Introduction

‘Fanhong’ is an overglaze pigment utilizing Fe2O3 as a chromophore, which can also be fired in an oxidizing atmosphere to produce monochromatic glaze. ‘Shengfanhong’ is an iron oxide powder obtained through the calcination, washing, and classification processes of iron vitriol (FeSO4·7H2O), and it serves as the core raw material for the production of ‘Fanhong’ porcelain. The origin of ‘Fanhong’ traces back to the Song Dynasty’s red-and-green polychrome wares, serving as a precursor to later multi-colored overglaze techniques. The term ‘Fanhong’ first appeared in the DaMingHuiDian: “In the second year of Jiajing (1523), Jiangxi kilns were ordered to replace underglaze copper red with deep iron oxide red in porcelain production.” In the mid-Ming Dynasty during the Chenghua period (1465–1487), the combination of underglaze blue and white with overglaze polychrome led to the production of uniquely styled ‘doucai’ porcelain, which, during this period, was known for its extremely thin body and rich colors, described as ‘exquisite quality and vibrant colors'1,2. In the Jiajing period of the Ming Dynasty (1522–1566), the Imperial Porcelain Factory in Jingdezhen began to use iron red glaze in place of copper red glaze. By the Kangxi period of the Qing Dynasty (1662–1722), ‘Fanhong’ evolved into a brilliantly saturated pigment, predominantly used in ‘wucai’ (five-color) and ‘doucai’ decorative schemes. However, after the Jiaqing period of the Qing Dynasty (1796–1820), the quality of the iron red overglaze color significantly declined, with only a slight improvement during the Guangxu (1875–1908) Emperor’s reign. The production techniques of ‘Shengfanhong’ are documented in The Science of Ancient Chinese Ceramics3 and Ceramic Decorative Materials4. The preparation of ‘Shengfanhong’ involves sequential steps: dehydration of FeSO4·7H2O, crushing and sieving, calcination, acid-leaching, and drying. The resultant material is then ground with water, purified through sedimentation, mixed with lead powder and ox glue, and finally fired twice to produce ‘Fanhong’ porcelains (Fig. 1)

Fig. 1: Photos of ancient Chinese ‘Fanhong’ porcelains from different periods.
Fig. 1: Photos of ancient Chinese ‘Fanhong’ porcelains from different periods.
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a Sample is from the Yuan Dynasty period (1271–1368), samples b, c are from Ming Dynasty period (1368–1644), b is from the mid of Ming Dynasty period, c is from the terminal of Ming Dynasty period, d–f are from Qing Dynasty period (1644–1911), d is from the mid of Qing Dynasty period, and e, f are from the terminal of Qing Dynasty period).

The traditional production process of ‘Shengfanhong’ involves three key steps: calcination, acid-leaching, and particle size classification of FeSO4·7H2O. During calcination, the crystal size and phase composition of FeSO4·7H2O undergo significant changes, directly influencing the chromatic properties of the final ‘Fanhong’ pigment. Hiroshi Asaoka et al.5 synthesized iron oxide powders by calcining FeSO4·7H2O, reporting Fe2O3 crystal sizes of 50 nm at 650 °C and 100 nm at 770 °C, with reduced calcination temperatures enhancing chromaticity. Hirofumi Inada et al.6 characterized a traditional red pigment via X-ray diffraction (XRD), confirming the dominance of α-Fe2O3 phase at 700 °C, while scanning electron microscopy (SEM) revealed α-Fe2O3 crystal sizes ranging from 30 to 100 nm. S. Kajihara et al.7 observed abundant sulfur-containing microcrystals in FeSO4·7H2O at 550 °C, which disappeared upon heating to 650 °C, yielding pure Fe2O3. P.K. Gallagher et al.8 identified Fe2O(SO4)2 as an intermediate during thermal decomposition of FeSO4·7H2O. R. Zboril et al.9,10,11 further demonstrated that FeSO4·7H2O transforms into various Fe2O3 polymorphs during heating, with α-Fe2O3 stabilized at 700 °C. The classification process of ‘Shengfanhong’ essentially involves sorting powder materials into different categories based on particle size distribution. In the study of factors influencing pigment coloration, particle size has been identified as a critical determinant12,13. The coarse particle size of ‘Shengfanhong’ powder leads to a highly rough surface in the resulting glaze. After firing, pronounced crystal precipitation occurs, which significantly affects the coloration of ‘Fanhong’ porcelain. Scholars have employed high-energy ball milling techniques14 to analyze the refractive index, reflectance, and chromaticity of mineral pigments with varying particle sizes. The results demonstrate that mineral pigments exhibit distinct coloration at different particle sizes, and the influence of particle size on coloration varies significantly depending on the material composition15,16. The crystal size of powdered pigments primarily governs the interaction between light and mineral pigment particles, with this effect exhibiting nonlinear characteristics17. Chen18 conducted a study on the grinding stability of iron oxide red (α-Fe2O3), revealing that mechanical force activation occurs during wet grinding. The relationship between ball milling time and particle size change is nonlinear—the particle diameter initially decreases but subsequently increases with prolonged grinding. This enlargement of particle size shifts the pigment’s hue toward yellow-blue tones while reducing both redness and brightness19,20,21.

