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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.^[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 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.^[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 prove 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:

K_{v}={\frac {CaO+MgO+FeO+MnO+AlK_{2}O}{Si_{2}O_{3}+Al_{2}O_{3}}}\,

where $K_{v}$ 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, the 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 the 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, the 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 the 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 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]

References

^ ^a ^b ^c Bachmann H. G.The Identification of slags from archaeological sites Institute of Archaeology, London, 1982.
^ Maldonado B. and Rehren T. "Early copper smelting at Itziparátzico, Mexico" in Journal of Archaeological Science 2009 vol 36.
^ 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.
^ Moore J. J. Chemical Metallurgy Butterworth-Heinemann, Oxford. Second edition, 1990 p152.
^ Craddock P. T. Early metal mining and production Edinburgh University Press, Edinburgh 1995.
^ "Archaeometallurgy" in Centre for Archaeology Guidelines [Brochure]. English Heritage, Wiltshire, 2001.
^ 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.
^ ^a ^b ^c ^d ^e ^f Hauptmann A. The archaeo-metallurgy of copper: evidence from Faynan, Jordan Springer, New York, 2007.
^ ^a ^b 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
^ 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.
^ The lower the Kv is, the higher the viscosity.
^ Raghunath, Sreekanth (April 2007). High-temperature viscosity measurements in slags (Thesis ed.). Brisbane, Australia: The University of Queensland.
^ 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. S2CID 95072612.
^ Donaldson C. H. "An experimental investigation of olivine morphology" in Contributions to mineralogy and petrology vol 57 p187–195, 1976.
^ 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.
^ 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.
^ Haustein M. et al "Dating archaeometallurgical slags using thermoluminescence" in Archaeometry 2003, 45:3 p519-530.

[Bachmann-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.

[craddock-5] Craddock P. T. Early metal mining and production Edinburgh University Press, Edinburgh 1995.

[archaeo-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.

[Hauptmann-8] ^ ^a ^b ^c ^d ^e ^f Hauptmann A. The archaeo-metallurgy of copper: evidence from Faynan, Jordan Springer, New York, 2007.

[Craddock-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. S2CID 95072612.

