Shell adzes were made across the Pacific, particularly on coral atolls which lack stones for toolmaking. This shell adze was made from a Tridacna gigas (giant clam) shell. The thickness of the adze and lack of anatomical features suggest that the adze was made from one of the thick folds on a very large shell. These shells can reach lengths up to 1.75 metres and weigh over 300 kg. The clams rely on their weight to anchor to the shallow sandy lagoon bottoms where they live. The adze lacks recognisable interstices that occur in the concavities between the folds, which suggests that it was oriented parallel to (rather than across) a fold.
Shell adzes were made by flaking in the early stages and finished by grinding. The process was similar to the manufacture of stone adzes, although controlled flaking in shell is difficult. Most shaping and preparation was completed by grinding. Tridacna shell may have been favoured for adzes in part because it has a relatively low organic content. One face of this adze is deeply pitted from bioerosion, suggesting that this surface was exposed to greater weathering than the opposite surface.
Tridacna shell is physically robust, with a tensile strength superior to medium quality basalt and on par with high quality basalt. The shell’s tensile strength is due to its carbonate crystal structure, as opposed to the largely amorphous silicate composition of basalt. Basalt is denser than shell however, and tends to wear less quickly as a result. An experimental study found that a shell adze could cut down a 20 cm diameter tree in about one hour, 43 minutes, or about 40 minutes longer than for a basalt adze. If both basalt and shell are available on an island, the basalt is marginally a more optimal choice for making adzes; however, if basalt must be imported, the shell becomes the most optimal choice by a wide margin. The exchange of basalt adzes across the Pacific was likely driven by social factors rather than considerations around optimal tool efficiency.
The people of Takuu Atoll in the North Solomons pretreated their Tridacna gigas shells prior to making them into adzes. The Takuu made very large ceremonial adzes from these shells. While most shell tools were extracted from the ocean and used directly, T. gigas clams intended for adzes were collected from the local reefs when small, and the live clams were transplanted to coral ‘gardens’ in the lagoon. There they were protected and allowed to grow to suitable size, a process that took many years. If the need for adze material was pressing, shells would be harvested and used immediately; however, Takuu craftsmen preferred to return a harvested shell to the coral garden and allow it to age underwater. The ageing process might take a generation or longer, and the shells were often used by the original collector’s descendants. Shell immersed in seawater was said to be better quality for working and more durable than fresh shell or shell that was aged on land. Specifically, shell pretreated in seawater was less likely to cleave along laminar planes when flaked or used. Adzes made from pretreated shells were also said to be resistant to ageing seen in adzes made from fresh shell. Scientific study of pretreated and fresh shells showed that the pretreatment process causes the aragonite mineral to recrystallise and/or reprecipitate to fill the pores between smaller crystals like those seen in fresh shells. This in effect strengthens the shell and decreases the likelihood of cleaving across the pore spaces, and decreases chemical erosion.
This adze is from Dauar, one of the three islands composing the Meriam (Mer or Murray) Islands in the Eastern Torres Strait Island, Queensland. The Meriam Islands’ most famous resident was Eddi Mabo, who became famous for suing the Queensland government for the return of his ancestral land. The successful 1992 lawsuit challenged the legal concept of terra nullius in Australia—the argument that the land was not legally possessed by Indigenous people prior to British conquest—thus opening the floodgates for Native Title claims across the continent.