with predominant hues ranging from celadon, celadon-grey, to celadon-yellow, characterised by a transparent vitreous quality that revealed the underlying ceramic body. Thicker glaze layers exhibited deeper olive-green to dark-green shades, while thinner areas manifested as pale celadon or celadon-grey tones.\u003c/p\u003e\n\u003cp\u003eIf the relative density of the glaze slurry is too low, the resulting glaze layer during application exhibits insufficient thickness, leading to coarse surface textures, poor gloss retention and ultimately thin-glaze defects. Conversely, excessively high slurry density may cause poor adhesion at angular intersections of the ceramic body, thereby promoting crack propagation during the drying and firing stages. This condition frequently manifests as glaze cracking (crazing) and thickened glaze accumulation on the finished surface. In addition, inadequate bonding between the glaze and body prior to or during firing can result in glaze spalling, characterised by partial or complete detachment of the vitreous layer. An excessively high moisture content in the ceramic body prior to kiln entry or excessive steam generation during firing can compromise the bonding integrity between the body and glaze, causing glaze detachment and pinhole defects. Inadequate dispersion of glaze slurry during application may cause localised thickening of the glaze layer. Upon melting, this thickened glaze migrates to form linear ridges or circular cord-like structures, which is termed glaze veining. Prolonged glazing duration leads to excessive absorption of slurry by the body, creating trapped air bubbles within the glaze matrix. Such conditions further promote the formation of closed pores within thickened glaze layers, ultimately manifesting as pinhole defects.\u003c/p\u003e\n\u003cp\u003eCategory V: Operational errors occurring during the kiln loading and firing stage. As saggar firing was not yet adopted during this period, open firing remained the predominant method. Kiln workers adapted their approach based on the kiln\u0026rsquo;s internal structure and spatial configuration as well as the types of vessels being produced. They applied appropriate glaze slurries to various greenware pieces, which were then dried and loaded into the kiln for single high-temperature firing. In the production process, a combined system of kiln posts with various supporting implements (including spurs, spacer beads, setters and other kiln furniture) was used, and these tools were repeatedly used to enhance firing efficiency and output (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e). Some artefacts exhibit sand adhesion on their bases likely resulting from direct contact with the kiln floor. In addition, a stacking method was implemented, where multiple vessels were fired in direct contact without intermediate supports, leaving residual fragments from stacked pieces within the vessels [\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eIn open firing without saggars, irregular loading practices during greenware arrangement and kiln stacking caused the body surfaces to readily adhere to debris such as grog particles, sand grains, glaze slag and kiln ash, manifesting as surface contamination defects. Furthermore, due to the open firing method, improper kiln stacking disrupts the balanced spacing between greenware bodies, causing displacement of the greenware pieces and resulting in direct contact between adjacent bodies. An excessively dense arrangement of products with reduced or eliminated inter-vessel spacing inevitably leads to mutual adhesion between the rim exteriors or belly sections of adjacent vessels, resulting in kiln adhesion defects. The absence of saggars during firing enables the backflow of flue gases within the kiln to directly erode the glaze surfaces, resulting in blackened and greyish discolouration characterised by smoke staining.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThrough examining defective specimens from the Xiangzhou kiln, it is evident that improper control of glaze and body materials by kiln workers during production stages\u0026mdash;including raw material preparation, drying, body trimming, decoration, glazing and firing\u0026mdash;readily leads to ceramic defects such as delamination, deformation, edge slumping, underfiring and glaze shrinkage. These defects resulted in low firing success rates and substantial quantities of sub-standard products, which were ultimately deemed unmarketable as quality commodities and systematically discarded in concentrated deposits near the kiln site.