μ-Raman study reveals the microstructure characteristics of dark spots on Qinghua porcelains from Jingdezhen imperial kiln

μ-Raman study reveals the microstructure characteristics of dark spots on Qinghua porcelains from Jingdezhen imperial kiln | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article μ-Raman study reveals the microstructure characteristics of dark spots on Qinghua porcelains from Jingdezhen imperial kiln Wenxuan Wang, Jian Zhu, Philippe Sciau, Jianxin Jiang, Zhongfeng Shi, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7799426/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 13 You are reading this latest preprint version Abstract The dark spots, as a characteristic of the Yuan and early Ming Qinghua productions, have a high research value regarding the source of cobalt pigment and the technology development of Qinghua porcelain. Aiming to comprehensively study the complex crystallites component caused by inhomogeneous pigment and glassy matrix. Eight sherds of Qinghua porcelain with dark spots unearthed in the Jingdezhen imperial kiln from Yuan dynasty (1271–1368 CE) to Ming Chenghua reign (1465–1487 CE) were analysed by stereomicroscope, Raman spectroscopy and scanning electron microscopy equipped with energy dispersive spectrometer. Taking the Ming Xuande reign (1426–1435 CE) as the turning point, we evidenced the Fe-richer spinels and Mn-richer spinels. Before Ming Xuande reign, the two types of Fe-richer spinels were identified as CoFe 2 O 4 and Fe-rich CoFe 2 O 4 with high-Mg substitutions. Surprisingly, Esseneite (CaFeAlSiO 6 ) were observed as well in Hongwu reign (1368–1398 CE) as the non-negligible constituent of dark spots. Then, MnFe 2 O 4 of high Mn and high Fe were found respectively in Xuande and Chenghua reign. By refining the phases of dark spots from different period, we propose an innovative identification method for the dating of imperial Qinghua porcelain. Moreover, it is proved that the compositions of dark spots correspond to component features of cobalt ores from different source that holds great promise for cobalt pigment and the technology development studies of Qinghua porcelain. Dark spots Qinghua porcelain Raman SEM-EDS Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Introduction Qinghua porcelain is one of the largest productions of porcelain varieties in the history of Chinese ceramics and had the greatest influence on the development of Chinese ceramics techniques and aesthetic orientation. Moreover, it had been a bridge connecting the east and west civilization because of its heavily dependence on imported cobalt ores and strong exports in international porcelain trade. Thus, related studies has consistently been a hot spot in archaeological issues [ 1 – 5 ]. As one of the criteria for the use of imported cobalt ores from the perspective of ceramics authentication, dark spots emerge at the high concentration area of cobalt blue area on Qinghua porcelain made by imperial kiln of the Yuan (1271–1368 CE) and Ming dynasty (1368–1644 CE), especially enriched in the Yongle (1403–1424 CE) and Xuande reign (1426–1435 CE) of the Ming dynasty. After the Xuande reign, with the improvement of the impurity removal process of Qinghua porcelain, dark spots gradually decreased and vanished in imperial porcelains [ 6 ].It is also known as “iron spot”, “beach spot” and “tin light” showing various forms and different macro-views [ 6 , 7 ]. The formation of dark spots is mainly caused by the excessive impurities of Fe, Mn in cobalt ores and firing conditions [ 8 ]. Hence, dark spots have a high research value regarding the source and formula of cobalt pigment and the technology development of Qinghua porcelain. The few researches work has been carried out on dark spots of Qinghua porcelain. Wu et al. analysed the structure of some specks on imperial Qinghua porcelains since 1999 [ 9 ]. The results showed that specks consist of Fe-rich and Mn rich crystallites. Then, magnetite (Fe 3 O 4 ) and jacobsite (MnFe 2 O 4 ) crystallites were found respectively in Hongwu (1368–1398 CE) and Xuande reign (1426–1435 CE) [ 10 ]. The further study identified the α-Fe and Fe 3 O 4 in Yuan dynasty, hematite (Fe 2 O 3 ) in Hongwu reign and MnFe 2 O 4 in Chenghua reign (1465–1487 CE) [ 11 ]. However, TEM used in these studies is a destructive testing method requiring lossy pretreatment which is not suitable for precious cultural relics. Raman spectroscopy is a powerful analytical technique with advantages of non-destructive sample preparation, high resolution and rapid measurement and has been proven to be useful in giving definite answers to phase identification of ceramic crystals [ 12 – 14 ]. Some scholars combined Raman spectroscopy with SEM-EDS to assess the phase of "iron spot". They evidenced the presence of dendrites of the cobalt ferrite (CoFe 2 O 4 ) in Qinghua porcelain from the Yuan dynasty [ 15 ] and the manganese oxide (Mn 2 O 3 , Mn 3 O 4 ), jacobsite (MnFe 2 O 4 ) with Co, Al, Ni in Qinghua porcelain from the middle and late Ming dynasty [ 16 , 17 ]. It should be pointed out that, in these studies, dark spots with single variety is unable to discuss the correspondence between crystallites morphology and different macroscopic identification features. The samples from the certain period also limited the clarification of the dark spots component changes caused by the improvements in the formulation or impurity process of cobalt blue pigments in different periods. Additionally, the complex crystallites component caused by inhomogeneous pigment and glassy matrix should be further explored. Therefore, in order to identify the various phases of different dark spots, stereoscopic microscope, scanning electron microscope equipped with energy dispersive spectrometer (SEM-EDS) and Raman spectroscopy (µ-Raman) were carried out on 8 sherds of Qinghua porcelain with dark spots excavated in imperial kiln from the Yongle dynasty (1271–1368 CE) to the Chenghua reign (1465–1487 CE) of Ming dynasty (1368–1644 CE). The study not only help to lay the scientific foundations and serve to the further studies like authentication of imperial Qinghua porcelain, but also could provide a new insight into cobalt ores provenance sourcing and technology evolution of Qinghua porcelain. Materials All the studied samples have been excavated from the imperial kiln at Zhushan, Jingdezhen and were provided by the Jingdezhen Institute of Ceramic Archaeology. The Qinghua porcelain produced in imperial kiln were selected as research objects not only because the imperial kiln represents the highest level of craftsmanship in Qinghua porcelain making, but because the imperial kiln has stable supply of cobalt pigment and the strictest manufacturing processes, which made the dark spots present stable change in micromorphology and microstructure from period to period. According to strata and stylistic study, 1 sherd can be dated to the Yuan dynasty (1271–1368 CE, Y-4) and 7 sherds can be dated to the Ming dynasty (1368–1644 CE) including 3 sherds (HW-2, HW-4) from Hongwu reign (1368–1398 CE), 2 sherds (YL-6, YL-15) from Yongle (1403–1424 CE), 2 sherds (XD-1, XD-5) from Xuande (1426–1435 CE) and 1 sherd (CH-8) from Chenghua reign (1465–1487 CE). 8 samples are presented in Fig. 1 . The sherds with dark spots were mounted in epoxy resin and polished to obtain the morphological characteristics and microstructure of dark spots on Qinghua porcelain by SEM-EDS. Stereomicroscope A Nikon SMZ1000 stereomicroscope and a Keyence VHX-600 digital microscope in the laboratory of the Institute of Ancient Vertebrates and Ancient Humanities, Chinese Academy of Sciences, were used to observe the morphological features of the dark spots. Micro-Raman spectrometry Micro-Raman spectrometry was performed in CEMES-CNRS (Toulouse) at room temperature using an XploRA MV 2000 Horiba spectrometer equipped with the 532nm excitation of a solid-state laser and confocal set-ups and charge coupled device detectors. A 2–3µm diameter laser spot was provided with a 100× microscope objective used to focus laser beam and collect scattered light. The single-grating set-up equipped with an edge filter to eliminate elastic scattering has a high optical throughput, enabling high sensitivity with short acquisition times. Scanning electron microscopy The electron microscopy investigations were performed in CEME-CNRS using a JEOL JSM 6460 LV scanning electron microscopy (SEM) operating at 20kV and equipped with an Oxford INCA PentaFETx3 energy dispersive X-ray spectrometer (EDS). Both imaging and elemental analyses were performed in high vacuum (HV) mode, working distance of 10 mm. The imaging was performed in backscattered electron mode. The electron acceleration was 10 kV for imaging and 15 kV for elemental analysis. Probing depths were estimated using CASINO software (Monte Carlo simulate of electron trajectory in solids). Since ceramics are not conductive, a thin layer of carbon covered the glaze surface, in order to avoid charging effect during the measurements. The carbon layer was limited to the analysed area whereas the rest of the shard was protected using tape. Results and Discussion The dark spots of 8 samples were observed under a stereomicroscope. As shown in Fig. 2 , what these specks have in common is that all dark spots are located inside the cobalt blue area, such as heavy brush strokes or turning points. This indicates that the necessary condition for the formation of dark spots is a high concentration of cobalt pigment. In addition, the vitreous lustre of all the samples was damaged by the crystallization resulting in the rough glaze surface and abrupt visual effect. It is the main reason why the dark spots were considered as a defect and then vanished with the development of manufacture. dark spots present varieties of macroscopic appearances with different shape and color. The dark spots of Y-4, YL-6, YL-15, CH-8 display frosted grey with no metallic lustre; A “tin light” with a metallic lustre can be observed in HW-2 and XD-1 samples. The HW-4 and XD-1 show the sparkling point-like flashes from a macro perspective; The dark spots on the XD-5 contain large brownish-yellow specks raised from glaze surface which even completely cover the blue of cobalt pigment. To further identify the crystalline phases of dark spots, Raman spectroscopy were conducted on 8 samples. The results demonstrated that crystals of dark spots are mainly composed of spinels, which has an essential contribution to the metallic color of dark spots. Comparing the Raman spectra of crystals at the dark