[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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@@ Line 1: / Line 1: @@
+{{Short description|Artefact of ancient iron production}}
-{{copy edit|date=January 2017}}
+{{Iron Age|expand=Related topics}}
-'''Ancient iron production''' refers to [[History of ferrous metallurgy|iron working]] in times from [[prehistory]] to the early [[middle ages]] where knowledge of the relevant technology is derived from [[Archaeology|archaeological]] investigation. [[Slag]], the byproduct of iron-working processes such as [[smelting]] or [[Smith (metalwork)|smithing]], is left at the iron-working site rather than being moved away as a product. It also [[weathering|weathers]] well and hence it is readily available for study. The size, shape, chemical composition and [[microstructure]] of slag are determined by the iron-working process used at the time of its formation.
+[[File:HedemueMilitaria.jpg|thumb|right|alt=Archaeometallurgical slag is slag discovered and studied in the context of archaeology]]
+'''Archaeometallurgical slag''' is [[slag]] discovered and studied in the context of [[archaeology]]. Slag, the byproduct of iron-working processes such as [[smelting]] or [[Smith (metalwork)|smithing]], is left at the iron-working site rather than being moved away with the product. As it [[weathering|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 ==
 {{see also|Bloomery}}
-The [[ore]]s used in ancient smelting processes were rarely pure metal compounds. Impurities were removed through the process of [[slag]]ging. [[Slag]] is the material in which the impurities from ores, [[gangue]], [[furnace]] lining and charcoal ash collect. Study of [[slag]] can reveal information about the smelting process used at the time of its formation.<ref name="Bachmann">Bachmann H. G.''The Identification of slags from archaeological sites'' Institute of Archaeology, London, 1982.</ref>
+The [[ore]]s used in ancient smelting processes were rarely pure metal compounds. Impurities were removed from the ore through the process of [[slag]]ging, which involves adding heat and chemicals. [[Slag]] is the material in which the impurities from ores (known as [[gangue]]), as well as [[Metallurgical furnace|furnace]] lining and charcoal ash, collect. The study of [[slag]] can reveal information about the smelting process used at the time of its formation.<ref name="Bachmann">Bachmann H. G.''The Identification of slags from archaeological sites'' Institute of Archaeology, London, 1982.</ref>
-Slag is also direct evidence of ancient smelting taking place in the location where it is found. Through slag analysis, archaeologists can reconstruct ancient human activities concerned with metal work such as its organization and specialization.(Maldonado et al. 2009).<ref>Maldonado B. and Rehren T. ''Early copper smelting at Itziparátzico, Mexico'' in ''Journal of Archaeological Science'' 36, 2009.</ref>
+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.<ref>Maldonado B. and Rehren T. "Early copper smelting at Itziparátzico, Mexico" in ''Journal of Archaeological Science'' 2009 vol 36.</ref>
-The contemporary knowledge of [[slag]]ging 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.<ref>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</ref>
+The contemporary knowledge of [[slag]]ging 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.<ref>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.</ref>
-[[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 (manufacture)|melt]] from contamination. The second is to accept unwanted liquid and solid components. Finally, [[slag]] can help to control the supply of refining media to the melt.
+[[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 (manufacturing)|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, lower [[specific gravity|density]] and higher [[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.<ref>Moore J. J. ''Chemical Metallurgy'' Butterworth-Heinemann Ltd, Oxford. Second edition 1990 p152.</ref>
+These functions are achieved if the slag has a low melting temperature, low [[specific gravity|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.<ref>Moore J. J. ''Chemical Metallurgy'' Butterworth-Heinemann, Oxford. Second edition, 1990 p152.</ref>
-Through chemical and mineralogical analysis of slag, factors such as the identity of smelted metal, the types of ore used and technical parameters such as working temperature, gas atmosphere and slag [[viscosity]] can be learned.
