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Case Study 4. The Technical Investigation of Eighth-, Ninth-, and Tenth-Century Statuettes from Indonesia

Case Study 4 Statuettes from Indonesia
  • Mathilde Mechling
  • David Bourgarit

Introduction

(figs. 504, 505, 506, 507, 508)

The technical study of a group of thirty-nine statuettes in the collection of the Musée national des arts asiatiques – Guimet, Paris, was the first comprehensive examination undertaken of Hindu and Buddhist statues from Indonesia. The wide array of visual and analytic methods utilized has provided a preliminary characterization of Indonesian bronze casting traditions. In addition, the study probed the possible exchange in technical know-how between the Indonesian islands and other regions of South and Southeast Asia, previously suggested on the grounds of stylistic connections. The research was part of a PhD project on Indonesian bronze sculpture combining traditional art historical research with technical studies.

The main questions

  • Is there consistency in technical features such as the facture and alloy composition of Indonesian bronze statuary? If so, how distinctive are these features?

  • Is it possible to refine the stylistic and chronological grouping of these objects by taking technical features into consideration?

  • Given the stylistic connections found within bronze statuary across multiple regions of Asia, might there also be connections in their material and casting features?

Casting technique

Lost-wax casts with numerous wax assemblies

(figs. 67, 54)

Naked-eye observation of the statuettes provides evidence that they were made by the technique. This is evident in the general nature of the modeling, and in particular in the decorative elements on the outer surface, whose undercuts suggest direct modeling.

And close examination of the hollow interior surfaces provides incontrovertible evidence that the figures were affixed to their pedestals before casting and that the bases were constructed from separately assembled wax sections: the uneven ridges at the level of the lotus blossoms and base are wax-to-wax joints.

Solid-cast figures

(figs. 126, 510)

The X-radiographs confirm that the figurines, including the largest ones (30 cm tall) are mostly solid, with open pedestals.

Figure 510 X-radiograph of frontal view. Statuette of Jambhala, central Java, first half of the 9th century, H. 28 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MG 3814). See Mechling et al. 2018.

Cold work

(figs. 276, 511)

Naked-eye and microscopic observation of the outer surfaces of many of the statuettes found crisply defined decorative details and textures that are characteristic of cold working of the metal. (Exactly how much detail was already present in the wax is hard to say.)

Figure 511 Detail showing engraving and punch marks. Kubera/Jambhala, Java, first half of the 10th century, H. 18 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MG 3625). See Mechling et al. 2018.

Engraving, as suggested by reflectance transformation imaging (RTI)

(fig. 241)

Reflectance transformation imaging (RTI; see II.2§2.3) allowed for close documentation and study of the incised patterns. Under certain lighting conditions, the interrupted, stepped pattern along the interior walls of each incision was brought into relief. These are characteristic of , formed by successive strikes on the graver as it is worked through the metal. The tapering ends of the incised lines are also consistent with engraving.

Metal punching confirmed by digital microscopy

(figs. 285, 286)

Close examination of marks (0.8 mm diameter) by digital microscopy (see II.2§2.1) revealed that the smooth, compressed surface inflected by the punch and the dimple of displaced metal around the edge resulted from cold working. In addition, on one statue, a 3D reconstruction of the dots and consequently of the active part of the punch showed a systematic defect in the sphere, clearly testifying to the use of the same tool for all the motifs investigated.

Figure 285 Digital microscopy detail of a punch mark (0.8 mm in diameter) on the lotus of a Buddha Vairocana. Note the smooth, compressed surface produced by the punch and the ridge of displaced metal around the edge. Buddha Vairocana, Indonesia, first half of the 10th century, H. 12 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MA 3475). See Mechling et al. 2018.

Metal engraving over preparatory drawing on wax confirmed by digital microscopy

(figs. 273, 512)

Digital microscopy was used to map the topography of various tool marks. The indented lines on several statuettes showed the V profiles and slightly raised edges (or burrs) along the tops characteristic of engraved rather than punch marks. In some instances, the presence of two lines very close to each other may testify to a preparatory drawing on the wax model.

Figure 512 Profile of an engraving made in the metal on the front piece of the cloth of the Statuette of Jambhala, as measured by digital microscopy. The vertical axis reports the depth and the horizontal axis the width of the engraving (both in µm). The surface of the bronze is represented by the horizontal green dotted line. The cross section of the engraving and its dimensions are represented by the red curve. The maximum depth appears to be ~0.6 mm (600 µm), and the width at the surface is ~2 mm (2000 µm). See the altered V shape of the imprint (blue arrow), which demonstrates that the engraving in the metal was made by following a line already in the wax model. Statuette of Jambhala, central Java, first half of the 9th century, H. 28 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MG 3814). See Mechling et al. 2018.

