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II. La fabrique du noir

Black coal cinder ceramics: an unusual technological choice in Sichuan Province, China

Céramiques à la cendre de charbon : un choix technique inhabituel dans la province du Sichuan, Chine
Chandra L. Reedy
p. 50-63

Résumés

Dans la province du Sichuan, en Chine, de nombreux ateliers de céramique installés dans l’historique rue des potiers de Yingjing, de même qu’un petit atelier de Gaoxian, produisent des céramiques noires grâce à l’ajout de cendres de charbon broyées à leur argile. Cette matière première unique en son genre est cuite dans un four dont la conception et le procédé de cuisson sont également très spécifiques. Les produits de cuisson à grains grossiers sont les plus communs, mais des services à thé à pâte fine et des lignes de produits inédites ont récemment vu le jour à Yingjing pour répondre aux besoins de nouveaux créneaux du marché. Les phases minérales haute température telles que mullite, cristobalite et verre observées dans des lames minces reflètent principalement celles également repérées dans les cendres de charbon. Le volume des pores fermés et ouverts et la structure des systèmes poreux diffèrent selon qu’il s’agit de céramiques à grains grossiers ou fins et sont bien adaptés aux fonctions auxquelles ces produits sont destinés. La variabilité des systèmes de pores dans le cas des céramiques à grains grossiers de Yingjing signale l’existence de différences subtiles sur le plan des matières premières, de la préparation des matériaux et des températures de cuisson, entre les ateliers ainsi que dans les nombreux cycles de production et produits de chaque atelier, par comparaison avec l’unique petit atelier de Gaoxian dont la production demeure très limitée.

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Texte intégral

Introduction

  • 1 Kerr, Wood, 2004, Parts 1, 2, 3; Rice, 2005, Chapter 11; Reedy, 2008, p. 144-146, 184-189.
  • 2 Rice, 2005, p. 334-335; Cartechini et al., 2021, p. 192; Quinn, 2022, p. 274-277.
  • 3 Chevalier et al., 1976; Nagy et al., 2000; Kerr, Wood, 2004, p. 296-301; Rice, 2005, p. 335-336; Ma (...)
  • 4 Reedy et al., 2017a, for the full cultural and technological details of this ceramic tradition.

1Black pottery usually indicates the significant presence of carbonized organic tempers or reduced iron oxides1. Some clay deposits contain organic matter; potters may also add it to improve the clay’s working, drying, and firing properties. Under oxidizing firing conditions, this organic matter can burn out or diffuse black carbon into the ceramic; under reducing conditions, it chars in place and creates a black color2. Iron oxides are also commonly found in clay deposits. Under reducing firing conditions magnetite and wüstite may form, resulting in a gray or black ceramic body3. Highly unusual pottery produced in Sichuan Province, China, uses a unique raw material to achieve its black color: a ground coal cinder additive. This is combined with a two-stage firing process that ends with an in-ground reducing kiln, resulting in a hard ceramic body that is very black from surface to interior4.

  • 5 Reedy et al., 2017a, p. 2045-2047, for comparison of the differences in extent of workshops in Ying (...)
  • 6 Sun, 2014, plates 73-79.
  • 7 Reedy et al., 2017a, for full discussion of the Gaoxian material and workshop practices, and the vi (...)
  • 8 Liu et al., 2019, report coal-ash slag with semi-reacted coal fragments and coal gangue in crucible (...)

2The current largest and most active center of this ceramic tradition is Yingjing, in west-central Sichuan, 170 kilometers southwest of the capitol city, Chengdu5. Many workshops are located along a street in the “old town” section known as Black Sand Pottery Village. In addition to workshops and kilns, the street is lined with sales rooms and with raw material storage and preparation areas. Oral histories trace the tradition back at least four generations. The forming and firing techniques were documented in 1939 by Sun Mingjing, a photographer who visited from Shanghai during the Japanese invasion and his photographs show similar steps and equipment to those used in Yingjing today6. A similar technology is practiced in a lone workshop in Gaoxian, in southeastern Sichuan, and in several workshops just over the Sichuan border in Guizhou Province, where historical sources trace the tradition back to the 17th century7. The use of a coal cinder additive in pottery production, and the associated kiln and firing processes, do not appear anywhere else in the world8.

  • 9 For examples, see Bureau of Culture, Radio and Television, Muli Tibetan Autonomous County, 2009; El (...)
  • 10 Nangsa, 2007.
  • 11 Zhou et al., 2012.

3The origins of this tradition are unknown. Black pottery is valued in Sichuan, especially in Tibetan communities here and in adjacent Yunnan Province. However, the Tibetan potters use organic-rich clay with reduced firing or an open firing concluding with burial in sawdust to create carbon that penetrates the vessels to produce black pottery9. Since Yingjing was once a central stop along the Tea Horse Road connecting Yunnan Province to Tibet, black pottery likely traveled in both directions as trade goods10. Coal cinders are readily available here to spark the idea of using it to create black pottery since Yingjing, Gaoxian, and Guizhou are coal-rich regions; coal fuels the pottery kilns, and until recently, coal-burning stoves were the main source of heat. Coal also fueled zinc smelting sites throughout the area, and coal ash was used as a temper for condensers11. Piles of burnt coal cinders would have been present in the past, and today’s coal-fueled industries make burnt coal easily available for innovative reuse in pottery workshops.

