Ancient iron production

Ancient iron production refers to iron working in times from prehistory to the early Middle Ages where knowledge of production processes is derived from archaeological investigation. Slag, the byproduct of iron-working processes such as smelting or smithing, is left at the iron-working site rather than being moved away with the product. It also weathers well and hence it is readily available for study. The size, shape, chemical composition and microstructure of slag are determined by features of the iron-working processes used at the time of its formation. Ancient iron production refers to iron working in times from prehistory to the early Middle Ages where knowledge of production processes is derived from archaeological investigation. Slag, the byproduct of iron-working processes such as smelting or smithing, is left at the iron-working site rather than being moved away with the product. It also weathers well and hence it is readily available for study. The size, shape, chemical composition and microstructure of slag are determined by features of the iron-working processes used at the time of its formation. The ores used in ancient smelting processes were rarely pure metal compounds. Impurities were removed from the ore through the process of slagging. Slag is the material in which the impurities from ores (known as gangue), as well as furnace lining and charcoal ash, collect. The study of slag can reveal information about the smelting process used at the time of its formation. The finding of slag is direct evidence of smelting having occurred in that place as slag was not removed from the smelting site. Through slag analysis, archaeologists can reconstruct ancient human activities concerned with metal work such as its organization and specialization. The contemporary knowledge of slagging gives insights into ancient iron production. In a smelting furnace, up to four different phases might co-exist. From the top of the furnace to the bottom, the phases are slag, matte, speiss, and liquid metal. Slag can be classified as furnace slag, tapping slag or crucible slag depending on the mechanism of production. The slag has three functions. The first is to protect the melt from contamination. The second is to accept unwanted liquid and solid impurities. Finally, slag can help to control the supply of refining media to the melt. These functions are achieved if the slag has a low melting temperature, low density and high viscosity which ensure a liquid slag that separates well from the melting metal. Slag should also maintain its correct composition so that it can collect more impurities and be immiscible in the melt. Through chemical and mineralogical analysis of slag, factors such as the identity of the smelted metal, the types of ore used and technical parameters such as working temperature, gas atmosphere and slag viscosity can be learned. Natural iron ores are mixtures of iron and unwanted impurities, or gangue. In ancient times, these impurities were removed by slagging. Slag was removed by liquation, that is, solid gangue was converted into a liquid slag. The temperature of the process was high enough for the slag to exist in its liquid form. Smelting was conducted in various types of furnaces. Examples are the bloomery furnace and the blast furnace. The condition in the furnace determines the morphology, chemical composition and the microstructure of the slag.