Concealed offerings revealed

Cavities filled with dense material

(fig. 513)

X-radiography has revealed in a number of statuettes a cavity extending from the top of the base up into the legs and the abdomen of the deity. This cavity is filled with a denser material than the metal of the statuette, making that area more radio-opaque (and consequently whiter in the film) and effectively masking all structural details. X-ray fluorescence analysis (XRF; see II.5§2.1) of the embedding alloy on several statuettes revealed them to be tin-based with smaller amounts of lead (90–95wt% tin, 5–10wt% lead). Such low-melting-temperature alloys are very radio opaque (i.e., difficult for X-rays to penetrate).

Consecration deposits visible from underside

(figs. 54, 514)

In one of the statuettes, damage to the pedestal resulted in the partial loss of the metal fill. One can catch a glimpse of what remains embedded in the remaining metal matrix: several folded metal foils, the rim of a silver coin, a stone bead, and several more unidentified objects. These constitute a deposit to consecrate the statue before it could be used for worship. Such addition of a consecration deposit embedded in a metal matrix would have been a common feature within the group of statuettes studied, though today few of these objects preserve their interior contents. Might there be a way to learn more about these hidden contents without further disrupting them?

Figure 514 Bottom view showing areas of the preserved conservation deposits poking out of the metal fills. Vairocana, Java, first half of the 10th century, H. 15.5 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MG 18290). See Mechling et al. 2018.

Consecration deposits revealed by neutron radiography and tomography

(figs. 401, 404, 403, 516)

Access to a neutron reactor allowed for the neutron radiography and tomography (see II.3§3) of several Javanese sculptures that still preserved their consecration materials embedded in metal matrices. This works because metals such as copper, tin, and lead may be X-ray opaque, but they are relatively transparent to neutrons.

Figure 403 Neutron radiography. This series of thirty-seven neutron radiographs was made as the sculpture was rotated through 180° on a turntable. Most consecration offerings (coins, beads, folded metal sheet) can be observed (white shapes under the seat). Vairocana, Java, first half of the 10th century, H. 15.5 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MG 18290). See Mechling et al. 2018.

The resulting images clarified the shape and location of the interior contents. Among other things, they enabled researchers to identify the typically Javanese piloncito-type cubic coins. Neutron radiography does not, however, allow for the precise characterization of materials that these consecration deposits were made of.

Figure 516 Annotated neutron radiograph of side view, showing more clearly than the X-radiograph (fig. 515) the contours of the sealing material (red overlay) and some of the consecration offerings. For an even better visualization of the consecration offerings, see tomographic images of a related figure (figs. 403, 404). Kubera/Jambhala, Java, first half of the 10th century, H. 18 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MG 3625). See Mechling et al. 2018.

Alloy composition

Bulk metal

(fig. 517)

Inductively coupled plasma atomic emission spectrometry (ICP-AES; see II.5§3.1) analysis of the bulk metal composition of the bronzes revealed that most of the thirty-nine statues analyzed are made of unleaded bronze alloys—copper-tin alloys with less than 2–3wt% lead (Pb). Zinc (Zn) is present only in traces. With some variations, median values for each time period are higher than 8wt%. Nine statues have relatively high (12–20wt%) tin (Sn) content.

Impurities in the metal

(fig. 518)

ICP-AES allowed us to determine that the main impurities found in these alloys are silver (Ag), arsenic (As), iron (Fe), nickel (Ni), sulfur (S), and antimony (Sb).

Summary of findings

Consistent features of Indonesian production

(figs. 504, 505, 506, 507, 508)

In all of the Indonesian statues investigated, even the largest ones, the figures themselves are mostly solid. All were made via lost-wax casting, with extensive modeling and assembling work on the wax. All were originally likely consecrated with sacred objects, and therefore have a cavity in the supporting pedestal or cushion.

The systematic use of a liquefied metal, namely a tin-based alloy, to seal the consecration deposits constitutes another consistent feature of Indonesian production. Copper-based sheets hammered over the base are often encountered for Buddhist statues in other regions, notably in Tibet () and China (figs. 225, 226).

Most statues are made of unleaded tin-bronze, with relatively high levels of tin. This is not very surprising given the vast and accessible resources of the Southeast Asian tin belt, running from central Burma down the Thai-Malay Peninsula to Bangka Island east of Sumatra, in the Indonesian archipelago.

