Thomas Begou

Frédéric Lemarquis

Antonin Moreau

Fabien Lemarchand

Holger Reus

Detlef Arhilger

Harro Hagedorn

Julien Lumeau
email: julien.lumeau@fresnel.fr

Application of static masking technique in Magnetron Sputtering Technology for the production of linearly variable filters

Keywords: Optical coatings, optical interference filters, linearly variable filters, deposition technology, magnetron sputtering

Introduction

Variable filters, in which the band pass central wavelength shifts along one direction of the component, is a promising way to simplify and to miniaturize spectral imaging systems which is of high interest especially for space applications where weight and compactness are of prime need. In fact, by combining a variable filter and a 2D array detector (CCD, CMOS camera, …), it is possible to realize simultaneously spectral analysis and imaging using pushbroom technique [START_REF] Mouroulis | Design of pushbroom imaging spectrometers for optimum recovery of spectroscopic and spatial information[END_REF].

From the design point of view, the possibly to obtain variable filters relies on the dependence of the spectral properties of a multilayer coating with the thicknesses of some of its layers, if not all. In the particular case of a Fabry-Perot filter, formed with two metallic mirrors, a simple change of the thickness of the cavity layer provides a shift of its centering wavelength. This simple structure has the advantage to naturally give broad rejection bands, but is not adequate to provide sharp transition band-pass and leads to high absorption losses. To improve this last point, a solution consists to use the so-called induced transmission filter approach where a metallic layer is placed at the minimum of the electric field distribution in the cavity of dielectric Fabry-Perot filter [START_REF] Piegari | Variable narrowband transmission filters with a wide rejection band for spectrometry[END_REF][START_REF] Piegari | Variable narrow-band transmission filters for spectrometry from space. 2. Fabrication process[END_REF][START_REF] Williams | Single-step fabrication of thin-film linear variable bandpass filters based on metal-insulator-metal geometry[END_REF]. However, the only way to produce a filter with arbitrary specified properties in terms of rejection, width and sharpness consists to use a standard all-dielectric approach formed with a multi-cavity Fabry-Perot structure associated with additional dielectric short and long wavelength pass blocking filters. In that case, the thickness of all the layers must be tuned by a common factor, resulting in a proportional wavelength shift, to produce a variable filter [START_REF] Lemarquis | 400-1000 nm all-dielectric linear variable filters for ultra compact spectrometers[END_REF][START_REF] Ko | Visible and near-IR range linear variable filter with two-dimensional modeling[END_REF].

Obviously, increasing the performance requires to increase the design complexity, and namely the layer count: a few layers for a metallic Fabry-Perot structure, to a few tens for an induced transmission filter, and to more than one hundred for an all dielectric solution.

There are mainly two technologies that have been investigated to produce variables filters. One consists in combining thin-film deposition with lithography and etching, but is limited to metal-dielectric approach or single cavity all-dielectric Fabry-Perot filters to avoid multiple etching processes [START_REF] Emadi | Fabrication and characterization of IC-compatible linear variable optical filters with application in a micro-spectrometer[END_REF]. The second consists in producing masks that are inserted into the deposition machine in order to generate, by the mean of a relative motion between the masks and the substrates, large and controllable thickness gradients.

Several masking geometries exist, resulting in different characteristics in terms of deposition rate and mass-production capability, but that also concern the curvature of iso-thickness lines on variable filters. Up to now, we only mentioned the thickness gradient as the main characteristic of a variable filter. However, one can define a "uniform" direction that is perpendicular to the "non-uniform" gradient direction, and iso-thickness lines along this uniform direction should ideally be perfectly straight. To achieve such a result, the substrate and the mask must have a relative translation movement, necessarily back and forth, the substrate being completely hidden behind the mask at both extremities. The main consequence of such a translation geometry is that only one filter is likely to be manufactured at a time, moreover with a very low deposition rate [START_REF] Piegari | Variable narrowband transmission filters with a wide rejection band for spectrometry[END_REF][START_REF] Piegari | Variable narrow-band transmission filters for spectrometry from space. 2. Fabrication process[END_REF].

On the contrary the use of a continuous rotation movement, which is compliant with most deposition machines equipped with a rotary substrate holder, allows a maximum deposition rate and the manufacturing of several filters at a time, but necessarily results in circular iso-thickness lines, the radius of curvature being equal to the distance to the rotation axis. These two cases are detailed in reference [START_REF] Abel-Tibérini | Masking mechanisms applied to thin-film coatings for the manufacturing of linear variable filters for two-dimensional array detectors[END_REF] which also reports on a more complex solution mixing both rotation and translation movements in order to benefit from the advantages of each of these exclusive solutions. The impact of the curvature of iso-thickness lines completely depends on filters applications. In case the filter is directly associated with a detector, a wavelength calibration can be performed to take account of this curvature. But it might be more critical if filters are associated with other dispersive elements [START_REF] Bernardi | Wedge filter imaging spectrometer[END_REF].

In our case, we are interested in the manufacturing of all dielectric variable filters that typically cover the spectral range of silicon detectors (400-1100 nm). Although this paper does not deal with the manufacturing of such filters, preliminary design studies allowed us to estimate the required layer count to about 140, for a total thickness up to 18 µm. Among the deposition machines that are available at Institut Fresnel, the Bühler HELIOS machine is considered as the most appropriate. First, its deposition process is highly stable, and particularly interesting for the deposition of complex coatings with a high layer count, as proved by the filters we manufactured for the IDEFIX, TARANIS or 3MI projects [START_REF] Lemarquis | Broadband antireflection coatings for visible and infrared ranges[END_REF][START_REF] Begou | High performance thin-film optical filters with stress compensation[END_REF][START_REF] Begou | Complex optical interference filters with stress compensation for space applications[END_REF] with more than 100 layers and 25 µm thick. Second, the HELIOS rotating substrate holder has a large diameter, between 460 and 740 mm, which gives, according to the position of the gradient on the substrate relatively to the substrate holder and using a simple masking technique, a 700 mm radius of curvature for variable filters, this value being large enough for our requirements.

The goal of this paper is to describe the masking system we developed for the manufacturing of variable filters with this machine, and the tests we performed to demonstrate the validity of the proposed approach. We will validate this process with the deposition of single layers of high and low refractive index materials (Nb 2 O 5 and SiO 2 ) and of two cavities band pass filters [START_REF] Begou | Linearly variable filters fabricated by magnetron sputtering technology[END_REF][START_REF] Begou | Non-linearly variable filters for spectro-imaging systems[END_REF].

Experimental set up to create a gradient 2.1 Deposition system description

The deposition technology used in this work is plasma assisted reactive magnetron sputtering (PARMS) with a HELIOS machine from Bühler Leybold Optics [START_REF] Scherer | UV-and VIS Filter Coatings by Plasma Assisted Reactive Magnetron Sputtering (PARMS)[END_REF]. The deposition parameters (for sputtering and plasma assistance) and geometry (targets, uniformity mask…) are identical to that are used with the standard configuration of a HELIOS machine. The only difference is described hereafter and consists in adding masks inserts within the actual shutter. The filters presented in the following parts are made using Nb 2 O 5 and SiO 2 respectively for high and low index materials.

Masking technology

In the Helios machine, substrates are located on a turntable, a few centimeters below the sputtering targets. The deposition is consequently intermittent and occurs only when a substrate is under a sputtering target, which approximately corresponds to a 120×120mm² area for each target. The deposited thickness during each turn is then proportional to the time required for the substrate to rotate in front of the target. If a mask is positioned between the target and the substrate, the deposited thickness is then proportional to the time required for the substrate to rotate in front of the open area of the mask, or in other words to the length of this open area. In its standards configuration, such masks are used to insure the best thickness radial uniformity (generally much better than 1% over 100 mm diameter). The idea developed for this study consists in developing masks that will produce an opposite effect, and generate a significant radial thickness gradient.

The developed technique consists in keeping uniformity masks below the target which guarantee a starting uniform distribution of sputtered material and to add new masks as close as possible to the substrates to control the deposition time of material along the radial direction on the substrate and thus generate the thickness gradient (Figure 1). However, for convenience reasons, we decided to implant these new masks directly on the shutter that is placed in between the targets and the substrates. But due to the few millimeters between the masks and the substrates, the design of the masks requires careful design and mounting. Fig. 1. Masking set up installed on the HELIOS deposition machine. At the bottom one can see the turntable where samples are inserted. On top, one can see the masks installed on the shutter position right in front of the magnetron sources. When lid is closed and turntable is rotating, each samples moves in front of each magnetron source and a thickness gradient is produced.

Shutter opened Shutter closed

Shutter

Gradient mask

Turn table

Uniformity mask

A new type of shutter and masks was designed, fabricated and then installed on the HELIOS machine [START_REF] Begou | Linearly variable filters fabricated by magnetron sputtering technology[END_REF][START_REF] Begou | Non-linearly variable filters for spectro-imaging systems[END_REF]. The shutter is made of titanium, like the original shutter, to limit the deformation related to thermal effects while the masks are made of aluminum to allow an easier machining during the development phase. Then, they should also be made of titanium as well as the fixing screws to further minimize thermal effects. The control programs of the machine have also been adapted to allow the use of the mask with new adapted positions.

Principle for the generation of a variable filter

In order to validate the principle of a thickness gradient generation, we designed a first set of masks, as illustrated in the Figure 2. The masks have 3 distinct zones:

 A uniform 46 mm-long and 55 mm-wide area to perform in-situ optical monitoring. This area allows to control the thickness deposited in the region having the thinnest material thickness (zone number 1 in figure 1).

 An area which width varies by a factor of 3 from 55 to 165 mm over a distance of 10 mm. It is this area that will be used to generate a thickness gradient with a ratio close to or equal to 3 (zone number 2 in figure 1).

 Another uniform area 19 mm-long, 165 mm-wide to avoid the shading effects associated with the edge of the mask (zone number 3 in figure 1).

Deposition of single layers with variable thicknesses

In order to precisely characterize the generated gradient, we have deposited, through the gradient mask described previously in figure 2:

 a 5 quarter-wave, at 500 nm, (i.e. ~272 nm thick) single layer of Nb 2 O 5 on a silica substrate,  a 5 quarter-wave, at 500 nm, (i.e. ~428 nm thick) single layer of SiO 2 on a silica substrate previously coated with a uniform 5 quarter wavelength at 500 nm layer of Nb 2 O 5 .