4The two-stage firing process (fig. 1 a-b) is very dramatic. It starts with oxidized firing in a coal-filled pit, with a heavy but movable ceramic-and-wire kiln cover levered over the pots. When the pots are orange-hot, the cover is levered off and they are pulled out with a hooked pole and placed into an adjacent in-ground brick-lined pit. Resinous organic material is added, resulting in immediate smoke and steam, and that pit is then covered to exclude air for a final reduced heating where the ceramic body becomes very black and a thin shiny ash glaze forms over the surface. Depending on the workshop and type of product, firing temperatures range between 900-1200°C. Visitors arriving to buy pottery enjoy watching the colorful glowing show as the pots are moved from the first to the second kiln. The process and kiln designs are unique to the coal cinder pottery tradition. This tradition is typically referred to in China as “black sand pottery”, implying that black iron-rich sand is mixed into the clay. Yet, the use of a coal cinder additive to create black pottery is as highly innovative and unique to this region as is the firing process.

Fig. 1 a-b. Firing and ash glazing processes of coal cinder ceramics

Fig. 1 a-b. Firing and ash glazing processes of coal cinder ceramics

a. In Yingjing, multiple pots are fired in a shallow brick-lined pit. The kiln lid is levered off when the pots are orange hot, so they can be moved to an adjacent deep ash-glazing pit. b. In Gaoxian, one pot is fired at a time in a hole in the coal pile, then moved into a shallow ash-glazing pit.

© C. L. Reedy.

  • 12 Reedy et al., 2017a, for more details on Yingjing and Gaoxian materials preparation methods and fir (...)

5Table 1 summarizes the materials preparation, firing regimes, composition, and appearance of the Yingjing and Gaoxian products. Both coarse wares and fine wares are made in Yingjing, and the single Gaoxian workshop is limited to two coarse ware products12.

Table 1. Main methods of coal cinder ceramic production

Table 1. Main methods of coal cinder ceramic production
  • 13 Reedy et al., 2017a.

6After initial research in 2014-2015 on the working practices and products at one Yingjing workshop and the Gaoxian one13, we returned in 2017-2018 to observe practices, collect raw materials and fired products, and interview potters, kiln managers, and shop managers all along the Yingjing pottery street. Between visits, the original Gaoxian workshop was relocated due to urban construction. The summary here includes analysis of raw materials and fired products from both Gaoxian workshop locations. The following sections summarize the research methods, range in visual appearance of products, and results of analyses of raw materials and ceramic products.

Methodology

7Three field visits were made to Yingjing and two to Gaoxian. Fieldwork included interviewing workshop owners/managers, workers involved in various steps of production (processing raw materials, designing products, fabricating vessels, decorating, firing, and finishing), storefront managers, and local cultural heritage officials. Workshop observations included procedures for obtaining raw materials (clay and coal cinders) and grinding and mixing them, fabricating objects and applying decorations, drying, firing, and finishing. At storefronts, we observed customer choices and interactions, display methods, product ranges, and the recent addition of web-based sales. Ceramic products were also observed in use.

  • 14 Reedy et al., 2017a, analyses performed by Pamela B. Vandiver at University of Arizona.
  • 15 Reedy, Reedy, 2022a for full discussion of the experiments that led to development of the scanning (...)
  • 16 The Dragonfly software package (version 2022.1) was selected because it has a very wide range of ca (...)

8Clay and coal cinders, unfired ceramics, ceramics that underwent the first phase of firing but not the glaze pit step, and fully fired ash-glazed ceramics were collected from ten workshops for materials analysis. The primary analytical method was thin-section petrography, using transmitted and reflected polarized light (Nikon 50i POL) to identify phases. Uncovered polished thin sections (27 x 46 mm, 30 µm thick) were mounted in blue-dyed epoxy so that pores could be distinguished. For Yingjing, 20 ceramic products and 35 raw material samples (clays and coal cinders) were studied in thin section, and for Gaoxian 10 ceramic products and 10 raw material samples. Mineral identifications were confirmed, and clay minerals identified, by X-ray diffraction (XRD) on three representative samples from Yingjing and three from Gaoxian (performed by MSE Supplies LLC, Tuscon, Arizona, USA). XRD patterns were obtained using powder samples of 0.5 g, ground to ≤ 75 µm particle size, on a Rigaku MiniFlex instrument with copper X-ray source, over 10-80 degrees at 2 degrees per minute. A previous publication reported results on additional analyses of a sample from Yingjing and one from Gaoxian (using scanning electron microscopy with energy-dispersive X-ray spectrometry, electron probe microanalysis, and differential thermal analysis14). Ceramic pore systems were studied by micro-CT imaging combined with 3D image analysis. Micro-CT scanning was done with a Rigaku GX-130 instrument with a micro-focus X-ray source 120 mm to center of rotation, with W target and a 7 Mp flat-panel X-ray detector (FPD). The sample remains stationary while the X-ray source illuminates it as the FPD circles 360° around it while collecting magnified projection images at view angles uniformly distributed around the object. The FPD active area size is 116.424 x 145.728 mm, with pixel dimensions 2552 x 2944, pixel size 49.5 µm, and distance of FPD to center of rotation 224 mm. Scans were conducted for 57 minutes at 130kV, 61 µA, spot size 5 µm for high resolution, with a 0.06 mm Cu and 0.5 mm Al filter combination15. For a study of pore systems, 0.5 cm3 fields of view were scanned and characteristics that potentially affect ceramic use properties were quantified with 3D image analysis using the Dragonfly software package by Object Research Systems16.