spots in each period, the samples can be divided into the following two categories according to the different chemical composition. Cobalt ferrite (CoFeO) The first categories can be subdivided into two groups. The crystals at the dark spots of Y-4 from the Yuan dynasty (1271–1368 CE)and YL-6, YL-15 from the Yongle reign (1403–1424 CE) are grouped together. As shown in Fig. 3 , it can be found that the Raman spectrum of dendrites has a good correspondence with the standard spectra of spinel-type cobalt ferrite (CoFe 2 O 4 ) referential to the literature and online RRUFF database [ 15 , 18 – 20 ]. Cobalt ferrite is a cubic ferromagnetic oxide with the inverse spinel structure AB 2 O 4 , which is a face-centred cubic structure belonging to space group \(\:{O}_{ℎ}^{7}\) (Fd3m) . A 2+ and B 3+ cations are located on the tetrahedral and octahedral sites, respectively. For inverse spinel B 3+ (A 2+ , B 3+ ) O 4 , the tetrahedral sites are occupied by half of the B 3+ and the octahedral sites are occupied by A 2+ and B 3+ . It is well known that 5 modes (A 1g + E g +3T 2g ) are Raman active in this cubic spinel [ 21 – 24 ]. For CoFe 2 O 4 , the E g T 2g (1), T 2g (2) and T 2g (3) mode of Raman scattering peak below 600 cm − 1 represents the symmetric bending, anti-symmetric bending and stretch and of the Fe-O chemical bond at octahedral sites, the translation motion of Fe-O respectively. The A 1g (1) above 600 cm − 1 correspond to the symmetric stretching vibration of Fe-O bonds, while the A 1g (2) mode is associated to the Co-O bonds at tetrahedral sites [ 15 ]. In the area of dark spots on Y-4, dendrites and snowflake-like crystals range from 1–40 µm can be observed under SEM (Fig. 3 a). The centre, branch and terminal of one dendrite were tested, and the obtained Raman spectrum is similar to the dark spots crystals on Yuan Qinghua according to previous study [ 15 ]. Taking the centre testing point as an example, 202.9cm − 1 , 305.3cm − 1 , 465.0cm − 1 , 558.3cm − 1 , 619.3 cm − 1 and 686.5cm − 1 corresponds to the T 2g (1), E g , T 2g (2), T 2g (3), A 1g (2) and A 1g (1) vibration modes of CoFe 2 O 4 . Among them, A 1g (2) can be noticed in the 614.1-621.0cm − 1 because the divalent cations (Co 2+ ) occupy the tetrahedral sites [ 19 ]. Therefore, it is speculated that the CoFe 2 O 4 crystals of Y-4 sample are partially disordered. Comparing the spectrum of each test point, all modes positions slightly shift. Wang has pointed out that different Fe/Co ratio have a great influence on the Raman vibration modes of CoFe 2 O 4 [ 15 ]. In the Co 3 O 4 -CoFe 2 O 4 -Fe 3 O 4 solid solution, with the decreasing of Fe/Co ratio, the modes tend to shift to the high wavenumber. Noteworthily, the intensity of A 1g (2) mode begin surging at Fe/Co = 2.5 until two A 1g modes with similar intensity approximately [ 25 ]. In addition, the effects of different laser wavelengths, powers, and crystal formation conditions also impact on each vibration mode [ 20 ]. In this research, the same test conditions are used, parameters such as laser wavelength, power and environmental factors are not considered here. Once solid solutions are involved, it is difficult to identify the precise composition only by Raman spectrum. For obtaining the chemical information, the components of the corresponding Raman test points were detected by SEM-EDS. The SEM-EDS semiquantitative results (Table S1 ) show that the crystals found in dark spots are not pure CoFe 2 O 4 . It should be noted that Si, an element not incorporated in the spinel crystals, occupies nearly 20%, indicating the larger spatial resolution of EDS with respect to spinel size. This phenomenon is particularly obvious on the fine dendrites. Thus, only the Fe/Co ratio of each test points were compared here to further understand the component changes of different crystals and different parts in same crystal. From the middle to the center then to the end of the dendrite, Raman modes slightly shift to low wavenumber, which is mainly manifested in the increase of Fe/Co ratio from 1.96 to 2.35. The reason for this dendrite segregation should cause by the inhomogeneity of the glaze. The dark spots of the YL-6 are mainly dendrites extending up to 100 µm and dendrites or snowflake-like crystals between nanoscale to 30 µm in length (Fig. 2 b). The YL-15 is dominated by dendrites range from 5 µm to 50 µm (Fig. 2 c). Raman analysis was performed on different positions of one large dendrite spine in YL-6 and small dendrites in YL-15. All Raman spectrum are consistent with that of Y-4. From the centre to the middle then to the end of the dendrite branch, the Raman peak position all move to the direction of low wavenumber with the Fe/Co increasing gradually due to the dendrite segregation. Combining the analysis results of Raman and SEM-EDS, the plot of positions of A 1g (1) and T 2g (2) modes versus the Fe/Co ratio were shown in Fig. 4 . Along with the increase of Fe/Co ratio, the peak positions of A 1g (1) and T 2g (2) began to move to the low wavenumber gradually except for Y-4 testing point 2. Although the range of Fe/Co ratio (1.89–2.35) in this group is too small to limit the discussion about the modes shift influenced by different chemical composition, the changing trend of the peak positions of A 1g (1) and T 2g (2) with Fe/Co ratio is consistent with previous studies [ 15 , 25 , 26 ]. In general, the dendrites with Fe/Co atomic ratio between 1.89 and 2.35 at the dark spots from this group are closest to CoFe 2 O 4 spinel in all samples. The relative stability of the Fe/Co ratio of dark spots on the samples in Yuan dynasty and Yongle reign not only reflects the standardized pattern of the Qinghua porcelain firing technology in Jingdezhen imperial kilns, but also reflects the stable supply of raw materials during these two periods, especially the imported cobalt ores. Because the imperial kilns in these periods relied heavily on imported cobalt ores rather than domestic asbolane, the stable import of cobalt ores was an important basis for the production of Qinghua porcelain in imperial kilns [ 27 ]. The second group is the HW-2 and HW-4 samples from the Hongwu period (1368–1398 CE) of the Ming Dynasty. The backscattered electron images reveal the crystalline blocks with a length of 5-150 µm formed in the dense crystals area, and two kinds of dendrites with different composition contrast in Fig. 5 . The low-contrast dendrites always precipitate around the high-contrast crystals, forming mat-like dendrites. Raman spectroscopy was carried out to identify the blocks and dendrites with high contrast in the two samples firstly. The spectrum of crystalline blocks (point 1 in Fig. 5 a and point 1,2,3 in Fig. 5 c) are close to that of dendrites (point 2,3,4 in Fig. 5 a) with high contrast. Except for the Raman modes approximately at 325cm − 1 and 600cm − 1 , the other peak positions show a good corresponding relationship with CoFe 2 O 4 . In case of HW-2, the Raman peak positions of the testing point a1 at one crystalline block centre located at 203.1cm − 1 , 325.4cm − 1 , 466.8cm − 1 , 603.7cm − 1 and 696.7cm − 1 corresponding to the T 2g (1), E g , T 2g (2), A 1g (2) and A 1g (1) vibration modes of CoFe 2 O 4 , respectively. However, the two A 1g symmetrical vibration modes have almost the same intensity. The E g vibration modes at 325.4cm − 1 and the A 1g (2) vibration mode at 603.7cm − 1 are different from the reported standard CoFe 2 O 4 Raman spectrum, also from the first group. The peak position of the E g and A 1g (2) vibration mode of HW-2 sample is lower than that of CoFe 2 O 4 standard spectrum by about 20-30cm − 1 . Although the Fe/Co ratio affect the position of vibration mode of CoFe 2 O 4 as mentioned before, no research reported a nearly 100cm − 1 difference between the two A 1g modes with same intensity. In order to further clarify the crystal structure, the chemical compositions of the corresponding Raman test points are analysed by SEM-EDS (Table S2 ). Compared to the first group with few Mg doping (0.4% on average), the crystals from second group are doped with more Mg (1.75–6.59%). According to the literature, there is no strong peak in the Raman spectrum of MgFe 2 O 4 around 600cm − 1 [ 23 ]. However, the peak positions of CoFe 2 O 4 move to the high wavenumber when part of Mg is doped in CoFe 2 O 4 . Among them, E g mode has the most significant blue shift of 20-30cm − 1 [ 28 , 29 ]. This better explains that the Raman E g mode peak position of the samples in the Hongwu period obviously shifted to 322.7-326.4cm − 1 . At the same time, blue shift are also observed in T 2g (2) and A 1g (1) modes to varying degrees. Moreover, Varshney [ 18 ] and Tong [ 30 ] have reported that the line widths of CoFe 2 O 4 Raman modes increase with the Mg doping, which is attributed to strong electron–phonon interaction and electronic disorder arising as random arrangement of Cations on the octahedral site. When the Mg /Co ratio increases from 0 to 1, the peak intensity ratio of A 1g (1) and A 1g (2) modes gradually decreases, then reaches the minimum and forms a similar double peak when the Mg /Co = 1. When the Mg/Co is higher than 1, the peak intensity ratio of two A 1g modes rises with the increase of Mg/Co. Finally, the A 1g (2) mode almost disappears as Co is completely replaced by Mg. Based on the chemical composition, the Mg/Co of HW-2 and HW-4 samples are between 0.86 and 1.09, so the spectrum present two A 1g modes with comparable intensities. A slight mode shift can be seen in the Raman spectrum of different test points on spinel of HW-2 and HW-4 according to different chemical composition. As shown in Fig. 6 , plot of the A 1g (1) and T 2g (2) positions versus the Fe/Co and Mg/Co ratio (at%) shows that the A 1g (1) and T 2g (2) shift to low wavenumber with the increase of the Fe/Co. As described above, with the increase of Mg content in CoFe 2 O 4 , spectrum could show a blue shift, in the opposite direction with the increasing of Fe. Nevertheless, the blue shift with the increase of Mg is not observed in this study considering owing to stable Mg/Co (0.86–1.09). Therefore, the shift of Raman modes is mainly influenced by Fe/Co rather than Mg/Co. Regarding our results, spectrum of the blocks and dendrites with high contrast in dark spots of HW-2 and HW-4 are consistent with an intermedia phase between CoFe 2 O 4 and Fe 3 O 4 with Mg substitutions. Additionally, the Raman spectrum of mat-like dendrite on HW-2 with low composition contrast (Fig. 5 b point5) belong to one of pyroxene, esseneite (CaFeAlSiO 6 ) with five peaks at 328.2, 524.4, 