The local transmission has been measured using a custom set-up that allows to map the transmission over the sample aperture [START_REF] Abel-Tiberini | Dedicated spectrophotometer for localized transmittance and reflectance measurements[END_REF]. The measurement has been performed along the x direction over 93 mm, with a step of 500 µm and a beam size of approximately 100 µm. To extract the thickness profile of both materials, reverse engineering has been used for each measured spectral transmission curve. The estimated error on this calculation is in the order of one nanometer. Figure 3 illustrates the absolute and relative evolution of the physical thicknesses of both materials along the x direction. At both extremities of the mask, sharp variations of the thickness can be noticed as shadowing effects associated with the mask. Then two distinct regions can be observed: one between 25 and 60 mm associated with the uniform zone of the mask and one between 65 and 80 mm associated with the gradient zone of the mask. Let us now analyze these two regions into details. Firstly, it can be noted the same mask gives almost identical thickness gradient for both materials, this last condition being required to guarantee the production of high performance variable filters. As a consequence, two identical masks can be used for both materials. Secondly, a comparison between the measured relative thicknesses evolutions and the expected variation derived from the shape of the masks is plotted in figure 3, and one can see that a mismatch exists. Indeed, the gradient of thickness is measured over a 18.5 mm range instead of the expected 10 mm and the ratio between the thickest and the thinnest regions is 2.22 instead of 3. This mismatch is probably due to shadowing effect linked to the distance between the mask and the substrate. This reveals a limitation of the considered masks geometry to achieve large gradients and large deposition rates. We observed that by decreasing the overall aperture of the mask, it is possible to better approach the expected gradient but with a significant decrease of the deposition rate. We decided in this work to keep this mask geometry and thus we opted for the calculation of calibration parameters to account for this mismatch.

A second mismatch can be seen also in the uniform region devoted to optical monitoring. The thickness was indeed expected to be constant, while one can observe a slight decrease (-0.25%.mm-1). To achieve such a result, the mask should have in this region the shape of an angular sector centered on rotation axis of the substrate holder. For an easier manufacturing of this test mask, this shape has been replaced by parallel edges, resulting in a slightly decreasing thickness, which is not considered as critical at this point.

Finally, considering this tooling factor but also due to the gradient observed in the supposedly uniform region in the optical monitoring region, one can show that the real gradient produced by the increase of the aperture of the mask is not 2.22 but closer to 2.64 (right part in Figure 3). Moreover, as seen in Figure 4, the deviation towards a linear gradient is less than ± 2% over a 14 mm range.

Despite these discrepancies, one can see that a ratio of 2 on the thickness can be achieved over a 12 mm aperture, which is within what is commonly proposed by linearly variable filters manufacturers [17,18].

Theoretical approach

One specificity of the proposed approach is that, due to the machine geometry, two distinct masks are required to produce the gradient of thickness, one for each material source. Therefore, to produce high quality optical filters, it is mandatory to be able to precisely position the masks in a way that both the high and low index materials thickness gradients will be perfectly superimposed on the substrates to be coated. To allow a precise relative positioning of the masks a preliminary manufacturing test has been planned with a specific 2-cavity band pass filter, one cavity being made of high index material, the other one of low index material: H L 2H L H L H 2L H. In case the two masks are miss-positioned, resulting in different thickness gradients for both materials, the centering wavelengths associated to each cavity should gradually sweep along the variable direction, resulting in a final distortion and collapse of the band pass profile. Figure 5 gives the spectral transmission profile of such a filter, centered at wavelength 500 nm and modeled using standard matrix formalism [START_REF] Macleod | Thin-Film Optical Filters[END_REF]. Assuming that the film thickness is directly proportional to the mask width, we can calculate the evolution of the spectral profile of the filter. This evolution is given in figure 6. The representation chosen in the left part of figure 6 allows to plot on the same graph the evolution of the transmission in a color level scale as a function of the wavelength and the x position. Below the 46 mm x-position, the spectral transmission profile of the filter is constant and identical to the one plotted in figure 5. From x = 46 mm to 56 mm, we can see (right part of figure 6) that the central wavelength of the filter varies from 500 nm to 1443.5 nm. Despite an increase of the thickness by a factor of 3 from the narrowest to widest part of the mask, the filter central wavelength only increases by a factor of 2.89. This is due to the fact that the central wavelength of a filter is not linked to physical thickness but to optical thickness and the decrease of refractive index with wavelength (dispersion).

These calculations were made considering the same thickness gradients for the high and low refractive index materials. Let us now consider the result considering some miss-positioning between the two masks. Considering the projection of the masks image for both targets on the sample to be coated, a misalignment (Δx) between the masks on both targets, for high and low refractive index materials shifts the two gradients of material along the x direction on the sample. A parametrical simulation has then been performed by varying Δx from -2.5 mm to 2.5 mm. These values were chosen as they seem to be representative of the mechanical tolerances associated with the masks and the opened shutter on the machine (see figure 1 and2). It can be easily shown that except at the gradient boundary, linear gradient is kept when masks are misaligned. But the most visible effect of a mask misalignment concerns the spectral performance of the filter, especially the shape of the band pass. Figure 7 gives the spectral dependence of the transmission when the filter is centered at 650 nm for different x misalignment values. As can be seen, the transmittance level in the band pass is severely affected and decreases as a function of the shift between both masks, from 100% down to 50%, whether this shift is positive or negative. An asymmetry of the band pass is also observed and this asymmetry depends on the sign of Δx. If the shift is negative, the amplitude of the first peak is higher than the one of the second peak. An opposite effect appears when the shift is positive, i.e. the amplitude of the second peak is higher than the one of the first peak. This effect is used to determine the relative position of the two masks: the position of the maximum (first or second peak) providing an information on what gradient starts first and the relative amplitude of the maxima of transmission, the exact distance between these two masks.

Deposition of a band pass Fabry-Perot filter

Mask alignment procedure

Gradient masks were first set in a position on the shutter that should be close to the optimal one. Then, the double cavity Fabry-Perot filter was deposited. To check the relative positions of both masks, the local transmission was measured on one point in the gradient region. As seen on the left part of figure 11, this measurement was compared with the theoretical transmission calculated with no shift Δx between both masks. The maximum of the measured transmission is less than that of the theoretical one and dissymmetry between both maxima is observed on experimental data. As demonstrated previously, this effect can be explained by a shift between the relative positions of both masks. If a -2 mm shift Δx is added in the simulation, a much better agreement between experimental and theoretical transmission curves is achieved, as seen on the right part of the figure 12. The residual mismatch between the curves can be easily explained by measurement uncertainties and especially the averaging of the variable transmittance over the measurement spot, which is not taken into account for the calculated profile. To align the masks correctly, a +2 mm shift was then physically applied on the position of the gradient mask under the low index material target. The same deposition of double cavity Fabry-Perot was carried out and the local measurement of transmission was again performed within the same region. The transmission is illustrated in figure 13 and, for this configuration, the experimental transmission matches with theory, confirming that the misalignment between the two masks has been corrected. These results therefore show that the proposed method allows to accurately predict the relative misalignment between the two masks and careful analysis allows to conclude that a precision of ±100 µm between the position of the two masks can be achieved. Simulation of various band pass filters showed that such a mismatch is small enough to secure high performance linear band pass filters. Fig. 13. Comparison between the local transmission measured on the sample and the theoretical one with a +2 mm shift of the mask under the low index material target.

Linearly variable filter characterization

Up to now, we only looked at one transmission spectrum measured in one arbitrarily chosen position in the gradient zone, for alignment purpose. Since the last profile shows that the alignment is now correct, this last filter can be considered as representative of our capability to manufacture variable filters. We can now focus on the characterization of its performance within the whole gradient zone. To extract the performance of the filter, the local transmission spectra was measured within the uniform and gradient zones with the same set up described in section 3 and the upper left par of figure 14 gives the spectral evolution of the transmission as a function of x. The left part of the figure 14 is a 3D plot of the evolution of the measured transmission as a function of the wavelength and the x position. Globally, the shape of this evolution is relatively close to the one plotted in figure 3. Similarly, to the spatial evolution of the transmission spectra measured in single layers, we observe some shadowing effects at the gradient masks openings boundaries (i.e. for x position close to zero and 60) resulting in sharp fluctuations of the transmission as well as two regions with either a uniform or linear gradient of the central wavelength of the filter. To accurately characterize the gradient zone, the central wavelength of the band pass was extracted for all the x positions and is plotted in the right part of figure 13. To compare this evolution with the theoretical one, the evolution of the central wavelength of the filter was simulated with the corrected parameters defined with single layers (see Figure 3). Using this model, one can see a very good agreement between theory and experiment confirming that the parameters used to model single layers allows an accurate modeling of more complex structures. Finally, as seen in the top left of figure 15, if we remove the transition zones within the gradient region, a filter with a band pass linearly varying from 550 nm to 1000 nm over a 12 mm distance (i.e. 37.5 nm.mm-1) was realized. The measured and theoretical spectral transmissions are also plotted for different x position on the filter, in figure 15, with a very good agreement from 500 nm to 1050 nm that is the wavelength range of the variable filter.

Conclusions

We have demonstrated in this paper an efficient method for variable thickness generation, compatible with a Bühler HELIOS type deposition machine, and allowing the fabrication of complex filters. Due to the machine geometry, this method relies on the use of two gradient masks, one for each material, that are fixed on the shutter. Because of shadowing effects resulting from the distance that exists between the shutter and the substrates to coat, the thickness gradient cannot be obviously related to the mask shape. For this purpose, measurement performed on variable single layers helped us to define a correction factor that allows now to design masks for a specified thickness gradient. We also developed a method that guarantees an accurate relative positioning of the two masks, based on the deposition of a simple 9-layer two-cavity Fabry-Perot filter, each material being used to form one of these cavities.

The wavelength shift measured on this filter is perfectly consistent with the results observed on single layers, the centering wavelength being approximately doubled within 13 mm.

Fig. 2 .

 2 Fig. 2. Photography and schematics of the first mask designed to generate variable filter.The maximum width of the masks is fixed by the wider aperture of the material sources (165 mm) in order to keep the deposition rate at its maximum, close to the one in classical uniform configuration of the HELIOS machine (≈ 0.4-0.5 nm.s -1 for both materials).

Fig. 3 .

 3 Fig. 3. Physical (left) and relative (right) thickness profiles for single layers of SiO 2 and Nb 2 O 5 .

Fig. 4 .