\u003c/p\u003e\u003cp\u003eArchaeological research has shown that these ceramics with firing defects enable the reconstruction of historical loading and firing techniques. As one of the northern folk kilns produced celadon during its early phases, the Xiangzhou kiln exhibited certain rudimentary aspects in its ceramic technology. The prevalence of various defects in its ceramic products logically aligns with this technological context. Simultaneously, the Xiangzhou kiln was capable of producing mature white porcelain characterised by pure white and dense bodies without slip application, featuring lustrous and immaculate glazes. Therefore, the ceramic technology of the Xiangzhou kiln occupied an advanced level among contemporaneous kilns.\u003c/p\u003e\u003cp\u003eBased on the Xiangzhou kiln ceramics discovered during the 2009 third excavation campaign\u0026mdash;primarily unearthed from bell-shaped ash pits at the site and demonstrating pronounced firing defects\u0026mdash;these pits are inferred to be discard deposits for storing rejected ceramics. This phenomenon shows the emergence of specialised production in porcelain manufacturing during this period.\u003c/p\u003e\u003cp\u003eAs one of the early celadon kiln sites in northern China, the Xiangzhou kiln provides crucial insights into ceramic production processes through the analysis of firing defects observed in its sub-standard products. By categorising and statistically examining these defects, researchers can reconstruct the production stages of the Xiangzhou kiln\u0026rsquo;s ceramic industry and deduce the specific techniques and technologies used in its historical manufacturing practices. The Xiangzhou kiln provides substantial physical evidence for research on the typology of early ceramics, technological evolution in firing techniques and development of decorative craftsmanship in the Xiangzhou region. This evidence markedly enhances our understanding of the distribution patterns of ceramic kiln sites in the area, particularly regarding the origins of celadon and white-glazed porcelain production. These findings hold crucial importance for investigating the production systems, spatial distribution of Sui Dynasty (581\u0026ndash;618 CE) kilns as well as the genesis and development of early northern celadon and white porcelain.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003e2025 General Project for Humanities and Social Sciences Research in Henan Provincial Higher Education Institutions (Project Approval Number: 2025-ZDJH-418)\u003c/p\u003e\u003cp\u003ePostgraduate Education Reform and Quality Improvement Project of Henan Province (Project Approval Number: YJS2025XQLH40)\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM.J. and X. wrote the main manuscript text and J.K. prepared figures 1-8. All authors reviewed the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLu, X. R. Ceramic Technology. Changsha: Hunan University Press, 2005, p. 187.\u003c/li\u003e\n\u003cli\u003eKong, D. M. Research on the Xiangzhou Kiln in Anyang and Related Issues. Yindu Academic Journal 1 (2014) 34\u0026ndash;38.\u003c/li\u003e\n\u003cli\u003eAnyang Municipal Institute of Cultural Relics and Archaeology. Kneading Earth into Gold: New Archaeological Discoveries on the Xiangzhou Kiln and Its Celadon Wares. Zhengzhou: Zhongzhou Ancient Books Publishing House, 2018, pp. 4\u0026ndash;9.\u003c/li\u003e\n\u003cli\u003eLi, H. 2016. Ancient Lime Kilns and Various Porcelain Relics Discovered at the Gongyi Kiln Site in Henan. China Cultural Relics News August 26, 2016, p. 8.\u003c/li\u003e\n\u003cli\u003ePeking University Center for Chinese Archaeology Research, Henan Provincial Institute of Cultural Relics and Archaeology. Excavation Brief Report on the Liujiamen Jun Kiln Site in Shanghou Town, Yuzhou City, Henan Province. Cultural Relics 11 (2003) 26\u0026ndash;52.\u003c/li\u003e\n\u003cli\u003eGuo, Z. Z, Su, D. Highlights of Liao and Jin Porcelains Excavated from the Chifeng Gangwayao Kiln Site. In Chinese Ceramic Studies (Vol. 11). Beijing: Forbidden City Press. 2005.\u003c/li\u003e\n\u003cli\u003eDepartment of Archaeology, Sichuan University; Guizhou Provincial Institute of Cultural Relics and Archaeology; Tianzhu County Cultural Relics Management Office.Excavation Brief Report on the Yuan Dynasty Kiln Site at Waguantan in Tianzhu County, Guizhou Province. Archaeology 3 (2016) 45\u0026ndash;67.\u003c/li\u003e\n\u003cli\u003eYu, H, Yang, Z. Z. Fundamentals of Ancient Ceramic Restoration. Shanghai: Fudan University Press, 2014, pp. 25\u0026ndash;29.\u003c/li\u003e\n\u003cli\u003eShi, Q, Guo, Z. M, Hao, J. J. Research and Application Considerations on Defects in Traditional Ceramics. Chinese Ceramics 6 (2007) 36\u0026ndash;38.