The Analysis of the Collection
An Overview of the Identified Stones
The Collection from a Material Point of View
The 2009 Survey of Khmer Quarries in Siem Reap and Preah Vihear Provinces
The Stone From The Quarry Of Thmâ Anlong
A comprehensive quantitative petrographic database of sandstones used by the Khmers for sculptural purposes would be a helpful tool for archaeologists, museum curators, and others interested in pursuing research on early stone usage, geologic source, and provenance of Southeast Asian stone materials. Toward that end, the Department of Scientific Research of The Metropolitan Museum of Art, in collaboration with the Department of Asian Art, is promoting the petrographic and geochemical study of Khmer sculptural production of the pre-Angkor (ca. seventh to ninth century) and Angkor (ca. tenth to thirteenth century) periods.
More than fifty Khmer sculptures from the Metropolitan Museum collection have been analyzed, together with sculptures from the National Museum of Cambodia and from other Western collections. This corpus of Khmer sculptures, including both well-provenanced and unprovenanced pieces, has been studied by means of minor-destructive and non-destructive analytical techniques (see fig. 1).
In addition, under memorandums of understanding signed between the Metropolitan Museum and the National Museum of Cambodia, and with the Authority for the Protection and Management of Angkor and the Region of Siem Reap (APSARA) of Cambodia, new sculpture and quarry samples have recently been added to this study, the aim of which is to provide a database of Khmer stone materials that has the potential to develop further with future contributions from scientists at other museums that have holdings of Khmer sculpture.
Several scientific works presenting overviews of Khmer stone petrography have been published. Among them, it is worth mentioning the early studies of stone materials of Angkor Wat by Delvert (1963) and Saurin (1954); the studies of the building materials of the Angkor complex by the Japanese research team headed by Etsudo Uchida (see, for instance, Uchida et al. 1998); the survey of about fifty sculptures from the Musée National des Arts Asiatiques–Guimet collection by Pierre Baptiste, published in 2001 (Baptiste et al., 2001); and the petrographic study of twenty-nine pre-Angkor to post-Angkor sculptures from the National Museum of Cambodia by Janet G. Douglas (Douglas, 2004; Douglas and Sorensen, 2007).
Additionally, petrographic descriptions of Cambodian stone sculptures have been published as appendices in special monographs and museum catalogues (see, for instance, Newman, 1997). The authors, though using different classification systems, describe three similar main types of sedimentary rocks (see Table 1).
The study of stone materials from the Metropolitan Museum collection is aimed at enlarging the existing database of Khmer stones by adding quantitative petrographic data, which will provide useful insight into the reconstruction of material usage and diffusion during the Khmer empire. Distinctive petrographic characteristics are described, which can be used by scientists and archaeologists in the attribution of sculptures with unknown provenance, and as guidelines for the identification of the most probable sites of material provenance.
|Author||Adopted rock name||Composition||Texture|
The collection of The Metropolitan Museum of Art covers pre-Angkor and Angkor period productions from Cambodia, but also from Thailand and Vietnam, and includes freestanding sculptures and architectural elements, such as lintels and columns. The sampled Khmer sculptures include the majority of the objects on view in the galleries and in storage. Small fragments of stone have been carefully sampled from areas affected by previous breaks or loss of material, and generally located at the back of arm or leg joints or at the bottom edges of architectural elements (see Fig. 2).
When possible, fragments include the exposed surface and the fresh stone interior.
Standard thin sections have been prepared from stone fragments embedded in epoxy resin. For each sample, the maximum number of detrital grains was counted and each grain classified and measured by means of a micrometric eyepiece using a polarized light microscope. Key petrographic parameters according to Pettijohn (1975) as well as the grain size distribution and key textural parameters were derived by means of computer software modified after Balsillie et al (2002).
All the analyzed stone samples from the Khmer collection at the Metropolitan are sandstones—clastic sedimentary rocks composed of a framework of sand-size grains set in a silty or clayey matrix and united by a cementing material. Many schemes of sandstone classification exist, and none of them is universally accepted by scientists. For the purposes of this review, we decided to adopt the schemes proposed by Pettijohn (1954) and Gazzi-Dickinson (Gazzi 1966; Dickinson 1970).
The first author divides sandstones in two main groups: those with less than 15% of matrix, called arenites, and those with more than 15% of matrix, called wackes. Despite the criticisms, this scheme is used here to favor a sharp visualization of the collection from a material point of view. According to this scheme, 57% of the collection can be classified as arenites while 43% are graywacke, a term used in this circumstance in its general sense of dark gray, immature sandstone with significant matrix content. By using the Gazzi-Dickinson classification, it is possible to detail further the nature of the stones present in the collection on the basis of the detrital grain composition. In this review we will focus on two types of arenites and two graywackes, which together make up 90% of the studied collection.