The coloration mechanism of ‘Fanhong’ porcelain constitutes a systematic issue, the calcination temperature of FeSO4·7H2O22, the particle size of the powder16,23, the quality ratio of ‘Shengfanhong’ and lead powder24,25,26,27, the secondary firing temperature28, and the thickness of the ‘Fanhong’ layer are all factors that restrict its coloration, where the traditional craftsmanship of ‘Shengfanhong’ critically influences its chromatic performance. Currently, academic research lacks a comprehensive scientific explanation for the traditional craftsmanship of ‘Shengfanhong’. The theory of tacit knowledge reveals the intangible dimensions of human expertise that are difficult to articulate—a phenomenon equally present in the traditional craftsmanship of ‘Shengfanhong’. Based on this, this study explains the scientific mechanism of the traditional craftsmanship of ‘Shengfanhong’ through experimental archeology and scientific testing methods, revealing the exquisite porcelain-making techniques of ancient people and scientifically quantifying this process.

Methods

Differential scanning calorimetry-thermogravimetric analysis (DSC-TG)

The raw material for the differential scanning calorimetric-thermogravimetric analysis was FeSO4·7H2O, and the thermal analyzer was supplied by Netzsch Instruments Manufacturing Co., Ltd. in Selb, Germany, model STA449C, with a resolution of 0.1 µg, and a temperature range from room temperature to 1650 °C. The mass of the experimental sample was 9.91 mg, with a heating rate of 10 °C/min in an Ar atmosphere. The testing temperature range was from room temperature to 900 °Cmeasuring the mass changes of FeSO4·7H2O and the temperature points of endothermic and exothermic reactions during heating.

X-ray diffraction (XRD)

The samples for XRD testing were FeSO4·7H2O powders calcined at different temperatures. The XRD was from Bruker AXS GmbH in Karlsruhe, Germany, model D8 Advance, with a Cu target and a power of 6.5 kW; the scanning step length was 0.01°, and the scanning speed was 0.05 s per step, testing the mineral composition of FeSO4·7H2O during heating.

X-ray photoelectron spectroscopy (XPS)

The samples for XPS testing were FeSO4·7H2O powders calcined at 125 °C, 300 °C, 600 °C. The model of the X-ray photoelectron spectrometer is Thermo Scientific Nexsa, the excitation source type is monochromatic aluminum target, the voltage is 12 kV, the power is 72 W, the vacuum degree is better than 5 × 10−9 mBar, the energy is 100 eV (full spectrum) and 50 eV (fine spectrum), and the work function is 4.20 eV. It was utilized to characterize the transformation in the valence states of iron ions throughout the calcination process.