Results and discussion

Ceramic products

9In Yingjing, potters report that the earliest products made here were coarse-grained utilitarian items such as chamber pots, wash basins, coal-burning stoves, and cooking pots. Today, only one workshop still makes coal-burning stoves, but most make cooking pots, some as their only product (fig. 2 a). Many other coarse-grained products from individual workshops meet specific market niches (fig. 2 b). Examples are medicine pots, incense burners, tablewares, steamers, Sichuan hotpots, and even art pottery. While some products are sold locally, many others go to customers in Chengdu, Chongqing, and elsewhere in China, as well as to visiting Chinese tourists; some workshops specialize in products for Tibet or North Korea. Fine-grained black teawares emerged as a new coal cinder ceramic product around 2000 and are now sold by multiple workshops (fig. 3 a). However, not everyone has added them because they require additional grinding equipment and a different clay mix. In Gaoxian, coarse-grained hotpots and one type of cooking pot are the only products (fig. 3 b). This workshop says that more than twenty years ago, there was a local market for other types of products, such as medicine pots and field pots, but that market died away, so they moved to specializing in primarily making their two types of cooking vessels. These are sold locally and in nearby Yibin City as a special local product, although some sales also go to Tibet and Xinjiang.

Fig. 2 a-b. Examples of Yingjing coarse wares with a shiny ash glaze. a. Cooking vessels; b. Medicine pots

Fig. 2 a-b. Examples of Yingjing coarse wares with a shiny ash glaze. a. Cooking vessels; b. Medicine pots

© C. L. Reedy.

Fig. 3 a-b. a. A Yingjing shop display including teapots, teacups, and art pottery; b. Hotpots, the main product of the Gaoxian workshop

Fig. 3 a-b. a. A Yingjing shop display including teapots, teacups, and art pottery; b. Hotpots, the main product of the Gaoxian workshop

© C. L. Reedy.

Raw materials

10In all workshops on the Yingjing pottery street, the raw materials are always ground coal mixed with local clay, but there are some variations in the type of clay or coal cinders, and in the proportions of these materials. Many workshops say they aim for a 50-50 mix, which will remain consistent no matter what type of product is to be made. However, some workshop managers claim that their specialized cooking wares, such as hot pots, are more durable because they have a special recipe, a proprietary ratio of clay to coal cinders. Other potters say they adjust the ratio if they are making innovative products, and sometimes experiment with formulations; none would reveal their special formulations for fear of competitors finding out and copying them. The exact proportion of clay-coal cinders does not affect the color of the fired product as much as grain size does, so better processing equipment allows a workshop to vary the color for specific product lines. For example, we observed in one workshop that a 50-50 mix from the same coal cinders supply was used for both fine teawares and for coarse soup pots, but the much more finely ground raw materials for teawares appeared blacker to the eye both in raw form and as finished products.

11In the single Gaoxian workshop, at our first visit, the potter reported that he used a 30-70 mix of clay to coal cinders. However, at our visit to his relocated workshop, the potter noted that he had improved the recipe of the material, so that it is finer and more durable than before; but he did not want to reveal the current mix, as it is a trade secret. Observing in the workshop, we saw him use three scoops of clay to one scoop of coal cinders, a much lower proportion of coal cinders than in Yingjing, which may have to do with his need to use only in-house coal cinders as he is not in a financial position to purchase them from external sources. While mechanical tests are yet to be performed, the potter notes that his products do easily break in transport, so he wraps them carefully for local customers and transporting them a long distance is difficult. The Yingjing potters do not report this problem, and we observed a large truck setting out for overland transport with unwrapped pots stacked high with only a little straw under each pot.

12Table 2 lists minerals that have been observed in the clays, coal cinders, and finished fired products. For coarse wares such as cooking and medicine pots, workshops use sandy yellow clay, rich in hematite and limonite. It is purchased from local farmers, and potters say it is abundant in the area. Since clay minerals cannot be characterized in thin section, the supplementary analytical techniques were crucial for their identification. Both illite and montmorillonite are found locally and used by potters. Construction projects near Yingjing often uncover clay deposits which also supply potters from the street. The yellow clay is sometimes layered with a white kaolinite clay with accessory sericite or plagioclase feldspar that serve as fluxes to help clay bodies sinter and vitrify.