The first arenite is classified as feldspathic arenite. It contains sub-rounded to angular, fine, and moderately well-sorted grains cemented predominantly by chlorite. Thin sections reveal a laminated structure with evident compaction causing pressure-solution phenomena and plastic deformation of micas and sedimentary rock fragments (see fig. 4a). Mono-, poly- and microcrystalline quartz are the most abundant framework grains. Feldspar is mostly plagioclase, while alkali feldspar is generally subordinate. Rock fragments are represented by chert, quartzite, phyllite, micaceous schist, siltstone, shale and intermediate to basic volcanic rocks, suggesting a mixed provenance influenced by a strong metamorphic source. Micas and detrital chlorite are present as flakes bent between the framework grains. The heavy mineral fraction comprises mostly hematite, magnetite, epidote, garnet, ilmenite, titanite, apatite, zircon and rutile.
The second arenite is classified as quartz arenite (see figs. 4b, 4c). It has very fine to medium, moderately well-sorted detrital grains. The framework grains are mostly composed of sub-rounded to rounded, mono- and microcrystalline quartz showing incipient quartz overgrowth and kaolinite cementation. Other accessory minerals are altered feldspar, which in some samples can reach 20% of the total framework, muscovite flakes, and layered opaque minerals, mostly represented by ilmenite and fine-grained iron oxides. The rare rock fragments include phyllite, siltstone, mudstone, and volcanic rocks. The heavy mineral fraction is poorly represented. In some cases, a diffuse film of hematite coats the grains and the interstices, and is responsible for a pink to reddish coloration of the stone.
The studied graywackes are compact, fine-grained and immature sandstone with a variable content of quartz, feldspar, and lithic fragments. Two main types are herein presented, which differ for the nature and amount of rock fragments present.
The first typology of graywacke is characterized by abundant rock fragments of volcanic, metamorphic, and sedimentary origin, and by abundant secondary calcite. Grains are sub-rounded to very angular, and poorly sorted (see Figs. 4d, 4e). Quartz is present in mono- and polycrystalline varieties, as well as chert and rare quartzite grains. Feldspar grains are predominant plagioclase and alkali feldspar. Feldspar is often altered to sericite, while calcite replacement of feldspars, as well as of quartz and lithic fragments, is abundant. The content in muscovite and biotite can vary up to 6% of the total framework, with biotite generally prevailing over muscovite. Accessory minerals include clinopiroxene, epidote, apatite, zircon, magnetite, titanite, ilmenite, rutile, iron oxides, and detrital chlorite flakes. Rock fragments content varies widely from 15% to 50%, but is generally high and reflects a metamorphic-volcanic provenance. Metamorphic rock fragment are phyllite, micaceous schist, and rare chlorite schist. Volcanic rock fragments have variable dimensions, from a few tenths of microns to up to one millimeter. They consist of andesitic to rhyolitic volcanic rock fragments in different proportions. Sedimentary rock fragments are less abundant, and include mostly very fine-grained argillite and shale. The matrix is composed of a fine-grained clay-sized assemblage of illitic composition, with subordinate calcite, chlorite, quartz, iron oxides, and heavy minerals. Close observation can reveal frequent pseudomatrix resulting from the breakdown of less competent volcanic and sedimentary lithic fragments. Calcite is present as interstitial cement, filling pore space together with chlorite, and abundantly replaces detrital grains of quartz, feldspar, and rock fragments.
The second identified graywacke is generally coarser and better sorted than other studied wackes (see fig. 4f). The matrix is abundant and rich in small feldspar laths. The framework is characterized by a relatively high proportion of feldspar grains (F=48±0.6%) and rock fragments (L=31±3%). The most distinctive feature is the presence of abundant igneous intermediate to basic brittle volcanic rock fragments (up to 90% of the entire lithic fraction), which easily break down into pseudomatrix. Albitized feldspar and hornblende are also distinctive of this particular sandstone (Douglas and Sorenson, 2007; Carò and Douglas, 2013). The relatively fresh volcanic grains suggest that the sediment underwent little transport after erosion from a preeminently volcanic source rocks.