+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 during the smelting process.<ref name="craddock">Craddock P. T. ''Early metal mining and production'' Edinburgh University Press, Edinburgh 1995.</ref> 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 be in liquid form.
+Natural iron ores are mixtures of iron and unwanted impurities, or ''[[gangue]]''. In ancient times, these impurities were removed by [[slag]]ging.<ref name="craddock">Craddock P. T. ''Early metal mining and production'' Edinburgh University Press, Edinburgh 1995.</ref> 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 [[furnace]]s, for example, the [[bloomery]] and the [[blast furnace]]. The condition in a furnace could determine the [[morphology (materials science)|morphology]], [[chemical composition]] and the microstructure of the slag.
+Smelting was conducted in various types of [[Metallurgical furnace|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.
-In the [[bloomery]] furnace, iron in a solid state was produced. This is because the [[bloomery]] process was conducted at a temperature higher than that at which the pure [[iron oxide]] could be reduced to iron metal, but lower than the [[melting point]] of iron metal.
+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 furnace]]s 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.
-Another process again was used to make tapped slag. Here, [[charcoal]] only was added to the furnace. It reacted with [[oxygen]], and generated [[carbon monoxide]], which reduced the iron ore to iron metal. The liquefied slag was separated from the ore, and was removed through the [[tapping arch]] of the furnace wall.<ref name="archaeo">"Archaeometallurgy" in ''Centre for Archaeology Guidelines'' [Brochure]. English Heritage, Wiltshire, 2001.</ref>
+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.<ref name="archaeo">"Archaeometallurgy" in ''Centre for Archaeology Guidelines'' [Brochure]. English Heritage, Wiltshire, 2001.</ref>
+In addition, the [[flux (metallurgy)|flux]] (purifying agent), the charcoal [[wood ash|ash]] and the furnace lining contributed to the composition of the slag.
-[[Slag]] morphology can also be determined by the method of removing the slag from the furnace. For instance, [[slag]] might be left inside the furnace rather than being tapped.
+Slag may also form during [[smithing]] and [[refining]]. The product of the bloomery process is [[Heterogeneity|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 furnace|open hearth]], the carbon is oxidized and removed from the iron. Liquid slag is formed and removed in this process.
-In addition, the [[flux (metallurgy)]] (purifying agent), the charcoal [[wood ash|ash]] and the furnace lining can all determine the composition of the slag.
-Slag may also form during [[smithing]] and [[refining]]. The product of the [[bloomer]] process is [[Heterogeneity|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 furnace|open hearth]], the carbon is oxidized and removed from the iron. Liquid slag is formed and removed in this process.
 == Slag analysis ==
@@ Line 39: / Line 38: @@
 === 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.<ref>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.</ref>
-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.
+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.
 === Bulk chemical analysis ===
 The chemical composition of slag can reveal much about the smelting process. [[X-ray fluorescence|XRF]] is the most commonly used tool in analysing the chemical composition of slag.<ref name="Hauptmann">Hauptmann A. ''The archaeo-metallurgy of copper: evidence from Faynan, Jordan'' Springer, New York, 2007.</ref> 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-SiO<sub>2</sub>-FeO-Al<sub>2</sub>O<sub>3</sub> simplified to CaO-SiO<sub>2</sub>-FeO<sub>2</sub>, giving a low and uniform melting point.<ref name="Hauptmann" /> {{rp|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.<ref name="Craddock">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</ref>
+Ancient [[slag]] composition is usually a quaternary [[eutectic]] system CaO-SiO<sub>2</sub>-FeO-Al<sub>2</sub>O<sub>3</sub> simplified to CaO-SiO<sub>2</sub>-FeO<sub>2</sub>, giving a low and uniform melting point.<ref name="Hauptmann" /> {{rp|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.<ref name="Craddock">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</ref>
 The melting temperature of slag can be determined by plotting its chemical composition in a [[ternary plot]].<ref>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.</ref>
 The viscosity of slag can be calculated through its chemical composition with equation:
+:<math>K_{v} = \frac{CaO + MgO + FeO + MnO + AlK_{2}O}{Si_{2}O_{3} + Al_{2}O_{3} } \,</math>