 4 Fig. 4. Relative shift between the experimental and theoretical thickness gradient for single layers of SiO 2 and Nb 2 O 5 in the linear gradient region (right).

Fig. 5 .

 5 Fig. 5. Transmittance of the double cavity Fabry-Perot when centered at 500 nm.

Fig. 6 .

 6 Fig. 6. Spectral evolution of the transmission (left) and of the central wavelength λC (right) of the double cavity Fabry-Perot filter along the x direction.

Fig. 7 .

 7 Fig. 7. Spectral evolution of the transmission of the double cavity Fabry-Perot filter when it is centered at 650 nm with different negative (left) and positive (right) values of Δx.

Fig. 12 .

 12 Fig. 12. Comparison between the local transmission measured on the sample and the theoretical one for different values of the shift between both masks used in the simulation Δx. (on the left part Δx = 0 mm and on the right part Δx = -2 mm).

Fig. 14 .

 14 Fig. 14. Measured spectral evolution of the transmission (left) and of the measured and theoretical central wavelength λC (right) of the double cavity Fabry-Perot filter along the x direction (linearly variable filter zone is in the blue part).

Fig. 15 .

 15 Fig. 15. Measured and theoretical central wavelength λC (top left) of the double cavity Fabry-Perot filter along the x direction and measured and theoretical spectral transmission for different x position (a, b and c).

Availability of data and material

Data can be provided upon request

Code availability (software application or custom code)

Not applicable

All the required tools seem now ready for the manufacturing of a more efficient all-dielectric variable filter, with a much more complex design since more than one hundred layers are required for a narrow band filter covering a similar spectral width with a reasonable rejection level.

Declarations Funding

The authors would like to acknowledge the French space agency (CNES) for the funding of this work.

Conflicts of interest/Competing interests

Not applicable

Authors' contributions

All the authors contributed equally to the work

Additional declarations for articles in life science journals that report the results of studies involving humans and/or animals

N/A

Ethics approval (include appropriate approvals or waivers)

N/A

Consent to participate (include appropriate statements)

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Minli Wang

Tian Wang
email: wangtian@sust.edu.cn

Fen Wang

Clement Hole

Jun Cao

Jianfeng Zhu

Pei Zhang

Pei Shi

Philippe Sciau
email: philippe.sciau@cemes.fr

Raman study of rusty oil spotted glaze produced in Linfen kilns (Shanxi province, AD 1115-1368)

Keywords: Linfen kilns, microstructure, hematite, ε-Fe 2 O 3, pseudobrookite

Abstract:

The Linfen kiln complex, one of the most representative of Shanxi kilns, is famous for its oil spotted glazed ware production. In this work, micro-Raman spectroscopy was applied to analyze the nature of the rusty oil spot pattern of a bowl fragment from Linfen kilns. Crystals of polygonal shape found in the spots at the glaze surface and around bubbles in the cross-section of the glaze were identified as hematite. Their distribution in the glaze suggests that nucleation seeds and small crystals are carried toward the surface by the rise of bubbles during the heating step of the firing. Raman spectra combined with EDS measurements also suggest that Fe ions in hematite crystals are partially substituted, causing the variation of band position and FWHM of the spectra. Dendritic ε-Fe 2 O 3 was identified in brown areas of the black glaze, as well as pseudobrookite crystals of tens of microns in the crosssection of the glaze. The degree of the glaze polymerization (A 500 /A 1000 ) was calculated at 1.85, indicating a firing temperature lower than 1300°C, consistent with the archaeological data.

Their popularity and preciousness led many northern kiln complexes, such as the Ding kilns (Hebei province), Hebi kilns (Henan province), Linfen kilns, Jiexiu kilns or Hunyuan kilns (Shanxi province), to compete to imitate Jian oil spotted wares. [START_REF] Zhang | The Science of Ancient Chinese Ceramics, Shanghai People's[END_REF] Among these northern complexes, the Linfen kilns were one of the most famous kilns due to their large production and high demand during the Jin and Yuan periods (AD 1115-1368). [START_REF] Zhang | The Science of Ancient Chinese Ceramics, Shanghai People's[END_REF] Many studies were carried out to understand the formation process and color variation of the oil spot pattern. [START_REF] Xu | [END_REF][5][6][7][8][9][10][11] It is believed that this pattern comes from local iron enrichment during the firing, which results in locally iron-supersaturated glaze leading to local crystallization of iron oxides, such as hematite (α-Fe 2 O 3 ), magnetite (Fe 3 O 4 ) [5,9] or ε-Fe 2 O 3 [START_REF] Xu | [END_REF]6] during cooling stage. The silvery shine of Jian oil spotted wares is hypothesized to come from ε-Fe 2 O 3 crystals of several micrometers in size organized in a periodic two dimensional fashion. [6] Studies about oil spotted glazes were mainly devoted to the more famous Jian wares [6,9] or replicated production [7,8,10,11] but few focused on Linfen wares and the rusty oil spot pattern.

Micro-Raman spectroscopy is a suited technique to investigate micro-crystalline phases in the glazes of ancient ceramics thanks to the micro-scale of the probe and its high sensitivity. This technique enable a precise discrimination of the different iron oxides, such as hematite, [12] maghemite (γ-Fe 2 O 3 ), [13] ε-Fe 2 O 3 [14] and magnetite [15] at very low laser power (less than 1 mW) and without any strong influence from glassy matrix. The non-destructiveness of this technique is also perfectly suited for analyzing precious and ancient artifacts.

In this work, a representative fragment of Linfen rusty oil spotted glazed ware was selected and analyzed to determine the chemical nature of the pattern and its surrounding black glaze using micro-Raman spectroscopy combined with XRF, optical microscopy and SEM-EDS. Based on the results, the formation process and chemical influence on the color of the oil spotted glaze are discussed.

| MATERIALS AND METHODS

| Sample description

The rusty oil spotted glazed fragment studied is presented in Figure 1a. It was excavated from Linfen kilns in Shanxi Province and can be dated from the Jin or Yuan Dynasty (AD 1115-1368) according to its archaeological context. It is characterized by a highly glossy black glaze decorated with rusty oil spots, which is DOI: 10.1002/jrs.6229 consistent with features of a typical oil spotted bowls from the Linfen kilns. [START_REF] Li | History of Science and Technology in China, Ceramics[END_REF]

| Analytical techniques and conditions

Optical microscopy equipped with an ultra-depth-of-field system was implemented to observe the oil spotted glaze surface. X-ray fluorescence spectroscopy was performed on the glaze surface and body. Scanning electron microscopy coupled with energy dispersive spectrometry and micro-Raman spectroscopy were implemented on the surface and a cross-section of the glaze. The preparation process of the cross-section is described elsewhere. [16]

| Optical microscopy equipped with an ultra-depth-of-field system

Optical imaging of the glaze surface was performed using a KEYENCE optical microscope equipped with a surface depth observation system (VHX-7000). Images were taken with a magnification ranging from x20 to x2500. Ring and coaxial light sources were used to observe the surface coloration and particle distribution in the glaze of the sample.

| X-ray fluorescence spectroscopy (XRF)

Elemental composition of the oil spotted ware was determined by energy dispersive X-ray fluorescence (EDXRF, XGT-7200, Horiba) equipped with an optical microscope to select the zones of interest. Measurements were carried out under vacuum and the X-ray tube voltage was 50 kV. Acquisition time was set to 120s for each point and the incident X-ray beam diameter was 1.2 mm. A minimum of three spectra was recorded on the glaze surface and the body to estimate their homogeneity.

| Scanning electron microscopy and energy dispersive spectrometry (SEM-EDS)

Prior to SEM-EDS analyses, the surface and cross-section of the sample were coated with a thin layer of gold to enhance their conductivity. The morphology and elemental composition of crystals in the glaze were investigated using a scanning electron microscope (FEI Apreo LoVac) equipped with an energy dispersive X-ray spectrometer analysis system (EDAX Octane Elect Super). EDS spatial resolution was around 0.2 µm. The Origin software was utilized to analyze the corresponding EDS spectra.

| Micro-Raman spectroscopy

Micro-Raman spectroscopy was performed on the surface and the cross-section of the glaze at room temperature using the Renishaw inVia spectrometer with a Peltier cooled CCD detector. 532 nm excitation was provided by a solid-state laser with a spot size of about 2-3 µm in diameter and a power inferior to 1 mW to optimize the signal/background ratio and avoid any thermal effect. Collection time of 100 s for each spectrum was chosen to obtain a refined resolution inferior to 1 cm -1 . Igor software was used for data treatment. The intelligent polynomial baseline was selected to subtract the photoluminescence background and the polynomial order DOI: 10.1002/jrs.6229 was defaulted to be 11. Minimum smoothing was performed to obtain the optimal spectral curves.

| RESULTS

| Elemental composition

Table S1 shows the elemental compositions recorded on the oil spots, surrounding black glaze and body of the sample. The main constituents of the glaze are SiO 2 (~69.1%), Al 2 O 3 (~14.6%), CaO (~4.9%) and Fe 2 O 3 (~5.1%). The MgO content is around 2.90% and K 2 O amount is about 1.85%. Small amounts of Na 2 O (~0.6%), TiO 2 (~0.8%) and MnO 2 (~0.1%) were also detected. The elemental composition recorded on the spots is relatively similar to the one of the black glaze, the only difference being the slightly higher iron concentration in the spots. The similarity can be explained by the fact that the analyzed area of XRF is around 1.2 mm, thus could not be precisely positioned on the oil spots, whose sizes vary from 0.4 to 1.4 mm (Fig. 1). The body contains more Al 2 O 3 and less SiO 2 , CaO and Fe 2 O 3 than the glaze.

Table 1 Analytical results (wt%) obtained by X-ray fluorescence on the glaze surface (standard variations in brackets) compared with ones of oil spotted glaze wares of Linfen kilns (Shanxi province), Jian kilns (Fujian province), Jiexiu kilns (Shanxi province) and Xiao kilns (Anhui province) [4]. The mole ratios of the glaze modifiers (∑[1/2Na2O+MgO+1/2K2O+CaO+1/2Fe2O3]) to network form (∑[1/2Al2O3+SiO2]) of OS glazes were calculated, abbreviated as ∑M/∑[Al]+[Si].

