6. CHAPTER SIX: A VISUAL STUDY OF DECORATIVE METALWORK AS A WAY OF UNDERSTANDING MANUFACTURING TECHNIQUES
6.5: ARCHAEOLOGICAL RECONSTRUCTIONS OF TECHNIQUES OF ANUFACTURE USED FOR DECORATIVE METAL WORK
6.5.3: ARCHAEOMETALLURGICAL RECONSTRUCTIONS OF TECHNIQUES USED TO FABRICATE DECORATIVE COPPER, BRONZE AND BRASS WORK
Figure 6.30 The photograph shows an example of poor welding between two strips of iron with different carbon content (Miller et al, 1995: 54) (Magnification 45x).
The results of poor welding on an iron bead are seen in the micrograph (below) where oxidation has burst the layers apart. This is seen in Figure 6.31 which shows a polished section through bead WAT 4, illustrating the laminated or layered structure (Miller et al, 1995: 54).
Figure 6.31 A micrograph showing a polished section through an iron bead (WAT 4) indicating its laminated structure (Miller et al., 1995: 54) (Magnification 7.2x).
6.5.3: ARCHAEOMETALLURGICAL RECONSTRUCTIONS OF TECHNIQUES USED TO
beginning of the Later Iron Age, when ornamental forms were made by pressure welding together thin strands of copper which after centuries of burial are now splitting apart (Childs, 1991c: 39). Figure 6.32 shows how the technique of layering copper exposed inadequacies as through time the stands have parted. It was concluded that the technical skills of the metal smiths of the time were inadequate for the process (Childs, 1991c: 39).
Figure 6.32 A micrograph showing layered copper strands detaching from the main form from corrosion (Childs, 1991c: 39).
The following example of copper investigation on ornamental forms, Figure 6.33 shows a substantial rod being formed as a bangle, while deeper examinations indicates that the metal smith pressure welded a thin copper strip to a thicker rod and then hammered around the circumference (Childs, 1991c : 39). The explanation indicates that pressure welding is difficult due to the speed with which the metal surfaces oxidize in the air and form a veneer which impedes the diffusion and bonding of the metal (citing Tylecote, 1969).
Figure 6.33 A photograph (left) showing the thick copper rod formed into a bangle and a micrograph (right) showing the inserted strand of copper hammered into its periphery (Childs, 1991: 39) (Magnification 12.5x)
Copper and iron were used for ornamental forms in the Middle Iron Age site of Mapungubwe (Meyer, 1998; Calabrese, 2000, Miller 2002). The copper decorative objects show that they were manufactured using similar techniques to those noted in the past and often left in a recrystallized annealed condition. Figure 6.34 shows a section of worked copper from a ring 19.0 mm in diameter and indicates that cold working took place, in which the original casting network of cuprite/copper eutectic had been drawn out longitudinally and given a slight torsional twist due to flattening of the sides (Miller, 2001: 92).
Figure 6.34 The micrograph shows how a network of cuprite/copper eutectic had been drawn out longitudinally (Miller, 2001: 92). (Magnification 240x).
The intensive archaeological studies at the Greefswald site produced numerous fragments of iron and copper wire-wound bracelets for arms and ankles (Calabrese, 2000; Miller, 2001). Miller (2001) found most of the copper wire for wire-wound bracelets to contained angular, recrystallized copper grains showing straight-sided annealing twins. Figure 6.35 is a micrograph of a polished longitudinal section through a copper wire-wound bracelet (M 1093) exhibiting thickened wire elements flattened on the outside, wrapped around a composite fibre and iron core (Miller, 2001). The copper wire used for this object indicates that a certain amount of cold working had taken place.
Figure 6.35 The micrograph shows a polished longitudinal section through a copper wire- wound bracelet (Miller, 2001: 92) (Magnification 7.5x).
A further micro image of a fragment of a copper wire-wound bracelet, Figure 6.36 shows the alternative shape of cut ribbon forming the object. Apart from the trapezoidal shapes (M 4001) placed closely against each other the corrosion rinds are packed around
residual cores of metal indicating corrosion (Miller, 2001). Many of the copper wire- wound bracelets from this site were found to have well preserved fibre cores, due to the antibiotic properties of copper (Miller, 2001: 90).
Figure 6.36 A micrograph of a polished longitudinal section through a copper wire-wound bracelet (M 400-1) showing corrosion rinds around residual cores of metal (Miller, 2001:
92) (Magnification 10x).
Many beads of copper and its alloys were retrieved from the site of Bosutswe in the Later Iron Age period, showing varying degrees of corrosion (Denbow & Miller, 2007). Their fabrication indicated that all beads consisted of single phase equiaxed copper grains with annealing twins, while their grain sizes varied from small to large. At times nearly a full range of grain sizes could be noted in a single object, indicating that different degrees of cold work had taken place before being annealed (Denbow & Miller, 2007: 287). Figure 6.37 shows a micrograph of the copper bead (B 133) with a cold worked join with some preferential corrosion attack in the cold worked areas. The image indicates that the beads were all cut from the “inside” of a hot-worked strip with a chisel and wrapped to keep the v-shape cut on the inside to form a neat join (Denbow & Miller, 2007: 287)
Figure 6.37 A micrograph showing a longitudinal section of an annealed copper bead from Bosutswe (Denbow & Miller, 2007: 286) (Magnification 11x).
