Comparative studies of “Fresh” and “Aged” Tridacna gigas shell: Preliminary investigations of a reported technique for pretreatment of tool material

Journal of Archaeological

Science 1990,17,329-345

Comparative Studies of “Fresh” and “Aged” Tridacna gigas Shell: Preliminary Investigations of a Reported Technique for Pretreatment of Tool Material Barbara G. Moir” (Received 18 September 1989, revisedmanuscript

accepted 27 November 1989)

X-ray diffraction analysis and scanning electron microscopy were employed to pursue qualitative ethnohistorical reports of physicochemical variability between “fresh” valves of Tridacna gigas, a giant clam, and valves that had been “aged” in seawater prior to reduction for use as tool material. Adze blades manufactured from the latter substance on pre-European Takuu Atoll were said to have been superior in workability and durability to blades made from fresh shell. Preliminary laboratory studies suggest that T. gigas shell material maintained in seawater following harvest of the animal undergoes a progressive transformation in the dimension, shape, and microstructural orientation of its aragonite crystals, resulting in greater thermodynamic stability and lower solubility of the material. Some apparent implications of this transformation for adze manufacture, use, and durability are discussed. ADZES, ARAGONITE, BIOGEOCHEMSTRY, INDO-PACIFIC, MINERALOGY, PRETREATMENT, RECRYSTALLIZATION, SHELL TECHNOLOGY, TRIDACNA GZGAS.

Keywords:

Introduction The “pretreatment” or treatment of material by craftsmen prior to tool manufacture has been reported for lithic materials in a variety of environmental/cultural contexts (see, e.g. Man, 1885; Robinson, 1938; Radcliffe-Brown, 1948; Goldschmidt, 1951; Malo, 1951; Crabtree & Butler, 1964; Montet-White, 1968; Purdy & Brooks, 1971; Collins, 1973; Mandeville, 1973; Collins & Fenwick, 1974; Purdy, 1975; Melcher & Zimmerman, 1977; Akerman, 1979; Schindler et al., 1982; Hanckel, 1985). To date, however, accounts of bivalve “aging” on Takuu Atoll appear to constitute the only extant description of the pretreatment of shell tool material. The laboratory studies described here were conducted to investigate the plausibility of ethnohistorical reports of significant physiocochemical differences between two types of tool material: (1) the shell of the giant clam Tridacna (Tridacna) gigas (Lint+) in its freshly harvested state, and (2) the shell of the same species following a period of “aging” in “Marianas/Pacific Studies Program, Northern Marianas College, Saipan, Commonwealth of the Northern Marianas Islands 96950,

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seawater. Statements to this effect were recorded during a 3-year study of the uses of this bivalve, in both pre-European and contemporary contexts, by the people of Takuu Atoll, a Polynesian outlier in the North Solomons Province of Papua New Guinea (see Moir, 1989a). Like all atoll dwellers, the early Takuu lacked local sources of the lithic materials commonly used throughout the Pacific for manufacturing durable and efficient cutting tools, including adzes. Artifactual and ethnohistorical data indicate that various species of bivalve (particularly Triducna spp.) and gastropod were exploited on Takuu for this purpose, and that T. gigas shell was used exclusively as the raw material both for large adze blades and for ceremonial “adzes” of exceptional size and workmanship. According to modern Takuu informants, direct and recent descendants of adze manufacturers, most shell tools were the end products of simple extraction-transformation sequences: the shell was collected from the local reef/lagoon environment and then modified to suit specific needs. Tridacnagigas adzes, however, were said to have been produced primarily from clams that had been gathered from the reef when small, then transplanted to coral head “gardens” in the lagoon for protection and further growth. These live clams became the property of the transplanters; they were tended and their harvest deferred for years until they had achieved the large sizes targeted for adze material. [Specimens of T. gigas, a true “giant clam”, have been measured at shell lengths of more than a metre and at shell weights surpassing 230 kg (Rosewater, 1965), but such individuals are rarely found in extant populations.] Takuu craftsmen reportedly utilized both “fresh” and “aged” T. gigas valves for adze manufacture, a newly harvested valve being transported to shore when there was a pressing need for tool material. The preferred behavioural option, however, was to overturn the valve and leave it on the coral head garden to age for as much time as the cultivator/ craftsman could afford, up to a generation or longer. Ultimately the aged material would be used either by the cultivator/craftsman or by his descendants. Complete immersion in seawater over a prolonged period was said to produce a better quality tool substanceand ultimately, both more efficient tools and finer ceremonial “adzes’‘-than fresh shell could provide. Takuu perceptions of dlflerential material quality

