♦Livre♦
Experiments in Egyptian Archaeology: Stoneworking Technology in Ancient Egypt
131. The tubular drilling of a rose granite block required the assembly of the four component parts of the drilling equipment: the 8 cm-diameter copper tube, the round wooden drill-shaft partly force-fitted into it, the driving bow and rope, and a capstone bearing in which to rotate the upper end of the drill-shaft (see Figure 6.6). The capstone took one hour to shape and hollow, using flint chisels and punches, from locally obtained hard sandstone. The top of the shaft was carved into a cone, with a rounded top; drilling experience demonstrated reduced friction if the top of the cone rotated in the apex of the bearing. This was lubricated with grease, in place of the likely ancient tallow. Preliminary tests in Manchester indicated that a very stiff bow-shaft was needed to place sufficient tension on the 1.3 cm-thick bow-rope, necessary to prevent slippage on the wooden drill-shaft.
A small area of the rose granite’s surface was prepared by pounding it with a dolerite hammer until it became flat and smooth. The end of the tube, smeared with red water-based paint (probably red ochre in ancient times), made a circular mark by pressing it on the stone’s surface. The dolerite hammer drove a flint chisel along the circular line to make a groove. (There is evidence in the Petrie Collection of such a circular groove in an unfinished, unprovenanced and uncatalogued alabaster vessel.) This groove allowed the tube to be located for the initial grinding operation, achieved by fastening two temporary stone weights to the top of the drill-shaft, which was continuously twisted, by hand, clockwise and anticlockwise on dry sand abrasive. Hand grinding continued until the groove attained a depth of 5 mm, a measurement at which the bow could spin the located tube without it jumping out.
131. A team of three workers operated the drill (Figure 4.21), one worker at each end of the bow to drive it, the third worker holding the capstone (first suggested in 1986).64 The bow-rope was sufficiently loosened to enable two complete turns to be made around the drill-shaft (Figure 4.22), which placed a bending stress upon the bow-shaft. This gave 50 cm of tight contact between the rope and the drill-shaft’s circumference. The 8 cm-diameter tube, with 1 mm thick walls, optimally required a total load of 2.5 kg acting upon its end-face. A greater load than this caused the drillers unnecessary work, and even bent the rigid bow-shaft, slackening the tension in the rope.
132. The workers’ normal reciprocating strokes, each approximately 50 cm in length, turned the drill-shaft at a rate of 120 revolutions/minute. The driller pushing the bow simultaneously assisted the other driller pulling it; these actions automatically reversed at the end of each stroke. Resisting the reciprocating strokes was not too difficult for the worker holding the capstone, although keeping it completely still was impossible. A small amount of dry sand, trickling around the tube as drilling progressed, found its way down to the cutting face. Later measurements showed that about 250 g were used by the saw, and the drill-tube, to remove 1 cm3 of the granite, similar to the Manchester results. Water in the sand abrasive made the drill-tube more difficult to turn and washed away the copper particles. Further, used wet sand powder, probably containing lime, is troublesome to remove from a tubular hole, rapidly setting as a rudimentary mortar if drilling is suspended for a short time. Dry sand powder was easy to remove; it stuck together inside the drill-tube and periodically could be withdrawn from the hole.65
134. The core was removed from the drilled granite by soundly hammering two adjacently placed flat tapered chisels vertically into the tapered slot: the slot and the chisels’ tapers fitted almost perfectly (Figure 4.24). The chisels acted on a short arc of the top of the core’s circumference, using its length as a lever. This forced the core over, causing the brittle granite at the bottom of the core, and directly below the chisels, to be placed under such tension that it parted completely, allowing the core to be extracted in a single piece (Figure 4.25). Horizontal striations similar to the ancient ones on rose granite were visible both in the wall of the hole (Figure 4.26), and upon the core.
♦Mégalithe♦
Abrasion - Forage tubulaire et Scie en Cuivre : Exp. d'Assouan