-:Kv=CaO+MgO+FeO+MnO+Alk<sub>2</sub>O/Si<sub>2</sub>O<sub>3</sub>+Al<sub>2</sub>O<sub>3</sub> where Kv is the index of viscosity.<ref>The lower the Kv is, the higher the viscosity.</ref>
+where <math>K_{v}</math> is the index of viscosity.<ref>The lower the Kv is, the higher the viscosity.</ref>
+With recent advances in rotational viscometry techniques, viscosities of iron oxide slags are also widely undertaken.<ref>{{cite book |last1=Raghunath |first1=Sreekanth |title=High-temperature viscosity measurements in slags |date=April 2007 |publisher=The University of Queensland |location=Brisbane, Australia |edition=Thesis |url=https://espace.library.uq.edu.au/view/UQ:158592}}</ref><ref>{{cite journal |last1=Chen |first1=Mao |last2=Raghunath |first2=Sreekanth |last3=Zhao |first3=Baojun |title=Viscosity Measurements of "FeO"-SiO2 Slag in Equilibrium with Metallic Fe |journal=Metallurgical and Materials Transactions B |date=June 2013 |volume=44 |issue=3 |pages=506–515 |doi=10.1007/s11663-013-9810-3 |s2cid=95072612 }}</ref> 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.<ref name="Craddock"/> 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.<ref name="Hauptmann" />{{rp|24}}
+In the early stages of smelting, the separation between melting metal and slag is not complete.<ref name="Craddock"/> 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.<ref name="Hauptmann" />{{rp|24}}
 === Mineralogical analysis ===
-The [[optical microscope]], [[scanning electron microscope]], [[X-ray diffraction]] and [[petrography|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.
+The [[optical microscope]], [[scanning electron microscope]], [[X-ray diffraction]] and [[petrography|petrographic analysis]] can be used to determine the types and distribution of minerals in slag. The minerals present in the 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 [[silicate]]s, [[oxide]]s and [[sulphide]]s. Bachmann classified the main [[silicate]]s in slag according to the ratio between metal [[oxide]]s and [[silica]].<ref name="Bachmann"/><ref name="Hauptmann"/>{{rp|171}}
+Slag minerals are classified as [[silicate]]s, [[oxide]]s and [[sulfide]]s. Bachmann classified the main [[silicate]]s in slag according to the ratio between metal [[oxide]]s and [[silica]].<ref name="Bachmann"/><ref name="Hauptmann"/>{{rp|171}}
-:::::Ratio  MeO : SiO<sub>2</sub>  silicate examples
+:::::Ratio MeO : SiO<sub>2</sub> silicate examples
-:::::::2  :  1     [[fayalite]]
+:::::::2 : 1 [[fayalite]]
-:::::::2  :  1     [[monticellite]]
+:::::::2 : 1 [[monticellite]]
-:::::::1.5 :  1     [[melilite]]
+:::::::1.5 : 1 [[melilite]]
-:::::::1  :  1     [[pyroxene]]
+:::::::1 : 1 [[pyroxene]]
-[[Fayalite]] (Fe<sub>2</sub>SiO<sub>4</sub>) 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.<ref>Donaldson C. H. "An experimental investigation of olivine morphology" in ''Contributions to mineralogy and petrology'' vol 57 p187–195, 1976.</ref><ref>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.</ref>
+[[Fayalite]] (Fe<sub>2</sub>SiO<sub>4</sub>) is the most common mineral found in ancient slag. By studying the shape of the fayalite, the cooling rates of the slag can be roughly estimated.<ref>Donaldson C. H. "An experimental investigation of olivine morphology" in ''Contributions to mineralogy and petrology'' vol 57 p187–195, 1976.</ref><ref>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.</ref>
 [[Fayalite]] reacts with [[oxygen]] to form [[magnetite]]:
@@ Line 75: / Line 77: @@
 :3Fe<sub>2</sub>SiO<sub>4</sub> + O<sub>2</sub>= 2FeO·Fe<sub>2</sub>O<sub>3</sub> + 3SiO<sub>2</sub>
 Therefore, the gas atmosphere in the furnace can be calculated from the ratio of [[magnetite]] to [[fayalite]] in the slag.<ref name="Hauptmann" />{{rp|22}}
-The presence of metal [[sulphide]]s suggests that a sulphidic ore has been used. Metal [[sulphide]]s survive the oxidizing stage before smelting and therefore may also indicate a multi-stage smelting process.
+The presence of metal [[sulfide]]s suggests that a sulfidic ore has been used. Metal [[sulfide]]s 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.<ref name="Bachmann"/>
 === Lead isotope analysis ===
-[[Isotope geochemistry| 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.<ref>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.</ref>
+[[Isotope geochemistry|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 the smelting process.<ref>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.</ref>