The main analytical results (wt%) reported in the literature [START_REF] Xu | [END_REF] on oil spotted glazes from different kilns are given in Table 1. The major elements found in the present sample are consistent with the recordings of wares from Linfen kilns. SiO 2 and Al 2 O 3 concentrations are around 68.7-68.8% and between 13.4-14.3%, respectively. The glaze presents a relatively high CaO concentration (4.3-4.7%). Only differences in K 2 O (relatively lower than the concentrations reported in the literature [START_REF] Xu | [END_REF] ) and MgO (slightly higher than the concentrations reported [START_REF] Xu | [END_REF] ) were observed. Compared with fragments from different kilns, fragments from Linfen, Jian and Xiao kilns present slightly higher SiO 2 (68.3-69.9%) than the ones from Jiexiu kilns (65.2%). Fragments from Linfen and Xiao kilns present slightly lower Al 2 O 3 amount (12.2-14.3%) than ones of Jian and Jiexiu kilns (15.1-15.7%). The glaze modifiers (Na 2 O, MgO, K 2 O, CaO ...) concentrations in wares from different kilns are more various from one to another. In order to precisely discuss their variations, the molar ratios of the glaze

modifiers (∑[1/2Na 2 O+MgO+1/2K 2 O+CaO+1/2Fe 2 O 3 ]) to the network formers (∑[1/2Al 2 O 3 +SiO 2 ]
) of oil spotted glazes were calculated. The ratio of the glaze from Linfen kilns calculated around 0.14 is consistent with previous reports averaging around 0.15. It can be noted that oil spotted glazes from Jiexiu and Xiao kilns (Anhui province) present a slightly higher ∑[M]/∑[Al]+[Si] ratio (~0.17) than the Jian wares, which is the lowest at ~0.11, indicating than the firing temperature needed to manufacture Jian wares might be slightly higher than the one needed for the wares from other kilns, according to the theory of silica-rich and modifier (Na, K, Ca,…)poor compositions corresponding to high firing temperature. [17,18] However, since the different ratios between the kilns are relatively similar and the analytical accuracy was not fully taken in account, one should be particularly cautious with these ratios.

Oil spotted glaze SiO 2 Al 2 O 3 Fe 2 O 3 CaO K 2 O MgO TiO 2 MnO Na 2 O P 2 O 5 ∑[M]/ ∑[Al]+[Si]

| Morphological analyses

Low magnification optical microscopic imaging of the glaze surface (Fig. 1a andb) reveals the relatively homogeneous distribution of the spots on the glaze. It also highlights that the black glaze contains many light brown areas. The spots are crimson in color and their size varies from 0.4 to 1.4 mm (Fig. 1b andc). Imaging at greater magnification (x2500) under a ring light source reveals that they are composed of many crimson polygonal-shaped crystals of several microns in size (Fig. 1d). Conversely, the light brown areas contain many small shiny crystals according to x2500 magnification imaging under a coaxial light source (Fig. 1e andf). SEM-BSE imaging and EDS analyses were performed on the glassy matrix, polygonal crystals in the oil spots and the shining crystals in the light brown areas (Fig. 2). Fig. 2a shows one oil spot in the glaze at low magnification. Higher magnification imaging of the crystals from this spot confirms that the small polygonal crystals constitute of the oil spot pattern (Fig. 2c). The shiny crystals found in the light brown zones of the glaze (Fig. 1f) are dendrites of several microns (Fig. 2e). EDS analyses were performed on the glassy matrix (Fig. 2b), the polygonal crystals in the oil spots (Fig. 2d) and the dendritic crystals in the brown areas (Fig. 2f). Analytical results shown in Table 2 highlight that the polygonal crystals contain mainly Fe, O and a little Ti, with a Ti/Fe ratio of about 0.07 (Fig. 2d and Table 2). The Ti concentration found in the crystals, albeit relatively low, is higher than in the glassy matrix, confirming that the crystals indeed contain Ti (Table 2). The dendritic crystals are composed of Fe and O (Fig. 2f and Table 2). As the glassy matrix is relatively rich in Si, Al, Ca, K and Mg (Fig. 2b andTable 2), the signals of other elements detected in the spectra of these crystals (Fig. 2d and Fig. 2f) probably come from the glassy matrix.

SEM-EDS analyses were also implemented on the cross-section of the glaze of the sample. Fig. S1a shows that the thickness of glaze is around 740 µm and that it contains many bubbles with size varying from 14 to 150 µm (Fig. S1b). Polygonal crystals of several microns, similar to the ones observed at the surface of the glaze, were also observed around the bubbles (Fig. S1c). Several different crystals of tens of microns were also discovered around some bubbles (Fig. 2g and Fig. S1d). The corresponding EDS results (Fig. 2h) show that these crystals contain mainly Fe and Ti with an atomic Ti/Fe ratio varying from 1.1 to 1.7 (labeled as Psb 1 , Psb 2 and Psb 3 in Table 2 for pseudobrookite). One large elliptical crystal was found in the crosssection of the glaze (Fig. 2i). According to the EDS analysis (Fig. 2j), it is rich in Si, Zr and O, with a Si/Zr ratio close to 1 (Table 2) a. Several star-shaped crystals (labeled as Star 1 and Star 2 , Table 2) were also observed in the cross-section of the glaze (Fig. 2k). They contain mainly Fe and a small quantity of Ti, with Ti/Fe ratios around 0.06 according to EDS analyses (Fig. 2l).

| Structural analyses

Micro-Raman spectroscopy was implemented to identify the different crystals observed both at the surface of the glaze and in its cross-section. Spectra recorded on the crimson polygonal crystals found in the rusty oil spots (Fig. 1d and Fig. 2c) and around the bubbles in the cross-section of the glaze (red cross in Fig. S1c) are characteristic of hematite, with typical peaks around 231 cm -1 , 297 cm -1 , 413 cm -1 and 1337 cm -1 (Fig. 3a). [12] An additional peak around 674 cm -1 , not typical of hematite, was also observed. This peak has been previously observed in Ti-and Al-substituted hematite. [19] Such attribution may be relevant to consider in this case as EDS measurements revealed the presence of Ti in these crystals. As for Al, its presence as a substituting element in the crystals cannot be confirmed by SEM-EDS analysis because the glassy matrix contains a significant level of Al and the analyzed volume cannot be limited to the crystals (Fig. 2d). The dendritic crystals found in the light brown zone of the glaze surface (Fig. 1f and Fig. 2e) have spectral characteristics of ε-Fe 2 O 3 phase (Fig. 3b), i.e., 123, 156, 179, 239, 309, 362, 384, 424, 452, 503, 578, 689, 749 and 1378 cm -1 , similar to spectra recorded on oil spotted wares from the Jian kilns. [6] Spectra recorded on an area without crystals at the glaze surface have two broadened bands at ~500 cm -1 and ~1000 cm -1 (Fig. 3c) which could be attributed to the bending and stretching modes of [SiO 4 ] tetrahedron, respectively. [18] The calculated area ratio of these bands is 1.85, suggesting that the firing temperature was lower than 1300°C. [16][17][18] The large crystals mainly located around the bubbles in the cross-section of the glaze (Fig. S1d and Fig. 2g) were identified as pseudobrookite (Fe 2 TiO 5 ) (Fig. 3d) by comparison with a spectrum (No. R050563) from the RRuFF database (Fig. 4), consistently with EDS analyses (Fig. 2h). The large elliptical crystal found in the cross-section of the glaze (Fig. 2i) presents typical Raman features of zircon (ZrSiO 4 ) (Fig. 3e), [20] which is consistent with the EDS-analyses (Fig. 2j). The star-shaped crystals found in the crosssection of the glaze (Fig. 2k) present similar Raman features (Fig. 3f) to the ones of polygonal crystals (Fig. 3a), including an additional peak around 684 cm -1 which were identified as hematite with probable substitutions. [19] In addition some crystalline clusters were observed in the cross-section of the glaze (Fig. S2) and were identified as a mixture of hematite (Fig. S2,pt01) [12] and pseudobrookite (Fig. S2,pt02). [19] A summary of the observed crystals at the surface and in the cross-section of the glaze are listed in the Table 3. To further study the influence of the Ti/Fe ratio on the Raman spectra of pseudobrookite crystals, analyses of the variation of the position and FWHM (Full Width at Half Maximum) of the bands at ca. 660-690 cm -1 were performed. A linear relationship between the position and FWHM of the bands could be observed (Fig. 5a). In addition, it was found that both the wavenumber and FWHM were higher the higher the Ti/Fe ratio measured on the crystals was. This suggests that a higher Ti/Fe ratio in the crystals results in a shift toward higher wavenumber and larger peaks. However, as the ratio was measured only for three crystals, the observed tendency has to be considered cautiously.

Crystalline type

Raman bands / cm 3 Crystal types detected by Raman spectroscopy in the surface and cross-section of the glaze of the Linfen oil spotted ware. The abbreviation "vw", "w", "s", "vs" in the brackets signifies "very weak", "weak", "strong" and "very strong" respectively, which indicate qualitatively the intensity of each band.

Similar further study was implemented with the additional peak (ca. 665-690 cm - 1 ) of the Raman spectra of hematite crystals of polygonal and star shape (Fig. 5b).