Colorimetric analysis

The samples for chromaticity analysis were ancient ‘Fanhong’ samples and ‘Fanhong’ experimental samples made from ‘Shengfanhong’ powder of different particle sizes, and the chromaticity meter used was an NF-333 portable spectrophotometric chroma meter with a wavelength range of 400–700 nm. All measurements were made under standard 10° observation conditions and D65 illuminant to test the relevant chromaticity parameters of different iron reds. L* represents the brightness of the object: 0–100, indicating from black to white; a* represents the red-green of the object: a positive value indicates red, and a negative value indicates green; b* represents the yellow-blue of the object: a positive value indicates yellow, and a negative value indicates blue.

Particle size testing

Traditional grinding methodology involved water-assisted manual trituration of ‘Shengfanhong’ powder using a mortar, typically requiring multi-hour processing by skilled artisans. The vertical planetary ball mill is used for modern ball milling technology of ‘Shengfanhong’ powder, and comes from Changsha Deke Instrument Equipment Co., Ltd., with the model of DECO-PBM-V-0.4 L. The experiment was configured with a mass ratio of powder:grinding balls:water = 1:1:1. Control groups were established with ball milling durations of: 20 min, 30 min, and 60 min, maintaining a constant rotational speed of 400 revolutions per minute. The laser particle size analyzer is used to test the particle size of ‘Shengfanhong’ powder, and from Malvern Instruments Limited in the UK, model Mastersizer2000, with a particle size range of 0.02–2000 microns and a scanning speed of 1000 times per second.

Results

Heating process of FeSO4·7H2O

In this experiment, industrial FeSO4·7H2O was used as the raw material, and the supplier of industrial FeSO4·7H2O was Wuhan Carnoss Co., Ltd. in Wuhan, China. This kind of industrial FeSO4·7H2O’s purity was 99% (Fig. 2a). The treatment process of FeSO4·7H2O involves three key steps. First, to address the difficulty in physical grinding caused by the strong hygroscopicity of the raw material, a gradient heating pretreatment method was adopted: dehydration was performed within the temperature range of room temperature to 120 °C. Through the thermal decomposition reaction (FeSO4·7H2O → FeSO4·nH2O + (7-n)H2O ↑ 22) facilitates gradual liberation of crystalline water molecules, accompanied by distinct phase transformation characteristics--the color of the crystal manifested by a progressive chromatic transition from initial pale green coloration(the absorption of specific wavelengths of light by the d–d electron transition of Fe2+) to grayish-white appearance (Fig. 2b), Second, to mitigate the agglomeration and caking of the dehydrated material, grinding was conducted using an agate mortar, and particle size control was achieved by sieving through a standard 250 μm mesh. Finally, considering the susceptibility of powder to oxidation and deterioration in air, the processed powder was stored in a vacuum drying oven (Fig. 2c). The physicochemical reactions during FeSO4·7H2O heating were characterized by DSC-TG analysis29, Fig. 2d revealed two distinct endothermic peaks at 75.6 °C and 122.7 °C (Peaks ① and ②), corresponding to mass losses of 17.66% and 20.74% (Stages I and II), respectively. XRD patterns before and after dehydration (Fig. 2e, f) confirm that the phase remains consistent with FeSO4·nH2O, with no new crystalline phases detected. It shows that the appearance of the endothermic peaks ① and ② corresponds precisely to the temperature node where FeSO4·7H2O undergoes two dehydration processes, without undergoing any phase transition, while I and II correspond to two distinct dehydration stages of FeSO4·7H2O. The transformation observed in Fig. 2e, f corresponds to the dehydration of FeSO4·7H2O to FeSO4·H2O, as evidenced by the incomplete removal of crystalline water. The single remaining water of crystallization is retained until its decomposition at around 250 °C22. This indicates that the mass loss in this stage is solely due to dehydration, without oxidative or phase transitions.