According to informants, these variables of (1) environment and (2) length of exposure were instrumental in enhancing the quality and utility of T. gigus shell as tool material. From the standpoint of environment, the aging (or “curing”) of T. gigas valves could be effected only in seawater. Fresh valves harvested and brought to shore would not undergo the same material transformation as their counterparts left submerged on coral heads; the physical quality of the former would in fact deteriorate, while that of the latter would be enhanced. The length of time for which a valve was maintained in a submarine environment was considered a relative measure of its eventual quality and utility as tool material. To the Takuu adze manufacturer, a “longer” period of submersion meant “better” workability of the raw material and durability of the final product. Aged shell was not more easily worked than fresh-on the contrary, working aged shell is reported to have been a more exacting and time-consuming process-but in its alleged resistance to cleavage along the laminar planes characteristic of fresh shell (see Cleghorn, 1980) aged T. gigas shell was considered a more plastic material, capable of yielding more varied forms and less susceptible to the production of “rejects”. Where durability was concerned, informants stated that the cutting edge of an adze blade manufactured from aged shell retained its sharpness for a longer period, hence needed regrinding less frequently than a blade of fresh shell. Overall, aged-shell blades

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were reputed to be far more resistant to the stresses of use, less likely to fracture in the course of woodworking. Variation in long-term durability or preservability, between tools made of fresh and of aged shell, was also reported. Over time and under conditions of storage, fresh-shell blades were said to undergo a superficial and often substantial physical deterioration to which the aged material apparently was not subject. Takuu craftsmen customarily stored their adze blanks, preforms, and spare blades in caches dug in their house floors; Takuu women followed the same practice in secreting their collections of ceremonial “adzes” (the family’s reserve of brideprice and grave goods). Over time, according to informants, buried blades of fresh shell would tend to “soften”, taking on a grey-white chalky appearance, their surfaces becoming finely pitted, with a resultant and corresponding decrease in their utility as tools. By contrast, blades fashioned from aged shell (including all ceremonial “adzes”) were said to retain their hardness, their original colour (often light brown to light yellow-brown), smooth surface texture, and faint lustre under any environmental conditions-including burial, exposure to air, rainwater seepage, and inundation by salt water. This reputed resistance to the effects of natural processes would have been an important advantage not only for tools cached in highly porous, alkaline atoll soils, but for ceremonial blades stored in the same manner and/or interred as burial accompaniments in shallow, sandy graves. Aged-shell tools were durable enough to be used, cached, and handed down from one generation to the next. Ceremonial “adzes” of the same substance were (and are today) more often to be found below ground than above-whether in a family cache or as grave goods-and their virtually unlimited use life in such contexts is attributed by the Takuu to the extreme durability of the aged material from which they were made. The present study

