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Cornaline de l’Inde

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Valentine Roux

I. Caractérisation et identification des techniques de fabrication des perles en roches dures

4. Drilling Chalcedony beads: a mechanical approach of Harappan techniques

Antonio Sela et Valentine Roux

Texte intégral

1Abstract. Starting at the technical stage of perforation, an object may be qualified as a bead, meaning it is an object that is pierced and threaded. At Cambay, the perforation of beads is carried out by traditional techniques, while utilizing a “modern” material: diamond. During the Harappan period, 12-centimeter long carnelian beads were pierced. An understanding of the techniques used to perforate these beads interests the study of both the general history of techniques and the techno-economic organization of the craft industry for which come into play as much problems of raw-material procurement (the abrasive) as the modes of fabrication (skills and time) of drills and beads.

2In fact, very few studies have been conducted on this subject. J.M. Kenoyer and M. Vidale (1992) have carried out the most elaborate experimental analyses to date. Inspired by the Cambay example, these studies resulted in hypotheses concerning the techniques of perforating Harappan chalcedony beads. The hypotheses distinguish two types of drills for which differences in efficacy are explained by the morphology of the bits and the material from which they are made. We propose a complementary approach consisting of a study of experimental results according to the classic mechanical principals of the materials. The properties of hardness, tenacity and resistance to abrasion are analyzed for the different types of drills envisioned by J.M. Kenoyer and M. Vidale. This analytical procedure allows us to obtain theoretical results that correspond to the experimental findings, which then permits an explanation of the latter based on the mechanical characteristics of perforation applied to the properties of the materials utilized.

3More precisely, our results suggest that the large “classic Harappan” beads (made of camelian and 7 to 12 cm long) were perforated thanks to the elongated drill bits of “ernestite” (denomination by J.M. Kenoyer 1992), a stone that contains titanium oxide, which is known for its exceptional perforating qualities. Much more than the morphology of the bits, this oxide is the determining element in the efficacy of the drill. Indeed, the “classic Harappan” beads could never have been perforated with drill bits made of silicate given, above all, the mechanical problems related to raw-material properties (different types of silicate). The geometry of the drill bits is the complementary element.

4We might ask ourselves to what degree the innovation of long drill bits of “ernestite” was related to a demand for exceptionally long beads, or if the discovery of the exceptional nature of stone containing titanium oxide led to the fabrication and demand for these unique beads. It is probably necessary to consider the appearance of these beads to be the result of a complex interaction between technical, environmental and cultural elements, an interaction stemming from the politico-religious context of the Indus civilization (see Chapter 7).

© Éditions de la Maison des sciences de l’homme, 2000

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