Fig. 2: Pretreatment process of FeSO4·7H2O and related test results.
Fig. 2: Pretreatment process of FeSO4·7H2O and related test results.
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a Raw material of FeSO4·7H2O. b grinding FeSO4·7H2O after heating at 120 °C. c The ground Powder. d DSC-TG of FeSO4·7H2O22. e XRD of iron vitriol. f XRD of dehydrated iron vitriol.

According to the records in refs. 3,4 regarding the calcination process of FeSO4·7H2O, this step adopts dynamic calcination to prepare ‘Shengfanhong’. The specific operational procedure is as follows: The pretreated powder is placed in an iron pot and subjected to gradient pyrolysis using an open flame. During calcination, real-time visual colorimetric monitoring is required to observe the phase transition characteristics of the material, corresponding to the historical description that “ Using an open flame to calcine FeSO4·7H2O until it turns slightly black, then transforms into orange–red upon cooling. “ The color evolution exhibits distinct stages: the initial grayish-white material gradually turns yellowish-brown during heating (Fig. 3a), when the color becomes slightly blackened (Fig. 3b), heating is terminated, upon cooling, it transforms into a reddish-orange hue (Fig. 3c). This transformation marks the completion of standard ‘Shengfanhong’ production. Comparative studies with modern calcination processes indicate that this state of ‘Shengfanhong’ achieves equivalent results to those obtained through contemporary 700 °C calcination techniques (Fig. 3d). In the calcination process of FeSO4·7H2O, open-flame heating is initiated from room temperature. during which process III corresponds to the thermal conversion of FeSO4 into Fe2(SO4)3, with an exothermic peak ③ observed at a critical temperature node of 567 °C where FeSO4 reacts with O₂ forming intermediate product Fe2O(SO4)2, and process IV represents subsequent decomposition into Fe2(SO4)3, Fe2O3 and SO38. The temperature node of the exothermic peak ④ is 661 °C, at which both Fe2(SO4)3 and Fe2O(SO4)2 undergo desulfurization to upon α-Fe2O3 (Fig. 2d). According to redox principles, oxidation of Fe2+ to Fe3+ occurs concomitantly with SO₄2− decomposition. XPS results further confirmed the valence transition of iron ions during the calcination process. After the dehydration of ‘Shengfanhong’, combined with XRD phase analysis, it was observed that both Fe2+ and Fe3+ coexisted in the powder at 300 °C, corresponding to the transformation from FeSO4 to Fe2(SO4)3. By 600 °C, the iron species were almost completely converted to Fe3+, with the phase composition consisting of Fe2(SO4)3 and α-Fe2O3. Further heating led to the complete transformation of the phase into α-Fe2O3 (Fig. 3e, f). This valence transition alters electronic configurations and associated absorption spectra, resulting in characteristic orange–red coloration upon formation of α-Fe2O3 particles. When the calcination temperature exceeds 700 °C, progressive chromatic shifts occur toward jujube-red and eventually black hues (Fig. 3d)30.

Fig. 3: The calcination process of ‘Shengfanhong’ and related test results.
Fig. 3: The calcination process of ‘Shengfanhong’ and related test results.
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Color changes observed during the experimental process in traditional craftsmanship: a FeSO4·7H2O at the initial heating stage; b FeSO4·7H2O heated to the blackened state; c transition to reddish-orange after cooling. d Images of calcined ‘Shengfanhong’ powder at different temperatures in modern craftsmanship. e XRD of calcined FeSO4·7H2O at different temperatures. f XPS of calcined FeSO4·7H2O at different temperatures