658.4, 764.3, 962.6cm − 1 [ 31 , 32 ]. The mat-like dendrites in HW-4 (Fig. 5 c point 4, 5) are also consistent with esseneite. The results of SEM-EDS further confirmed the identification. As a vital component of dark spots, it is first discovered in Qinghua porcelain samples. A growth mechanism for CaFeAlSiO 6 is proposed here. After the growth of CoFe 2 O 4 on top of glaze surface, Ca, Al, Si is squeezed into the liquid phase around them. Then, CaFeAlSiO 6 started to grow on both sides of CoFe 2 O 4 and become the dendrites. Similar crystallization mechanism was also found in other ancient porcelain samples, such as the pinholes in brown glazed stoneware from Yaozhou kiln [ 33 ] Manganese ferrite (MnFe 2 O 4 ) The second category is shown in Fig. 7 including XD-1 and XD-5 from the Xuande reign and CH-8 from the Chenghua reign. The dark spots of XD-1 can be observed under the backscattering mode with a variety of crystal forms (Fig. 7 a). Massive polycrystals aggregates as crystalline blocks reaching up to 50 µm. The dendrites surrounding the blocks are parallel to each other and periodically arranged so that the incident light is reflected at a certain angle, thereby causing the metallic lustre on a macro view, which is commonly known as “tin light” in ancient ceramics authentication. The dark spots of XD-5 show the developed reticular dendrites with its width of trunk up to 10–20 µm are interspersed with high-contrast polyangular grains (Fig. 7 b). The CH-8 sample presents a large number of dendritic crystallites from nano-scale to 60 µm (Fig. 7 c). The Raman spectrum were recorded on several positions. All spectrum are dominated by A 1g peak ranging from 624.5 to 648.6 cm − 1 . It is similar to the reported Mn and Fe spinel with the anti-spinel structure of the cubic crystal system which usually forms a solid solution between jacobsite (MnFe 2 O 4 ), hausmannite (Mn 3 O 4 ) or even magnetite (Fe 3 O 4 ) [ 22 , 34 ]. For jacobsite, the one main dominated A 1g peak of Raman spectra appear at 620–640 cm − 1 which represents the stretching vibration of the Mn-O chemical bond at the octahedral position in the spinel structure [ 17 , 35 – 37 ]. In our cases, A 1g peak are slightly high for jacobsite. Different compositions of jacobsite could influence the Raman vibration mode. Some scholars have pointed out that if the proportion of Fe in jacobsite increases, the A 1g mode shift towards high wavenumbers. As jacobsite and magnetite are isostructural, the A 1g move to 626 cm − 1 [ 35 , 36 ]. With more substitution of Fe, jacobsite transform into magnetite (Fe 3 O 4 ) with the A 1g position around 670 cm − 1 [ 38 ]. Relatively, the substitution of Fe with Mn leads to the shifting of the spectral line to 630–665 cm − 1 , eventually forming hausmannite ((Mn 3 O 4 ) [ 16 , 39 , 40 ]. Namely, no matter which side of Fe 3 O 4 -MnFe 2 O 4 -Mn 3 O 4 the jacobsite is biased to, the A 1g mode will move in the direction of high wavenumbers. Thus, it is difficult to reveal the specific crystal compositions solely by Raman spectrum. The component of the corresponding Raman test point is estimated by SEM-EDS. The results (Table S3 ) display that the main constitutive elements of the crystal are Mn (up to 21 at%) and Fe (up to 20 at%). The crystals in XD-1 and XD-5 with a Fe/Mn ratio lower than 2 are considered as MnFe 2 O 4 –Mn 3 O 4 and the crystals in CH-8 with a Fe/Mn ratio superior to 2 belong to MnFe 2 O 4 –Fe 3 O 4 . On the basis of SEM-EDS measurements, the scatter plot of Fe/Mn ratio (at%) versus A 1g (1) positions (Fig. 8 ) further reveals the effect on the Raman vibration mode caused by different compositions of Fe 3 O 4 -MnFe 2 O 4 -Mn 3 O 4 solid solution. Taking the Fe/Mn ratio equal to 2 as the turning point, the A 1g mode positions first move to the low wavenumber and then to the high wavenumber with the increase of the Fe/Mn ratio. These findings are accordant with the previously reported Raman modes changes caused by different components of jacobsite. Furthermore, a shoulder peak at approximately 670–680 cm − 1 gradually emerges which might be explained by the increasing of Co. The significant proportions of Co (0.67 ~ 10.67 at%) were found, which suggests that a complex phase of mixed Co-Mn ferrites is involved in samples from the Xuande and Chenghua reign. It is reported that, from MnFe 2 O 4 to CoFe 2 O 4 , red shift in the phonon mode and change in the relative intensity between the two vibration modes A 1g (1) and A 1g (2) can be observed [ 41 – 43 ]. The A 1g peak with composition x = 0.4 (Co x Mn 1−x Fe 2 O 4) begin to show a split, which further intensifies with the increase of Co 2+ in the mixed ferrite [ 44 ]. The crystals formulae were calculated based on the chemical composition of Raman testing points by EDS (Table 1 ). Except for spectrum of XD-1 point1(Co 0.4 Mn 1.8 Fe 0.8 O 4 ) and point 2(Co 0.4 Mn 1.9 Fe 0.7 O 4 ), all the rest of spectrum with Co ratio higher than 0.4 show a A 1g (2) peak with a shoulder A 1g (1), which is in accord with earlier studies. In conclusion, the crystallites in dark spots of XD-1, XD-5 and CH-8 are reasonable to be identified as Co-Mn ferrites. Table 1 Crystals formulae and Co/Mn obtained from Chemical composition of Raman testing points by EDS (at%) Sample code Crystal Formula Co/Mn CH-8-3 Co 0.8 Mn 0.5 Fe 1.7 O 4 1.50 CH-8-2 Co 0.7 Mn 0.5 Fe 1.8 O 4 1.16 XD-5-2 Co 0.7 Mn 0.9 Fe 1.4 O 4 0.83 CH-8-1 Co 0.5 Mn 0.8 Fe 1.7 O 4 0.68 XD-5-3 Co 0.7 Mn 1.4 Fe 0.9 O 4 0.47 XD-1-3 Co 0.5 Mn 1.5 Fe 1.0 O 4 0.35 XD-5-1 Co 0.6 Mn 1.6 Fe 0.8 O 4 0.33 XD-1-2 Co 0.4 Mn 1.8 Fe 0.8 O 4 0.22 XD-1-1 Co 0.4 Mn 1.9 Fe 0.7 O 4 0.20 Moreover, the structure of the developed reticular dendrites in XD-5 assigns to anorthite according to RRUFF database [ 45 ] (Fig. 9 ). The dominating and sharp modes at 508 cm − 1 with corresponding peculiar shoulder peak around 482 cm − 1 are observed. Interestingly, they can also be seen in the spectrum b1 and b3 of XD-5 (Fig. 7 ). A similar crystallization mechanism such as CaFeAlSiO 6 and Mg doped CoFe 2 O 4 -Fe 3 O 4 in HW-2 and HW-4 applies equally here. From the perspective of dark spots, Fe/Mn variation rule of the crystallization composition is consistent with that of cobalt pigment source in imperial kiln from Yuan dynasty to the Chenghua reign of Ming dynasty. It illustrates that the crystallization composition of dark spots is largely affected by components of cobalt ores. The Fe-rich cobalt imported from Persia was used in imperial kiln from Yuan Dynasty to Yongle reign of the Ming Dynasty so that almost all of the crystallization in dark spots are Fe-rich spinels and none of manganiferous crystal is found. Significantly, the surging Fe content becomes a prominent feature of dark spots in Hongwu reign of Ming dynasty, which might be caused by a change in the formulation or treatment of cobalt blue pigments. As the beginning of Ming dynasty, the Hongwu reign had not yet fully controlled northern China [ 46 ]. The “maritime prohibition” policy greatly restricted imports. Non-government trade was also strictly forbidden [ 47 ]. Therefore, it is possible that craftsmen were forced to change the formula of cobalt pigment because of the disruption of imported cobalt ores. Considering the consistency and inheritance of craftsmanship in the Jingdezhen imperial kiln, it has a fairly high probability that the Qinghua porcelain production process in the Hongwu reign was inherited from Yuan dynasty. Accordingly, we demonstrate that the dramatic increase of Fe should be caused by the recipe change of blue pigment rather than the purification of raw material or firing process which would only reduce the content of impurities like Fe, Mn but not increase. Owing to the lack of imported cobalt ore, it was possible to mix imported cobalt ore with iron-bearing mineral, resulting in the characteristics of dark blue and “iron spot” with high-Fe in Hongwu reign. From the Xuande reign (1426–1435 CE), the domestic cobalt ores with characteristic of high-Mn were begun to use [ 48 ]. Thus, the crystallization in dark spots in Xuande and Chenghua reign are Co-Mn spinel ferrites. However, the Fe content of the Chenghua sample is higher than that of the Xuande samples, which might relate to the use of domestic cobalt ores from different regions. Conclusion In this study, the necessity of the combining use of µ-Raman and corresponding chemical composition analysis in complicated and various historical relics is emphasized. Based on µ-Raman and SEM-EDS, we identified the various phases of crystals in the dark spots of Qinghua porcelain manufactured in imperial kilns from the Yuan dynasty to the middle Ming dynasty. CoFe 2 O 4 with Fe/Co around 2 were found in dark spots of the Yuan and Ming Yongle reign when the imported cobalt ore with the high-Fe and low-Mn content was consistently supplied in imperial kiln. During the Ming Hongwu period, the dark spots had various crystal forms and different chemical compositions. Except for an intermedia phase between CoFe 2 O 4 and Fe 3 O 4 with Mg substitutions, the esseneite (CaFeAlSiO 6 ) were also observed in dark spots. The high-Fe content is the characteristic of dark spots in this period, which is probably because of the mixture of imported cobalt ores and iron-bearing minerals. From the Ming Xuande reign, according to the use of domestic cobalt ore with different composition, the main crystal phase on dark spots in Xuande and Chenghua reigh assign to Mn richer and Fe richer Co-Mn ferrites, respectively. Anorthite is also a main part of dark spots in Xuande reign, which proves that dark spots are not only related to Fe, Mn. The characteristics of dark spots before and after Ming Xuande reign is entirely different, which could be a identification criterion to date the imperial Qinghua porcelain. Moreover, this study provides a new perspective on cobalt ores provenance sourcing and technology evolution of imperial Qinghua porcelain. Declarations Competing interests No competing interests associated with this study. Funding This study was sponsored by the Shaanxi Provincial Department of Education Key Laboratory Research Project (20JS147) and Humanities and Social Sciences Research Project from the Ministry of Education of China (MOE, No.19YJAZH130). Author Contribution JZ helped devise the research questions addressed in this paper. PS and WXW led the μ-Raman experiments and analysis of data. ZFS, ZZW and WXW led the SEM-EDS experiments. JXJ provided samples and helped with the sample selection. WXW wrote the article. All authors read and approved the final manuscript. 