Thermal aspects of the smelting of iron ore in reconstructed South African Iron Age furnaces

SYNOPSIS A study was made of the smelting process in two experimental furnaces patterned on Iron Age furnaces excavated at Melville Koppies Nature Reserve, Johannesburg. In a number of smelting experiments, the influences of air supply, temperature, time of operation, fuel consumption, and other operational factors were investigated. Metallurgical and metallographic aspects of the furnace operation and smelting products are discussed, and the experiments are compared with similar research work conducted in Europe and Africa. SAMEVATTING Daar is 'n studie gemaak van die smeltproses in twee eksperimentele oonde wat gebou is volgens die patroon van oonde uit die Ystertydperk wat in die Natuurreservaat Melville Koppies, Johannesburg, uitgegrawe is. Die invloed van die lugtoevoer, temperatuur, bewerktyd, brandstofverbruik en ander bedryfsaspekte is in 'n aantal smelteksperimente ondersoek. Die metallurgiese en metallografiese aspekte van die oondbedryf en smeltprodukte word bespreek, en die eksperimente word vergelyk met deeglike navorsingswerk wat in Europa en Afrika gedoen is. Introduction The work described in this paper is a continuation of the archaeo-metallurgical investigations undertaken over the past ten years at the Archaeological Research Unit of the University of the Witwatersrandl-3. One of the previous studies on the correlation of slag characteristics and operational furnace temperatures had led to the conclusion that further work in this field, based on smelting experiments, would be desirable3. The present paper reports on such experiments, and on the results obtained by temperature measurements in furnaces and by the construction of temperature profiles. Since slag is the principal, and often the only, preserved material from ancient smelting sites, various methods were investigated by which information on the thermal characteristics of the smelting process could be obtained from samples of slag: chemical/physical analysis, phase-diagram studies, determination of liquidus temperatures, and metallographic examinations. In addition, the problem of preheating ofthe air injected into such furnaces was investigated