Browsing by Author "Miller, Duncan"
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- ItemOpen AccessCasting platinum jewellery alloys: the effects of casting variables on fill and porosity(2005) Miller, Duncan; Keraan, Tauriq; Park-Ross, Penny; Husemeyer, Victoria; Brey, Ali; Khan, Irshad; Lang, CandyComparisons are made between platinum-copper and platinum-ruthenium alloys used for jewellery to evaluate the effects of casting variables. The effects of flask temperatures, investments, and centrifugal speeds on microstructure, percentage fill, and porosity were examined over a range of temperatures. Optimum conditions and materials for successful casting of high quality platinum jewellery alloys, using a Hot Platinum induction melting and casting machine, are described. Suitable choice of investment materials and rotational speeds produced good grid fills with Pt-5%Cu and Pt-5%Ru alloys. Metal porosity was more difficult to control, due to the inherently chaotic nature of the casting process, but casting into a relatively cool mould minimised the probability of bad porosity for both alloys. Pt-5%Ru was found to be successful as a casting alloy when used with induction melting technology. It displayed superior uniformity, hardness and colour, compared with cast Pt-5%Cu alloy.
- ItemOpen AccessIndigenous metal melting and casting in Southern Africa(South African Archaeological Society, 2010) Miller, DuncanMetal casting involves melting a metal charge in a crucible, and pouring it into a mould with a predetermined shape. This is not generally thought of as an important aspect of metal working in pre-European southern Africa, but it played a role in the second millennium AD for producing ingots, blanks for rings and bangles, and rare objects of probable ritual significance. Casting was restricted to the non-ferrous metals, like copper and tin and their alloys, because indigenous bloomery iron technology could not produce large quantities of molten iron for casting. Asfar as we know, gold was not cast into moulds, although beads were fashioned by punching holes through spherical globules produced by melting. This paper presents metallographic and chemical analyses of the various products of metal casting, and summarizes what is known about indigenous casting technology in southern Africa.
- ItemOpen AccessIron age metal working at the Tsodilo Hills, Northwestern Botswana(1992) Miller, DuncanThis study documents the metal working technology employed at two major Iron Age archaeological sites in southern Africa. The research involved the description and analysis of two large metal working assemblages with a total of 2922 metal artefacts, fragments of ore, and slag, from the sites of Divuyu (6th 8th century AD) and Nqoma (7th - 10th century AD, with a later 17th - 19th century AD occupation) in the Tsodilo Hills, northwestern Botswana. This is the first systematic description and metallographic analysis of a large collection of Early Iron Age metal artefacts from southern Africa. The artefacts were small, mainly delicate items of copper and iron jewellery, and tools possibly used in their manufacture. They were classified, described, and sampled selectively for metallographic, petrographic, and chemical analysis. Seventy artefacts were studied in detail, from which the fabrication technology employed at these sites was reconstructed. During the Early Iron Age forging, and probably also smelting, of iron took place at both sites. The smelting products were inhomogeneous iron and steel, with typical fayalitic slag, characteristic of indigenous bloomery iron production. The forging was done in an oxidising hearth and the technique used was poor, with no deliberate control over carbon content, the mechanical properties of the steel, or heat treatment other than annealing. Fabrication involved hammering square wire and flat sheets, which were cut into strips for beads, clips, chains, and fibre-cored wound ornaments. Numerous finger rings were made from crude round iron wire. Copper was worked in the same way, generally leaving the metal in its annealed state. Significant chemical variation in the copper artefacts and iron slag inclusions indicated that diverse ore sources were involved. There were stylistic similarities between individual artefacts from the Tsodilo Hills and Early Iron Age material from the Upemba Depression in Zaire, as well as with a copper chain from Broederstroom in the Transvaal. Comparison of the fabrication technology with Later Iron Age material suggested that local indigenous iron and copper working technology has changed little since its introduction in southern Africa.
- ItemOpen AccessOrder hardening of platinum alloys(1999) Towle, Nicholas Richard; Miller, Duncan; Lang, CandyThe hardening behaviour of three cold-worked platinum alloys, Pt 5 at% Mo, Pt 5 wt% Ru and Pt 5 wt% Cu, has been investigated through a systematic series of heat treatments. All three of the experimental alloys showed a hardness increase during annealing within a specific temperature range. The hardness of the Pt-Mo and Pt-Ru alloys was found to increase rapidly at annealing temperatures above the recrystallisation temperature, with the final hardness similar to the original coldworked hardness. The hardness of Pt-Cu showed an increase of up to 30% at low annealing temperatures of between 200°C and 500°C. In addition, the Pt-Cu alloy also showed the increased hardness found in Pt-Mo and Pt-Ru at high annealing temperatures, but the hardness increase was not to the same extent. Specimens subjected to the annealing treatments were studied by means of optical, scanning electron and transmission electron microscopy, in order to determine the effect of annealing on microstructure and structural order. Resistivity, XRD and OTA techniques were employed in order to study the mechanisms of ordering with temperature, but these techniques did not produce any significant results. It was concluded that the most likely cause for the hardness increase observed in all three experimental alloys was due to a change in structural order upon annealing. The Pt-Mo and Pt-Ru alloys hardened through an increase in short-range order at annealing temperatures above the recrystallisation temperature. The Pt-Cu alloy hardened through the development of long-range order on annealing between 200°C and 500°C. This increase in hardness was in· addition to the high dislocation density in the alloy specimen due to prior cold-work.
- ItemOpen AccessTechnological, social and economic aspects of gold production and use by the iron age people of Southern Africa(2001) Desai, Nirdev; Miller, DuncanThis dissertation addresses technological, social and economIC aspects of gold production and use in the Late Iron Age of southern Africa. The topic is approached in two ways. The first is to define the fabrication technology employed in producing gold artefacts. The second is to use trace element fingerprinting to try to determine which geological deposits were exploited by gold miners of this period. Three assemblages exist that allow these questions to be addressed; Mapungubwe (1 ath - 13th century AD), Great Zimbabwe (12th - 15th century AD) and Thulamela (l4th - 17th century AD). Previous descriptions of the fabrication technology of southern African gold exist, but this is the first, systematic study of all three assemblages. The fabrication technology reconstruction used three lines of analysis; visual inspection with the naked eye, microscopy of the surfaces, and microhardness testing and metallography of selected polished samples. Fifty eight specimens were studied from Mapungubwe, two hundred and sixty eight pieces from Great Zimbabwe and fifteen from Thulamela. Trade and socio-economic effects of southern Africa's Later Iron Age are discussed in the light of the now available trace element analysis and fabrication technology of the gold artefacts studied here. No tools for working gold have been found, and inferences have been made by modelling them on tools for copper and iron working. The basic toolkit consisted of a blade, hammer, chisel, a punch and an anvil. There were four basic artefact types; wire, beads, foil and tacks. There was no significant stylistic change in artefact types and the number of artefact types in the three assemblages. Cold working and annealing were standard practices in fabrication. Ten finished artefacts types have been identified; wrapped, rolled and punched beads, foil, strips cut from foil, tacks, straight and coiled wire, rod sections and links. Other gold artefacts were recovered but were either offeuts or in the process of being made into one of the ten types described above. These are prills, discs and offcuts. Trace element groups were based on grouping the samples by similarities in the signature profiles. Identification was on the basis of the presence and absence of metallic impurities. It was deduced that alluvial gold mining was practised alongside reef gold mining. Mixing of gold ores occurred. Alloying was not intentionally practised. Identification of the gold sources would require further analysis of unworked material from potential geological sources.