Table 2. Minerals observed in raw materials versus fired ceramics

Yingjing

Clay

Coal cindersc

Fired ceramicse

Illite or montmorillonite, and sometimes minor kaolinitea
Quartz (fine-grained)
Hematite
Limonite
Albite
Sericite
Chlorite

Quartz
Cristobalite
Glass
Mullite
Anorthite
Muscovite
Hematite
Pyrite
Spinel

Quartz
Cristobalite
Glass
Mullite
Anorthite
Hematite
Pyrite
Spinel

Gaoxian

Clay

Coal cindersd

Fired ceramicsf

Illite (humic rich) or
kaolinite (humic rich)
b
Quartz (fine-grained)
Hematite
Anatase
Chalcedony
Calcite
Sericite

Quartz
Cristobalite
Chalcedony
Glass
Mullite
Anorthite
Hematite
Anhydrite
Sillimanite

Quartz
Cristobalite
Glass
Anorthite
Mullite
Hematite
Anatase
Rutile
Anhydrite
Sillimanite

a. Clay is collected from many fields and construction sites in and surrounding Yingjing; some deposits are layered with kaolinite, and some potters add imported kaolinite to finer wares.
b. The single pottery workshop in Gaoxian selected clay from two different sources during the study, both were reported as being deep deposits adjacent to coal sources and hence are humic rich.
c. In Yingjing, coal cinders come from a wide variety of sources, mostly external.
d. In Gaoxian, cinders come only from the workshop’s burnt coal from its own ceramic kiln.
e. A wide range of products are made in Yingjing, including various cooking wares and finer teawares.
f. A narrow range of cooking wares are the only products made in Gaoxian.

13Some potters carefully sort the yellow and white clays (called “ordinary” versus “best quality” clays) and use the white alone for finer products; but most do not try to separate them (the white is often streaked through with the yellow) and simply grind them together. Some potters use white clay to coat the insides of bowls, plates, and other tablewares before firing, to create a smoother inner surface. A few potters import kaolinite clay from the nearby town of Mingshan to mix with local clay for fine teawares. However, other potters shun any imports, saying that local clays are more successful in creating fine surface details.

14In the original Gaoxian workshop location, the potter collected a ball clay himself from a nearby source close to coal deposits, paying the landowner. That quartz-rich illitic clay is black, high in humic content. In the new location, the potter purchases black ball clay from Yibin City, requesting organic-rich clay found adjacent to coal deposits. This clay is kaolinite with carbonaceous material, quartz, anatase, and hematite.

15A few Yingjing workshops recycle coal cinders from their kilns for ceramic raw material, but this is a minor source; most of it comes from industrial suppliers in Yingjing, Chengdu and Chongqing. Many workshops separate cinders into “qualified” (for use as a raw material) and “unqualified” (to be discarded). One workshop has three levels of “qualified” (low, medium and high) and uses them for different types of products. Some workshops meet with suppliers in advance to show them what they want, to ensure receipt of mainly “qualified” material. There are large quantities of coal cinders discarded (from the kilns and from rejected “unqualified” purchased cinders) and local cultural heritage officials expressed concerns about the need to collect and dispose of coal cinders from the pottery street as toxic waste. In contrast, the single workshop in Gaoxian uses only in-house coal cinders from its kiln and has no leftover waste for disposal.

  • 17 Starna, 1999, for descriptions and micrographs of coal cinders.

16Geoscientists have identified conditions that form two types of coal cinders (fig. 4 a-b): (1) incomplete combustion of coal at 600-1000°C results in highly carbonaceous cinders that retain clay fragments, micas, and quartz; lack mullite; and are rich in hematite with some iron sulfides; and (2) temperatures between 1000-1300°C break down the clays and micas and create carbonaceous cinders with extensive iron-containing mullite, moderate amounts of anorthite and quartz or cristobalite, and hematite and iron sulfide prills that often react to form iron spinel amid a glassy matrix17.

Fig. 4 a-b. Thin sections mounted in blue epoxy (plane polarized light) of Yingjing coal cinders deemed “qualified” and similar to Type 1, with layered clay fragments and quartz in a carbonaceous matrix (a); and “unqualified” and similar to Type 2, with extensive glass filled with mullite needles within a porous carbonaceous matrix containing prills of hematite and iron sulfide and some iron spinel (b)

Fig. 4 a-b. Thin sections mounted in blue epoxy (plane polarized light) of Yingjing coal cinders deemed “qualified” and similar to Type 1, with layered clay fragments and quartz in a carbonaceous matrix (a); and “unqualified” and similar to Type 2, with extensive glass filled with mullite needles within a porous carbonaceous matrix containing prills of hematite and iron sulfide and some iron spinel (b)

17Thin sections of coal cinders from Yingjing workshops show variation in what is sought after as “qualified” raw materials (with the supplementary XRD on a few selected samples particularly useful to confirm the identification of the often-fine grained mullite and cristobalite). Some select material is closest to Type 1, and others material closest to Type 2. The workshop that divides their “qualified” material into three categories chooses Type 1 for the best and medium and Type 2 for the lowest quality, using that for coarse wares. Cinders in all workshops are often very inhomogeneous, with outer edges that were exposed to higher temperatures often glassy and filled with iron-containing mullite and dendritic spinel, even for Type 1. Gaoxian coal cinders generally fall midway between Type 1 and 2 but vary considerably.