The test results of DSC, XRD, and XPS provide a reasonable physicochemical explanation for key processes in FeSO4·7H2O’s thermal treatment: dehydration, oxidation of divalent iron ions to trivalent iron ions, and decomposition reactions of sulfate ions, thereby scientifically validating how heating transforms FeSO4·7H2O into red pigment. The control of calcination temperature exhibits distinct empirical characteristics. Ancient kiln workers had to repeatedly extract samples during the firing process and visually compare them with pre-selected ‘Fanhong’ porcelain specimens, this rooted in perceptual skills and the intergenerational transmission of visual experience through master-apprentice traditions. As the temperature further increases, the test results of XRD and XPS show that the conversion of Fe2+ to Fe3+ and the decomposition of iron sulfates lead to a color transition in ‘Shengfanhong’ powder from gray to pale yellow, orange–red, and eventually deep red. The orange–red hue described in historical records corresponds to a calcination temperature of approximately 700 °C under modern processes22. The empirical observation where craftsmen cease firing upon watching slight blackening, later cooling into orange–red, results from superimposed thermal radiation and reflection spectra effects during high-temperature processing. At precisely controlled calcination temperatures around 700 °C, where α-Fe2O3 has been completely formed (Fig. 3e)31, its hexagonal close-packed crystal structure demonstrates maximum reflectance in red spectral regions32. At this temperature, thermal radiation occurs according to blackbody radiation theory, where high-temperature objects emit electromagnetic waves while simultaneously reflecting external light33. Under high temperatures, thermal radiation intensity may surpass reflected light intensity and superimpose onto reflection spectra, causing slight blackening in appearance. After cooling, the thermal radiation of the material disappears, and the reflection spectrum dominates the coloration. At this point, the strong reflection of red light by α-Fe2O3 appears, and the color returns to the original red color of the material34. To further verify the influence of surface thermal radiation of the powder on the coloration, samples were divided into two cooling methods: one portion was cooled in ambient air at 20 °C, while another portion was cooled in water maintained at the same room temperature. As established principles dictate, water cooling achieves significantly faster heat dissipation than air cooling at identical temperatures—a phenomenon experimentally confirmed by observing a more rapid chromatic transition from black to red in water-quenched powder compared to air-cooled powder. In addition, this process is reversible, and α-Fe2O3 does not undergo crystal transformation at 700 °C26, these results conclusively demonstrate that high-temperature darkening originates from transient superposition effects between thermal radiation and reflection spectra.

Rinsing process of ‘Shengfanhong’

The prepared ‘Shengfanhong’ powder is transferred into a beaker, and room-temperature clean water is added while stirring rapidly and uniformly with a wooden stick. Under agitation, the mixture in the beaker gradually clarifies, at which point the undissolved particles in the supernatant are removed by decantation. The addition of cold water is then stopped, and hot water is used for eight or nine additional washings until all alum liquor is completely eliminated. In ancient practices, tea infusion was typically employed to test whether the alum liquor had been fully removed. As shown in Fig. 4, if alum liquor remains, the liquid in the cup turns black; conversely, if the liquid’s color remains unchanged, it indicates complete removal of alum liquor, marking the completion of the rinsing process and confirming that the ‘Shengfanhong’ has achieved the required purity. Before classifying the ‘Shengfanhong’, it must undergo thorough grinding. After grinding, an appropriate amount of clean water is added to the ‘Shengfanhong’ to form a thin slurry consistency.

Fig. 4: Rinsing process of ‘Shengfanhong’.
Fig. 4: Rinsing process of ‘Shengfanhong’.
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Rinsing process of 'Shengfanhong' (a Rinsing 'Shengfanhong', b Mixing 'Shengfanhong' into a slurry, c Tea infusion, d The untreated alum water turns black after being added to the tea water)