Ethnohistorical evidence suggests that T. gigus tools were made and used on Takuu through the early years of the 20th century (see Moir, 1989a), apparently after this technology was abandoned elsewhere in the Pacific. In 1978 I interviewed the only Takuu craftsman then living who had manufactured and used T. gigasadzes; in 1978-9 and 198 l-3 I recorded numerous second-hand accounts (from older informants who had received the information from their grandfathers and great-grandfathers) of the aging of T. gigas shell and its implications for tool manufacture and durability. However, I was unaware of any geochemical process that might account for the sort of material transformation described by the Takuu as taking place within a time span of fewer than 100 years. Archaeologists and others who have recovered T. gigus valves, and/or artifacts made from this material, have referred to some specimens as “fossil” or “fossilized” (Gill, 1885; Willey, 1896; Finsch, 1914; Oestergaard, 1935; Rosewater, 1965; Poulsen, 1970; Kirch & Rosendahl, 1973, 1976; Kay & Switzer, 1974; Taylor, 1978; Cleghorn, 1980; Specht, 1982; Hocart in Russell, 1972), “subfossilized” (Finsch, 1914; Poulsen, 1970), and “recrystallized” Triducnu (Chappell & Polach, 1972). These terms have had different connotations in different contexts of use, but in general they have been employed as referents to geochemical transformation processes that occur over the course of millenia, rather than in the decades cited by the Takuu. With the objective of investigating whether there might be mineralogical, microstructural, and chemical differences between “fresh” and “aged” T. gigus shell, it was decided to subject samples of each (including adze blades said to have been fashioned from the latter substance) to X-ray diffraction analysis and scanning electron microscope study. These efforts are described and their results discussed below.

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ventral margin

Figure section

1. Interior represented

I umbo

margin

hinge line

view of left valve of Triducna (Tridacna) in sampled adze blades is outlined.

gigas

(Linni).

Valve

Materials and Methods

X-ray diffraction analysis was undertaken to identify and compare the mineral compositions of T. gigas valves in two different size/age classes, and of T. gigas adze blades of undetermined antiquity which had been manufactured on Takuu, reportedly of “aged” shell. A 22-cm specimen of T. gigus had been collected live at Takuu Atoll and its valves returned to the University of Hawaii. For comparative purposes a pair of 5%cm T. gigm valves, said to have been obtained live, were made available by a collector. Three adze blades fashioned from the hinge area of large T. gigus valves, donated by individuals on Takuu, were also sampled (see Figure 1). In preparing samples of these specimens for scanning electron microscope (SEM) examination (see below), a sufficient quantity of crystalline powder was produced to allow Xray diffraction analysis by the powder diffraction method (see Parkes, 1986). For each of the five specimens tested-a small valve, a large valve, and three adze blades-from 3 to 5 mg of powder were mixed with Duco cement and acetone (1: 100 by volume), then analysed in a North American Phillips X-ray diffractometer under the following conditions: Radiation: Cu K2a radiation under 30 kV 15 mA Slits: 1 - 1 - 0.4, time constant: 2 Scanning speed: 2 degrees min-’ Chart speed: 20 mm min ~ ’ Scanning electron microscopy was employed to obtain a detailed view of the internal microstructure of the specimens that had been subjected to X-ray diffraction analysis. In preparation, a core sample was cut vertically-from exterior to interior surfacesthrough the umbonal region (see Figure 1) of each of the two T. gigus valves. The core cut from the larger valve was 83 mm in length and 27 mm in diameter. From its centre a longitudinal slice 10mm thick was removed; this was polished with a 0.3-u alumina suspension. Two lo-mm3 samples were detached 30-40 mm from the exterior end of the core section, and a further two 40-50 mm from the exterior end. The core of the smaller valve was 27 mm in diameter and only 25 mm in length. From its centre a longitudinal slice 10mm thick was removed; this was polished with a 0.3-u alumina suspension and cut into four lo-mm3 samples.

