51148 S5003129

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_1B a boat la a furnace by a fIow of gas of controlled composltlon. Experimental smelting and reaults

(a> Copper

The flrst experiments on anclent copper smelting may be traced back to 1894 when Cushing (1894) reported on the operatlon of a prlmltlve furnace based on archaeologlcal evldence from the Salado Valley, Arizona. The furnace remalns under investigation were attributed to the Pueblo Indiana (possibly AD 1300-1400). The copper ore was flrst roasted ln a separata, open flre. The smelting emperiment used a shallow plt or bowl furnace with natural draft; lt was started by bullding a large charcoal flre over the clay-lined plt. The roasted ore was slowly added. The resulting Products were copper prills or 'buttons' mixed with slag. The smelted copper was then remelted into ingots for further working.

In 1912 Gowland reported the use of a similar 'hole-in-the-ground* furnace constructed ln a laboratory at the Royal School of Mines, London (Gowland 1912, 242) (Fig. 1). The purpose of his experiments was to prove that copper ore and tin ore could be smelted together to produce a bronze. Gowland claimed success on every attempt. During an experiment, 7 kg copper ore (30* copper), 4.5 kg tin ore (20% tin) and 3.4 kg limestone were charged with 4.5 kg charcoal. The furnace was blown with a single 2.5 cm diameter tuyere, but the volume of air was not specified. Nothing is sald about the air source. Aft er a short period of preheat, the mixed ores and flux were charged in layers with the charcoal. The operation of the furnace was presumably based on Gowland's (1899) observations of copper smelting in Japan and Korea. The quantity or quality of the recovered bronze was not reported. Too few details were published for these experiments to be of much scientlfic value today.

During 1939 and 1940 Coghlan reported on several experiments using a charcoal fired 'bonfire' or bowl furnace with natural draft. The flrst experlments demonstrated that an open flre alone was unsuitable for smelting copper. By building a smali kiln-like or covered crucible chamber within the charcoal Coghlan (1939, 1939/1940) was then able to produce smali beads of copper from crushed malachite. This led him to believe copper smelting may have been flrst discovered in ceramic kilns. Apparently the optimum products from smelting attempted in a ceramic klin are only copper prills embedded ln slag.

Furtber experimentation with copper smelting simulations using chal-copyrite was reported by Bohne (1968). The multiple-step process under lnvestlgation was based on archaeologlcal evidence from Austria. Work was also conducted on a large scalę with a shaft furnace having slag tapping capabllities. The furnace measured about 20 cm in diamter, 1 m high and was bullt from clay and Stones. It operated with one tuyere, an iron pipę 25 mm ln diameter placed through the tap hole. An industrlal blower with a flow regulator was used to provide the blast. According to Bohne (1968, 50). modem analyses of furnace temperatures, airflow rates, and top gas composltlon were avoided for authenticity, in order to somehow force lntultlve Solutions of problems encountered during the smelting experiments. The chalcopyrite ore, from Mitterberg, was flrst roasted and the sulphur concentration was reduced by roastlng from 16.17% to 6.65%. There were many unsuccessful attempts to operate the shaft furnace. The roasted copper ore was mlxed with various amounts of limestone and fluorlte flux in the empirlcal approach to slag compositlon. Charcoal was the fuel. The shaft furnace was used only for the produciion of mat te. Bo slag was tapped. Even ln the furnace, the matte and slag dld noc aeparata suff1-ciently to form dlstinct layers aa expacted. The matte/slag alitirt needed to be removed from the furnace, crushed and haad-aorted. The matte was agaln roasted, then reduced ln another furnace. Thls aecond, bowl, furnace was slmply a clay-llned plt or hearth fliled with charcoal. lt measured 50 cm ln diameter and was 20 cm deep. It also used the iron tuyere. The finał products of thls smelting step were copper prills embedded ln slag. The metal prills were about 95 to 96% copper, accom-panled with Iron as the impurity. Chemical analysls of the slag gave 33.28% slllca, 33.38% ferrous oxlde, 0.621 copper and a tracę of selphur. lt was suggested that ingots could be produced by remeltlng large quantltles of copper prills. Further reflnlng sieps would be necessary for the smelted copper, called 'Schwarzkupfer*.

An excellent serles of twenty copper smelting experlments. based om archaeologlcal and ethnographlc evldence from che Transvaal ln Southern Afrlca, was conducted by Frlede and Steel (1975). Three types of furnaces were tested, startlng with a tali furnace bullt of modern flrebrlck. lt measured 18 cm ln diameter and about 42 cm in height. The dimensions and operation were based on ethnographlc evldence from the Kaonde tribe of Zimbabwe (Chaplin 1961). A single tuyere was positloned through the flrst course of flrebrlck, approximately 5 cm above the hearth. Inltially an air compressor was used, blowing 60 to 120 lltres/min. A pair of bag-bellows, constructed after Żulu examples, were found to dellver only 60 to 90 lltres/min worked at aaximua speed. The measured airflow was quite less than an estimate based on bag volumes; five lltres at 70 double strokes/min eąualled 350 lltres/min. The Iow efficiency was attributed to 'poor craftsmanship* and *inexperlenced men worklng the bellows* (Frlede & Steel 1975, 226). The diameter of the tuyere was varied from 2.5 to 4.0 cm internal diameter, but the effects were not reported. To begia a smelting simulation, the hearth was covered with wood ash, sticks and dry leaves. Charcoal, screened to sizes between 10 and 20 mm, was added and the airblast started. A thermocouple placed through the furnace top was used to measure internal temperatures. Halachite ore containlng 17.5% copper was layered with charcoal. The Phalaborwa malachite was consldered to be self-fluxing. No input weights are glven. The blast was increased untll the bed reached 800 to 900*C. Horę charcoal was added. After three hours of firing the blast was increased to raise the bed temperaturę to about 1000*C. Then the blast was stopped and the furnace allowed to cool. To remove the smelting products the furnace top had to be broken down.

'The conditlons of the smelting procedurę was varied ln a number of experiments, but in each case only a smali quantity of reduced copper, in the form of tiny globules and of thin irregular-shaped copper layers coatlng the surface of the slag, were obtained' (Frlede k Steel 1975, 224). The furnace needed to be rebuilt for each experiment.

The copper prills collected from the experiments were remelted in a smali crucible furnace. Thls furnace was a simple design, based on archaeologlcal descriptions by Wagner and Gordon (1929) and ethnographlc evidence recorded by Chaplin (1961) and Stayte (1931). A shalIow clay-llned plt 20 cm in diameter was bullt, surrounded with Stones. Two tuyeres were used with the compressor or bellows. The crucible was about 8 cm in diameter. It was filled with the smelted copper prills and dry leaves, then covered with a ceramic lid. The crucible was supported by four vertlcally placed stones and positloned in the charcoal in front of the tuyere. A temperaturę of 1100 to 1200*C was maintained for about 30 minutes. Many problems were encountered ln removing the crucible