18In Yingjing, while the traditional method of grinding both coal cinders and clay was an animal-pulled milling stone, today both are milled in a jaw crusher. For fine products such as teawares, a second processing through a flour mill achieves a finer grain size. The two raw materials are then mixed on the grinding room floor, wetted, and then further mixed and de-aired in a pug mill (for coarse wares) or a ball mill (for fine wares). In Gaoxian, the coal cinders and clay are ground together (originally in a grain mill but since very recently in a pug mill), then wetted and taken directly for potting.

Mineralogy of fired ceramics

19The carbonaceous material of incompletely combusted coal is finely powdered during the grinding and mixing process, and obscures thin sections. Some of the small quartz grains that could have entered with the clay can be seen, along with the phases seen in the coal cinders. Unfired sherds show this mixing well, with clearly two populations of quartz grain size, finer from the clay and larger from the coal cinders, along with the high-temperature minerals and glass coming in with the cinders. The Yingjing fired ceramic thin sections are also seen to reflect primarily the mineralogy of the added coal cinders, whether Type 1 or 2. Coal cinder particles displaying similar mineralogy are readily visible in mixed raw materials, dried but unfired ceramics, ceramics fired but never placed into the glaze pit, and those fully fired and undergoing the ash glaze formation. This indicates that the firing process does not affect the coal cinders drastically. Comparing mixed raw materials, dried but unfired ceramics, ceramics fired but never entering the glaze pit, and those fully fired and undergoing the ash glaze formation process indicates that the coal cinder particles are readily visible throughout these stages, with their original mineralogy intact. The clay matrix sinters and vitrifies in the kiln, cementing in the chunks of coal cinders. The cinder chunks now incorporated into the ceramic matrix are seen to contain their original quartz (often cracked), cristobalite, patches of glass (sometimes with incipient mullite needles), fully developed mullite within glass (fig. 5 a), occasional twinned anorthite blades, hematite prills, iron sulfides, and spinel; these phases reflect the range of minerals observed in the coal cinders.

Fig. 5 a-b. Thin section of a Yingjing coarse ceramic (a) showing well-developed mullite in glass from use of Type 2 coal cinders, plane polarized light; and of a Gaoxian product (b) with round reddish hematite prills and smaller and more lustrous TiO2 (reflected light)

Fig. 5 a-b. Thin section of a Yingjing coarse ceramic (a) showing well-developed mullite in glass from use of Type 2 coal cinders, plane polarized light; and of a Gaoxian product (b) with round reddish hematite prills and smaller and more lustrous TiO2 (reflected light)
  • 18 Shoval et al., 2011.

20In Gaoxian, the mineralogy reflects the coal cinders but also the change in clay sources between the two workshop locations. All products can have quartz, glass, anorthite, mullite, or hematite while those of the relocated workshop also have some anatase, and rutile (fig. 5 b). As is the case here, kaolinite often contains accessory anatase; during firing anatase can begin to transform to rutile at 1100°C18. Here too the supplementary XRD analysis was helpful because the very fine-grained anatase and rutile are difficult to discern within this matrix and cannot be distinguished from each other optically. One sample from the relocated workshop also indicated the presence of sillimanite in some coal cinders and also in at least one ceramic sample. This will require further study with additional samples, as will the occasional presence of anhydrite.

21The uneven ash glazes of Yingjing coarse products are thicker and more evenly distributed on the surface than those of Gaoxian, reflecting differences in the glaze pit designs and time during which the ceramics are kept in the pit. These “glazes” that form as the hot ceramic surface encounters the burning resinous organic material inside the hot pit. They are composed of very porous glass which incorporates fine-grained quartz, ash particles, and incipient anorthite. The fine grained teawares lack glaze, as they receive only the first firing (in saggars). As they are made with very finely-ground coal cinders, only small chunks of quartz, cristobalite, and glass are visible within very fine black powdered carbon.

Ceramic pore systems

  • 19 Cultrone et al., 2004 and Buchner et al., 2021, for representative studies using micro-CT for asses (...)
  • 20 Coletti et al., 2016, for a good comparative review of techniques for characterizing porosity in ce (...)

22Micro-CT, often coupled with 3D image analysis, has been used for characterizing porosity in bricks and ceramics for almost twenty years now19. Many studies have compared this approach to other porosity analysis techniques; all have concluded that comparisons are difficult because different techniques are based on different physical principles, each has limitations, no one method can characterize the entire pore system and its range of pore sizes well, and they provide varying types of quantitative and visual data20. Micro-CT with 3D image analysis to interpret results was chosen here because it is non-destructive, so the same sample can be reused for other analyses if desired; unlike thin-section petrography or other 2D imaging methods the full 3D pore networks are visible, giving more realistic measurements; and a good range of quantitative data can be obtained along with qualitative visual data that highlights exactly where on each sample the results are located.