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Figure 2. X-ray diffraction intensities of aragonite and calcite samples of two “fresh” T. gigus valves (V-22, V-58).

valve

peaks for powdered

A vertical section 10 mm thick was detached from the butt end of each of the three adze blades; the butts of these blades represent the umbonal extreme of the hinge lines of the T. gigas valves from which they were fashioned (see Figure 1). The sections were polished with a 0.3-u alumina suspension, and each was cut into lo-mm3 samples which were identified separately as to source and sample number. Two samples each of the 22-cm valve (V-22) and the three adze blades were etched for 10 min, and two each for 15 min, in a solution of 7% EDTA (ethylene-diamene tetraacetic acid), pH adjusted to 8.05 by NaOH, in order to enhance the surface detail of the samples. The four samples of the 58-cm valve (V-58) were etched for 15 min in a solution of 7% EDTA, pH adjusted to 7-96 by NaOH. All samples were then rinsed in distilled water, dried in air, and mounted on SEM viewing stubs using conductive colloidal graphite paint. Prior to SEM examination the samples were vacuum coated with gold-palladium, in an anode-type sputter coater, for 3 min. Observations were made of the samples at different magnifications, using an International Scientific Instruments SS40 scanning electron microscope. For further study, photomicrographs were produced with Polaroid Type 55 positive-negative film. Results

Figure 2(a) illustrates the X-ray diffraction intensity of aragonite and calcite peaks for the powdered sample of the 22-cm T. gigas valve (V-22). The mineral composition of this sample is almost exclusively aragonitic. Figure 2(b) depicts the X-ray diffraction intensity of aragonite and calcite peaks for the 58-cm valve (V-58); this sample is composed of more than 99% aragonite with a trace of 5-6 mol % magnesium calcite.

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,d

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calcite

aragonite

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araqonite

Figure 3. X-ray diffraction intensities samples of three adze blades reportedly

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of aragonite and calcite peaks for powdered manufactured from “aged” T. @gas valves.

In Figure 3 the corresponding data for the powdered samples of the three T. gigas adze blades are given. The mineral composition of each sample is more than 95% aragonite and a few % 5-6 mol % magnesium calcite. Figure 4 presents scanning electron micrographs of a vertical section from the umbo of V-22 (the smaller/younger valve), taken at two different magnifications (a) and (b); Figure 5, of a vertical section from the umbo of V-58 (the larger/older valve), sampled 3&40 mm from the valve exterior; Figure 6, of a vertical section (cut through the umbonal extreme of the valve hinge) of an “aged” T. gigas adze blade. The micrographs illustrate observable differences in the morphology, apparent surface area, and orientation of aragonite crystals, when the two “fresh” valve specimens are compared with each other and with the adze specimen. Figure 4 shows this sample of the young T. gigas umbo to be composed of fairly small aragonite needles, l-5 u in length, arranged in the crossed lamellar pattern noted for other Triducna species (Kennedy et al., 1969; Kobayashi, 1969; Taylor et al., 1973; R. Radtke et al., unpubl. data). By contrast, the microstructure of the adze specimen represented in Figure 6 is characterized by densely aggregated platy crystals in a visibly homogeneous arrangement. [Photomicrographs of the other “aged” adze specimens, not shown here, are remarkably similar in visible features to the present Figure 6; see Moir (1989~: figures 28, 29, 30).] Figure 5, of the umbonal region of the older T. gigus valve, appears to fall between these two structural endpoints (young T. gigus valve, and “aged” adze blade). These differences were immediately apparent when the samples were magnified x 193 [Figures 4(b), 5(b), 6(b)]: In Figure 4(b) the microstructure of the young T. gigus valve umbo is seen to be crossed lamellar in orientation (Boggild, 1930; Kennedy et al., 1969; Kobayashi, 1969; Carter & Clark, 1985; Shimamoto, 1986), with well-defined first- and second-order lamels. In Figure 5(b), sampled from the umbo of the older T. gigus valve and representing shell layers 304l mm from the valve exterior, the crystal orientation is virtually undetectable. This is interpreted as representing a progressive loss of first-order structural arrangement relative to layers of the same core section but sampled 40-50 mm from the valve exterior.