In the rinsing process for ‘Shengfanhong’ preparation, the degree of soluble salt elimination is directly related to the color stability of ‘Shengfanhong’ powder. However, historical texts only vaguely mention “thorough washing” without specifying the rationale behind using cold water, hot water, or tea infusion, it means that craftsmen relied on observing changes in water turbidity to determine the endpoint. Calcined FeSO4·7H2O undergoes sequential rinsing with cold and hot water, respectively, the former removes undissolved particles from the supernatant, while the latter eliminates residual sulfate groups from powder particles. When treated with hot water, Fe2(SO4)3 reacts with water and hydrolyzes to form a reddish brown colloidal precipitate of Fe(OH)3 and H₂SO₄. The chemical equation is: Fe2(SO4)3 + 6H2O → 2Fe(OH)3 + 3H2SO4, and multiple rinses remove the colloidal precipitate of Fe(OH)3 and H₂SO₄. Tea infusion was employed to verify the complete removal of alum liquor residues. In the field of analytical chemistry, a colorimetric detection method can be established through the chromogenic reactions between tannic acid and metal ions. When using tea infusion as an indicator system to verify whether alum water remains residual iron species (mainly containing Fe2+), if there are uncleaned ferrous salts in the system, they are easily dissolved and oxidized to Fe3+ in the aqueous phase. At this stage, tannic acid (C76H52O46), abundantly present in tea solution, undergoes specific complexation with Fe3+, forming black-colored iron tannate (Fe(C76H52O46)) through coordination chemistry principles. It is a classic example of a metal-organic ligand chromogenic reaction. Experimental observations demonstrate that solution coloration transitions from amber to deep ink-black within 30 s, with chromatic intensity positively correlating to Fe3+ concentration as governed by Lambert-Beer’s law fundamentals for colorimetric detection35. Therefore, the use of tea infusion is used for the inspection of trivalent iron salts, and the color change of tea infusion is used to characterize whether the impurities in alum water have been removed completely35. This visually manifests as the “black water” observed by craftsmen, serving as an indicator of incomplete impurity removal.

Classification process of ‘Shengfanhong’

The classification process applied to rinsed ‘Shengfanhong’ powder primarily aims to achieve standardized particle size control through physical classification methods, thereby meeting specific granularity requirements for ancient ‘Fanhong’ pigment preparation. In this classification process, two aluminum trays are first aligned linearly with the container (beaker) holding ‘Shengfanhong’, connected by an absorbent medium (such as straw paper) to create a fluid transfer channel. This setup allows the suspended slurry in the beaker to flow along the medium into the first aluminum tray. During this procedure, particle sedimentation occurs in the first aluminum tray while supernatant moisture continues migrating via capillary action through the straw paper into the second aluminum tray. After sufficient settling time, the slurry sedimented at the bottom of the first aluminum tray is selected as the raw material for the ancient ‘Fanhong’ due to its optimal particle size distribution characteristics. This sediment undergoes secondary rinsing to eliminate residual soluble impurities. After sun drying treatment, the entire classification process is completed (Fig. 5a, b). Parallel experiments employed modern ball milling technology to quantify particle size distribution of the same ‘Shengfanhong’ powder across discrete processing durations (20, 30, and 60 min). Simulated samples were fabricated using these differentially processed powders (Fig. 5c), to systematically investigate how particle size variations affect ‘Fanhong’ porcelain coloration characteristics, along with a chromatic comparison between samples prepared from differently sized ‘Shengfanhong’ fractions and ancient reference samples, as shown in Fig. 5d, the L*, a*, and b* values of the simulated ‘Fanhong’ porcelain for ‘Shengfanhong’ powder at 20 min, 30 min, and 60 min ball milling time are as follows: 42.23, 35.32, 30.36, and 43.91, 35.39, 31.07, and 39.78, 37.49, 32.11. Through particle size comparison, the granulation of traditionally classified ‘Shengfanhong’ powder is similar to that of modern ball-milled ‘Shengfanhong’ powder processed for 20–30 min, with an average particle size of approximately 7 μm. Simultaneously, the experiment conducted particle size analysis on straw paper-filtered ‘Shengfanhong’ powder samples and post-filtration fractions obtained through straw paper sieving (as shown in Fig. 5e), Particle size distributions corresponding to each milling condition are characterized in Fig. 5f.

Fig. 5: Classification process of ‘Shengfanhong’ and related test results.
Fig. 5: Classification process of ‘Shengfanhong’ and related test results.
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a Schematic diagram of the classification process of ‘Shengfanhong’. b The actual operation diagram of the classification process for ‘Shengfanhong’. c Experimental simulation samples of the same ‘Shengfanhong’ powder under different ball milling times. d The chromaticity of experimental simulation samples of the same ‘Shengfanhong’ powder under different ball milling times. e Particle size distribution of ‘Shengfanhong’ powder on and through the straw paper in the classification process (Ⅰ. ‘Shengfanhong’ powder on the straw paper; Ⅱ. ‘Shengfanhong’ powder through the straw paper). f Particle size distribution of the same ‘Shengfanhong’ powder under different ball milling times (Ⅰ. 20 min; Ⅱ.30 min; Ⅲ.60 min)).