  • 21 Reedy, Reedy, 2022b.
  • 22 Reedy et al., 2017a.

23Micro-CT scans were done on six Yingjing coarse cooking pot sherds, two Yingjing fine teaware sherds, and four Gaoxian sherds (two from the original workshop and two from the relocated one). The results are summarized in Table 3. The 3D image analysis of the scans (fig. 6) shows Yingjing coarse wares have an average volume porosity (open and closed pores) of 17.23%; however, measurements vary from 13.64-22.84%, likely reflecting variations in the mineralogical composition of various batches and sources of clay and coal cinders, materials preparation, and firing temperatures from one workshop and production run to another that could lead to differences in sintering and vitrification behavior of the ceramic matrix. Gaoxian ceramics present a consistent porosity volume of about 17%, in spite of the workshop relocating, likely because the potter and his wife are sole operators of the production and firing process and make only two products, so techniques will vary little. The Yingjing fine wares are much less porous than the coarse wares from either Yingjing or Gaoxian. Sometimes lower porosity is related to higher firing temperatures21 but since there is no evidence for that in Yingjing, low porosity is likely due to the much finer grinding of raw materials (reducing the amount of porosity that tends to develop around large particles as they pull away from the clay matrix during drying and firing) and use of a different clay mix that includes kaolin, which will sinter and vitrify at a lower temperature22. There is currently no evidence that the selection of Type 1 versus Type 2 coal cinders influences porosity more than particle size. Many workshops do not try to sort their cinders by quality, and those that do vary from each other in how they make decisions as to which is higher quality and are often inconsistent in their choices. However, some workshops do say they endeavor to set aside for their fine ware products cinders that we have found to be Type 1, so the relationship of coal cinder type to porosity would be interesting to pursue.

Fig. 6. Micro-CT and image analysis views of pores in a Yingjing coarse ware with 17.07% volume porosity (closed and open)

Fig. 6. Micro-CT and image analysis views of pores in a Yingjing coarse ware with 17.07% volume porosity (closed and open)

Left, 3D reconstruction of a region of interest taken from the scan of a 0.5 cm3 field of view; center, pores (blue) segmented from matrix (yellow) using a classical machine learning model (Random Forest – Activation Map 2); right, pores color-coded by aspect ratio showing a morphological difference between the surface-connected pores and the interior pores.

Table 3. Yingjing and Gaoxian ceramic pore systems

Yingjing (coarse wares)

Sherd

No.

% vol.

porosity

Mean elongation,

µma

Mean sphericity,

µmb

% pores unconnectedc

Max pore

connectivityd

YJc1.1

15.63

0.61

4.11

6

15

YJc1.2

15.68

0.63

3.3

3

30

YJc3.1

18.44

0.62

3.81

4

19

YJc3.2

22.84

0.65

3.51

4

17

YJc8.1

13.64

0.61

4.53

7

14

YJc8.2

17.07

0.62

4.15

5

16

Yingjing (fine wares)

YJf1.1

1.76

0.61

5.2

22

5

YJf1.2

3.95

0.59

5.47

19

8

Gaoxian (coarse wares)

Gao1.1

17.22

0.64

3.34

3

18

Gao1.2

17.14

0.63

3.68

3

19

Gao2.1

17.34

0.63

3.74

3

22

Gao2.2

16.75

0.63

3.58

3

17

a. Minimum orthogonal Feret diameter/maximum Feret diameter is used as a measure of pore elongation.
b. Volume/surface area ratio (volume in relation to outside surface area of the pores) is an indication of sphericity since for a given volume the ratio is maximized for spheres.
c. The % of pores in the system that are isolated and unconnected to any other pores.
d. The maximum number of other pores to which any pore in the system is connected.

24Having sufficient porosity is useful for the cooking vessels that contain larger coal cinder particles. These particles can expand during heating and contract during cooling without exerting pressure that might crack the vessel because of the many pores that absorb the strain. In contrast, high porosity is not needed for teawares and other fine ware products that are used mainly for serving food and drinks.

  • 23 Reedy, Reedy, 2022b.
  • 24 Rice, 2005, p. 367-368.

25Other pore variables are affected by an increased level of vitrification in the fine wares. Previously connected pores are separated as vitrification proceeds, and newly formed ones have a more spherical shape typical of isolated secondary pores formed during release of gases during vitrification. As a result, pore values indicating elongation are lower for the fine wares than for the coarse ones, while those indicating sphericity are higher. The percentage of a vessel’s pores that are unconnected is also related to degree of vitrification of the body that takes place during firing23. Here the data show an average of 3% unconnected pores for Gaoxian ceramics, 4.8% for Yingjing coarse ones, and 20.5% for Yingjing fine wares (fig. 7 a-b). A higher percentage of connected pores is advantageous for cooking wares24, to allow heat to permeate the ceramic body evenly and completely. Small and unconnected pores are better for a teacup, to keep the tea hot but not allow the heat to pass through the ceramic wall and burn the hand of the drinker.