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2","display":"","copyAsset":false,"role":"figure","size":1177105,"visible":true,"origin":"","legend":"\u003cp\u003eThe\u003cstrong\u003e \u003c/strong\u003eStereomicroscope images of dark spots on imperial Qinghua porcelain.\u003c/p\u003e","description":"","filename":"Figure2.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/0cc71f2b6148daa48400c7ca.jpg"},{"id":94508630,"identity":"908b93d1-4a8c-4c87-a017-8c7f36fc4359","added_by":"auto","created_at":"2025-10-28 16:32:44","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1172932,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra recorded on dendrites of sample Y-4 (a), YL-6 (b) and YL-15 (c) respectively with laser spot positions are indicated in the corresponding SEM images.\u003c/p\u003e","description":"","filename":"Figure3.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/c8fd361bedd1c152359ff152.jpg"},{"id":94508745,"identity":"fcea78bf-4256-4027-bed9-d156cfed96eb","added_by":"auto","created_at":"2025-10-28 16:33:10","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":123629,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical representations of A\u003csub\u003e1g\u003c/sub\u003e(1) and T\u003csub\u003e2g\u003c/sub\u003e(2) positions versus the Fe/Co ratio (at%) from “iron spot” on Y-4, YL-6, YL-15.\u003c/p\u003e","description":"","filename":"Figure4.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/0c6dca5899c8c73135152568.jpg"},{"id":94508988,"identity":"b875711a-ce6d-4381-aa81-ea94fe82f183","added_by":"auto","created_at":"2025-10-28 16:33:23","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1144813,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images and Raman spectra recorded on crystallizations of sample HW-2 (a, b) and HW-4 (c) respectively with laser spot positions are indicated in the corresponding SEM images.\u003c/p\u003e","description":"","filename":"Figure5.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/0878aed4777f55026bdf07c9.jpg"},{"id":94508842,"identity":"ae7bbe96-4c18-4220-8891-066d83193c73","added_by":"auto","created_at":"2025-10-28 16:33:17","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":209778,"visible":true,"origin":"","legend":"\u003cp\u003ePlot of the A\u003csub\u003e1g\u003c/sub\u003e(1) and T\u003csub\u003e2g\u003c/sub\u003e(2) positions versus the Fe/Co and Mg/Co ratio(at%) from “iron spot” on HW-2, HW-4.\u003c/p\u003e","description":"","filename":"Figure6.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/ecb1d4cd2d296774be9a3c94.jpg"},{"id":94508780,"identity":"425ec16a-1f32-4e85-95eb-158b4b22e921","added_by":"auto","created_at":"2025-10-28 16:33:13","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1150406,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra recorded on crystallizations of sample XD-1 (a), XD-5 (b) and CH-8 (c) respectively with laser spot positions indicated in the corresponding SEM images.\u003c/p\u003e","description":"","filename":"Figure7.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/2c90fd873b9dd4b3b6ba597f.jpg"},{"id":94508679,"identity":"78d75b4d-4a9e-4cb5-968f-5f654cdea126","added_by":"auto","created_at":"2025-10-28 16:32:51","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":145513,"visible":true,"origin":"","legend":"\u003cp\u003ePlot of Fe/Mn ratio (at%) and position of Raman A\u003csub\u003e1g\u003c/sub\u003e mode of MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e from “iron spot” on XD-1, XD-5 and CH-8\u003c/p\u003e","description":"","filename":"Figure8.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/872b3f277b5caf65d0582aa4.jpg"},{"id":94508623,"identity":"d0252a2f-76df-4675-8c11-541fc7c2c14f","added_by":"auto","created_at":"2025-10-28 16:32:41","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":648899,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image (left) and Raman spectrum of the reticular crystals in the XD-5 (right)\u003c/p\u003e","description":"","filename":"Figure9.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/859d823481cf2340d2aea91f.jpg"},{"id":94598706,"identity":"c75ee68c-790a-48ed-9d34-2a9c38354947","added_by":"auto","created_at":"2025-10-28 18:55:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8373788,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/7a3b80d6-e4f9-4428-b5d4-73fd4737c68f.pdf"},{"id":94508632,"identity":"f4fe10f5-8b47-4eac-96b4-993b2dcd55ac","added_by":"auto","created_at":"2025-10-28 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16:33:12","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":14516,"visible":true,"origin":"","legend":"","description":"","filename":"TableS3.docx","url":"https://assets-eu.researchsquare.com/files/rs-7799426/v1/a2c8aea6f68664e5537ecca3.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"μ-Raman study reveals the microstructure characteristics of dark spots on Qinghua porcelains from Jingdezhen imperial kiln","fulltext":[{"header":"Introduction","content":"\u003cp\u003eQinghua porcelain is one of the largest productions of porcelain varieties in the history of Chinese ceramics and had the greatest influence on the development of Chinese ceramics techniques and aesthetic orientation. Moreover, it had been a bridge connecting the east and west civilization because of its heavily dependence on imported cobalt ores and strong exports in international porcelain trade. Thus, related studies has consistently been a hot spot in archaeological issues [\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. As one of the criteria for the use of imported cobalt ores from the perspective of ceramics authentication, dark spots emerge at the high concentration area of cobalt blue area on Qinghua porcelain made by imperial kiln of the Yuan (1271\u0026ndash;1368 CE) and Ming dynasty (1368\u0026ndash;1644 CE), especially enriched in the Yongle (1403\u0026ndash;1424 CE) and Xuande reign (1426\u0026ndash;1435 CE) of the Ming dynasty. After the Xuande reign, with the improvement of the impurity removal process of Qinghua porcelain, dark spots gradually decreased and vanished in imperial porcelains [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].It is also known as \u0026ldquo;iron spot\u0026rdquo;, \u0026ldquo;beach spot\u0026rdquo; and \u0026ldquo;tin light\u0026rdquo; showing various forms and different macro-views [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The formation of dark spots is mainly caused by the excessive impurities of Fe, Mn in cobalt ores and firing conditions [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Hence, dark spots have a high research value regarding the source and formula of cobalt pigment and the technology development of Qinghua porcelain.\u003c/p\u003e\u003cp\u003eThe few researches work has been carried out on dark spots of Qinghua porcelain. Wu et al. analysed the structure of some specks on imperial Qinghua porcelains since 1999 [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The results showed that specks consist of Fe-rich and Mn rich crystallites. Then, magnetite (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) and jacobsite (MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) crystallites were found respectively in Hongwu (1368\u0026ndash;1398 CE) and Xuande reign (1426\u0026ndash;1435 CE) [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The further study identified the α-Fe and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e in Yuan dynasty, hematite (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) in Hongwu reign and MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e in Chenghua reign (1465\u0026ndash;1487 CE) [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. However, TEM used in these studies is a destructive testing method requiring lossy pretreatment which is not suitable for precious cultural relics. Raman spectroscopy is a powerful analytical technique with advantages of non-destructive sample preparation, high resolution and rapid measurement and has been proven to be useful in giving definite answers to phase identification of ceramic crystals [\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Some scholars combined Raman spectroscopy with SEM-EDS to assess the phase of \"iron spot\". They evidenced the presence of dendrites of the cobalt ferrite (CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) in Qinghua porcelain from the Yuan dynasty [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] and the manganese oxide (Mn\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Mn\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e), jacobsite (MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) with Co, Al, Ni in Qinghua porcelain from the middle and late Ming dynasty [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. It should be pointed out that, in these studies, dark spots with single variety is unable to discuss the correspondence between crystallites morphology and different macroscopic identification features. The samples from the certain period also limited the clarification of the dark spots component changes caused by the improvements in the formulation or impurity process of cobalt blue pigments in different periods. Additionally, the complex crystallites component caused by inhomogeneous pigment and glassy matrix should be further explored.\u003c/p\u003e\u003cp\u003eTherefore, in order to identify the various phases of different dark spots, stereoscopic microscope, scanning electron microscope equipped with energy dispersive spectrometer (SEM-EDS) and Raman spectroscopy (\u0026micro;-Raman) were carried out on 8 sherds of Qinghua porcelain with dark spots excavated in imperial kiln from the Yongle dynasty (1271\u0026ndash;1368 CE) to the Chenghua reign (1465\u0026ndash;1487 CE) of Ming dynasty (1368\u0026ndash;1644 CE). The study not only help to lay the scientific foundations and serve to the further studies like authentication of imperial Qinghua porcelain, but also could provide a new insight into cobalt ores provenance sourcing and technology evolution of Qinghua porcelain.\u003c/p\u003e"},{"header":"Materials","content":"\u003cp\u003eAll the studied samples have been excavated from the imperial kiln at Zhushan, Jingdezhen and were provided by the Jingdezhen Institute of Ceramic Archaeology. The Qinghua porcelain produced in imperial kiln were selected as research objects not only because the imperial kiln represents the highest level of craftsmanship in Qinghua porcelain making, but because the imperial kiln has stable supply of cobalt pigment and the strictest manufacturing processes, which made the dark spots present stable change in micromorphology and microstructure from period to period.