Beyond the main function: An experimental study of the use of hardwood boomerangs in retouching activities

Abstract

Retouched lithic tools result from the functional modification of their edges following knapping operations. The study of the later stages of the reduction sequence is fundamental to understanding the techno-functional features of any toolkit. In Australia, a gap exists in the study of the chaîne opératoire of lithic tools shaped or re-shaped through percussion retouching. In our previous works (Martellotta EF., 2021, Martellotta EF., 2022), we have presented evidence for the use of hardwood boomerangs for retouching purposes in Australian Aboriginal communities. Through a detailed experimental protocol, the present study demonstrates how boomerangs can function as retouchers. We found that the use-wear generated on the boomerang's surface during retouch activity is comparable to retouch-induced impact traces observed on Palaeolithic bone retouchers, as well as to experimental bone retouchers generated in our replication experiments. Finally, we explore the role that microscopic lithic chips embedded in the retouchers' surface play in the formation process of retouching marks. Our results address the need for a deeper investigation of percussion retouching techniques in Australian contexts, opening the possibility that uncommon objects-such as boomerangs-could be used for this task. This concept also highlights the broader topic of the highly diverse multipurpose application of many Indigenous tools throughout Australia. At the same time, the study reveals a deep functional connection between osseous and wooden objects-a topic rarely investigated in archaeological contexts.

Fig 1. Retouching session 2 using boomerangs.

(a, b) tangential movement of retouch; (c) use area B1_1 and retouched lithic flake F56; as this boomerang is painted, the ventral face of the flake shows residues of paint oriented according to the percussion movement of retouch; (d) flint chip embedded within retouch impact traces at the initial stage of retouch.

Fig 2. Results of retouching session 2 on the boomerang B1 (plano-convex cross-section and location of use areas).

(a) location of use area B1_1, (i) use area B1_1 (1x), (ii) retouched flake F56; (b) location of use area B1_2, (i) use area B1_2 (1x), (ii) retouched flake F59. (c) location of use area B1_3, (i) use area B1_3 (1x), (ii) retouched flake F51; (d) location of use area B1_4, (i) use area B1_4 (1x), (ii) retouched flake F90.

Fig 3. Results of retouching session 2 on the boomerang B2 (plano-convex cross-section and location of use areas).

(a) location of use area B2_1, (i) use area B2_1 (1x), (ii) retouched flake F100; (b) location of use area B2_2, (i) use area B2_2 (1x), (ii) retouched flake F71. (c) location of use area B2_3, (i) use area B2_3 (1x), (ii) retouched flake F17; (d) location of use area B2_4, (i) use area B2_4 (1x), (ii) retouched flake F83.

Fig 4. Results of retouching session 2 on the boomerang B3 (bi-convex cross-section and location of use areas).

(a) location of use area B3_1, (i) use area B3_1 (1x), (ii) retouched flake F36; (b) location of use area B3_4, (i) use area B3_4 (1x), (ii) retouched flake F76; (c) location of use area B3_5, (i) use area B3_5 (1x), (ii) retouched flake F40. (d) location of use area B3_2, (i) use area B3_2 (1x), (ii) retouched flake F73; (e) location of use area B3_3, (i) use area B3_3 (1x), (ii) retouched flake F03; (f) adjacent use areas B3_2 and B3_3.

Fig 5. Results of retouching session 2 on the boomerang B4 (bi-convex cross-section and location of use areas).

(a) location of use area B4_1 (i) use area B4_1 (1x), (ii) retouched flake F60; (b) location of use area B4_2(i) use area B4_2 (1x), (ii) retouched flake F35. (c) location of use area B4_3, (i) use area B4_3 (1x), (ii) retouched flake F81; (d) location of use area B4_4(i) use area B4_4 (1x), (ii) retouched flake F41.

Fig 6

3D analysis of a single linear impression on bone retoucher (left) and boomerang (right). (a) colour 2D image of a linear impression on bone retouchers (10x); (b) image of the same stigma with 3D height data; (c) V-shaped profile measurements and cross-section internal surface measured in different portions of the linear impression on bone retoucher; the deep penetration of the use mark in the bone surface can be observed. (d) colour 2D image of a linear impression on boomerang (10x); (e) image of the same stigma with 3D height data; (f) V-shaped profile measurements and cross-section internal surface measured in different portions of the linear impression on boomerang; the deep penetration of the use mark in the wooden surface can be observed.

Fig 7

3D analysis of a single punctiform impression on bone retoucher (left) and boomerang (right). (a) colour 2D image of a punctiform impression on bone retoucher (10x); (b) image of the same stigma with 3D height data; (c) U-shaped profile measurements and cross-section internal surface measured in different portions of the punctiform impression on bone retoucher; we can observe how the punctiform mark does not penetrate deeply in the bone surface. (d) colour 2D image of a punctiform impression on boomerang (3x); (b) image of the same stigma with 3D height data; (c) U-shaped profile measurements and cross-section internal surface measured in different portions of the punctiform impression on boomerang; we can observe how the punctiform mark penetrates deeply in the wooden surface, but it remains shallower than the linear impression.

Fig 8

3D analysis of a single notch on bone retoucher (left) and boomerang (right). (a) colour 2D image of the notch on bone retoucher (10x); (b) image of the same stigma with 3D height data; (c) profile measurements and cross-section internal surface measured in different portions of the notch on bone retoucher; the analysis shows that the notch is composed of the superposition of linear and punctiform impressions, penetrating deeply in the bone surface. (d) Colour 2D image of the notch on boomerang (3x); (b) image of the same stigma with 3D height data; (c) profile measurements and cross-section internal surface measured in different portions of the notch on boomerang; the analysis shows that the notch is composed of the superposition of linear and punctiform impressions, penetrating very deeply in the wooden surface.

Fig 9. Images of other marks identified within the use areas on boomerangs.

(a) striations associated with punctiform and linear impressions (3x); (b) scraping marks interacting and intersecting with linear and punctiform impressions (3x); (c) peeling-like use-wear (3x); (d) tool-edge scratches interacting and intersecting with linear impressions (3x); (e) flint chip embedded in linear mark on boomerang surface (10x); (f) flint chip embedded in linear marks on bone retoucher surface (R6_2) (3x).

Fig 10. Flint chips embedded within the use areas on boomerangs (10x).

(a) embedded flint chip mostly intact, showing paint residues owing to the use of a painted surface of the boomerang for retouching; (b) elongated flint chip creating a linear impression; (c) embedded flint chip fractured in its extremities following repeated percussion: punctiform impressions are more likely to be created as a consequence of these fractures; (d) embedded flint chip fractured in most of its surface due to repeated percussion: the micro-flakes resulting create punctiform impressions; (e) small, thin flint chip showing how each fracture of the chip itself from repeated percussion creates new impressions on the boomerang surface; (f) as a result of repeated percussion, several embedded flint chips are completely fractured leaving punctiform impressions and notches in the wooden surface.