In the classification process for ‘Shengfanhong’ preparation, the particle size of the powder directly affects the final coloration of ‘Fanhong’ porcelain. Craftsmen traditionally relied solely on straw paper filtration and empirical judgment to select ‘Shengfanhong’ powder from the first aluminum tray; its core mechanism involves hydrodynamic sorting of raw alum red powder particles. Through a multi-stage gradient sieving system, the primary grading phase achieves separation of coarse particle fractions, and target particle-size-range powder forms a dominant enrichment zone within the first aluminum tray. The first aluminum tray not only collects sediment with optimal particle size distribution but also facilitates secondary impurity removal as supernatant liquid flows into the second aluminum tray during sedimentation. Under modern techniques, by comparing the results of chromaticity tests on ancient samples from previous work, it can be seen that the coloration of ‘Fanhong’ basically conforms to the rule of the influence of ball milling time on coloration in the experiment. Comparative analysis revealed that the particle size distribution obtained through traditional classification processes corresponds approximately to that achieved after 20–30 min of modern ball milling treatment. Chromaticity tests reveal that the coloration of ‘Fanhong’ porcelain follows a normal distribution relative to particle size distribution. The optimal effect of ‘Fanhong’ porcelain is achieved with 20–30 min of ball milling; the coloration of the simulated sample in the experiment is similar to that of the ancient samples (e) and (f) in Fig. 1, with a bright orange-red color, while prolonged milling resulted in darker hues. Therefore, ancient craftsmen selected ‘Shengfanhong’ powder from the first aluminum tray for making ‘Fanhong’ porcelain.

Discussion

This study deciphers the traditional production of ‘Shengfanhong’ by elucidating three core mechanisms:

1. Scientific basis of calcination temperature control: the color evolution during calcination—from green (FeSO4·7H2O) to gray (dehydrated FeSO4), and finally to orange–red (α-Fe2O3)—is governed by phase transitions and iron oxidation (Fe2+ to Fe3+). The empirical endpoint, where slightly blackened material turns orange–red upon cooling, corresponds to α-Fe2O3 formation at ∼700 °C, The phenomenon that craftsmen observed the powder turning slightly black during FeSO4·7H2O calcination and halted firing results from the superposition effect of high-temperature thermal radiation and reflected spectra.

2. Chemical verification of rinsing process: sequential rinsing with cold and hot water removes insoluble impurities and sulfate residues via hydrolysis. The traditional tea-infusion test functions as a highly effective colorimetric method, where tannic acid complexes with residual Fe3+ to form black iron-tannate, providing a visual and quantitative indicator of purification completeness.

3. Rationality of particle size classification process: the traditional hydraulic classification using straw paper and aluminum trays consistently produces ‘Shengfanhong’ powder with an optimal particle size distribution, equivalent to modern ball milling for 20–30 min. This specific size range yields a bright orange–red glaze, while deviations result in suboptimal coloration, scientifically affirming the efficacy of the ancient empirical selection from the first aluminum tray.

Collectively, this work demystifies the tacit knowledge embedded in ‘Shengfanhong’ production, demonstrating that its traditional excellence arose from an empirically optimized interplay of thermal chemistry, reaction engineering, and particle science. It should be noted that this study has certain limitations. A primary constraint is the inherent challenge in fully replicating the authentic historical tools and production environment used by ancient artisans. While experimental archeology provides a robust approximation, these uncontrollable variables may introduce discrepancies between our laboratory-scale reproductions and the original craftsmanship, potentially affecting direct comparisons of the final pigment’s properties.