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[In micrographs of the latter, not depicted here, relicts of first-order lamels are discernible; see Moir (1989~: figure 26(c)).] Figure 6(b) reveals the microstructure of the “aged’‘-shell adze (sampled from the umbonal extreme of the valve hinge) to be fine grained and homogeneous (Carter & Clark, 1985) with no visible evidence of first-order structural arrangement. At a higher level of magnification following:

( x 3 100) these crystal fabrics appear to consist of the

Figure 4(a): fine aragonite needles in second-order lamels, aligned in opposing directions in adjacent first-order lamels; Figure 5(a): round-edged aggregates of platy aragonite, varying in shape and dimension, with no apparent orientation; in part these may overlie more elongate but also irregularly shaped crystals whose orientation is unidentified; Figure 6(a): aragonite crystals appearing as subequant, polyhedral and round-edged plates, densely aggregated and homogeneous in orientation; crystal boundaries are often indistinct.

Discussion The determination of the mineral composition (100% aragonite) of the young T. gigas valve was consistent with such findings for other tridacnid species (Taylor et al., 1973) and with its having been collected live. For both the older, larger valve and the three T. gigus adzes sampled, it is suggested that the very small percentages of magnesium calcite revealed in Figures 2(b) and 3 are the result of uptake from seawater, following removal of the animal and oxidation of the conchiolin matrix in which the shell’s aragonite crystals occur. It is noted that tropical surface seawater is highly supersaturated with respect to a 5-6 mol % magnesium calcite. With regard to the adze blades, if the calcite representation were the result of a partial aragonite-to-calcite inversion it would be reasonable to expect a greater degree of variation in the calcite content of the samples. Instead we find a very small and consistent amount of magnesium calcite, together with a high percentage of aragonite, in all three adze specimens. The scanning electron micrographs of the valve and adze samples appear to mark a progressive transformation in crystal dimension, shape, and orientation-from a young/ small T. gigas valve, to older layers of a more mature/larger valve, to the material from which the adze blade was manufactured. The transformation apparently involves a decrease in surface:volume ratio; a coarsening, or distortion of crystal shape from fine needles to polyhedral and round-edged plates; and the alteration of crystal fabric from a crossed lamellar to a homogeneous structure. The observed differences in the samples are interpreted as suggestive of an isomineralic (aragonite-to-aragonite) transformation of the crystal fabrics, effected through one or both of the following processes: (1) aggrading recrystallization, wherein aragonite grains starting from widely spaced nuclei (wide compared to grain size, i.e. l-5 u apart) grow to replace the original mass of tiny needles or plates and to occupy the pores between them; Folk (1965) describes this process for aragonite needles in the course of inversion to calcite; (2) solution of the smallest aragonite grains or projections, which are highly soluble because of their high surface:volume ratio (i.e. the surface free energy is a significant part of the total free energy of the crystal); the aragonite is then reprecipituted as overgrowths

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Figure 7. Successive stages in “porphyroid neomorphism” (either recrystallization or inversion), a type of “aggrading neomorphism”. (Adapted from Folk, 1965.)

