Archaeometallurgical slag is slag discovered and studied in the context of archaeology. 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. As it weathers well, 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.
Overview
The ores used in ancient smelting processes were rarely pure metal compounds. Impurities were removed from the ore through the process of slagging, which involves adding heat and chemicals. 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.
Slag formation
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.
The bloomery furnace produced iron in a solid state. This is because the bloomery process was conducted at a temperature lower than the melting point of iron metal. Carbon monoxide from the incomplete combustion of charcoal slowly diffused through the hot iron oxide ore, converting it to iron metal and carbon dioxide.
Blast furnaces were used to produce liquid iron. The blast furnace was operated at higher temperatures and at a greater reducing condition than the bloomery furnace. A greater reducing environment was achieved by increasing the fuel to ore ratio. More carbon reacted with the ore and produced a cast iron rather than solid iron. Also, the slag produced was less rich in iron.
A different process was used to make "tapped" slag. Here, only charcoal was added to the furnace. It reacted with oxygen, and generated carbon monoxide, which reduced the iron ore to iron metal. The liquefied slag separated from the ore, and was removed through the tapping arch of the furnace wall.
In addition, the flux (purifying agent), the charcoal ash and the furnace lining contributed to the composition of the slag.
Slag may also form during smithing and refining. The product of the bloomery process is heterogeneous blooms of entrapped slag. Smithing is necessary to cut up and remove the trapped slag by reheating, softening the slag and then squeezing it out. On the other hand, refining is needed for the cast iron produced in the blast furnace. By re-melting the cast iron in an open hearth, the carbon is oxidized and removed from the iron. Liquid slag is formed and removed in this process.