The petrographic characterization of the stone materials sheds new light on the Khmer collection in The Metropolitan Museum of Art. The study reveals that it is possible to group the sculptures according to strong petrographic similarities, while the integration of stylistic considerations and the comparison with published petrographic data about stone usage in Cambodia help to contextualize this result. The entire corpus of pre-Angkor sculptures (ca. seventh–ca. ninth century), characterized by a diversity of styles and iconographies, is unified by similar stone materials (see fig. 5).
These early Khmer deities, mostly originating from the southern provinces of Cambodia, are carved from graywackes that display a generally well-preserved, dark grayish to dark greenish, sometimes highly polished surface. This sandstone was rarely used for buildings, although a similar type of stone has been found in the early sanctuary of Ta Keo, built in the late tenth to early eleventh century.
Conversely, the feldspathic arenite is characteristic of the Angkor decorative statuary, such as guardian deities and lions flanking the main approaches to the sanctuaries and is consistent with a similar stone described by other authors (Delvert, 1963; Uchida et al, 1998; Baptiste et al, 2001; Douglas, 2004). This lithotype, which has also been called grey or green sandstone, arkose, or gray to yellowish brown sandstone, is occurring extensively in architectural elements and stone blocks of the Angkor period (Delvert, 1963; Uchida et al, 1995), and has been also identified in most of the decorative sculptures of Angkor styles collected in the Musée Guimet (Baptiste et al, 2001) and at the National Museum of Cambodia (Douglas, 2004). This stone, which constitutes the main subhorizontal tablelands of Northern Cambodia, was the preferred building material during the highly centralized Angkor kingdom.
According to the existing data, the stone used for the images of deities placed inside the sanctuaries continued to be different from building and decorative stone material also during the Angkorian period. During the early stages of the Angkor empire, sculptures in the styles of Kulen (800–875), Preah Ko (875–900), and Bakheng (900–925) were carved from the same graywacke as was used during the pre-Angkor period, while some sculptures in the style of Banteay Srei (ca. 976) were carved from a peculiar green to bluish graywacke. According to the existing petrographic data, Baphuon and Angkor Wat-style sculptures of reduced size were specifically carved from a very fine, compact, feldspar-rich sandstone (see fig.3). A specific role in the Angkor sculptural production is played by a volcanic-rich graywacke. This sandstone, markedly immature and rich in volcanic rock fragments, was predominantly used during the reign of Jayavaraman VII for Bayon style statuary (ca. 1180–ca. 1230) (see fig. 5) (Douglas and Sorenson, 2007; Carò and Douglas, 2013).
Finally, quartz arenite of different color and grain size is found predominantly in decorated architectural elements of pre-Angkor and Angkor period. This result is in agreement with the occurrences found in the Musée Guimet collection where this stone is used in architectural elements, lintels and columns of different periods (Baptiste et al, 2001). According to these data, this particular lithotype represents the favorite stone material for the architectural element of intricate details. In this case, the choice of a specific stone seems to have been influenced mostly by technical motivations rather than geographical or historical reasons.
A field trip to Cambodia was organized in January 2009 in cooperation with APSARA Authority to acquire and analyze rock samples from documented quarries in the provinces of Siem Reap and Preah Vihear, Northern Cambodia (see fig. 6). Jurassic sandstones constitute the main sub-horizontal tablelands of Northern Cambodia. The Khorat Plateau in the north, the Phnom Kulen in the south, and the Tbêng Mountain in the east are the broadest outcrops of these Mesozoic sedimentary rocks in the considered area. These sandstone outcrops also constitute numerous mesas and buttes scattered in the Quaternary sediments and are frequently exposed along the riverbeds of seasonal streams in Siem Reap and in Preah Vihear provinces (see fig. 7).
These Jurassic sedimentary units, notably the Lower-Middle Jurassic sediments known as Red Terrain (Terrain Rouge) and Upper Jurassic conglomerates and sandstones known as Upper Sandstones (Grès Supérieures), are thought to have provided much of the building and sculptural material in the area during the whole Khmer empire, although Triassic formations have also been exploited for early statuary and building materials (Delvert, 1963; Uchida et al., 1995; Baptiste et al., 2001; Douglas, 2004).