on larger aragonite grains, a process thought to cease when all of the pores have been filled (Folk, 1965). While the petrographic literature makes little mention of fine-to-coarse aragonite transformation, other examples of grain-size change within a single mineral species have been described. In the fields of sedimentary and metamorphic petrology, the term “recrystallization” has been applied to the transformation of carbonate crystal fabric (crystal volume, shape, and microstructural orientation) without change in mineralogy (Folk, 1965; Dodd, 1966; Spry, 1969; Bathurst, 1975). Most often cited as examples of this process are calcite-to-calcite transformations. Metallurgists have used the term “recrystallization” to denote the alteration of strained to unstrained crystals of a given metal through heating (Mehl, 1948; Cottrell, 1955; Chalmers, 1959). Further annealing results in “grain growth” or “grain coalescence” (Harker & Parker, 1945; Walker, 1945; McLean, 1957), wherein unstrained metal crystals consume others and coalesce to produce a coarser fabric. Folk (1965) introduced the inclusive term “neomorphism” to describe all transformations between one mineral and itself or a polymorph, whether inversion (aragonite-tocalcite, calcite-to-aragonite) or recrystallization, whether crystals grow or diminish in size or merely change in shape. For geologic materials, grain size usually changes from small/ fine to larger/coarse, a process Folk has termed “aggrading neomorphism” (see also Bathurst, 1975). One of the ways in which aggrading changes may occur is through the gradual consumption of small crystals by larger crystals growing from widely spaced nuclei, and by the coalescence of the new grains to form a completely different mosaic (see Figure 7). It is suggested that an in situ process analogous to Folk’s “porphyroid” recrystallization may account for apparent changes in the crystal fabric of T. gigas shell, from relatively young layers of “fresh” shell [see Moir (1989~: figure 26(b)], to somewhat older layers of the same sample [Figure 5(a)], to shell that reportedly was maintained (“aged”) in seawater for years to decades following harvest of the animal [Figure 6(a)]. Alternatively, or perhaps coactively, the smallest aragonite grains or crystal projections may have dissolved, and the aragonite reprecipitated inorganically as platy overgrowths on larger aragonite crystals. This is one possible interpretation of Figure 5(a) (R. Folk, pers. comm.), and is suggested by the relatively high solubility of crystals with a high surface:volume ratio. Surface seawater is supersaturated with respect to both aragonite and a variety of magnesium calcites (Allegre & Michard, 1974; Brownlow, 1979), and aragonite may be precipitated when calcite growth is inhibited (Curl, 1962). During inorganic precipitation the presence of certain ions in solution-particularly Mg2+, which is abundant in seawater-favours the production of aragonite and magnesium calcite and inhibits the formation ofcalcite (Kitano, 1964; Simkiss, 1964; Krauskopf, 1967; Bathurst, 1968; Folk, 1974). That the (possibly) reprecipitated aragonite in the present samples appears to

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overlie extant crystals [see Figures 5(a) and 6(a)] may be accounted for by the tendency of calcium carbonate nuclei to promote the represented polymorph by epitaxy, or oriented overgrowth (Curl, 1962). Such overgrowth and continued coalescence of transformed crystals might have produced the unusual mosaic ofplaty aggregates visible in Figure 6(a). The driving force for these and other neomorphic transformations, as described qualitatively by Folk (1965) and more quantitatively by Fyfe & Bischoff (1965) and Chave & Schmalz (1966), is a lowering of the total free energy of the system (i.e. of a portion of the shell). As crystals grow in size (and/or coalesce into aggregates) and their surface:volume ratio decreases, the contribution of surface energies involving unsatisfied atomic bonds at surfaces, edges, and corners becomes insignificant compared with the total internal lattice energy of the crystals. As the crystals give up progressively more surface energy with loss of surface area, they become progressively more stable and move toward chemical equilibrium. Aragonite is thermodynamically less stable than calcite, another polymorph of calcium carbonate, at the temperatures and normally low pressures that occur near the earth’s surface (Goldsmith, 1959; Chave, 1964; Allegre & Michard, 1974; Brownlow, 1979). When the apparent surface area of the aragonite needles of the young, “fresh” T. gigas valve [Figure 4(a)] is compared with that of the platy aggregates found in the “aged” material [Figure 6(a)], the lower surface:volume ratio of the latter suggests that aragonite in this recrystallized form is thermodynamically more stable than aragonite needles-and that, in a stability continuum, aggregated plates of aragonite may lie between the needle-like form of this mineral and coarser calcite crystals. The process of aragonite stabilization most often cited in the literature is that of inversion, or transformation to calcite. But inversion is generally accepted as taking place in nature over the course of tens of thousands or millions of years (Stehli, 1956; Hallam & O’Hara, 1962; Weyl, 1964; Fyfe & Bischoff, 1965; Dodd, 1966; Chappell & Polach, 1972) while the craftsmen of Takuu reportedly “aged” aragonitic T. gigas valves in seawater for periods of fewer than 100 years. On the basis of the chemical arguments presented above, it is proposed that recrystallization of aragonite needles to aragonite platy aggregates constitutes a stage in the thermodynamic stabilization of this mineral. When it occurs it may precede inversion to calcite (see Folk, 1965), but it is not conceived as being a prerequisite to calcitization (Bathurst, 1975). It is further suggested that aragonite-to-aragonite recrystallization may be effected within a far shorter time span than aragonite-to-calcite inversion. If we accept that the large T. gigas valve used in these studies was collected live (“fresh”), we may infer that aragonite-to-aragonite recrystallization (or at least solution and reprecipitation) begins during the life of the clam, in older shell layers (Figure 5) at some distance from the mollusc’s calcium carbonate-secreting mantle. Further, if we accept that the three T. gigas adze blades sampled were manufactured from valves that had been “aged” in seawater over a period of some years or decades, it is reasonable to conclude that the transformation proceeds (see Figure 6bi.e. the shell’s mineral substance becomes progressively more stable-when the valve is maintained in a submarine environment following removal of the animal. This last inference is supported by the observation that, while aragonite will eventually invert to calcite at atmospheric temperatures and pressures, it may persist as metastable aragonite for a long period of time if it remains in contact with a solution similar to that from which it precipitated (Cloud, 1965ti.e. seawater. When exposed to the moist atmosphere or to rainwater, a shell composed of unstable aragonite needles would begin to dissolve; but a shell whose aragonite needles had recrystallized to coarser platy aggregates-to crystals with a lower surface:volume ratio and less surface energy-would be significantly less soluble under such conditions (K. Chave, pers. comm.).