\u003c/p\u003e\u003cp\u003eAccording to strata and stylistic study, 1 sherd can be dated to the Yuan dynasty (1271\u0026ndash;1368 CE, Y-4) and 7 sherds can be dated to the Ming dynasty (1368\u0026ndash;1644 CE) including 3 sherds (HW-2, HW-4) from Hongwu reign (1368\u0026ndash;1398 CE), 2 sherds (YL-6, YL-15) from Yongle (1403\u0026ndash;1424 CE), 2 sherds (XD-1, XD-5) from Xuande (1426\u0026ndash;1435 CE) and 1 sherd (CH-8) from Chenghua reign (1465\u0026ndash;1487 CE). 8 samples are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The sherds with dark spots were mounted in epoxy resin and polished to obtain the morphological characteristics and microstructure of dark spots on Qinghua porcelain by SEM-EDS.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eStereomicroscope\u003c/h2\u003e\u003cp\u003eA Nikon SMZ1000 stereomicroscope and a Keyence VHX-600 digital microscope in the laboratory of the Institute of Ancient Vertebrates and Ancient Humanities, Chinese Academy of Sciences, were used to observe the morphological features of the dark spots.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eMicro-Raman spectrometry\u003c/h3\u003e\n\u003cp\u003eMicro-Raman spectrometry was performed in CEMES-CNRS (Toulouse) at room temperature using an XploRA MV 2000 Horiba spectrometer equipped with the 532nm excitation of a solid-state laser and confocal set-ups and charge coupled device detectors. A 2\u0026ndash;3\u0026micro;m diameter laser spot was provided with a 100\u0026times; microscope objective used to focus laser beam and collect scattered light. The single-grating set-up equipped with an edge filter to eliminate elastic scattering has a high optical throughput, enabling high sensitivity with short acquisition times.\u003c/p\u003e\n\u003ch3\u003eScanning electron microscopy\u003c/h3\u003e\n\u003cp\u003eThe electron microscopy investigations were performed in CEME-CNRS using a JEOL JSM 6460 LV scanning electron microscopy (SEM) operating at 20kV and equipped with an Oxford INCA PentaFETx3 energy dispersive X-ray spectrometer (EDS). Both imaging and elemental analyses were performed in high vacuum (HV) mode, working distance of 10 mm. The imaging was performed in backscattered electron mode. The electron acceleration was 10 kV for imaging and 15 kV for elemental analysis. Probing depths were estimated using CASINO software (Monte Carlo simulate of electron trajectory in solids). Since ceramics are not conductive, a thin layer of carbon covered the glaze surface, in order to avoid charging effect during the measurements. The carbon layer was limited to the analysed area whereas the rest of the shard was protected using tape.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eThe dark spots of 8 samples were observed under a stereomicroscope. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, what these specks have in common is that all dark spots are located inside the cobalt blue area, such as heavy brush strokes or turning points. This indicates that the necessary condition for the formation of dark spots is a high concentration of cobalt pigment. In addition, the vitreous lustre of all the samples was damaged by the crystallization resulting in the rough glaze surface and abrupt visual effect. It is the main reason why the dark spots were considered as a defect and then vanished with the development of manufacture. dark spots present varieties of macroscopic appearances with different shape and color. The dark spots of Y-4, YL-6, YL-15, CH-8 display frosted grey with no metallic lustre; A \u0026ldquo;tin light\u0026rdquo; with a metallic lustre can be observed in HW-2 and XD-1 samples. The HW-4 and XD-1 show the sparkling point-like flashes from a macro perspective; The dark spots on the XD-5 contain large brownish-yellow specks raised from glaze surface which even completely cover the blue of cobalt pigment.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo further identify the crystalline phases of dark spots, Raman spectroscopy were conducted on 8 samples. The results demonstrated that crystals of dark spots are mainly composed of spinels, which has an essential contribution to the metallic color of dark spots. Comparing the Raman spectra of crystals at the dark spots in each period, the samples can be divided into the following two categories according to the different chemical composition.\u003c/p\u003e\n\u003ch3\u003eCobalt ferrite (CoFeO)\u003c/h3\u003e\n\u003cp\u003eThe first categories can be subdivided into two groups. The crystals at the dark spots of Y-4 from the Yuan dynasty (1271\u0026ndash;1368 CE)and YL-6, YL-15 from the Yongle reign (1403\u0026ndash;1424 CE) are grouped together. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, it can be found that the Raman spectrum of dendrites has a good correspondence with the standard spectra of spinel-type cobalt ferrite (CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) referential to the literature and online RRUFF database [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eCobalt ferrite is a cubic ferromagnetic oxide with the inverse spinel structure AB\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, which is a face-centred cubic structure belonging to space group \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{O}_{ℎ}^{7}\\)\u003c/span\u003e\u003c/span\u003e \u003cem\u003e(Fd3m)\u003c/em\u003e. A\u003csup\u003e2+\u003c/sup\u003e and B\u003csup\u003e3+\u003c/sup\u003e cations are located on the tetrahedral and octahedral sites, respectively. For inverse spinel B\u003csup\u003e3+\u003c/sup\u003e (A\u003csup\u003e2+\u003c/sup\u003e, B\u003csup\u003e3+\u003c/sup\u003e) O\u003csub\u003e4\u003c/sub\u003e, the tetrahedral sites are occupied by half of the B\u003csup\u003e3+\u003c/sup\u003e and the octahedral sites are occupied by A\u003csup\u003e2+\u003c/sup\u003e and B\u003csup\u003e3+\u003c/sup\u003e. It is well known that 5 modes (A\u003csub\u003e1g\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;E\u003csub\u003eg\u003c/sub\u003e +3T\u003csub\u003e2g\u003c/sub\u003e) are Raman active in this cubic spinel [\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. For CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, the E\u003csub\u003eg\u003c/sub\u003e T\u003csub\u003e2g\u003c/sub\u003e(1), T\u003csub\u003e2g\u003c/sub\u003e(2) and T\u003csub\u003e2g\u003c/sub\u003e(3) mode of Raman scattering peak below 600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represents the symmetric bending, anti-symmetric bending and stretch and of the Fe-O chemical bond at octahedral sites, the translation motion of Fe-O respectively. The A\u003csub\u003e1g\u003c/sub\u003e(1) above 600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to the symmetric stretching vibration of Fe-O bonds, while the A\u003csub\u003e1g\u003c/sub\u003e(2) mode is associated to the Co-O bonds at tetrahedral sites [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn the area of dark spots on Y-4, dendrites and snowflake-like crystals range from 1\u0026ndash;40 \u0026micro;m can be observed under SEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The centre, branch and terminal of one dendrite were tested, and the obtained Raman spectrum is similar to the dark spots crystals on Yuan Qinghua according to previous study [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Taking the centre testing point as an example, 202.9cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 305.3cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 465.0cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 558.3cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 619.3 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003eand 686.5cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the T\u003csub\u003e2g\u003c/sub\u003e(1), E\u003csub\u003eg\u003c/sub\u003e, T\u003csub\u003e2g\u003c/sub\u003e (2), T\u003csub\u003e2g\u003c/sub\u003e (3), A\u003csub\u003e1g\u003c/sub\u003e(2) and A\u003csub\u003e1g\u003c/sub\u003e(1) vibration modes of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. Among them, A\u003csub\u003e1g\u003c/sub\u003e(2) can be noticed in the 614.1-621.0cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e because the divalent cations (Co\u003csup\u003e2+\u003c/sup\u003e) occupy the tetrahedral sites [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Therefore, it is speculated that the CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e crystals of Y-4 sample are partially disordered.\u003c/p\u003e\u003cp\u003eComparing the spectrum of each test point, all modes positions slightly shift. Wang has pointed out that different Fe/Co ratio have a great influence on the Raman vibration modes of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. In the Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e solid solution, with the decreasing of Fe/Co ratio, the modes tend to shift to the high wavenumber. Noteworthily, the intensity of A\u003csub\u003e1g\u003c/sub\u003e(2) mode begin surging at Fe/Co\u0026thinsp;=\u0026thinsp;2.5 until two A\u003csub\u003e1g\u003c/sub\u003e modes with similar intensity approximately [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. In addition, the effects of different laser wavelengths, powers, and crystal formation conditions also impact on each vibration mode [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. In this research, the same test conditions are used, parameters such as laser wavelength, power and environmental factors are not considered here.\u003c/p\u003e\u003cp\u003eOnce solid solutions are involved, it is difficult to identify the precise composition only by Raman spectrum. For obtaining the chemical information, the components of the corresponding Raman test points were detected by SEM-EDS. The SEM-EDS semiquantitative results (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) show that the crystals found in dark spots are not pure CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. It should be noted that Si, an element not incorporated in the spinel crystals, occupies nearly 20%, indicating the larger spatial resolution of EDS with respect to spinel size. This phenomenon is particularly obvious on the fine dendrites. Thus, only the Fe/Co ratio of each test points were compared here to further understand the component changes of different crystals and different parts in same crystal. From the middle to the center then to the end of the dendrite, Raman modes slightly shift to low wavenumber, which is mainly manifested in the increase of Fe/Co ratio from 1.96 to 2.35. The reason for this dendrite segregation should cause by the inhomogeneity of the glaze.