Fig. 7 a-b. Pore networks imaged by graph representation where spheres represent pores and lines between them pore connections

Fig. 7 a-b. Pore networks imaged by graph representation where spheres represent pores and lines between them pore connections

Within a particular system, pores range from light to dark by increasing connectivity and connections are colored purple, blue, green or orange by increasing length. a. A Gaoxian ceramic with 17.34% pores and connectivity ranging 0-22, but only 3% of the total are unconnected so isolated pores are difficult to discern. b. A Yingjing fine ware with 1.76% pores and connectivity ranging 0-5, with 22% of the total unconnected so isolated pores are abundant and clearly visible.

26In contrast, a higher maximum connectivity is associated with larger additives that begin to crack or melt but do not fully vitrify. Partial dissolution of some grains, cracking of larger quartz or glass particles from coal cinders, and other firing effects on these coarse particles can create many locations where intraparticle pores are in contact with each other and with some interparticle pores, raising the connectivity values as a consequence. The average maximum connectivity for Yingjing and Gaoxian coarse wares is 19, while that of the fine wares is 6.5, reflecting differences in grinding of their raw materials.

Conclusions

27Adding a large amount of ground coal cinders to clay is a unique way of attaining pottery with a black color, but the resulting ceramics are well suited to their functions. In Yingjing, there are many workshops of varying sizes. All make the coarser utilitarian wares. Cooking pots for soup or stews are the most popular, but medicine pots, incense burners, hotpots, rice steamers, and coal-burning stoves are also made for specialized markets. Some Yingjing workshops also make fine-grained teawares. Gaoxian has a lone workshop that makes only coarse-grained hotpots and cooking pots; their cinders come only from the coal used to fire their kiln. The Yingjing workshops, with larger production, need to purchase most of their cinders. Different workshops choose different cinder types. All who make fine wares grind their clay and cinders much more finely for these products, and do not add the ash glaze that is ubiquitous for coarse ware products. Thin sections are obscured by finely powdered carbon from unreacted coal in the cinders, but many particles are still visible; most phases are also seen in the cinder raw material. In Gaoxian, the major change between clay sources that occurred when the workshop relocated is reflected in differing thin section mineralogy. Pore variables in the Gaoxian workshop coarse wares are very consistent, but with multiple workshops and higher volume production of coarse wares in Yingjing, greater variation is seen in pore amount, size, shape, and connectivity values. The porosity and structure of pore systems in coarse wares differs from the fine wares due to differences in particle size and amount of vitrification of the matrix, with each best suited to intended functions.

Fieldwork was funded by the National Science Foundation (grant 1339530) and Wenner-Gren Foundation for Anthropological Research (grant 9247). Fieldwork arrangements were coordinated by the Sichuan Provincial Institute of Cultural Relics and Archeology, and I was joined in the field in various years by Ting He, Yanyu Wang, Pamela B. Vandiver and Ying Xu.

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Notes

1 Kerr, Wood, 2004, Parts 1, 2, 3; Rice, 2005, Chapter 11; Reedy, 2008, p. 144-146, 184-189.

2 Rice, 2005, p. 334-335; Cartechini et al., 2021, p. 192; Quinn, 2022, p. 274-277.

3 Chevalier et al., 1976; Nagy et al., 2000; Kerr, Wood, 2004, p. 296-301; Rice, 2005, p. 335-336; Maritan et al., 2006 and 2020; Cartechini et al., 2021, p. 192; Quinn, 2022, p. 274-277.

4 Reedy et al., 2017a, for the full cultural and technological details of this ceramic tradition.

5 Reedy et al., 2017a, p. 2045-2047, for comparison of the differences in extent of workshops in Yingjing versus Gaoxian; the video by Bin indicates a scope in Guizhou Province smaller than that of Yingjing but larger than Gaoxian. It is likely that other workshop centers existed in the past in this general region, but evidence has not yet been found.

6 Sun, 2014, plates 73-79.

7 Reedy et al., 2017a, for full discussion of the Gaoxian material and workshop practices, and the video by Bin for the Guizhou Province workshops.

8 Liu et al., 2019, report coal-ash slag with semi-reacted coal fragments and coal gangue in crucibles used for lead-silver smelting in 12th-13th century China.

9 For examples, see Bureau of Culture, Radio and Television, Muli Tibetan Autonomous County, 2009; Elliott, 2011; Reedy et al., 2017b.

10 Nangsa, 2007.

11 Zhou et al., 2012.

12 Reedy et al., 2017a, for more details on Yingjing and Gaoxian materials preparation methods and firing equipment and regimes.

13 Reedy et al., 2017a.

14 Reedy et al., 2017a, analyses performed by Pamela B. Vandiver at University of Arizona.

15 Reedy, Reedy, 2022a for full discussion of the experiments that led to development of the scanning protocols including choice of field of view size for pore system characterization.