Toward a chronological and stylistic serialization supported by metal composition

(figs. 518, 519)

Despite the common technical features apparent in the whole study group, the lowest levels of tin were found exclusively in the statues attributed to the first half of the ninth century. Tin content increased markedly in the statues attributed to the early tenth century, despite several exceptions to the rule. Some periods are marked by very specific and homogeneous alloy composition and impurity patterns. This is particularly true for two groups, namely the group attributed to the second half of the ninth century and the group attributed to the early tenth century.

Figure 518 Main impurities in the metal of the Javanese statuettes from the Musée national des arts asiatiques – Guimet, Paris. One bar represents one statuette, sorted by chronological group (wt%, ICP-AES analysis on drillings). See Mechling et al. 2018.

Alloy connections between Java and mainland Southeast Asia

(figs. 509, 521, 522)

The high tin content may pertain to technical connections with contemporaneous casting traditions in mainland Southeast Asia, namely the Mon culture of Dvāravatī in central and northeast Thailand (notably the Prasat Hin Khao Plai Bat II bronzes in Buriram province). High-tin bronze alloys are also found in the Khmer bronze-casting tradition for objects from various periods (the first to the tenth century). Conversely, coeval bronze statuary from the Indian subcontinent shows much lower tin content, except some examples from Sri Lanka (Polonnaruwa) and northeast India (Nalanda).

Figure 509 Front view. Vairocana, Java, first half of the 10th century, H. 15.5 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MG 18290). See Mechling et al. 2018.
Figure 521 Example of a Thai statuette cast in an unleaded high-tin bronze (16wt% Sn, 0.4wt% Pb, ICP-AES analysis). Buddha in Vitarkamudrā, Mon culture of Dvāravatī, central Thailand, 7th–8th century, H. 19 cm (Musée national des arts asiatiques – Guimet, Paris, inv. MA 3785). See Bourgarit et al. 2003; Mechling et al. 2018.

Characterization of consecration deposits thanks to neutron tomography

(fig. 401)

Neutron tomography has proven a successful nondestructive technique to learn more about the consecration deposits sealed inside the statuettes. Such deposits are important archaeological artifacts and indicators of the ritualistic rules overlaying the creation and use of these religious images. Few of the known contemporaneous statues from India or other parts of Southeast Asia retain their consecration deposits, but there is ample evidence that this practice was widespread in Indonesia. Bronzes such as these also form a crucial link with the later and better-documented Tibetan tradition of consecrating Buddhist images.

Synopsis of technical parameters

This case study is based on a technical study carried out by Mathilde Mechling in collaboration with David Bourgarit and Brice Vincent as part of her PhD project on Indonesian bronze sculpture. The examinations and analyses were integrally carried out for free at the C2RMF as a service provided to French museums, in this case the Musée national des arts asiatiques – Guimet. Neutron tomography was also carried out for free at Reactor Orphée – Laboratoire Léon Brillouin, CEA-CNRS, Paris, as a service for public research. Main operating conditions are reported in . For more on the analytical techniques, please refer to volume II.

The approximate time required to carry out the whole study, excluding publication, was more than sixty days. The study involved:

  • daylight and UV photography: Anne Maigret (C2RMF)

  • digital microscopy: Mathilde Mechling (PhD candidate), David Bourgarit and Dominique Robcis (C2RMF)

  • X-radiography: Elsa Lambert (C2RMF)

  • neutron tomography: Frédéric Ott (CNRS-CEA)

  • bulk metal analysis by ICP-AES: Nathalie Gandolfo and David Bourgarit (C2RMF)

  • XRF: David Bourgarit (C2RMF)

Further questions

  • Would comparative data from either similar objects or copper and tin ore deposits make it possible to determine the provenance of the metal?

  • Is there physical evidence pointing conclusively to the use of either the direct or indirect process to form the wax model?

  • Can the fabrication of the hosting cavity be further elucidated?

  • Can it be determined whether the statuettes were horizontally or vertically? Since horizontal casting is known to have been utilized in South India, Sri Lanka, and Tibet, is it a feature that might help to determine intercultural exchanges?

  • Can a comprehensive quantification of tool marks (depth and angle of the grooves) by digital microscopy yield interesting clues regarding know-how in certain workshops?

  • Can the identification of items in consecration deposits serve as a means of tracing the regional exchange of materials, and if so, can it be done nondestructively, avoiding the deconsecration of the religious image?

Further resources

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