\u003c/p\u003e\u003cp\u003eThe dark spots of the YL-6 are mainly dendrites extending up to 100 \u0026micro;m and dendrites or snowflake-like crystals between nanoscale to 30 \u0026micro;m in length (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). The YL-15 is dominated by dendrites range from 5 \u0026micro;m to 50 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). Raman analysis was performed on different positions of one large dendrite spine in YL-6 and small dendrites in YL-15. All Raman spectrum are consistent with that of Y-4. From the centre to the middle then to the end of the dendrite branch, the Raman peak position all move to the direction of low wavenumber with the Fe/Co increasing gradually due to the dendrite segregation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eCombining the analysis results of Raman and SEM-EDS, the plot of positions of A\u003csub\u003e1g\u003c/sub\u003e(1) and T\u003csub\u003e2g\u003c/sub\u003e(2) modes versus the Fe/Co ratio were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Along with the increase of Fe/Co ratio, the peak positions of A\u003csub\u003e1g\u003c/sub\u003e(1) and T\u003csub\u003e2g\u003c/sub\u003e(2) began to move to the low wavenumber gradually except for Y-4 testing point 2. Although the range of Fe/Co ratio (1.89\u0026ndash;2.35) in this group is too small to limit the discussion about the modes shift influenced by different chemical composition, the changing trend of the peak positions of A\u003csub\u003e1g\u003c/sub\u003e(1) and T\u003csub\u003e2g\u003c/sub\u003e(2) with Fe/Co ratio is consistent with previous studies [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. In general, the dendrites with Fe/Co atomic ratio between 1.89 and 2.35 at the dark spots from this group are closest to CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e spinel in all samples. The relative stability of the Fe/Co ratio of dark spots on the samples in Yuan dynasty and Yongle reign not only reflects the standardized pattern of the Qinghua porcelain firing technology in Jingdezhen imperial kilns, but also reflects the stable supply of raw materials during these two periods, especially the imported cobalt ores. Because the imperial kilns in these periods relied heavily on imported cobalt ores rather than domestic asbolane, the stable import of cobalt ores was an important basis for the production of Qinghua porcelain in imperial kilns [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe second group is the HW-2 and HW-4 samples from the Hongwu period (1368\u0026ndash;1398 CE) of the Ming Dynasty. The backscattered electron images reveal the crystalline blocks with a length of 5-150 \u0026micro;m formed in the dense crystals area, and two kinds of dendrites with different composition contrast in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The low-contrast dendrites always precipitate around the high-contrast crystals, forming mat-like dendrites.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eRaman spectroscopy was carried out to identify the blocks and dendrites with high contrast in the two samples firstly. The spectrum of crystalline blocks (point 1 in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and point 1,2,3 in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec) are close to that of dendrites (point 2,3,4 in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea) with high contrast. Except for the Raman modes approximately at 325cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 600cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the other peak positions show a good corresponding relationship with CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. In case of HW-2, the Raman peak positions of the testing point a1 at one crystalline block centre located at 203.1cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 325.4cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 466.8cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 603.7cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 696.7cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponding to the T\u003csub\u003e2g\u003c/sub\u003e(1), E\u003csub\u003eg\u003c/sub\u003e, T\u003csub\u003e2g\u003c/sub\u003e(2), A\u003csub\u003e1g\u003c/sub\u003e(2) and A\u003csub\u003e1g\u003c/sub\u003e(1) vibration modes of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, respectively. However, the two A\u003csub\u003e1g\u003c/sub\u003e symmetrical vibration modes have almost the same intensity. The E\u003csub\u003eg\u003c/sub\u003e vibration modes at 325.4cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the A\u003csub\u003e1g\u003c/sub\u003e(2) vibration mode at 603.7cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are different from the reported standard CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e Raman spectrum, also from the first group. The peak position of the E\u003csub\u003eg\u003c/sub\u003e and A\u003csub\u003e1g\u003c/sub\u003e(2) vibration mode of HW-2 sample is lower than that of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e standard spectrum by about 20-30cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Although the Fe/Co ratio affect the position of vibration mode of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e as mentioned before, no research reported a nearly 100cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e difference between the two A\u003csub\u003e1g\u003c/sub\u003e modes with same intensity.\u003c/p\u003e\u003cp\u003eIn order to further clarify the crystal structure, the chemical compositions of the corresponding Raman test points are analysed by SEM-EDS (Table \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e). Compared to the first group with few Mg doping (0.4% on average), the crystals from second group are doped with more Mg (1.75\u0026ndash;6.59%). According to the literature, there is no strong peak in the Raman spectrum of MgFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e around 600cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. However, the peak positions of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e move to the high wavenumber when part of Mg is doped in CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. Among them, E\u003csub\u003eg\u003c/sub\u003e mode has the most significant blue shift of 20-30cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. This better explains that the Raman E\u003csub\u003eg\u003c/sub\u003e mode peak position of the samples in the Hongwu period obviously shifted to 322.7-326.4cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. At the same time, blue shift are also observed in T\u003csub\u003e2g\u003c/sub\u003e(2) and A\u003csub\u003e1g\u003c/sub\u003e(1) modes to varying degrees. Moreover, Varshney [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] and Tong [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] have reported that the line widths of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e Raman modes increase with the Mg doping, which is attributed to strong electron\u0026ndash;phonon interaction and electronic disorder arising as random arrangement of Cations on the octahedral site. When the Mg /Co ratio increases from 0 to 1, the peak intensity ratio of A\u003csub\u003e1g\u003c/sub\u003e (1) and A\u003csub\u003e1g\u003c/sub\u003e (2) modes gradually decreases, then reaches the minimum and forms a similar double peak when the Mg /Co\u0026thinsp;=\u0026thinsp;1. When the Mg/Co is higher than 1, the peak intensity ratio of two A\u003csub\u003e1g\u003c/sub\u003e modes rises with the increase of Mg/Co. Finally, the A\u003csub\u003e1g\u003c/sub\u003e(2) mode almost disappears as Co is completely replaced by Mg. Based on the chemical composition, the Mg/Co of HW-2 and HW-4 samples are between 0.86 and 1.09, so the spectrum present two A\u003csub\u003e1g\u003c/sub\u003e modes with comparable intensities.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eA slight mode shift can be seen in the Raman spectrum of different test points on spinel of HW-2 and HW-4 according to different chemical composition. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, plot of the A\u003csub\u003e1g\u003c/sub\u003e (1) and T\u003csub\u003e2g\u003c/sub\u003e (2) positions versus the Fe/Co and Mg/Co ratio (at%) shows that the A\u003csub\u003e1g\u003c/sub\u003e (1) and T\u003csub\u003e2g\u003c/sub\u003e (2) shift to low wavenumber with the increase of the Fe/Co. As described above, with the increase of Mg content in CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, spectrum could show a blue shift, in the opposite direction with the increasing of Fe. Nevertheless, the blue shift with the increase of Mg is not observed in this study considering owing to stable Mg/Co (0.86\u0026ndash;1.09). Therefore, the shift of Raman modes is mainly influenced by Fe/Co rather than Mg/Co. Regarding our results, spectrum of the blocks and dendrites with high contrast in dark spots of HW-2 and HW-4 are consistent with an intermedia phase between CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with Mg substitutions.