Archaeometallurgical slag
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.
Slag is the material in which the impurities from ores (known as gangue), as well as furnace lining and charcoal ash, collect.
Through slag analysis, archaeologists can reconstruct ancient human activities concerned with metal work such as its organization and specialization.
From the top of the furnace to the bottom, the phases are slag, matte, speiss, and liquid metal.
This is because the bloomery process was conducted at a temperature lower than the melting point of iron metal.
[6] In addition, the flux (purifying agent), the charcoal ash and the furnace lining contributed to the composition of the slag.
The analysis of slag is based on its shape, texture, isotopic signature, chemical and mineralogical characteristics.
Analytical tools like Optical Microscope, scanning electron microscope (SEM), X-ray Fluorescence (XRF), X-ray diffraction (XRD) and inductively coupled plasma-mass spectrometry (ICP-MS) are widely employed in the study of slag.
[7] Furthermore, the macro-analysis of slag heaps can prove an estimated total weight which in turn can be used to determine the scale of production at a particular smelting location.
[8] Through chemical analysis, the composition of the charge, the firing temperature, the gas atmosphere and the reaction kinetics can be determined.
Ancient slag composition is usually a quaternary eutectic system CaO-SiO2-FeO-Al2O3 simplified to CaO-SiO2-FeO2, giving a low and uniform melting point.
[8] : 21 In some circumstances, the eutectic system was created according to the proportion of silicates to metal oxides in the gangue, together with the type of ore and the furnace lining.
[11] With recent advances in rotational viscometry techniques, viscosities of iron oxide slags are also widely undertaken.
[12][13] Coupled with phase equilibria studies, these analysis provide a better understanding of physico-chemical behaviour of slags at high temperatures.
[9] Hence, the main, minor and trace elements of metal in the slag can be indicators of the type of ore used in the smelting process.
The type of ore and flux used in the smelting process can be determined if there are elements of un-decomposed charge or even metal pills trapped in the slag.
Bachmann classified the main silicates in slag according to the ratio between metal oxides and silica.
They are an indicator of a high calcium content in the ore.[1] Lead isotope analysis is a technique for determining the source of ore in ancient smelting.
However, the complex composition of slag can make this technique difficult unless the crystal elements can be isolated.