Evidence of multiple episodes of quarrying is present near Beng Mealea at the foot of Phnom Kulen and in the riverbed of the Mealea River, where Lower–Middle Jurassic sandstones are exposed in horizontal layers (Delvert, 1963). No other quarry remains have yet been documented in Siem Reap province, but previous studies of Khmer stone material suggest that other sources have been exploited through time, from both the Lower and Upper Jurassic units (Delvert, 1963; Uchida et al., 1995, 1996; Uchida and Maeda, 1998; Uchida and Ando, 2001; Douglas, 2004). In Koh Ker, Preah Vihear province, rock carvings on a sandstone outcrop known as Ang Khna are thought to be realized on the vertical surfaces of an abandoned quarry (Jacques and Lafond, 2004). Moriai (2002) reports an important pit quarry in a riverbed about eight kilometers north of Koh Ker, where massive sandstone monoliths that are five meters long are still present on the ground, while in a recent fieldtrip we identified at least two other quarries and multiple sandstone outcrops with similar attitudes scattered in the area.
Lower–Middle Jurassic Red Terrain is widely distributed in northern Cambodia, while it is absent in the central and southern areas of the country (see fig. 7). This sequence mostly occurs in isolated outcrops and comprises conglomerates, sandstones, and siltstones of sub-continental origin that resulted from the massive deposition following the regional Indosinian Orogeny of the Late Triassic period.
In northern Cambodia, this Jurassic unit is covered by thin, unconsolidated quaternary deposits in the form of older and younger alluvium. As a consequence, this sandstone is typically exposed in numerous riverbeds in Siem Reap and Preah Vihear provinces (Delvert, 1963; Moriai, 2001; Jacques and Lafond, 2004).
Samples from the Lower–Middle Jurassic continental unit were collected from a quarry located in the commune of Beng Mealea, forty-five kilometers east of Angkor (see fig. 8). This site comprises two quarry sites, locally known as Thmâ Anlong (see fig. 8) and Veal Vong. Together with O Thmâ Dap and Phnom Bei, these sites form a well-known vast quarrying district in the Kulen area (Delvert, 1963). The sandstone is exposed in an area that shows remains of ancient open quarrying near the foothills of Phnom Kulen, two kilometers west of O Thmâ Dap. The remains of the quarry suggest that stone blocks, about one meter in height, were removed with chisels or picks following the horizontal bedding of the sandstone terraces (see fig. 9). These field evidences might find a match in a scene depicted on the bas-relief of the southern gallery of the Bayon temple (see fig.10), in which workers are shown chiseling and wedging materials from the ground (Higham, 2002).
This quarry is characterized by feldspathic arenites, according to the Gazzi-Dickinson classification, or arkosic arenites according to Pettijohn. The framework of this sandstone is composed of very fine to fine, moderately well-sorted, sub-angular to rounded grains, which show an evident laminated structure. In both of the samples, grains are well packed and frequently show pressure-solution phenomena and plastic deformation, the latter mostly affecting rock fragments and micas. The sandstone is cemented by a combination of chlorite and calcite, while quartz and feldspar partial overgrowths are also frequent.
Mono- and polycrystalline quartz is the most abundant fraction, followed by plagioclase and alkali feldspar. The rock fragment fraction composes about five percent of the entire framework volume and is represented by volcanic, metamorphic, and sedimentary rocks. The heavy mineral fraction is mostly represented by opaque hematite, magnetite, epidote, garnet, ilmenite, titanite, apatite, zircon and rutile. Occasional detrital chlorite flakes have been found.
The collected data are in agreement with other published studies (Delvert, 1963, Uchida et al., 1998; Kučera et al., 2008), and confirm the affinity between the stone used for the construction of the Angkor temples and the litotypes exposed at the eastern foothill of Kulen Mountain.
This research is supported by an Andrew W. Mellon Fellowship.
Several staff members from other departments at The Metropolitan Museum of Art continue to contribute their knowledge, comments, and suggestions to this project, including John Guy, Curator of South and Southeast Asian Art, Kurt Behrendt, Assistant Curator, and Donna Strahan, Conservator at the Sherman Fairchild Center for Object Conservation.
The cooperation of several people and institutions in Cambodia has also made this project possible. We are grateful to the Ministry of Culture, particularly H. E. Chuch Phoeurn, Secretary of State Culture and Fine Arts; at the Apsara Authority, H. E. Bun Narith, Director-General, and H. E. Khun Neay Khuon, Deputy Director-General; at the National Museum of Cambodia, Mr. Hab Touch, Director, and Mr. Bertrand Porte, EFEO; and at Apsara Authority, Mr. Im Sokrithy.
Janet Douglas, Conservation Scientist at the Department of Conservation and Scientific Research of the Freer Gallery of Art / Arthur M. Sackler Gallery, Smithsonian Institution, strictly cooperates in the project by sharing knowledge, data, and samples.
Special thanks are due to Sieng Sotham, Director of the Department of Geology, Ministry of Industry, Mines, and Energy in Phnom Penh.
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