Archaeometallurgy-slag analysis-non-ferrous metallurgy
Encyclopedia
Archaeometallurgy is the study of the history and prehistory of metals and their use through humans. Slag analysis is a technique used in archaeometallurgy.
is the necessary material to collect impurities from ores, gangue
, the furnace
lining and charcoal ash, which can yield numerous aspects of information about smelting process (Bachmann 1982). Slag is also direct evidence of ancient smelting activities. Through slag analysis, archaeologist can reconstruct ancient human activities concerned with metal and try to discuss the organization and specialization in metal production (Maldonado et al 2009).
In modern view, slag should have three functions. The first is to protect the melt from contamination. The second is to accept unwanted liquid and solid components. Finally, it can help to control the supply of refining media to the melt. In order to approach these objectives, a good slag should have lower melting temperature, lower gravity and better fluidity
to ensure a liquid slag which can be separated well with metal melting. Moreover, slag should maintain the correct composition so that it can collect more impurities and be immiscible in the melt (Moore1990, 152). Yet this is not always the case for ancient slag. In the investigating of ancient metallurgy the intention and constraints of ancient people should be carefully considered.
There are only several kinds of non-ferrous metal
that have ever been used by ancient people. Slag analysis is usually applied in the research about smelting and refining processes of copper, tin, lead and zinc. Slag can be created in many steps of metal production. Based on the mechanism of creation, slags can be catalogued as furnace slag, tapping slag, crucible
slag, etc. In a smelting furnace, as many as four different phases could co-exist. From top to bottom, they are slag, matte, speiss, and liquid metal (Thornton 2009). The separation between these phases would not be perfect and therefore, three other phases can usually be observed in the slag if they ever existed. Through chemical and mineralogical analysis of the slag, things can be learnt like the identity of metal smelted, kinds of ore used and technical parameters such as working temperature, gas atmosphere, slag viscosity
, etc.
, 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.
is needed to get the right propertion (Craddock 1989). Plotting result of chemical analysis in the ternary phase diagram, the melting temperature of slag can be learnt. This is a quite typical procedure in slag analysis and has nearly been used in every case of this investigation. For example, Chiarantini et al's (2009) analyzed the copper smelting slags from Populonia, southern Tuscany and estimated the melting point of these slags were about 1125-1250°C. Another quite important parater, viscosity of slag, can also be calculated through its chemical composition with equation:
Additionally, composition of slag can determine the metal smelted and provide information about ores. In early stage of smelting, separation between melting metal and slag is usually not complete. Thus, the smelted non-ferrous metal would usually be rich in the slag created in this process. For example, copper slag often contain more than 0.1% copper in it. For slag rich in lead and sometimes tin with over 100ppm of silver, it might be considered as the waste of silver production (Craddock 1989). The main, minor and trace elements of slag can be indicators about type of the ore used in smelting as well(Hauptmann 2007, 24). For example, presence of sulfur usually suggests that sulphidic ores had been used (Bachmann 1982,16)
Fayalite (Fe2SiO4) is the most common mineral in ancient slag. Through the shape of fayalite, cooling rates of the slag can be roughly estimated (Donaldson 1976, cited in Etller et al 2009). In addition, fayalite can react with oxygen to form magnetite:
Therefore, the gas atmosphere in furnace can be calculated through the ratio of magnetite/fayalite in slag (Hauptmann 2007, 22). Metal sulphides usually suggest that sulphidic ores has been used. These metal sulphides survived from oxidizing stage before smelting and therefore may also indicate a multi-stage smelting process. Pure metals are always hoped to be removed from slag by smelter yet this process is never complete. Left metal prills can tell archaeologists what kind of metal was extracted from ore.
Mineral composition can also be used to estimate the chemical composition primarily. For example, monticellite
and pyroxene
would form when fayalite can not take more CaO into it. They could be the indicator of relatively high calcium content (Bachmann 1982).