Major discrepancies between the star-shaped and polygonal crystals can be observed, with a peak centered between 673 and 686 cm -1 and with an FWHM varying from 47 to 72 cm -1 for the polygonal crystals (red triangles in the orange circle, Fig. 5b) and more various features for the star-shaped crystals (blue squares, Fig. 5b). For the latter, the peaks of most of crystals locate at higher wavenumbers (centered at 684-689 cm -1 ) and have larger FWHM values (69-84 cm -1 ) (blue squares in the green circle, Fig. 5b). Four star-shaped crystals also observed present relatively low FWHM value (48-57 cm -1 ) and Raman shifts than the other. Two of them are in the same region than the polygonal crystals (blue squares in the orange circle, Fig. 5b) and the peaks of the other two at even lower wavenumbers (668 and 670 cm -1 ) (green circle, Fig. 5b). Unfortunately, no clear relation among those features and the Ti/Fe ratio can be highlighted. The small size of these crystals and the presence of Ti in the glassy matrix surrounding them prevent accurate measurement of their low Ti concentration allowing the linkage of their Ti/Fe ratio to their FWHM or wavenumber shifts. However, the shifts in wavenumber and the FWHM variation of the main hematite peaks (the broadening of the doublet at 231-297 cm -1 and the shits of the bands at ~413 cm -1 and ~1300-1400 cm -1 ) caused by the level of substitution of Fe ions was significant and consistent with previous works. [21,22]

| Discussion

According to the morphology and structure of the hematite crystals found in the oil spots and around the bubbles, a possible formation mechanism of the oil spot pattern is proposed in Fig. 6. Hematite nucleation seeds are first formed at the gasliquid interface of the bubbles inside the glaze during the firing at high temperature (<1300 °C) (Fig. 6a). They progressively accumulate around the growing bubbles, which move within the glassy matrix toward the surface carrying with them the small hematite crystals. During this step, some iron oxides might react with titanium oxides present within the glaze to form pseudobrookite crystals observed around the bubbles (Fig. 6b). The rest that reach the surface grows to give the spots of different sizes that characterize the rusty oil spot pattern (Fig. 6c). Contrary to the oil spotted ware from the Jian kilns, [6] which display dendritic structure hypothesized to come from a dissolution of the iron oxides during firing followed by a re-crystallization during cooling, the polygonal shape of the crystals of this sample suggest that their growth began during the heating step. In addition, the spots of this Linfen ware fragment are flower-shaped with well-defined boundary (Fig. 1b andc) instead of being round, such as those found on Jian wares. [6] The crimson color of the rusty oil spotted pattern is probably the result of the concentration of polygonal hematite crystals of several microns in the spots. The dark shade of the crystals is typical of the diffuse color of hematite crystals of this size. [23] The substitution of some Fe ions by Ti ions and other elements might also influence the color of the hematite crystals: Ti [19] has been reported to usually darken the color of hematite, however probable Al substitution which could not be verified due to the high Al concentration in the glassy matrix could counterbalance this darkening. The global crimson aspect of the spots can be explained by the diversity of crystalline orientation of the crystals, which prevents highly reflective effects and favors the crimson diffused color of the crystals.

The brown areas found in the black glaze may be explained by the presence of the rare ε-Fe 2 O 3 polymorph. It has been already documented in several black-to-brown Chinese glazes, such as oil spotted and hare's fur wares from the Jian kilns (southern Song dynasty), [6,25] sauce wares from the Yaozhou kilns (Shaanxi province, northern Song dynasty), [26] purple-gold wares from the Jingdezhen kilns (Jiangxi province, Qing dynasty) [27] and sauce glaze from the Qilizhen kilns (Jiangxi province, southern Song dynasty). [28] Zircon was also found in the glaze and was also observed in other glazes from Yaozhou hare's fur wares [29] and celadons. [16] This very stable mineral with a high melting point (>2430 °C) is common in sediments such as those used to produce the wares, so it probably comes from the raw materials used. The EDS spectrum (Fig. 2h) recorded on the crystals shows that it is pure, similarly to the ones found in Yaozhou hare's fur wares [27] and celadons. [16]

| Conclusions

The surface and cross-section of a Linfen rusty oil spotted bowl sherd were analyzed by micro-Raman spectroscopy, XRF and SEM-EDS. The glaze surface is composed of the oil spot pattern on brown and black areas, which present all three similar elemental composition, except for a slightly higher Fe concentration in the spots. The spots are composed of polygonal-shaped hematite crystals. Ti-substitution has been detected in the hematite crystals (according to EDS results). Al-substitution is also probable, however its high concentration in the glassy matrix does not allow certainty. The accumulation and diversity of orientation of the substituted hematite crystals may explain the crimson color of the oil spots. Similar crystals were also observed in the cross-section of the glaze, at the interface between bubbles and glaze, suggesting that hematite seeds are first formed in the glaze during the heating step and are transported toward the surface by the bubbles rather than directly formed on the glaze surface. Mixtures of hematite and pseudobrookite crystals, as well as isolated pseudobrookite crystals were observed in the cross-section of the glaze, suggesting that pseudobrookite crystals might come from a reaction between hematite and titanium oxides. Besides, the brown areas of the glaze has been associated with the presence of small dendritic ε-Fe 2 O 3 crystals.

Figure 1

 1 Figure 1 Oil spotted fragment excavated in Linfen kilns (a), with the corresponding optical observation of the oil spot pattern under X20 (b), X200 (c) and X2500 (d) magnification (ring light source) and the light brown zone of the glaze under X100 (e, ring light source) and X2500 (f, coaxial light source) magnification

Figure 2

 2 Figure 2 SEM-BSE images of one oil spot under low magnification (a), particles on the glaze surface (c and e) and in the cross-section (g, i and k) at higher magnification. EDS spectra recorded on the glassy matrix in a black area (b) and on the different crystalline structures observed by SEM-BSE (d, f, h, j and l).

Figure 3

 3 Figure 3 Raman spectra recorded on hematite (a) and ε-Fe2O3 (b) crystals (corresponding to the crystals shown in Fig. 2c and e, respectively) and black area of the glaze surface (c). Raman spectra recorded on pseudobrookite (d), zircon (e) and hematite (f) crystals found in the cross-section of the glaze (corresponding to the crystals shown in Fig. 2g, i and k, respectively).

Figure 4

 4 Figure 4 Comparison of the Raman spectrum recorded on one pseudobrookite crystal in the cross-section of the glaze with the spectrum of pseudobrookite (Fe2TiO5, No. R050563) from the RRuFF database.

Figure 5

 5 Figure 5 (a) Graphical representation of the position versus the full width at half maximum (FWHM) of the pseudobrookite peak around 680 cm-1 observed in the cross-section of the glaze. Ti/Fe ratios of three pseudobrookite crystals were calculated from the corresponding EDS results. (b) Graphical representation of the position versus FWHM of the hematite peak around 680 cm -1 . Peaks from spectra recorded on crystals from the surface of the glaze are indicated asred triangles and on crystals from the cross-section as blue squares.

Figure 6

 6 Figure 6 Schematic diagram of growth process of oil spot pattern on the black glaze.

Table 2

 2 Analytical

results (atom%) obtained by EDS spectrometry on crystals observed on the glaze surface and the cross-section by SEM. Chemical compositions of polygonal and dendritic crystals as well as the glassy matrix at the surface were obtained from the EDS spectra shown in Fig.

2d, 2f and 2b

respectively. Chemical composition of ellipse, Psb1 and Star1 crystals in the glaze cross-section were obtained from the EDS spectra of Fig.

2h

, 2j and 2l respectively.

Acknowledgement

This work has been financially supported by China National Key R&D Program (No. 2019YFC1520202 and No. 2019YFC1520103), China National Natural Science Foundations (No. 51972201 and No. 51902192), Shaanxi Natural Science Basic Research Projects (No. 2021JZY-014 and No. 2020JQ-721) and Silicate Cultural Heritage Protection and Utilization Innovation Team of Shaanxi Province (No. 2020TD-008). It was also supported by the PHC CAI YUANPEI project (No° 46284PB). It was performed in the framework of the research collaboration agreement (CNRS No. 186116) between the French National Centre for Scientific Research and the Shaanxi University of Science and Technology.


Tian Wang
email: wangf@sust.edu.cn

Clement Hole

Zhao Ren

Pei Zhang

Pei Shi

Jianfeng Zhu

Hongjie Luo

Fen Wang

Philippe Sciau
email: philippe.sciau@cemes.fr

Morphological and structural study of crystals in black-to-brown glazes of Yaozhou ware (Song

Keywords: Yaozhou kilns, Brown glaze, Microstructure, Raman microscopy, ε-Fe 2 O

come

Introduction

Black-to-brown ware, which encompasses all ceramics with iron-colored glazes displaying different shades of brown and black, played and still plays an important role in the Chinese ceramic history [START_REF] Li | History of science and technology in China, Ceramics[END_REF]. From current knowledge, the first black-to-brown glazed ceramics were manufactured during the Eastern Han Dynasty (AD in the Yue kiln complex (Zhejiang province). Throughout the following periods, they experienced major technological advances to reach what is considered as their maturity under the Song Dynasty (AD 960-1279) [START_REF] Ling | A preliminary study on ancient black glazed porcelain in China[END_REF][START_REF] Feng | Histroy of Chinese ceramics[END_REF]. During the Song period, almost every Chinese kiln complex produced at least some black-to-brown wares. In addition to the famous Southern Jian kilns (Fujian province), several northern complexes, such as Pingding kilns, Jiexiu kilns (Shanxi province), Cizhou kilns, Ding kilns, Peicheng kilns, Dangyangyu kilns (Hebei province) or Yaozhou kilns (Shaanxi province) also manufactured various types of blackto-brown ware [START_REF] Feng | A photo dictionary of Chinese ancient ceramics[END_REF]. One of the most representative kiln complex is Yaozhou: famous for its northern celadon production, this site also manufactured great quantities of black-tobrown ware from Tang to Yuan Dynasty [START_REF] Chen | Study of Yaozhou celadon and black wares (Tang dynasty)[END_REF][START_REF] Wang | [END_REF].

In spite of the large production of black-to-brown glazes, they were relatively under studied in comparison with celadon or blue-and-white porcelain for instance. Orvar Karlberk [START_REF] Orvar | Notes on the wares from the Chiao Tso potteries[END_REF] first found that the plain brown glaze (commonly referred to as sauce glaze) is composed of a surface brown layer sitting on an underlying black layer. Zhang et al. [START_REF] Zhang | The science of ancient Chinese ceramics, Shanghai People's[END_REF] reported that this brown surface layer contains dendritic hematite (α-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) crystals using optical microscopy and laboratory XRD. These crystals, associated with maghemite (γ-Fe 2 O 3 ), were reported in other types of black-to-brown wares, such as plain black glaze, hare's fur glaze, oil spot glaze and tea dust glaze [START_REF] Zhang | The science of ancient Chinese ceramics, Shanghai People's[END_REF][START_REF] Li | Studies on the microstructure of the black-glazed bowl sherds excavated from the Jian kiln site of ancient China[END_REF][START_REF] Zhang | Scientific research on crystallization black glaze of high iron[END_REF][START_REF] Ma | Integrated analysis of a black-glazed porcelain bowl in Tushan Kiln dated back to Song Dynasty, China[END_REF]. These studies mainly based on laboratory XRD, the results of which were consistent, formed the basis of knowledge about black-to-brown ceramics and were not much complemented with additional studies for decades. However XRD patterns obtained in laboratory can be hard to interpret because of the intense background coming from the glassy matrix, which makes the process of identifying minor crystalline phases much harder. In a 2014 study based on synchrotron radiation micro-XRD combined with Raman spectroscopy, Dejoie et al. showed that the dendritic crystals found in the oil spot patterns of Jian ware were not hematite as previously reported, but in fact the metastable ε-Fe 2 O 3 polymorph, which Raman spectrum was also identified for the first time [START_REF] Dejoie | Learning from the past: Rare ε-Fe 2 O 3 in the ancient black-glazed Jian (Tenmoku) wares[END_REF].

