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.

Main article: Ferrous metallurgy § Iron smelting and the Iron Age

Overview
See also: Bloomery

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.[1]

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.[2]

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.[3]

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.[4]

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.[5] 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 higher than that required for reduction of pure iron oxide to iron metal, but a temperature lower than the melting point of iron metal.

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.[6]

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.

Slag analysis

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.

Macro-analysis

The first step in the investigation of archaeometallurgical slag is the identification and macro-analysis of slag in the field. Physical properties of slag such as shape, colour, porosity and even smell are used to make a primary classification to ensure representative samples from slag heaps are obtained for future micro-analysis.

For example, tap slag usually has a wrinkled upper face and a flat lower face due to contact with soil.[7]

furthermore, the macro-analysis of slag heaps can proved an estimated total weight which in turn can be used to determine the scale of production at a particular smelting location.

Bulk chemical analysis

The chemical composition of slag can reveal much about the smelting process. XRF is the most commonly used tool in analysing the chemical composition of slag.[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. In other instances, a flux was required to achieve the correct system.[9]

The melting temperature of slag can be determined by plotting its chemical composition in a ternary plot.[10]

The viscosity of slag can be calculated through its chemical composition with equation:

Kv=CaO+MgO+FeO+MnO+Alk2O/Si2O3+Al2O3 where Kv is the index of viscosity.[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.

In the early stages of smelting, separation between melting metal and slag is not complete.[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.[8]:24

Mineralogical analysis

The optical microscope, scanning electron microscope, X-ray diffraction and petrographic analysis can be used to determine the types and distribution of minerals in slag. The minerals present in slag are good indicators of the gas atmosphere in the furnace, the cooling rate of the slag and the homogeneity of the slag. 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.

Slag minerals are classified as silicates, oxides and sulfides. Bachmann classified the main silicates in slag according to the ratio between metal oxides and silica.[1][8]:171

Ratio MeO : SiO2 silicate examples
2 : 1 fayalite
2 : 1 monticellite
1.5 : 1 melilite
1 : 1 pyroxene

Fayalite (Fe2SiO4) is the most common mineral found in ancient slag. By studying the shape of the fayalite, cooling rates of the slag can be roughly estimated.[14][15]

Fayalite reacts with oxygen to form magnetite:

3Fe2SiO4 + O2= 2FeO·Fe2O3 + 3SiO2

Therefore, the gas atmosphere in the furnace can be calculated from the ratio of magnetite to fayalite in the slag.[8]:22

The presence of metal sulfides suggests that a sulfidic ore has been used. Metal sulfides survive the oxidizing stage before smelting and therefore may also indicate a multi-stage smelting process.

When fayalite is replete with CaO, monticellite and pyroxene form. They are an indicator of a high calcium content in the ore.[1]

Lead isotope analysis

Lead isotope analysis is a technique for determining the source of ore in ancient smelting. Lead isotope composition is a signature of ore deposits and varies very little throughout the whole deposit. Also, lead isotope composition is unchanged in smelting process.[16]

The amount of each of the four stable isotopes of lead are used in the analysis. They are 204Pb, 206Pb, 207Pb and 208Pb. Ratios: 208Pb/207Pb, 207Pb/206Pb and 206Pb/204Pb are measured by mass spectrometry. Apart from 204Pb, the lead isotopes are all products of the radioactive decay of uranium and thorium. When ore is are deposited, uranium and thorium are separated from the ore. Thus, deposits formed in different geological periods will have different lead isotope signatures.

238U →206Pb
235U →207Pb
232Th→208Pb

For example, Hauptmann performed lead isotope analysis on slags from Faynan, Jordan. The resulting signature was the same as that from ores from the dolomite, limestone and shale deposits in the Wadi Khalid and Wadi Dana areas of Jordan.[8]:79

Physical dating

Ancient slag is difficult to date. It has no organic material with which to perform radiocarbon dating. There are no cultural artifacts like pottery shards in the slag with which to date it. Direct physical dating of slag through thermoluminescence dating could be a good method to solve this problem. Thermoluminescence dating is possible if the slag contains crystal elements such as quartz or feldspar. However, the complex composition of slag can make this technique difficult unless the crystal elements can be isolated.[17]

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See also

References
  1. Bachmann H. G.The Identification of slags from archaeological sites Institute of Archaeology, London, 1982.
  2. Maldonado B. and Rehren T. "Early copper smelting at Itziparátzico, Mexico" in Journal of Archaeological Science 2009 vol 36.
  3. Thornton C. P. et al "The production of speiss (iron arsenide) during the Early Bronze in Iran" in Journal of Archaeological Science 2009, vol 36, p308-316.
  4. Moore J. J. Chemical Metallurgy Butterworth-Heinemann, Oxford. Second edition, 1990 p152.
  5. Craddock P. T. Early metal mining and production Edinburgh University Press, Edinburgh 1995.
  6. "Archaeometallurgy" in Centre for Archaeology Guidelines [Brochure]. English Heritage, Wiltshire, 2001.
  7. Tumiati S. et al "The ancient mine of Servette (Saint-Marcel, Cal d’Aosta, Western Italian Alps): a mineralogical, metallurgical and charcoal analysis of furnace slags" in Archaeometry, 2005 vol 47 p317 to 340.
  8. Hauptmann A. The archaeo-metallurgy of copper: evidence from Faynan, Jordan Springer, New York, 2007.
  9. Craddock P. "The Scientific investigation of early mining and smelting" in Henderson J. (Ed.) Scientific analysis in archaeology Oxford University Committee for Archaeology, Oxford, Institute of Archaeology, Los Angeles and the UCLA Institute of Archaeology. Distributed by Oxbow Books, 1989, p178-212
  10. Chiarantini L. et al "Copper production at Baratti (Populonia, southern Tuscany) in the early Etruscan period (9th–8th centuries BC)" in Journal of Archaeological Science vol 36 p1626-1636, 2009.
  11. The lower the Kv is, the higher the viscosity.
  12. Raghunath, Sreekanth (April 2007). High-temperature viscosity measurements in slags (Thesis ed.). Brisbane, Australia: The University of Queensland.
  13. Chen, Mao; Raghunath, Sreekanth; Zhao, Baojun (June 2013). "Viscosity Measurements of "FeO"-SiO2 Slag in Equilibrium with Metallic Fe". Metallurgical and Materials Transactions B. 44 (3): 506–515. doi:10.1007/s11663-013-9810-3.
  14. Donaldson C. H. "An experimental investigation of olivine morphology" in Contributions to mineralogy and petrology vol 57 p187–195, 1976.
  15. Ettler V. et al "Mineralogy of medieval slags from lead and silver smelting" in Towards estimation of historical smelting conditions in Archaeometry vol 51:6 p987-1007, 2009.
  16. Stos-Gale Z. ,A. "Lead isotope studies of metals and the metal trade in the Bronze Age Mediterranean" in Henderson J. (Ed.) Scientific Analysis in Archaeology Oxford University Committee for Archaeology, Institute of Archaeology, Los Angeles, UCLA Institute of Archaeology. 1989 p274-301. Distributed by Oxbow Books.
  17. Haustein M. et al "Dating archaeometallurgical slags using thermoluminescence" in Archaeometry 2003, 45:3 p519-530.
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