Moreover, in the early stage of metallurgy, the original charges of furnace may not decompose completely and therefore some component of ores or flux would remain in slag. These material can be discerned by petrographic analysis and help to identify ores and fluxing agent used in the process. Hauptmann (2007, 171) has ever investigated copper slag from Faynan, Jordan and interpreted quartz inclusions in the slag as a charge coming from a quartz-rich host rock.
analysis is an useful technique to determine the ore source of ancient smelting activities. Lead isotope composition is the signature of certain metal ores deposit and varies very little throughout the whole deposit. It has also proved that lead isotope composition will pass unchanged through smelting process (Stos-Gale 1989). Therefore, the metal rich non-ferrous slag is usually proper material for this analysis. On one hand, it can provide information about origin of ores used in the smelting site and facilitate the research about production organization. On the other hand, it can play a significant role in tracing raw material source of certain metal artefact.
The content of four stable isotopes of lead are usually used in this analysis. They are 204Pb, 206Pb, 207Pb and 208Pb. Ratios: 208Pb/207Pb, 207Pb/206Pb and 206Pb/204Pb are measured through mass spectrometry and widely employed to express the character of certain ore deposit. Except 204Pb, other lead isotopes are products of radioactive decay of uranium
and thorium
. During deposit forming process, uranium and thorium would be separated from ores. Thus, deposits formed in different geological period would have various lead isotope signature.
Hauptmann (2007, 79) analysed lead isotopic composition of slags from Faynan, Jordan and the result revealed the same signature as the ores from the Dolomite-Limestone-Shale Units in Wadi Khalid and Wadi Dana.
material like charcoal stratigraphic associated with slags. Direct physical dating of slag through thermoluminescence dating could be a good method to solve this problem. Thermoluminescence dating
(TL dating), which is often used in investigation about pottery age, is actually measuring the time since last heating event of the sample. Agencies which can emit luminescence signals are crystals like quartz and feldspar.
Slag has good potential for TL dating because it forms through high temperature process and usually contains crystals that can emit luminescence signal. However, the complex composition of slag always affect the quality of TL measurement. Haustein (2003) suggested that if quartz separated from out from slag was used as dating material, the problem could be solved.
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) 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.
Overview
Ores used in ancient smelting process is rarely pure metal compounds and the worthless material has to be removed through slagging. SlagSlag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...
is the necessary material to collect impurities from ores, gangue
Gangue
In mining, gangue is the commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit. The separation of mineral from gangue is known as mineral processing, mineral dressing or ore dressing and it is a necessary and often significant aspect of mining...
, the furnace
Furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
lining and charcoal ash, which can yield numerous aspects of information about smelting process (Bachmann 1982). Slag is also direct evidence of ancient smelting activities. Through slag analysis, archaeologist can reconstruct ancient human activities concerned with metal and try to discuss the organization and specialization in metal production (Maldonado et al 2009).
In modern view, slag should have three functions. The first is to protect the melt from contamination. The second is to accept unwanted liquid and solid components. Finally, it can help to control the supply of refining media to the melt. In order to approach these objectives, a good slag should have lower melting temperature, lower gravity and better fluidity
Fluidity
Fluidity may refer toIn science*reciprocal of viscosity*Cognitive fluidity*Membrane fluidity*Sexual fluidityOthers*Fluidity *Dark Fluidity – a literature magazine*Empire Fane ship...
to ensure a liquid slag which can be separated well with metal melting. Moreover, slag should maintain the correct composition so that it can collect more impurities and be immiscible in the melt (Moore1990, 152). Yet this is not always the case for ancient slag. In the investigating of ancient metallurgy the intention and constraints of ancient people should be carefully considered.
There are only several kinds of non-ferrous metal
Non-ferrous metal
In metallurgy, a non-ferrous metal is a metal that is not ferrous, that is, any metal, including alloys, that does not contain iron in appreciable amounts...
that have ever been used by ancient people. Slag analysis is usually applied in the research about smelting and refining processes of copper, tin, lead and zinc. Slag can be created in many steps of metal production. Based on the mechanism of creation, slags can be catalogued as furnace slag, tapping slag, crucible
Crucible
A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...
slag, etc. In a smelting furnace, as many as four different phases could co-exist. From top to bottom, they are slag, matte, speiss, and liquid metal (Thornton 2009). The separation between these phases would not be perfect and therefore, three other phases can usually be observed in the slag if they ever existed. Through chemical and mineralogical analysis of the slag, things can be learnt like the identity of metal smelted, kinds of ore used and technical parameters such as working temperature, gas atmosphere, slag viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, etc.