Since this discovery, this ferric oxide was identified in several types of Chinese glazes, such as in the colored wares from Changsha kilns (Tang Dynasty) [START_REF] Shen | Micro-structural study of colored porcelains of Changsha kiln using imaging and spectroscopic techniques[END_REF], purple-gold wares from the Jingdezhen kilns (Qing Dynasty) [START_REF] Liu | The morphology and structure of crystals in Qing Dynasty purple-gold glaze excavated from the Forbidden City[END_REF], brown ware from the Ding kilns (Song Dynasty) [START_REF] Sciau | Les glaçures de céramiques chinoises colorées au fer : un matériau historique à fort potentiel en science de la matière ?[END_REF], sauce wares from the Yaozhou [START_REF] Wen | The colouring mechanism of the brown glaze porcelain of the Yaozhou Kiln in the Northern Song Dynasty[END_REF] and Qilizhen kilns (Song Dynasty) [START_REF] Wang | Three-dimensional microstructure of ε-Fe 2 O 3 crystals in ancient Chinese sauce glaze porcelain revealed by focused ion beam scanning electron microscopy[END_REF]. The presence of ε-Fe 2 O 3 crystals in these glazes opens up interesting perspectives, not only in the field technical history of ancient ceramics, but also in the field of modern materials science with the synthesis of pure ε-Fe 2 O 3 [START_REF] Tuček | ε-Fe 2 O 3 : An advanced nanomaterial exhibiting giant coercive field, millimeter-wave ferromagnetic resonance, and magnetoelectric coupling[END_REF].

To our knowledge, the crystalline organization of ε-Fe In this work, four fragments representative of different types of black-to-brown wares from the Yaozhou kilns were selected. The morphology of the precipitated crystals, their structure and distribution in the glazes were studied using imaging and spectroscopic techniques (optical microscopy, XRF, SEM-EDS, TEM-EDS, µRS and UV-vis-IR). ε-Fe 2 O 3 crystals were identified not only in the roughly homogeneous in color sauce glaze, but also in other types of black-to-brown glazes, such as brown glaze with decorations, plain black glaze and black glaze with brown spots.

Experimental details

Sample descriptions

Four fragments of black-to-brown ceramic produced in the Yaozhou kilns (referenced BG-01, BG-02, BG-03 and BG-04) were selected and are shown in Fig. 1. They were excavated from the Huangpu archaeological site of Tongchuan town (Shaanxi province) and provided by the Yaozhou kiln museum. All the samples are dated from the Northern Song dynasty (960-1127 AD) according to the archaeological layer they were found in.

They present brown colors of different shade (from light to dark brown) and tint (sauce to black hue). BG-01 has a homogeneous glossy brown glaze, which is typically referred to as "sauce glaze". Its color is very similar to persimmon, which explains why it is also locally known as "persimmon glaze" [START_REF] Wang | [END_REF]. BG-02 has a dark brown glaze with lighter stripes, the latter being called sauce decoration [START_REF] Wang | [END_REF]. This is another typical representative Yaozhou brown-to-black ware production. BG-03 has a black and thick outer glaze while its inner glaze is dark brown and relatively thin. The inner surface also presents topography peculiarities probably due to the hand casting process. BG-04 has a brownspotted dark glaze (Fig. 1 enlarged insert map) and homogeneous brown glaze surrounding the foot.

In order to investigate the depth of the glaze, cross-sections of each fragment was made following the preparation method described in detail in reference [START_REF] Wang | Micro-structural study of Yaozhou celadons (Tang to Yuan Dynasty): probing crystalline and glassy phases[END_REF].

XRF quantitative analyses

X-ray fluorescence (XRF) was operated using a spectrometer from HORIBA XGT-7200V, Japan at 50 kV-1 mA, with a beam size of 1.2 mm diameter. The resolution was 138.3 eV at Mn K α line. Spectra were recorded during approximately 120 s directly at the surfaces of the samples. Spectra of the sample bodies were acquired, as well as spectra of the glaze of BG-01, the dark brown glaze and the lighter stripe of BG-02, the outer Images were taken with a magnification range from x20 to x2500. Ring and coaxial light sources were applied to observe the surface coloration and particle distribution in the glazes of the samples.

SEM-EDS analyses

Prior to the analyses, the selected samples (both surface glazes and cross-sections)

were coated with a thin layer of platinum to enhance their conductivity. The morphological and elemental analyses of the glaze were conducted using a FEI Apreo LoVac scanning electron microscope (SEM) equipped with an EDAX Octane Elect Super energy dispersive X-ray spectrometer (EDS). Punctual elemental analyses were performed with a beam size of about 0.2 µm and quantified afterwards thanks to internal standards.

TEM-EDS analyses

Prior to the analyses, two thin sections of sample BG-01 were prepared by ''Lift-out'' technique [START_REF] Ph | Applications of focused ion beam for preparation of specimens of ancient ceramic for electron microscopy and synchrotron X-ray studies[END_REF][START_REF] Sciau | Transmission electron microscopy: emerging investigations for cultural heritage materials[END_REF] and fixed on a copper grid using a Helios NanoLab 600i focused ion beam (FIB)-SEM from Thermofisher equipped with a Ga ion source. Elemental analyses were performed using a CM20FEG scanning transmission electron microscope (STEM) from Philips equipped with a Brücker SDD EDS detector. EDS analyses and elemental mapping were recorded at 200kV at magnification ranging between 50000 and 150000 and data was processed using the Esprit 1.9 software.

Spectroscopic techniques

Raman spectroscopy analyses

Raman investigations were mainly performed using a Renishaw inVia spectrometer from Renishaw. Raman excitation was provided by a cw 532-nm solid state laser. The laser spot size was around 2-3 µm diameter. Laser power of 1.31 mW at x100 magnification was employed to analyze crystals in the glaze surface and cross-section, to optimize the signal/background ratio and avoid any thermal influence on the crystal spectrum. Spectra were recorded in the 100-2000 cm -1 range.

To assess the impact of crystalline orientation on the Raman spectra, spectra were also recorded using an XploRA spectrometer from Horiba. Raman excitation at 532 nm was provided by a vertically-polarized DPSS (Diode Pumped Solid State) Nd-YAG laser.

Spectra were recorded in the 50-1730 cm -1 range. Spectra were recorded on a crystal tilted of -20°, 0°, +30°, +65°, +125°, +160° and +220° from its initial position. https://doi.org/10.1016/j.jeurceramsoc.2021.05.025

UV-vis-NIR spectroscopy analyses

Diffuse reflectance ultraviolet-visible-near-infrared spectroscopy (UV-vis-NIR) was performed using a Cary 5000, Varian, Palo Alto, USA spectroscope operated at room temperature. The reflectance spectra were collected from 200 to 800 nm (UV-vis) and 800 to 2000 nm (NIR) with a beam size of 0.6 mm. The spectra were analyzed with the Cary WinUV software supplied with the instrument. The measurements were operated directly at the surface of ceramic fragments on the zones previously analyzed by XRF. To make comparison with the UV-vis and NIR absorption spectra easier, the diffuse reflectance measurements were converted into absorption using the Kubelka-Munk remission function defined by f(R)=(1-R) 2 /2R (R is the reflectance). Data manipulation was carried out using the Origin software.

Results and discussion

Global Elemental composition

The elemental compositions of the glazes measured by XRF are given in Table 1. The results are consistent with Yaozhou black-to-brown productions with SiO 2 , Al 2 O 3 and Fe 2 O 3 in the range of 63-71%, 14-19% and 4-8% respectively [START_REF] Wang | [END_REF][START_REF] Li | Composition and structure of ancient Chinese rabbit hair black glaze[END_REF]. The Al 2 O 3 /SiO 2 ratios are generally around 0.25, except for the BG-01 (0.30-0.31) and the inner brown glaze of BG-03 (0.28-0.31). However the smaller thickness of the inner-glaze of BG-03 may explain this higher ratio, as the Al 2 O 3 -richer body of the ceramic piece (c. 33%, Table S1) could have been in the relatively high interaction volume of XRF during the measurement. concentrations than black glazes, which is consistent with previous study [START_REF] Zhang | The science of ancient Chinese ceramics, Shanghai People's[END_REF].

Morphological analyses

The glaze of BG-01 (Fig. S1a) is roughly homogeneously brown in color with many large particle clusters. Optical examination under X2000 magnification (Fig. S1b) and SEM-imaging (Fig. 2a) shows that these clusters are composed of a big globular particle surrounded by dendritic crystals of about ten microns. On the other hand the homogeneous areas contain very smaller light-reflecting crystals, which revealed to be smaller dendrite crystals of 2-3 microns homogeneously distributed in the whole glaze according to SEM-imaging (Fig. 2b).

Fig. S2 highlights the significant color difference between the bright brown stripes and darker background of the BG-02 glaze (Fig. S2a). Bubbles with sizes varying from a few to about sixty microns in diameter were observed in the dark brown glaze under x200 magnification (Fig. S2b) but no particle was found. On the contrary, large particle clusters similar to the one observed on BG-01, as well as small brown crystals were found in the lighter stripes (Fig. S2c). They have a dendritic structure and are homogeneously distributed across the stripe and with sizes varying from a few to about ten microns (Fig. S2d and Fig. 2c).

The outer black glaze of BG-03 contains irregular brown particles (Fig. S3a), which are composed of crystals ranging from several to tens of microns (Fig. S3b), which can be classified as two types according to the differences of their colors under coaxial light:

bright white or multicolored. They seem to have different shapes, which is verified with SEM-imaging (Fig. 2d ande): the bright white crystals are dendrites and the multicolored ones triangles. The inner brown glaze is rich in bubbles with size varying from several to ca. forty microns (Fig. S3c) and contains shining crystals distributed heterogeneously (Fig. S3d). The crystals are irregular in shape and size, varying from micrometric crystals to two-dimensional dendritic structures of about 3-4 µm (Fig. 2f) or highly organized dendritic structures of about 6-7 µm (Fig. 2g).