\u003c/li\u003e\n\u003cli\u003eFeng, X. M. Chinese Ceramics Dictionary. Beijing: Cultural Relics Publishing House, 1998, pp. 391\u0026ndash;395.\u003c/li\u003e\n\u003cli\u003eLi, J. J. Ceramic Technology. Beijing: China Light Industry Press, 2006, pp. 495\u0026ndash;510.\u003c/li\u003e\n\u003cli\u003eXu, S. Y, Xu, K. Chinese Ceramics Dictionary. Beijing: China History and Culture Press, 2013, p. 443.\u003c/li\u003e\n\u003cli\u003eTerminology of Defects in Daily-Use Ceramics, pp. 6\u0026ndash;10.\u003c/li\u003e\n\u003cli\u003eFang, K. K. Application of Modern Experimental Techniques in Xiangzhou Kiln Ceramics [D]. Zhengzhou: Zhengzhou University. 2017.\u003c/li\u003e\n\u003cli\u003eAnyang Municipal Institute of Cultural Relics and Archaeology. Kneading Earth into Gold: New Archaeological Discoveries on the Xiangzhou Kiln and Its Celadon Wares. Zhengzhou: Zhongzhou Ancient Books Publishing House, 2018, p. 53.\u003c/li\u003e\n\u003cli\u003eAnyang Municipal Institute of Cultural Relics and Archaeology. Kneading Earth into Gold: New Archaeological Discoveries on the Xiangzhou Kiln and Its Celadon Wares. Zhengzhou: Zhongzhou Ancient Books Publishing House, 2018, p. 52.\u003c/li\u003e\n\u003cli\u003eMing, Z. F, Kong, D. M, Jiao, P. Discussion on Issues Related to Xiangzhou Kiln Celadon Wares in Anyang: A Case Study Based on the 2009AYTYGJT1 Dataset. Yellow River, Loess, and Yellow People, 14 (2018) 31\u0026ndash;40.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"humanities-and-social-sciences-communications","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"palcomms","sideBox":"Learn more about [Humanities \u0026 Social Sciences Communications](http://www.nature.com/palcomms/)","snPcode":"41599","submissionUrl":"https://submission.springernature.com/new-submission/41599/3","title":"Humanities and Social Sciences Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Nature AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Xiangzhou Kiln, Fired defects, Fired process, Causes of defects","lastPublishedDoi":"10.21203/rs.3.rs-7310003/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7310003/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study examines 9 pieces of white-glazed and 247 pieces of celadon artefacts from the Sui Dynasty, unearthed during the third excavation of the Xiangzhou kiln in 2009. Based on the burial environment and morphological characteristics of these artefacts, firing defects in the samples\u0026rsquo; body and glaze were observed, recorded and analysed. Eight types of defects were identified in the foetal body, with underfiring defects accounting for the largest proportion. Meanwhile, 12 types of defects were found in the glaze surfaces, with spot defects being the most prevalent. The firing defects indicate that the Xiangzhou kiln possessed a certain degree of originality in raw material preparation, moulding, drying and firing. In addition, it could produce relatively mature, transparent, white-glazed porcelain. Consequently, its porcelain-making process should be at a high level during the same period.\u003c/p\u003e","manuscriptTitle":"Archaeological Observation of Firing Defects in Xiangzhou Kiln Porcelain","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-18 10:28:35","doi":"10.21203/rs.3.rs-7310003/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-05T04:36:25+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-05T04:26:57+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-14T11:30:16+00:00","index":"","fulltext":""},{"type":"submitted","content":"Humanities and Social Sciences Communications","date":"2025-08-06T12:47:24+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"humanities-and-social-sciences-communications","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"palcomms","sideBox":"Learn more about [Humanities \u0026 Social Sciences Communications](http://www.nature.com/palcomms/)","snPcode":"41599","submissionUrl":"https://submission.springernature.com/new-submission/41599/3","title":"Humanities and Social Sciences Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Nature AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"ae5194d0-a9c9-499d-9ee4-0aa5d2ff3445","owner":[],"postedDate":"August 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":53169795,"name":"Humanities/History"},{"id":53169796,"name":"Social science/History"},{"id":53169797,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-05-05T03:23:48+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-18 10:28:35","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7310003","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7310003","identity":"rs-7310003","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}