\u003c/p\u003e\u003cp\u003eAdditionally, the Raman spectrum of mat-like dendrite on HW-2 with low composition contrast (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb point5) belong to one of pyroxene, esseneite (CaFeAlSiO\u003csub\u003e6\u003c/sub\u003e) with five peaks at 328.2, 524.4, 658.4, 764.3, 962.6cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The mat-like dendrites in HW-4 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec point 4, 5) are also consistent with esseneite. The results of SEM-EDS further confirmed the identification. As a vital component of dark spots, it is first discovered in Qinghua porcelain samples. A growth mechanism for CaFeAlSiO\u003csub\u003e6\u003c/sub\u003e is proposed here. After the growth of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e on top of glaze surface, Ca, Al, Si is squeezed into the liquid phase around them. Then, CaFeAlSiO\u003csub\u003e6\u003c/sub\u003e started to grow on both sides of CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and become the dendrites. Similar crystallization mechanism was also found in other ancient porcelain samples, such as the pinholes in brown glazed stoneware from Yaozhou kiln [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eManganese ferrite (MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e)\u003c/h2\u003e\u003cp\u003eThe second category is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e including XD-1 and XD-5 from the Xuande reign and CH-8 from the Chenghua reign. The dark spots of XD-1 can be observed under the backscattering mode with a variety of crystal forms (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). Massive polycrystals aggregates as crystalline blocks reaching up to 50 \u0026micro;m. The dendrites surrounding the blocks are parallel to each other and periodically arranged so that the incident light is reflected at a certain angle, thereby causing the metallic lustre on a macro view, which is commonly known as \u0026ldquo;tin light\u0026rdquo; in ancient ceramics authentication. The dark spots of XD-5 show the developed reticular dendrites with its width of trunk up to 10\u0026ndash;20 \u0026micro;m are interspersed with high-contrast polyangular grains (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb). The CH-8 sample presents a large number of dendritic crystallites from nano-scale to 60 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe Raman spectrum were recorded on several positions. All spectrum are dominated by A\u003csub\u003e1g\u003c/sub\u003e peak ranging from 624.5 to 648.6 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. It is similar to the reported Mn and Fe spinel with the anti-spinel structure of the cubic crystal system which usually forms a solid solution between jacobsite (MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e), hausmannite (Mn\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) or even magnetite (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. For jacobsite, the one main dominated A\u003csub\u003e1g\u003c/sub\u003e peak of Raman spectra appear at 620\u0026ndash;640 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e which represents the stretching vibration of the Mn-O chemical bond at the octahedral position in the spinel structure [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. In our cases, A\u003csub\u003e1g\u003c/sub\u003e peak are slightly high for jacobsite. Different compositions of jacobsite could influence the Raman vibration mode. Some scholars have pointed out that if the proportion of Fe in jacobsite increases, the A\u003csub\u003e1g\u003c/sub\u003e mode shift towards high wavenumbers. As jacobsite and magnetite are isostructural, the A\u003csub\u003e1g\u003c/sub\u003e move to 626 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. With more substitution of Fe, jacobsite transform into magnetite (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) with the A\u003csub\u003e1g\u003c/sub\u003e position around 670 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Relatively, the substitution of Fe with Mn leads to the shifting of the spectral line to 630\u0026ndash;665 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, eventually forming hausmannite ((Mn\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Namely, no matter which side of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-Mn\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e the jacobsite is biased to, the A\u003csub\u003e1g\u003c/sub\u003e mode will move in the direction of high wavenumbers. Thus, it is difficult to reveal the specific crystal compositions solely by Raman spectrum.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe component of the corresponding Raman test point is estimated by SEM-EDS. The results (Table \u003cspan refid=\"MOESM3\" class=\"InternalRef\"\u003eS3\u003c/span\u003e) display that the main constitutive elements of the crystal are Mn (up to 21 at%) and Fe (up to 20 at%). The crystals in XD-1 and XD-5 with a Fe/Mn ratio lower than 2 are considered as MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026ndash;Mn\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and the crystals in CH-8 with a Fe/Mn ratio superior to 2 belong to MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026ndash;Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. On the basis of SEM-EDS measurements, the scatter plot of Fe/Mn ratio (at%) versus A\u003csub\u003e1g\u003c/sub\u003e(1) positions (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) further reveals the effect on the Raman vibration mode caused by different compositions of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-Mn\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e solid solution. Taking the Fe/Mn ratio equal to 2 as the turning point, the A\u003csub\u003e1g\u003c/sub\u003e mode positions first move to the low wavenumber and then to the high wavenumber with the increase of the Fe/Mn ratio. These findings are accordant with the previously reported Raman modes changes caused by different components of jacobsite. Furthermore, a shoulder peak at approximately 670\u0026ndash;680 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e gradually emerges which might be explained by the increasing of Co. The significant proportions of Co (0.67\u0026thinsp;~\u0026thinsp;10.67 at%) were found, which suggests that a complex phase of mixed Co-Mn ferrites is involved in samples from the Xuande and Chenghua reign. It is reported that, from MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e to CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, red shift in the phonon mode and change in the relative intensity between the two vibration modes A\u003csub\u003e1g\u003c/sub\u003e(1) and A\u003csub\u003e1g\u003c/sub\u003e(2) can be observed [\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. The A\u003csub\u003e1g\u003c/sub\u003e peak with composition x\u0026thinsp;=\u0026thinsp;0.4 (Co\u003csub\u003ex\u003c/sub\u003eMn\u003csub\u003e1\u0026minus;x\u003c/sub\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4)\u003c/sub\u003e begin to show a split, which further intensifies with the increase of Co\u003csup\u003e2+\u003c/sup\u003e in the mixed ferrite [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The crystals formulae were calculated based on the chemical composition of Raman testing points by EDS (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Except for spectrum of XD-1 point1(Co\u003csub\u003e0.4\u003c/sub\u003eMn\u003csub\u003e1.8\u003c/sub\u003eFe\u003csub\u003e0.8\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) and point 2(Co\u003csub\u003e0.4\u003c/sub\u003eMn\u003csub\u003e1.9\u003c/sub\u003eFe\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e), all the rest of spectrum with Co ratio higher than 0.4 show a A\u003csub\u003e1g\u003c/sub\u003e(2) peak with a shoulder A\u003csub\u003e1g\u003c/sub\u003e(1), which is in accord with earlier studies. In conclusion, the crystallites in dark spots of XD-1, XD-5 and CH-8 are reasonable to be identified as Co-Mn ferrites.\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\u003eCrystals formulae and Co/Mn obtained from Chemical composition of Raman testing points by EDS (at%)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\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\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSample code\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCrystal Formula\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCo/Mn\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCH-8-3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.8\u003c/sub\u003eMn\u003csub\u003e0.5\u003c/sub\u003eFe\u003csub\u003e1.7\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.50\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCH-8-2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.7\u003c/sub\u003eMn\u003csub\u003e0.5\u003c/sub\u003eFe\u003csub\u003e1.8\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.16\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eXD-5-2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.7\u003c/sub\u003eMn\u003csub\u003e0.9\u003c/sub\u003eFe\u003csub\u003e1.4\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.83\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCH-8-1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.5\u003c/sub\u003eMn\u003csub\u003e0.8\u003c/sub\u003eFe\u003csub\u003e1.7\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.68\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eXD-5-3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.7\u003c/sub\u003eMn\u003csub\u003e1.4\u003c/sub\u003eFe\u003csub\u003e0.9\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.47\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eXD-1-3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.5\u003c/sub\u003eMn\u003csub\u003e1.5\u003c/sub\u003eFe\u003csub\u003e1.0\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.35\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eXD-5-1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.6\u003c/sub\u003eMn\u003csub\u003e1.6\u003c/sub\u003eFe\u003csub\u003e0.8\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.33\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eXD-1-2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.4\u003c/sub\u003eMn\u003csub\u003e1.8\u003c/sub\u003eFe\u003csub\u003e0.8\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.22\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eXD-1-1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCo\u003csub\u003e0.4\u003c/sub\u003eMn\u003csub\u003e1.9\u003c/sub\u003eFe\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.20\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\u003eMoreover, the structure of the developed reticular dendrites in XD-5 assigns to anorthite according to RRUFF database [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e] (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). The dominating and sharp modes at 508 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with corresponding peculiar shoulder peak around 482 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are observed. Interestingly, they can also be seen in the spectrum b1 and b3 of XD-5 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). A similar crystallization mechanism such as CaFeAlSiO\u003csub\u003e6\u003c/sub\u003e and Mg doped CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e in HW-2 and HW-4 applies equally here.