Slag analysis
Analysis of slag is mainly based on its shape, texture, isotopic signature, chemical and mineralogical characters. Analytical tools like Optical MicroscopeOptical microscope
The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the...
, scanning electron microscope (SEM
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
), X-ray Fluorescence (XRF), X-ray diffraction (XRD) and inductively coupled plasma-mass spectrometry (ICP-MS
ICP-MS
Inductively coupled plasma mass spectrometry is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 1012...
) are widely employed in the study of slag.
Macro analysis
The first step in the investigation of archaeometallurgical slag is usually the identification and macro analysis of slag in the field. Physical properties of slag such as shape, colour, porosity and even smell can be used to identify slags in archaeological site and catalogue them primarily. For example, tap slag usually has wrinkling upper face and flat lower face which contacted with soil (Tumiati 2005). This primary catalogue can be quite useful in designing sampling strategy which should make sure that samples collected for micro analysis can represent the character and variety of slag heaps. In addition, total weight of slag heaps could also be estimated through macro analysis and then the production scale might be determined.Bulk chemical analysis
Chemical composition of slag can reveal a lot of crucial information about smelting process and XRF is the most common tool used in this study. Hauptmann (2007, 20) stated that through the composition of slag, four smelting parameters could be detected. They are the composition of charge, the firing temperature, the gas atmosphere and the reaction kinetics. In order to obtain low melting slag, most ancient slag composition is consistent with the eutectics parts of quaternary system CaO-SiO2-FeO-Al2O3. In most cases this system can be simplified to CaO-SiO2-FeO2 (Hauptmann 2007, 21). In some areas, proportion of silicates to metal oxides in gangue with ore and furnace lining is consistent with this eutectics parts and form slag by itself. Otherwise, fluxFlux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
is needed to get the right propertion (Craddock 1989). Plotting result of chemical analysis in the ternary phase diagram, the melting temperature of slag can be learnt. This is a quite typical procedure in slag analysis and has nearly been used in every case of this investigation. For example, Chiarantini et al's (2009) analyzed the copper smelting slags from Populonia, southern Tuscany and estimated the melting point of these slags were about 1125-1250°C. Another quite important parater, viscosity of slag, can also be calculated through its chemical composition with equation:
- Kv=CaO+MgO+FeO+MnO+Alk2O/Si2O3+Al2O3 where Kv is the index of viscosity.
Additionally, composition of slag can determine the metal smelted and provide information about ores. In early stage of smelting, separation between melting metal and slag is usually not complete. Thus, the smelted non-ferrous metal would usually be rich in the slag created in this process. For example, copper slag often contain more than 0.1% copper in it. For slag rich in lead and sometimes tin with over 100ppm of silver, it might be considered as the waste of silver production (Craddock 1989). The main, minor and trace elements of slag can be indicators about type of the ore used in smelting as well(Hauptmann 2007, 24). For example, presence of sulfur usually suggests that sulphidic ores had been used (Bachmann 1982,16)
Mineralogical analysis
Optical microscope, SEM, and XRD can tell us the identity and distribution of minerals in slag. Minerals present in slag are often good indicators of gas atmosphere in furnace, cooling rate of slag and homogeneity of slag. If there is un-decomposed charge trapped in slag, more information about ore and flux can be learnt. Slag minerals are generally catalogued as silicates, oxides and sulphides. Moreover, metal pills are always concerned in this study as well. Bachmann (1982) classified main silicates in slag according to the ration between metal oxides and silica.-
-
-
-
- Ratio MeO : SiO2 silicate examples
-
- 2 : 1 fayalite
- 2 : 1 monticellite
- 1.5 : 1 melilites
- 1 : 1 pyroxene
-
- Ratio MeO : SiO2 silicate examples
-
-
-
Fayalite (Fe2SiO4) is the most common mineral in ancient slag. Through the shape of fayalite, cooling rates of the slag can be roughly estimated (Donaldson 1976, cited in Etller et al 2009). In addition, fayalite can react with oxygen to form magnetite:
- 3Fe2SiO4 + O2= 2FeO·Fe2O3 + 3SiO2
Therefore, the gas atmosphere in furnace can be calculated through the ratio of magnetite/fayalite in slag (Hauptmann 2007, 22). Metal sulphides usually suggest that sulphidic ores has been used. These metal sulphides survived from oxidizing stage before smelting and therefore may also indicate a multi-stage smelting process. Pure metals are always hoped to be removed from slag by smelter yet this process is never complete. Left metal prills can tell archaeologists what kind of metal was extracted from ore.