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Some Apparent Implications of Shell Aging for Adze Material

Such propositions may be extended to include tools manufactured from “fresh” T. gigas shell (aragonite needles) and “aged” material (coarser crystal aggregates). A decrease in the surface:volume ratio of aragonite crystals and the corresponding loss of free energy would have the effect of lowering the solubility of the crystals-an effect of importance to the Takuu, who stored T. gigas adzes in subterranean caches subject to rainwater seepage. If a fresh-shell adze and an aged-shell adze were deposited together in such a cache, leaching would act selectively on the fresh-shell adze. This conclusion is supported by data indicating that marine skeletons composed of the unstable carbonate minerals (aragonite, magnesium calcites) are significantly more soluble in natural waters than those composed of the stable polymorph, calcite (Chave et al., 1962; Chave, 1964; Rolfe & Brett, 1969; Stenzel, 1971; Bottjer, 1985). So too within a single mineral, aragonite that recrystallizes from unstable needles to the more stable, platy crystal aggregates becomes less soluble; tools manufactured from the latter material are more likely to resist leaching and to be preserved over time (K. Chave, pers. comm.). When informants stated that a T. gigas valve aged in seawater “becomes harder”, they explained that it would not break as easily as fresh shell; nor would aged-shell adzes fracture as readily, either in manufacture or in use. It was also remarked that adzes (including those used solely in ceremonial contexts) made from the aged material were far more durable over time, less likely to be adversely affected by placement under ground. While the studies described here did not include tests to measure the relative hardness of fresh and aged T. gigas shell, it is possible to gain at least a qualitative appreciation of informants’ statements by examining the crystal mosaics visible in the micrographs. The homogeneous microstructure and the density of crystal aggregation seen in Figure 6 [and in corresponding micrographs for two other adze samples, not shown here but presented in Moir (1989a: figures 28 and 29)] suggest this reportedly aged material would tend to resist fracture where fresh shell, with needle-like crystals arranged in lamels, would be susceptible. By recrystallization and/or solution and reprecipitation these platy aggregates appear to have filled the intercrystalline pores once occupied by organic conchiolin, and to have produced a homogeneous, densely crystalline material likely to be somewhat better suited to controlled knapping (Crabtree, 1972). A very common analogue to this process-analogous in form, but not time-is the recrystallization of fine-grained limestone (calcite) to marble (calcite). Of the two, only marble lends itself to the sculpture of statues and monuments of three-dimensional complexity. By contrast, as mentioned previously, adzes manufactured from fresh shell were said by the Takuu to deteriorate over time and under conditions of cache storage. The surface of fresh-shell blades would become finely pitted and chalky in appearance, and the tools themselves would become increasingly subject to fracture during use. These empirical observations on the part of the Takuu are supported by evidence for dissolution pitting of calcium carbonate material (Cloud, 1965; Aller, 1982), and for the development of a porous, loosened microstructure and chalky appearance in shell that has lost its conchiolin matrix (Lewy, 1981). It would be necessary to examine the microstructure of fresh-shell blades using the SEM in order to estimate the degree to which these processes affect such tools over time, but the apparently porous mosaic of the large and presumably fresh valve [Figure 5(a)] suggests it would constitute a tool material of lesser quality than the densely homogeneous aged shell. Conclusion