16 The Dragonfly software package (version 2022.1) was selected because it has a very wide range of capabilities and is free for academic or nonprofit users conducting research. Available from Object Research Systems at [http://www.theobjects.com/company/index.html]. For a full discussion of all pore variables, how they were calculated using the Dragonfly software, and why they were selected, see Reedy, Reedy, 2022a and 2022b.

17 Starna, 1999, for descriptions and micrographs of coal cinders.

18 Shoval et al., 2011.

19 Cultrone et al., 2004 and Buchner et al., 2021, for representative studies using micro-CT for assessing the porosity of ceramic materials.

20 Coletti et al., 2016, for a good comparative review of techniques for characterizing porosity in ceramic materials.

21 Reedy, Reedy, 2022b.

22 Reedy et al., 2017a.

23 Reedy, Reedy, 2022b.

24 Rice, 2005, p. 367-368.

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Table des illustrations

Titre Fig. 1 a-b. Firing and ash glazing processes of coal cinder ceramics
Légende a. In Yingjing, multiple pots are fired in a shallow brick-lined pit. The kiln lid is levered off when the pots are orange hot, so they can be moved to an adjacent deep ash-glazing pit. b. In Gaoxian, one pot is fired at a time in a hole in the coal pile, then moved into a shallow ash-glazing pit.
Crédits © C. L. Reedy.
URL http://journals.openedition.org/techne/docannexe/image/17263/img-1.jpg
Fichier image/jpeg, 452k
Titre Table 1. Main methods of coal cinder ceramic production
URL http://journals.openedition.org/techne/docannexe/image/17263/img-2.jpg
Fichier image/jpeg, 708k
Titre Fig. 2 a-b. Examples of Yingjing coarse wares with a shiny ash glaze. a. Cooking vessels; b. Medicine pots
Crédits © C. L. Reedy.
URL http://journals.openedition.org/techne/docannexe/image/17263/img-3.jpg
Fichier image/jpeg, 460k
Titre Fig. 3 a-b. a. A Yingjing shop display including teapots, teacups, and art pottery; b. Hotpots, the main product of the Gaoxian workshop
Crédits © C. L. Reedy.
URL http://journals.openedition.org/techne/docannexe/image/17263/img-4.jpg
Fichier image/jpeg, 564k
Titre Fig. 4 a-b. Thin sections mounted in blue epoxy (plane polarized light) of Yingjing coal cinders deemed “qualified” and similar to Type 1, with layered clay fragments and quartz in a carbonaceous matrix (a); and “unqualified” and similar to Type 2, with extensive glass filled with mullite needles within a porous carbonaceous matrix containing prills of hematite and iron sulfide and some iron spinel (b)
URL http://journals.openedition.org/techne/docannexe/image/17263/img-5.jpg
Fichier image/jpeg, 948k
Titre Fig. 5 a-b. Thin section of a Yingjing coarse ceramic (a) showing well-developed mullite in glass from use of Type 2 coal cinders, plane polarized light; and of a Gaoxian product (b) with round reddish hematite prills and smaller and more lustrous TiO2 (reflected light)
URL http://journals.openedition.org/techne/docannexe/image/17263/img-6.jpg
Fichier image/jpeg, 860k
Titre Fig. 6. Micro-CT and image analysis views of pores in a Yingjing coarse ware with 17.07% volume porosity (closed and open)
Légende Left, 3D reconstruction of a region of interest taken from the scan of a 0.5 cm3 field of view; center, pores (blue) segmented from matrix (yellow) using a classical machine learning model (Random Forest – Activation Map 2); right, pores color-coded by aspect ratio showing a morphological difference between the surface-connected pores and the interior pores.
URL http://journals.openedition.org/techne/docannexe/image/17263/img-7.jpg
Fichier image/jpeg, 392k
Titre Fig. 7 a-b. Pore networks imaged by graph representation where spheres represent pores and lines between them pore connections
Légende Within a particular system, pores range from light to dark by increasing connectivity and connections are colored purple, blue, green or orange by increasing length. a. A Gaoxian ceramic with 17.34% pores and connectivity ranging 0-22, but only 3% of the total are unconnected so isolated pores are difficult to discern. b. A Yingjing fine ware with 1.76% pores and connectivity ranging 0-5, with 22% of the total unconnected so isolated pores are abundant and clearly visible.
URL http://journals.openedition.org/techne/docannexe/image/17263/img-8.jpg
Fichier image/jpeg, 825k
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Chandra L. Reedy, « Black coal cinder ceramics: an unusual technological choice in Sichuan Province, China »Technè, 55 | 2023, 50-63.

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Chandra L. Reedy, « Black coal cinder ceramics: an unusual technological choice in Sichuan Province, China »Technè [En ligne], 55 | 2023, mis en ligne le 16 novembre 2024, consulté le 22 mars 2025. URL : http://journals.openedition.org/techne/17263 ; DOI : https://doi.org/10.4000/techne.17263

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Auteur

Chandra L. Reedy

Professor, Center for Historic Architecture and Design, University of Delaware, Newark (clreedy[at]udel.edu).

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