A slightly lower magnification of a copper bead from the same locality as the former indicates the full bead showing the strip of metal with tapered wedge-shaped ends cut with the aid of a sharp object. Figure 6.38: indicates more clearly that a hammered sheet was cut into strips for beads which were made by cutting short lengths of copper ribbon and bending them around with the cut bevels on the inside to create a relatively smooth
join (Miller 2002). The several collections of copper beads from Iziko Museum revealed similarities to the form and manufacture to those discussed above (SAMAE 9910, 9911, 9913, 9147, 9326, 11837).
Figure 6.38 The micrograph shows an etched section through a wrapped copper bead (B 159) from Bosutswe (Miller, 2002: 1124) (Magnification 10x).
Until recently the production of bronze; an alloy of copper and tin has been sparsely documented and a few objects have been metallographically studied (Friede, 1975, Miller, 1992; Miller et al., 1995; Hall et al., 2006; Bandama, 2013). The fabrication technology used for working bronze was similar to that for working with copper, and in many body ornamental objects bronze has been used in multi-metalled composite wire- wound bracelets (Miller, 2002). The earliest appearance of bronze in an archaeological Middle Iron Age site is recorded at Mapungubwe in the form of a substantial curved bar shaped like a bucket handle and a few fragments of wire-wound bracelets (Miller, 2002:
1102). The specimens were all 6% tin in copper and when polished reached a golden colour, similar to the appearance of gold and most likely, the social context of this cultural group, enjoyed a similarly elevated position as gold for status enhancement (Miller, 2002:
1124). From the Middle to Later Iron Aged site of Bosutswe bronze detritus and beads were recovered and metallographically analysed (Miller, 2001, 2003; Denbow & Miller, 2007). Figure 6. 41 shows an etched section through a bronze nodule (B 3) from Bosutswe, consisting of 8% tin in copper. The copper grains are rounded, with irregular intergranular island of the tin-rich delta eutectoid, indicating slow cooling taking place in a molten droplet. The black areas are voids (Miller, 2002; 1124)
Figure 6.39 The micrograph shows an etched section through a bronze nodule (B 3) from Bosutswe (Miller, 2002: 1124) (Magnification 257x).
The technology for making bronze beads was similar to that employed for iron, copper and gold. The bronze objects from Bosutswe indicated varying degrees of corrosion.
Some alloyed beads showed that a full range of grain sizes could be found in a single object, indicating that different degrees of cold-work had occurred before the final anneal (Denbow & Miller, 2006: 287). All the beads had copper oxide or copper sulphide inclusions, most often both. These were found in concentric bands which appeared to be from hammering the spherical smelted prills into flat strips before cutting lengths for beads (Denbow & Miller, 2007). Most beads had indications of cold-work in the form of slip bands and grain deformation near their joints, see Figure 6. 40: indicating that annealing (probably during hot working) was a deliberate part of the fabrication process (Denbow & Miller, 2007). The bead illustrated below shows an etched longitudinal section of 7% tin bronze (B 92) and an open join with preferential corrosion attack in cold worked areas (Denbow & Miller, 2007).
Figure 6.40 The micrograph shows etched longitudinal section of a bronze bead showing open join with preferential corrosion attack in cold-worked areas (Denbow & Miller, 2007:
286) (Magnification 7.2x).
The example of a Middle Iron Age wire-wound bronze bracelet from Mapungubwe, seen below; Figure 6. 43 had a composition of 6% tin in copper (Miller, 2002). It was reported that there were no other detectable elements present. The alloy had homogeneous microstructures with angular, recrystallized, single-phase copper grains with annealing twins. The structure resulted from hot working and annealing to form a sheet, from which
strips were cut from one side only. Figure 6. 41 shows that the strips were wound around a fibrous core (Miller, 2002). The micrograph shows a section of a polished longitudinal section showing typical trapezoidal cross-sections of the strip, wound with the pinched edges on the inside. The core is well preserved. The manufacture of the bronze wire- wound bracelets did not differ from the copper examples examined throughout the Iron Age (Miller, 2002: 92)
Figure 6.41 A micrograph of a polished longitudinal section through a bronze wire-wound bracelet (M 1175) showing trapezoidal cross-section of the strips wound with the pinched edges on the inside (Miller, 2001: 91) (Magnification 15x).
Brass is not an indigenous African product, although it was remelted by metal workers and made into a number of ornamental forms, sparse attention has been paid to this alloy (Maggs & Miller, 1995, Denbow & Miller 2007, Thondhlana & Martinón-Torres, 2009). An assessment of a brass mesh strip from the surface levels (B 432) at Bosutswe is dated to the Later Iron Age. A metallographic study indicated that the fragment of mesh wire consisted of 35 % zinc weight (Denbow & Miller 2007). Figure 6. 42 shows a micrograph of brass showing recrystallized and heavy worked grain structures (Scott, 1991). The sample examined by Denbow & Miller (2007) showed fine, uniform, single- phase wire with equiaxed recrystallizing grains and annealing twins. The wire had been hot-worked, drawn, and then annealed before being woven into a complex regular mesh.
The example was considered to be a foreign import, as there is no record of indigenous zinc mining in southern Africa (Miller 2002).
Figure 6.42 A micrograph of a cross-section of brass showing recrystallized and heavy worked grain structures (Scott, 1991: 97).
6.5.4: ARCHAEOMETALLURGICAL RECONSTRUCTIONS OF TECHNIQUES USING