The brown glaze near the foot of BG-04 (Fig. S4a) has a similar morphology to the brown stripes of BG-02 (Fig. S2c), with clearly visible dendritic crystals of about ten microns (Fig. 2i andS4b). Globular crystals surrounded by dendritic crystals of about ten microns, such as those found in BG-01 were also observed in this area (Fig. 2h). The brown spots on the dark glaze were found to contain many small shining crystals (Fig. S4d) of irregular shape with sizes averaging around one micron (Fig. 2j).

To complement these surface analyses, SEM-imaging was performed on crosssections. Fig. 3a shows that the crystals are distributed in three layers in BG-01 sample.

The first layer (Layer 1) is close to the surface and continues up to 1-2 µm deep. It https://doi.org/10.1016/j.jeurceramsoc.2021.05.025

Similar morphological structures were observed in the brown stripe of BG-02 (Fig. 3c and d), however with respective thicknesses much larger than those of BG-01. Such layered structures were already reported in a study of sauce glaze from Qilizhen kilns [START_REF] Wang | Three-dimensional microstructure of ε-Fe 2 O 3 crystals in ancient Chinese sauce glaze porcelain revealed by focused ion beam scanning electron microscopy[END_REF].

This layered organization was however not observed in the two other samples, which have no sauce glaze zone, and in the darker area of BG-02. There, the crystals are only distributed in one layer at the glaze surface.

Characterization of crystalline structures

A combination of micro-Raman measurements and EDS analyses were performed to precisely identify the various crystals previously described.

The globular crystals found in BG-01 and BG-04 samples (Fig. 2a) contain Si, Zr and O (Fig. 4a) and their Raman spectra are well characteristic of zircon (Fig. 5a). Dendriticshaped crystals (Fig. 2b, Fig. 3a-b, Fig. 3d-g) and irregularly-shaped crystals (Fig. 3h) contain mainly iron according to the EDS analyses (Fig. 4d red curve), the other elements found being very likely to come from the glassy matrix surrounding them. They were both identified as ε-Fe 2 O 3 crystals by Raman spectroscopy (Fig. 5b). Unfortunately, Raman study of comparable compounds has not been reported in the literature to our knowledge, which prevents us from going further in the interpretation of the results.

Figure 5 Raman spectra recorded in glaze surfaces of the four samples ((a) zircon, (b) ε-Fe2O3, (c) magnesioferrite, (d) hematite, (e) anatase), in the cross-section of BG-01 (analyzed areas are 5 µm (f) and 19 µm (g) away from the glaze surface, respectively) and in the cross-section of BG-02 (analyzed areas are 5 µm (h) and 35 µm (i) away from the glaze surface, respectively).

Several particles found in the dark brown glaze of BG-02 were identified as hematite according to Raman spectroscopy (Fig. 5d). The additional peak found around 671 cm -1 can be explained by partial substitution of Fe by Al and Ti [START_REF] Ohsaka | Raman spectrum of anatase, TiO 2[END_REF]. A global frequency shift of hematite spectra was also observed (Fig. S6), which are consistent with Fe substitutions.

Additionally anatase particles were observed in the brown glaze near the foot of BG-04 (Fig. 5e). The different species detected by SEM-EDS and Raman spectroscopy in the glaze surfaces of four samples (BG-01, BG-02, BG-03, BG-04) are summarized in Table 2. Raman spectroscopy was also performed on the cross sections of BG-01 and BG-02 samples to characterize the crystals found in the different layers. Spectra recorded on the https://doi.org/10.1016/j.jeurceramsoc.2021.05.025 crystals found in Layer 2 of BG-01 (Fig. 5f) and BG-02 (Fig. 5h) display the characteristics of the ε-Fe 2 O 3 phase, such as the spectra recorded on the crystals found at the surface (Fig. 5b). As for crystals found in Layer 3 (Fig. 3) (19 µm bellow the surface for BG-01 and 35 µm for BG-02), some differences were noticed. Crystals in Layer 3 of BG-01 (Fig. 5g) were still identified as ε-Fe 2 O 3 although the Raman peaks were broadened [START_REF] Sanchez | Sol-gel synthesis and micro-Raman characterization of ε-Fe 2 O 3 micro-and nanoparticles[END_REF]. Raman spectra of crystals in Layer 3 of BG-02 display a strong peak at 671 cm -1 and two smaller peaks at 310 and 545 cm -1 (Fig. 5i), which can be attributed to the A 1g , E g and T 2g modes of magnetite respectively [START_REF] Shebanova | Raman study of magnetite (Fe 3 O 4 ): laser-induced thermal effects and oxidation[END_REF]. Magnetite clusters were also reported in the glaze of a brown Ding ware deep under layers containing only ε-Fe 2 O 3 [START_REF] Sciau | Les glaçures de céramiques chinoises colorées au fer : un matériau historique à fort potentiel en science de la matière ?[END_REF]. Two broad peaks are also present at c. 500 and 1000 cm -1 which could be explained with the bending and stretching modes of the [SiO 4 ] n-tetrahedron of the glassy matrix [START_REF] Wang | Micro-structural study of Yaozhou celadons (Tang to Yuan Dynasty): probing crystalline and glassy phases[END_REF]. Fig. 6a-c shows that the positions of the three peaks vary from 235 to 245 cm -1 , 685

to 699 cm -1 and 727 to 774 cm -1 respectively with varying crystalline sizes. These ranges encompass the positions of the peaks found in previous studies on oil spotted Jian ware [START_REF] Dejoie | Learning from the past: Rare ε-Fe 2 O 3 in the ancient black-glazed Jian (Tenmoku) wares[END_REF]. However shift toward higher wave number with increasing crystalline size was observed for crystals in the inner brown glaze of BG-03 and the brown glaze near the foot of BG-04, whereas the contrary is observed for BG-01 and the black glaze of BG-03.

In addition the spectra recorded on the bright brown stripes of BG-02 show no significant variation of the peak positions with the crystalline size.

The correlation of the broadening of all peaks with the Raman shift presented in the Fig. 6d-f show a general increase of the broadening with the increase of the Raman shift.

However when looking separately at the samples, this correlation is only noticeable in the brown glaze of BG-03.

The shifts and line broadenings of Raman peaks recorded on crystals can generally be attributed to particle size effect [START_REF] Sanchez | Sol-gel synthesis and micro-Raman characterization of ε-Fe 2 O 3 micro-and nanoparticles[END_REF] or ion substitutions [START_REF] Gupta | Raman study of stoichiometric and Zndoped Fe 3 O 4[END_REF]. Crystalline orientation can also influence the Raman spectra [START_REF] Marshall | Polarized Raman spectra of hematite and assignment of external modes[END_REF] by extinguishing some peaks, which can be seen as shifts for structures displaying complex Raman features with many peaks such as ε-

Fe 2 O 3 .
In this study, analyzed crystals are micrometric, which makes the effect of the particle size negligible, as it is significant only for crystals in the nanometer to tens of nanometers range. This is confirmed by the absence of coherent trend linking the size of the crystals studied here with the shifts.

Raman spectra recorded on 2D organized ε-Fe 2 O 3 crystals of BG-01 at different angles (Fig. S7) show that the broadening and the intensity of the Raman bands indeed varies with the angle, however no significant position shift of P1 (236 cm -1 ), P2 (686 cm -1 )

and P3 (750 cm -1 ) was observed. This suggests that the peak shifts observed for crystals of different sizes (Fig. 6a-c) is not due to their different orientations.

These results suggest that the peak shifts observed are due to ionic substitution in the ε-Fe 2 O 3 crystals. TEM-EDS investigation was implemented on two thin sections of sample BG-01 to verify this hypothesis. One thin section make in a large 2D organized crystals and another in smaller dendrites in order to compare their composition.

Elemental mapping shows that the 2D organized crystals contain Al, Ti and Mg in addition to Fe (Fig. 7a) whereas smaller dendrites seem to contain less Ti and Al and much less Mg (Fig. 7b). To highlight the observed differences, quantitative elemental analyses (atomic %) on crystals and glaze areas were carried out. The atomic % of the main elements of interest found on the red areas highlighted in the pictures are given in the tables (Fig. 7), other elements being O, K and minor amounts of Na, Cr and Mn. Measurements performed on several other crystals and glaze areas gave similar results. They confirm the presence of Al, Mg and Ti in bigger 2D organized crystals (Fig. 7a). The smaller dendritic crystals are also Al and Ti substituted (Fig. 7b) and the comparison of Mg/Si ratios between the glaze and the crystals show that the latter are also enriched in Mg (0.17 in the crystals > 0.05 in the glaze), such as the bigger crystals, which seem logical to compensate the excess of charges brought by the Ti ions. In addition it can be noted that the two types of crystals have different substitution rates. It is higher in bigger crystals than in smaller. The difference in composition between both types of crystals is coherent with the Raman observations.

Further investigation using UV-Vis-NIR spectroscopy to compare the absorptive features of the brown and darker areas of the glazes was performed. Since absorption bands situated in the 200 to 800 nm range give information about the coordination of trivalent iron and the bands in the 800 to 2000 nm range about coordination of bivalent iron [START_REF] Shi | Effect of phase separation on the Jian ware blue colored glaze with iron oxide[END_REF], spectra were separated in these two ranges for easier interpretation (Fig. 8 and Fig. 8S).

Fig. 8a displays the UV-vis absorption spectra from 200 to 800 nm recorded on the brown areas of each sample. Four main absorption bands were detected (labeled bands I, II, III and IV). Band I is broad (270-350 nm) and hard to locate precisely due to the noise and the strong background at the lower wavelength. Band II (400-450 nm) is visible both in BG-01 and BG-04 can be attributed to a ligand field transition of octahedrally coordinated Fe 3+ , which is typically observed in the spectra of a wide variety of iron oxides (hematite, maghemite, geothite...) [START_REF] Sherman | The electronic structures of Fe 3+ coordination sites in iron oxides: Applications to spectra, bonding, and magnetism[END_REF][START_REF] Sherman | Electronic Spectra of Fe 3+ Oxides and Oxide Hydroxides in the Near IR to Near UV[END_REF]. Bands III (480-450 nm) and IV (625-675 nm), the latter being highlighted with second derivative analysis although not obviously visible on the figure, of the four samples can be associated respectively with https://doi.org/10.1016/j.jeurceramsoc.2021.05.025 tetrahedral and octahedral coordination of Fe 3+ [START_REF] Sherman | The electronic structures of Fe 3+ coordination sites in iron oxides: Applications to spectra, bonding, and magnetism[END_REF][START_REF] Sherman | Electronic Spectra of Fe 3+ Oxides and Oxide Hydroxides in the Near IR to Near UV[END_REF]. The observed absorptive features are consistent with the ε-Fe 2 O 3 structure, which contains both tetrahedrally and octahedrally coordinated Fe 3+ ions. According to the optical and SEM imaging showing that there are a great proportion of crystals at the surface, the STEM-EDS analysis showing that iron is much more concentrated in the crystals than the glaze and due to the low penetration depth of the beam, the influence of Fe 3+ ions in the glassy matrix is low.