-The amount of each of the four stable [[isotope]]s of [[lead]] are used in the analysis. They are <sup>204</sup>Pb, <sup>206</sup>Pb, <sup>207</sup>Pb and <sup>208</sup>Pb. Ratios: <sup>208</sup>Pb/<sup>207</sup>Pb, <sup>207</sup>Pb/<sup>206</sup>Pb and <sup>206</sup>Pb/<sup>204</sup>Pb are measured by mass spectrometry. Apart from <sup>204</sup>Pb, the [[lead]] [[isotope]]s 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 [[Geology|geological periods]] will have different [[lead]] [[isotope]] signatures.
+The amount of each of the four stable [[isotope]]s of [[lead]] are used in the analysis. They are <sup>204</sup>Pb, <sup>206</sup>Pb, <sup>207</sup>Pb and <sup>208</sup>Pb. Ratios: <sup>208</sup>Pb/<sup>207</sup>Pb, <sup>207</sup>Pb/<sup>206</sup>Pb and <sup>206</sup>Pb/<sup>204</sup>Pb are measured by mass spectrometry. Apart from <sup>204</sup>Pb, the [[lead]] [[isotope]]s are all products of the [[radioactive decay]] of [[uranium]] and [[thorium]]. When ore is deposited, [[uranium]] and [[thorium]] are separated from the ore. Thus, deposits formed in different [[Geology|geological periods]] will have different [[lead]] [[isotope]] signatures.
 :::::::<sup>238</sup>U →<sup>206</sup>Pb
@@ Line 90: / Line 92: @@
 :::::::<sup>232</sup>Th→<sup>208</sup>Pb
-For example, Hauptmann performed [[Isotope geochemistry|lead isotope analysis]] on slags from Faynan, [[Jordan]]. The resulting signature was the same as that from ores from the [[Dolostone|Dolomite limestone shale]] deposits in the Wadi Khalid and Wadi Dana areas of [[Jordan]].<ref name="Hauptmann" />{{rp|79}}
+For example, Hauptmann performed [[Isotope geochemistry|lead isotope analysis]] on slags from Faynan, [[Jordan]]. The resulting signature was the same as that from ores from the [[Dolomite (rock)|dolomite]], [[limestone]] and [[shale]] deposits in the Wadi Khalid and Wadi Dana areas of [[Jordan]].<ref name="Hauptmann" />{{rp|79}}
 === Physical dating ===
-Ancient [[slag]] is difficult to date. It has no organic material with which to perform [[radiocarbon dating]]. There are no cultural artefacts 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.<ref>Haustein M. et al "Dating archaeometallurgical slags using thermoluminescence" in ''Archaeometry'' 2003, 45:3 p519-530.</ref>
+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.<ref>Haustein M. et al "Dating archaeometallurgical slags using thermoluminescence" in ''Archaeometry'' 2003, 45:3 p519-530.</ref>
-<! Belongs elsewhere
-== Early copper smelting slag ==
-At dawn of copper smelting, the shape and composition of copper smelting slag vary considerably from modern impression of slag. The traditional way to separate slag of non-ferrous metallurgy as smelting slag and crucible slag is not applicable anymore. Charges are only partially decomposed and the whole slag has never been fully molten. It can not be tapped out of furnace and would solidified in it as furnace slag. People have to crush it into pieces to recover metal prills embedded in it. Moreover, gas atmosphere in early furnaces is much less reducing and fayalite may not be able to form in this condition. The whole texture of slag could be quite inhomogeneous and less vitrified (Craddock 1995, 126-127).<ref>Craddock, P., 1995, ''Early Metal Mining and Production'', Edinburgh: Edinburgh University Press Ltd</ref>
-These slags are usually nut-size and easily overlooked by excavator yet are the most crucial indicator and evidence of early copper smelting activities. Hauptmann (2007, 157)<ref name="Hauptmann" /> listed several cases of this finding. For example, slag granules from [[Catal Huyuk]] ([[Anatolia]]) are usually thought to be the first evidence of copper ore smelting.-->
-== Conclusion ==
-Slag analysis can always be a useful tool for archaeologists in reconstructing ancient metal production process and detecting ancient human activities in this process. Primarily, through macro analysis, slag should be catalogued to facilitate sample selection and production scale may also be estimated. Then, chemical and mineralogical composition of slag can be investigated through micro analysis. Various parameters of metal production process can be estimated. Measuring of lead isotope would link slag with both ores and metal objects. Physical dating is quite useful in determining the precise age of slag. Finally, early copper smelting slag is always worth more attention though it may look quite different from normal slags.
 == See also ==
 * [[Archaeometallurgy]]
 * [[Bog iron]]
+* [[Iron metallurgy in Africa]]
+* [[Iron Age]]
 == References ==

Iron Age
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Related topics Iron Age metallurgy Archaeometallurgical slag
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