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFrom the perspective of dark spots, Fe/Mn variation rule of the crystallization composition is consistent with that of cobalt pigment source in imperial kiln from Yuan dynasty to the Chenghua reign of Ming dynasty. It illustrates that the crystallization composition of dark spots is largely affected by components of cobalt ores. The Fe-rich cobalt imported from Persia was used in imperial kiln from Yuan Dynasty to Yongle reign of the Ming Dynasty so that almost all of the crystallization in dark spots are Fe-rich spinels and none of manganiferous crystal is found. Significantly, the surging Fe content becomes a prominent feature of dark spots in Hongwu reign of Ming dynasty, which might be caused by a change in the formulation or treatment of cobalt blue pigments. As the beginning of Ming dynasty, the Hongwu reign had not yet fully controlled northern China [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. The \u0026ldquo;maritime prohibition\u0026rdquo; policy greatly restricted imports. Non-government trade was also strictly forbidden [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. Therefore, it is possible that craftsmen were forced to change the formula of cobalt pigment because of the disruption of imported cobalt ores. Considering the consistency and inheritance of craftsmanship in the Jingdezhen imperial kiln, it has a fairly high probability that the Qinghua porcelain production process in the Hongwu reign was inherited from Yuan dynasty. Accordingly, we demonstrate that the dramatic increase of Fe should be caused by the recipe change of blue pigment rather than the purification of raw material or firing process which would only reduce the content of impurities like Fe, Mn but not increase. Owing to the lack of imported cobalt ore, it was possible to mix imported cobalt ore with iron-bearing mineral, resulting in the characteristics of dark blue and \u0026ldquo;iron spot\u0026rdquo; with high-Fe in Hongwu reign. From the Xuande reign (1426\u0026ndash;1435 CE), the domestic cobalt ores with characteristic of high-Mn were begun to use [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. Thus, the crystallization in dark spots in Xuande and Chenghua reign are Co-Mn spinel ferrites. However, the Fe content of the Chenghua sample is higher than that of the Xuande samples, which might relate to the use of domestic cobalt ores from different regions.\u003c/p\u003e\u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, the necessity of the combining use of \u0026micro;-Raman and corresponding chemical composition analysis in complicated and various historical relics is emphasized. Based on \u0026micro;-Raman and SEM-EDS, we identified the various phases of crystals in the dark spots of Qinghua porcelain manufactured in imperial kilns from the Yuan dynasty to the middle Ming dynasty. CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with Fe/Co around 2 were found in dark spots of the Yuan and Ming Yongle reign when the imported cobalt ore with the high-Fe and low-Mn content was consistently supplied in imperial kiln. During the Ming Hongwu period, the dark spots had various crystal forms and different chemical compositions. Except for an intermedia phase between CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with Mg substitutions, the esseneite (CaFeAlSiO\u003csub\u003e6\u003c/sub\u003e) were also observed in dark spots. The high-Fe content is the characteristic of dark spots in this period, which is probably because of the mixture of imported cobalt ores and iron-bearing minerals. From the Ming Xuande reign, according to the use of domestic cobalt ore with different composition, the main crystal phase on dark spots in Xuande and Chenghua reigh assign to Mn richer and Fe richer Co-Mn ferrites, respectively. Anorthite is also a main part of dark spots in Xuande reign, which proves that dark spots are not only related to Fe, Mn. The characteristics of dark spots before and after Ming Xuande reign is entirely different, which could be a identification criterion to date the imperial Qinghua porcelain. Moreover, this study provides a new perspective on cobalt ores provenance sourcing and technology evolution of imperial Qinghua porcelain.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eCompeting interests\u003c/h2\u003e\u003cp\u003eNo competing interests associated with this study.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThis study was sponsored by the Shaanxi Provincial Department of Education Key Laboratory Research Project (20JS147) and Humanities and Social Sciences Research Project from the Ministry of Education of China (MOE, No.19YJAZH130).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eJZ helped devise the research questions addressed in this paper. PS and WXW led the μ-Raman experiments and analysis of data. ZFS, ZZW and WXW led the SEM-EDS experiments. JXJ provided samples and helped with the sample selection. WXW wrote the article. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThanks to the Jingdezhen Ceramic Archaeological Institute for providing samples, and especially Shurong Wu for sample selection. We also thank Tian Wang and Ariane Pinto for their aid in data analysis.\u003c/p\u003e\u003ch2\u003eAvailability of data and materials\u003c/h2\u003e\u003cp\u003eThe authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials and upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRawson J. The lotus and the dragon: sources of Chinese ornament. Orientations. 1984;15:22\u0026ndash;36.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi J. History of Science and Technology in China, Ceramics Volume. 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Export and Characteristics of Jingdezhen Porcelain in Middle Ming Dynasty. J northwest Minzu Univ Philos Soc Sci Ed. 1987;3:5.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhong Y, Qin D, Li K. Export and Characteristics of Jingdezhen Porcelain in Middle Ming Dynasty. Cult Relics. 2020;11:49\u0026ndash;66.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBao Z, Yuan H, Wen R, Chen K. The fast and direct characterization of blue-and-white porcelain glaze from Jingdezhen by laser ablation-inductively coupled plasma mass spectrometry. Anal Methods. 2015;7:5034\u0026ndash;40.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"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":"npj-heritage-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"hsci","sideBox":"Learn more about [Heritage Science](http://heritagesciencejournal.springeropen.com)","snPcode":"40494","submissionUrl":"https://submission.nature.com/new-submission/40494/3","title":"npj Heritage Science","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Dark spots, Qinghua porcelain, Raman, SEM-EDS","lastPublishedDoi":"10.21203/rs.3.rs-7799426/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7799426/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe dark spots, as a characteristic of the Yuan and early Ming Qinghua productions, have a high research value regarding the source of cobalt pigment and the technology development of Qinghua porcelain. Aiming to comprehensively study the complex crystallites component caused by inhomogeneous pigment and glassy matrix. Eight sherds of Qinghua porcelain with dark spots unearthed in the Jingdezhen imperial kiln from Yuan dynasty (1271\u0026ndash;1368 CE) to Ming Chenghua reign (1465\u0026ndash;1487 CE) were analysed by stereomicroscope, Raman spectroscopy and scanning electron microscopy equipped with energy dispersive spectrometer. Taking the Ming Xuande reign (1426\u0026ndash;1435 CE) as the turning point, we evidenced the Fe-richer spinels and Mn-richer spinels. Before Ming Xuande reign, the two types of Fe-richer spinels were identified as CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe-rich CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with high-Mg substitutions. Surprisingly, Esseneite (CaFeAlSiO\u003csub\u003e6\u003c/sub\u003e) were observed as well in Hongwu reign (1368\u0026ndash;1398 CE) as the non-negligible constituent of dark spots. Then, MnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e of high Mn and high Fe were found respectively in Xuande and Chenghua reign. By refining the phases of dark spots from different period, we propose an innovative identification method for the dating of imperial Qinghua porcelain. Moreover, it is proved that the compositions of dark spots correspond to component features of cobalt ores from different source that holds great promise for cobalt pigment and the technology development studies of Qinghua porcelain.\u003c/p\u003e","manuscriptTitle":"μ-Raman study reveals the microstructure characteristics of dark spots on Qinghua porcelains from Jingdezhen imperial kiln","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-27 19:40:31","doi":"10.21203/rs.3.rs-7799426/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-10-29T18:27:45+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-29T16:40:08+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-29T01:33:32+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-23T16:47:20+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-23T01:49:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"19630042102485797044193662827193611221","date":"2025-10-15T15:13:31+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"338328922660116000741897351376492278835","date":"2025-10-14T23:37:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"254050783740381659061207893547795960282","date":"2025-10-13T14:50:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"116491178809724267230542582164089618364","date":"2025-10-13T05:23:51+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-13T05:16:55+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-11T19:15:45+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-11T19:14:49+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj heritage science","date":"2025-10-07T12:08:48+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-heritage-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"hsci","sideBox":"Learn more about [Heritage Science](http://heritagesciencejournal.springeropen.com)","snPcode":"40494","submissionUrl":"https://submission.nature.com/new-submission/40494/3","title":"npj Heritage Science","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ad46a4b1-83f3-446c-92b2-f1efdc501e71","owner":[],"postedDate":"October 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-22T13:23:35+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-27 19:40:31","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7799426","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7799426","identity":"rs-7799426","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}