Mineral composition can also be used to estimate the chemical composition primarily. For example, monticellite
Monticellite
Monticellite and kirschsteinite are gray silicate minerals of the olivine group with compositions CaMgSiO4 and CaFeSiO4, respectively. Most monticellites have the pure magnesium end-member composition but rare ferroan monticellites and magnesio-kirschsteinite are found with between 30 and 75 mol.%...
and pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
would form when fayalite can not take more CaO into it. They could be the indicator of relatively high calcium content (Bachmann 1982).
Moreover, in the early stage of metallurgy, the original charges of furnace may not decompose completely and therefore some component of ores or flux would remain in slag. These material can be discerned by petrographic analysis and help to identify ores and fluxing agent used in the process. Hauptmann (2007, 171) has ever investigated copper slag from Faynan, Jordan and interpreted quartz inclusions in the slag as a charge coming from a quartz-rich host rock.
Lead isotope analysis
Lead isotopeIsotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
analysis is an useful technique to determine the ore source of ancient smelting activities. Lead isotope composition is the signature of certain metal ores deposit and varies very little throughout the whole deposit. It has also proved that lead isotope composition will pass unchanged through smelting process (Stos-Gale 1989). Therefore, the metal rich non-ferrous slag is usually proper material for this analysis. On one hand, it can provide information about origin of ores used in the smelting site and facilitate the research about production organization. On the other hand, it can play a significant role in tracing raw material source of certain metal artefact.
The content of four stable isotopes of lead are usually used in this analysis. They are 204Pb, 206Pb, 207Pb and 208Pb. Ratios: 208Pb/207Pb, 207Pb/206Pb and 206Pb/204Pb are measured through mass spectrometry and widely employed to express the character of certain ore deposit. Except 204Pb, other lead isotopes are products of radioactive decay of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
. During deposit forming process, uranium and thorium would be separated from ores. Thus, deposits formed in different geological period would have various lead isotope signature.
-
-
-
-
-
-
- 238U →206Pb
- 235U →207Pb
- 232Th→208Pb
-
-
-
-
-
Hauptmann (2007, 79) analysed lead isotopic composition of slags from Faynan, Jordan and the result revealed the same signature as the ores from the Dolomite-Limestone-Shale Units in Wadi Khalid and Wadi Dana.
Physical dating
Date of slag is usually thought to be a complex problem for there is few chronological indicators like pottery or radiocarbon datingRadiocarbon dating
Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...
material like charcoal stratigraphic associated with slags. Direct physical dating of slag through thermoluminescence dating could be a good method to solve this problem. Thermoluminescence dating
Thermoluminescence dating
Thermoluminescence dating is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated or exposed to sunlight...
(TL dating), which is often used in investigation about pottery age, is actually measuring the time since last heating event of the sample. Agencies which can emit luminescence signals are crystals like quartz and feldspar.
Slag has good potential for TL dating because it forms through high temperature process and usually contains crystals that can emit luminescence signal. However, the complex composition of slag always affect the quality of TL measurement. Haustein (2003) suggested that if quartz separated from out from slag was used as dating material, the problem could be solved.
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).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) listed several cases of this finding. For example, slag granules from Catal Huyuk (Anatolia
Anatolia
Anatolia is a geographic and historical term denoting the westernmost protrusion of Asia, comprising the majority of the Republic of Turkey...
) are usually thought to be the first evidence of copper ore smelting.
Conclusion
Slag analysis can always be an useful tool for archaeologist 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
- archaeometallurgyArchaeometallurgyArchaeometallurgy is the study of the history and prehistory of metals and their use through humans. It is a sub-discipline of archaeology and archaeological science...
- Archaeometallurgy – slag analysis – ferrous metallurgy