Spectra recorded in darker areas of the glaze (Fig. 8b) show three bands (labeled 1, Raman and EDS analysis. The other bands can be associated to Fe 3+ transitions. In the darker areas, much less crystals were observed, making the contribution of Fe 3+ ions dispersed in the glassy matrix higher.

The NIR absorption spectra recorded on the samples (Fig. S8) all show similar features: a broad band between 1025 and 1350 nm, which may be two broad bands convoluted together because of the shoulder noticeable around 1200 nm on few spectra, and a smaller band between 1400 and 1450 nm. They can be attributed to octahedrally coordinated Fe 2+ ions [START_REF] Shi | Effect of phase separation on the Jian ware blue colored glaze with iron oxide[END_REF]. The presence of Fe 2+ ions in the glazes of the samples reveals that the glaze vitrified under reducing conditions. However, the presence of the Fe 3+ -bearing crystals at the surface shows that the cooling was under an oxidizing atmosphere.

Conclusions

The rare ε-Fe 2 O 3 polymorph was found in the typical sauce glaze sample (BG-01) as well as all the other types of brown-to-black glazes from the Yaozhou kilns studied, such as the brown stripes of the sauce-decorated ware (BG-02), the inner brown glaze of a two-colored brown ware (BG-03) and the brown ring surrounding the foot and the brown spots of a brown-spotted black glaze (BG-04).

https://doi.org/10.1016/j.jeurceramsoc.2021.05.025

Well-developed dendritic structures were observed in the lighter brown areas of the samples, whereas almost no crystals were observed in the darker areas of BG-02 and irregularly shaped ones were noticed on the dark glaze of BG-03. Cross-section analyses revealed the presence of crystalline-layered structures in BG-01 and the brown stripes of BG-02, similar to those reported in sauce glaze from the Qilizhen kilns [START_REF] Wang | Three-dimensional microstructure of ε-Fe 2 O 3 crystals in ancient Chinese sauce glaze porcelain revealed by focused ion beam scanning electron microscopy[END_REF]. ε-Fe 2 O 3 is present in the different layers of BG-01 and the top layers of BG-02, but magnetite was found in the deeper layer of BG-02, as reported in a brown Ding ware [START_REF] Sciau | Les glaçures de céramiques chinoises colorées au fer : un matériau historique à fort potentiel en science de la matière ?[END_REF]. Such structures were not observed on the differently colored samples and the crystals were only in one layer near the surface. Mg-, Ca-and Fe-rich spinel crystals were also detected in irregular brown areas of the outer black glaze of BG-03.

These identifications, coupled with the fact that the glazes are generally brown on the surface and black in depth, suggest that the brown coloration is closely linked with the presence of ε-Fe 2 O 3 and the darker color in depth to the presence of magnetite clusters. The shades of brown seem also be related to the different nature, distribution and characteristics of the crystals in the glaze.

Furthermore Raman investigation showed varying peak shifts, which suggested that the ε-Fe 2 O 3 crystals of different sizes are stabilized by cationic substitutions. TEM-EDS analyses confirmed the presence of Al-, Mg-and Ti-substitutions in the crystals of the sauce glaze sample.

2 O 3

 23 polymorph was studied in detail in examples of sauce glaze (Yaozhou kilns and Qilizhen kilns), purple-gold glaze (Jingdezhen kilns) and oil spot and hare's fur glazes (Jian kilns). The existence of ε-Fe 2 O 3 phase in other types of brown-to-black glazes, which also contain Fe-bearing crystals, still remains to be verified. https://doi.org/10.1016/j.jeurceramsoc.2021.05.025

Figure 1

 1 Figure 1 Photos of the four analyzed samples from Yaozhou kiln site (Song Dynasty).

  contains long crystals and is separated from the second layer (Layer 2) by a crystaldepleted layer of 1-2 µm. Layer 2 contains sub-micrometric crystals. Another crystaldepleted layer is found between Layer 2 and 3. Layer 3 contains crystals larger than the ones found in Layer 2, which decrease in size deeper in the glaze.

Figure 2

 2 Figure 2 Particles of different shapes in glazes of samples observed by SEM-BSE: (a) and (b) sauce glaze of BG-01; (c) brown stripe decors of BG-02; (d) and (e) outer black glaze of BG-03; (f) and (g) inner brown glaze of BG-03; (h) and (i) brown circle of BG-04; (j) brown spots in black glaze of BG-04.

Figure 3

 3 Figure 3 SEM observation of cross-sections of BG-01 (a) and the brown stripe zone of BG-02 (c) and the corresponding iron concentration by EDS (b and d).

Figure 4

 4 Figure 4 (a) Comparison of the elemental compositions of dendrite crystal (red curve) and triangle crystal (blue curve) in the outer black glaze of BG-03; (b) Representative EDS spectrum of circularparticles of tens of microns diameter observed in BG-01.

Figure 6

 6 Figure 6 Graphical representations of crystal sizes versus P1, P2 and P3 peak positions (a, b and c) and the peak positions versus full width at half maximum (FWHM) (d, e and f) for all samples.

Figure 7

 7 Figure 7 STEM-EDS elemental mapping of thin sections of 2D organized dendrites (a) and smaller dendrites (b) and EDS analysis of a crystal and the near glaze (red areas on the STEM picture). Percentages given in the composition tables are completed with O, K, Na, Cr and Mn to obtain 100%.

3 and 6 )

 6 similar to the band I, III and IV recorded on the brown areas. However other features can be noticed, such as band 2 (450-480 nm) in BG-03, band 4 (545-590 nm) in BG-02 and band 5 (590-615 nm) of BG-03 and BG-04. Band 2 in particular can be associated to a transition of a Mg-Fe pair of the Mg-and Fe-rich spinel characterized by

Figure 8

 8 Figure 8 UV-vis spectra with main absorption features of the glaze recorded from 200 to 800 nm: (a) brown areas of the glazes of BG-01 (blue curve), BG-02 (black curve), BG-03 (Green curve), BG-04 (red curve) and (b) the darker areas of the glazes of BG-02 (black curve), BG-03 (green curve), BG-04 (red curve).

Table 1

 1 The glaze of BG-01 has a higher K 2 O and a lower CaO concentration compared to the other samples. The concentrations of the main coloring elements (Ti and Fe) vary relatively little between glazes. In general brown glazes have higher TiO 2 and Fe 2 O 3

	Sample Glaze	Na 2 O	MgO	Al 2 O 3	SiO 2	K 2 O	CaO	TiO 2	MnO 2	Fe 2 O 3	SiO 2 / Al 2 O 3
			0.49	2.68	19.13	62.85	4.08	3.17	1.25	0.07	7.09	0.30
	BG-01	Sauce glaze	0.72 0.23	1.52 1.32	19.38 19.85	63.62 64.37	4.00 4.04	3.23 3.16	1.24 1.23	0.07 0.06	7.00 6.73	0.30 0.31
		Bright	1.07	3.30	14.26	64.01	3.35	5.59	0.69	0.09	8.54	0.22
		brown	0.07	1.59	16.37	68.02	3.80	4.19	0.69	0.06	5.93	0.24
	BG-02	stripes Dark	0.29 0.78	0.87 4.28	19.04 13.66	66.54 64.44	4.02 3.38	3.97 6.84	0.80 0.58	0.05 0.09	5.06 6.69	0.29 0.21
		brown	0.60	4.08	14.03	64.74	3.18	6.85	0.58	0.08	6.60	0.22
		glaze	0.48	3.80	14.48	65.63	3.20	6.29	0.61	0.08	6.19	0.22
		Inner	0.01	2.09	18.58	65.43	3.47	4.88	1.17	0.06	5.11	0.28
		brown	0.61	1.40	19.85	63.63	3.47	5.22	1.30	0.05	5.24	0.31
	BG-03	glaze Outer	0.01 0.28	2.12 2.37	18.54 13.53	64.96 69.37	3.48 3.50	4.81 5.38	1.36 0.86	0.06 0.07	5.46 5.29	0.29 0.20
		black	0.77	2.86	10.86	71.83	3.47	5.06	0.56	0.07	5.20	0.15
		glaze	0.01	3.50	13.36	66.88	3.48	6.57	0.82	0.08	5.97	0.20
	BG-04	Brown circle Black glaze	0.93 0.28 1.43 0.01 0.68	2.64 3.13 3.38 2.13 2.67	17.62 16.88 16.88 16.80 16.75	63.41 64.26 62.53 68.15 67.72	3.85 3.54 3.58 3.55 3.41	4.30 4.19 4.39 4.31 4.21	1.17 1.17 1.16 0.74 0.68	0.09 0.09 0.09 0.08 0.07	6.98 7.30 7.42 4.98 4.51	0.28 0.26 0.27 0.25 0.25
			0.56	3.20	16.62	67.01	3.37	4.53	0.69	0.08	4.67	0.25

Analytical results (wt%) obtained by X-ray fluorescence on the surfaces of glazes of four samples (BG-01, BG-02, BG-03, BG-04). Three analyzed points were carried out in each area to estimate the homogeneity. https://doi.org/10.1016/j.jeurceramsoc.2021.05.025

Table 2

 2 Crystal

types detected by SEM-EDS and Raman spectroscopy in the glaze surfaces of four samples (BG-01, BG-02, BG-03 and BG-04).

Acknowledgement

We warmly thank Dr. Cécile Marcelot from CEMES CNRS, for her essential assistance in the recording of the STEM-EDS data. This work has been financially supported by China National Key Research and Development Plan (No. 2019YFC1520300), China National Natural Science Foundations (No. 51972201 and No. 51902192) and Shaanxi Natural Science Basic Research Projects (No. 2021JZY-014, No. 2020JQ-721 and No. 2018JQ5071). It was performed in the framework of the research collaboration agreement (CNRS No. 186116) between the French National Centre for Scientific Research and the Shaanxi University of Science and Technology.