It is emphasized that the studies described here were preliminary explorations, undertaken with the goal of pursuing qualitative statements by informants. They have perhaps

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generated more questions than they have answered. Further investigations using the valves of Triducna spp. and perhaps others, by archaeologists, zoologists, geochemists, and others to whom problems of material transformation and preservation are relevant, would expand upon these initial findings and in all likelihood would bring them into clearer focus. For T. gigas, more rigorous sampling procedures in future efforts would include the live collection of a large T. gigus specimen (to ensure “freshness” of the shell); the replication of the aging process, to the extent possible, over periods of time ranging from a few years to decades (to test for diachronic variability in microstructural alteration); and the sampling and microscopic examination of multiple shell layers and valve areas. Archaeologists interested in comparing the physical properties of fresh and aged T. gigus shell might subject these materials to hardness tests, and to experiments in tool manufacture and use. Such efforts may be expected to yield quantitative data for comparison with the qualitative information available to date. Ethnohistorical evidence from Takuu for the “aging” of T. gigus valves argues that early atoll craftsmen within the distribution of this species were not inevitably as restricted in their range of local tool materials as might be inferred from the lack of lithic resources in their environment. It implies that such craftsmen recognized the mutable physical properties of T. gigus shell, perceived that an advantageous transformation could be effected within an accessible time frame, and manipulated the raw material so as to induce that transformation. It is possible, even likely, that this strategy for altering the physical properties of fresh T. gigus shell was practised not only on Takuu but elsewhere in the Indo-Pacific, an area encompassing the post-Pleistocene distribution of this species (Rosewater, 1965; Taylor, 1971, 1978) and noted for the widespread use of Triducnu spp. valves in early tool manufacture (Moir, 19896). The apparent uniqueness of the strategy may be attributed to differential retention of such knowledge in the ethnographic and historical records: shell adze manufacture and use evidently persisted for longer on Takuu (until early in the 20th century) than elsewhere in the region. That other island craftsmen as well may have so enhanced the quality of this important material resource, is an aspect of prehistoric economy that might fruitfully be explored by archaeologists concerned with shell technology, resource use and management, and material transformation processes. Acknowledgements

Grateful acknowledgement is made of the generous assistance of Keith Chave (Department of Oceanography, University of Hawaii), who both encouraged and facilitated these laboratory studies. Thanks are also extended to Jim Cowen and Naomi Okazaki (Scanning Electron Microscope Laboratory), to Richard Radtke, Joann Sinton, and Denys VonderHaar (Hawaii Institute of Geophysics), to Stephen Smith (Hawaii Institute of Marine Biology), and to Lisa Hacskaylo (Department of Anthropology), all of the University of Hawaii. Robert Folk and Lynton Land (University of Texas) and John Taylor (British.Museum, Natural History) kindly reviewed and commented upon the photomicrographs and other laboratory data. The University of Papua New Guinea, the Institute of Papua New Guinea Studies, and the National Museum and Art Gallery (P.N.G.) made possible the field research on Takuu Atoll from which these studies evolved. The support of the (U.S.) National Science Foundation and the Wenner-Gren Foundation for Anthropological Research is also acknowledged. References

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