♦Livre♦
Experiments in Egyptian Archaeology: Stoneworking Technology in Ancient Egypt
37. Ancient smelting furnace-holes were probably lined with stones before filling with charcoal and ore, the finished hole measuring approximately 30 cm in diameter, with a height of about 25 cm. Furnaces of these dimensions have been examined by B. Rothenberg at a chalcolithic copper smelting site at Timna in the Negev desert.51 Their efficiency was examined by R.F. Tylecote and P.J. Boydell,52 who constructed experimental furnaces fired with charcoal. The controlled admission of air into a melting furnace is crucial if its interior is to reach the temperature necessary to melt the metal contained in a crucible; copper requires a temperature of 1,083°C. For bronzes containing varying amounts of tin, lower temperatures are sufficient. For example, a bronze containing 10 per cent tin has a melting point some 80°C less than pure copper. Tylecote and Boydell discovered that an air flow of 200 l/minute, delivered through a tuyère, raised the furnace temperature to 1,300°C,53 more than enough to smelt ore, or melt copper in a crucible. The test furnace, of similar dimensions to the Timna furnaces, enabled an air flow of 200 l/minute to melt 1 kg of copper. This furnace also possessed a maximum melting capacity of 2 kg of bronze, when operated with an air flow of 600 l/minute.
38. Some experiments were conducted with a reconstructed blowpipe manufactured from a bamboo cane, but in ancient Egypt blowpipes would have been constructed from the bamboo-like common reed: tomb artists depicted blowpipes with clearly defined leaf-joints.54 Tomb representations also show that two types of blowpipe were in use. In the Eighteenth Dynasty tomb of Rekhmire at Thebes,55 a jeweller’s blowpipe is about 60 cm long, whereas a drawing in the Twelfth Dynasty tomb of Pepionkh depicts furnace blowpipes of approximately 1.5 m in length (Figure 2.20).56 The experimental cane possessed an average external diameter of just over 2 cm, and a length of 56 cm (Figure 2.21). Workers crucially adapted a reed into a tube by jabbing a slimmer, sharpened reed, or stick, through the leaf-joint partitions and uniting the previously separate hollow sections. The experimental cane was adapted in a similar fashion.
56. The project furnace for casting coppers and bronzes into open sand moulds was constructed from sheet metal riveted together to form a hood, a flue and a base, which contained a lining of firebricks (Figure 2.48). This lining formed a space for the fuel equal in volume to the average capacity of the ancient furnaces examined by Rothenberg at Timna in the Negev desert. When fully filled with fuel, a bowl-shaped furnace measuring 30 cm in height and 25 cm in diameter was created. An electric blower supplied air through a steel pipe connected to the furnace. The air flow rate could be adjusted from a minimum of 200 l/minute to a maximum of 600 l/minute (Figure 2.49). The maximum flow rate allowed the furnace to reach an operating temperature of approximately 1,500°C, the minimum flow rate producing a temperature somewhat in excess of 1,200°C. Three modern silicon carbide crucibles were available for use.
