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Cornaline de l’Inde

Valentine Roux

II. Habiletés impliquées dans la taille des perles en roches dures

5. Skills involved in the knapping of chalcedony beads: Motor and cognitive characteristics of a complex situated action

Blandine Bril, Valentine Roux y Gilles Dietrich

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1Abstract. Compared to pre-Harappan stone beads, Harappan beads are characterized by a great diversity of shapes, as well as by their large dimensions. They include pieces whose length can reach 12 centimeters, whereas the length of pre-Harappan pieces is never over 3 centimeters.

2Now, it is a general rule that apprentices learn by reproducing what the master teaches. They never learn by innovating whatever the context of apprenticeship. Therefore, if a change occurs in the skills involved in craftsmanship, this change should be regarded as a signal that reflects a rupture with the model transmitted by the tradition. This is to say, a discontinuity in the evolution of the technical system, and consequently, modifications in the environment (Shennan 1989).

3Applied to Harappan stone beads, our hypothesis is the following: compared to pre-Harappan production, Harappan stone-bead production would imply new skills and may thus be significant in relation to the transformations that occurred in Harappan society around 2500 bc.

4In order to characterize the different skill-levels developed by the Harappan craftsmen, we took Cambay as a referential model. In Cambay, the bead-manufacturing workshops are divided into two categories: 1) those specialized in the manufacture of high-quality beads, and 2) those specialized in the manufacture of low-quality beads. The craftsmen who produce the high-quality beads are able to knap all types of beads of varied shapes and dimensions. The duration of their apprenticeship period is ten years. The craftsmen who produce the low-quality beads knap beads with irregular shapes, and with dimensions smaller than 3 centimeters. The duration of their apprenticeship is three years.

5In order to measure knapping skills “universally” in terms of apprenticeship duration and to assess to what extent observable performances reflected the real abilities of the craftsmen, a method of field experimentation was designed based on principles found in psychology.

6Field experimentation refers to the construction of a situation, based on a task familiar to the subject, that allows a better control of the different parameters under study. The design of the experiment was based on the notion of transfer. That means that the level of expertise can be inferred from the capacity to transfer one's skill from a well-known situation to a new situation. Different levels of novelty were thus introduced, either new raw material (glass which presents the advantage of being a homogeneous material, unlike chalcedony) or new shape (or length), or both. We proposed the hypothesis that the degree of success in the case of modified situations reflects the ability to adjust to task constraints, which is considered as a means of measuring the level of expertise. A highly skilled craftsman should be able to transfer his planning and motor abilities to new situations.

7In total, we worked with twelve craftsmen divided between the two levels of performance: a) group 1 was comprised of craftsmen who produced high quality beads daily, b) group 2 was composed of craftsmen who produced low quality beads daily.

8Each craftsman was asked to knap 80 roughouts of different shapes, dimensions and materials (chalcedony and glass).

9The knapping process was recorded so that the course of action as well as the characteristics of the elementary movements and their sequencing could be analyzed. Two types of recording device were employed:

  • a video camera situated in the axis of the movement, which recorded the entire knapping activity;
  • an uniaxial accelerometer attached to the hammer head and connected to a laptop computer that measured the acceleration of the hammer.

10The analysis could then be based on three types of data: the finished products (the beads), the succession of subgoals (the plan of action) used by the craftsman, and the structure of the elementary movement.

11The finished products were analyzed using a program for pattern recognition (automatic analysis of each contour). This analysis is based on a comparison between an ideal shape and the knapped product. Several parameters were chosen: parameters indexing the overall shape of the bead in terms of quantity of raw material and absolute dimensions; parameters indexing sphericity, and parameters indexing the profile of the bead contours. The plan of action followed by the craftsmen was analyzed using a time-series procedure. The succession of subgoals was noted as was their duration. The structure of the elementary movement was analyzed in terms of movement duration and variation of the acceleration of the hammer.

12The results obtained confirm our hypothesis. First, group 1 produced glass beads of a much higher quality than group 2. Second, by suppressing disturbances caused by the heterogeneous nature of the stone, it was possible to bring out characteristics that clearly distinguish the two groups and which therefore could be interpreted only in terms of competence. We found that competence was not characterized by the plan of action. In fact, in most cases the sequences (succession of subgoals) do not enable us to distinguish between the subjects of groups 1 and 2, and do not distinguish the level of expertise. Instead, it is the elementary actions, interpreted by reference to the finished products, that appear to be determinant, as shown by the values of acceleration of the hammer. A high level of expertise (group 1) is characterized by an acceleration that is adapted to the local constraints (hardness of the raw material, dimensions of the flake to be detached) while minimizing energy. The level of expertise could thus be defined, not by the planning abilities, but by the mastering of the dynamics of the interaction between the subjects and their environment in the realization of the action.

13These results should explain the variation in apprenticeship duration depending on the type of bead to be knapped. In fact, the knapping of the different types of beads requires different types of flakes, which imply variations in the elementary actions. Consequently, the mastering of the knapping of beads of all shapes and dimensions requires long years of practice. The most competent knappers, i.e. the ones who knap high-quality beads of all shapes and dimensions, had apprenticeship periods often years. Recent psychological studies have shown that ten years is the minimum time required to become an expert in an activity, be it purely cognitive or motor (e.g. chess or sport, Ericson & Lehman 1996). Ten years to become an expert in knapping all types of beads (i.e. to produce the elementary gestures which allow the knapping of beads of all dimensions and shapes) thus appears to be a cross-cultural regularity that can be applied independently of the cultures under study.

14In the context of Harappan stone beads, this regularity enables us to distinguish between the craftspeople who knap small stone beads and those who knap long beads, i.e. between pre-Harappan and Harappan production. Manufacture of the so-called Harappan classical beads necessitates particular skills and a long period of apprenticeship. It should be significant of major transformations (see Chapter 7).

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

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