Archaeometallurgical slag is

slag Slag is a by-product of smelting ( pyrometallurgical) ores and used metals. Broadly, it can be classified as ferrous (by-products of processing iron and steel), ferroalloy (by-product of ferroalloy production) or non-ferrous/base metals (by-pr ...

discovered and studied in the context of

archaeology Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landscape ...

. Slag, the byproduct of iron-working processes such as

smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a c ...

smithing A metalsmith or simply smith is a craftsperson fashioning useful items (for example, tools, kitchenware, tableware, jewelry, armor and weapons) out of various metals. Smithing is one of the oldest metalworking occupations. Shaping metal with a ...

, 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 Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...

of slag are determined by features of the iron-working processes used at the time of its formation.

Overview

The

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...

s used in ancient smelting processes were rarely pure metal compounds. Impurities were removed from the ore through the process of

ging, which involves adding heat and chemicals.

Slag Slag is a by-product of smelting ( pyrometallurgical) ores and used metals. Broadly, it can be classified as ferrous (by-products of processing iron and steel), ferroalloy (by-product of ferroalloy production) or non-ferrous/base metals (by-pr ...

is the material in which the impurities from ores (known as

gangue In mining, gangue () is the commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit. It is thus distinct from overburden, which is the waste rock or materials overlying an ore or mineral body t ...

), as well as furnace lining and charcoal ash, collect. The study of

can reveal information about the smelting process used at the time of its formation.Bachmann H. G.''The Identification of slags from archaeological sites'' Institute of Archaeology, London, 1982. 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

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.

can be classified as furnace slag, tapping slag or

crucible A crucible is a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures. While crucibles were historically usually made from clay, they can be made from any material that withstands t ...

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,

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 Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

and high

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inter ...

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 Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also applies ...

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

can be learned.

Slag formation

Natural iron ores are mixtures of iron and unwanted impurities, or ''

''. In ancient times, these impurities were removed by

ging.Craddock P. T. ''Early metal mining and production'' Edinburgh University Press, Edinburgh 1995. 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 A bloomery is a type of metallurgical furnace once used widely for smelting iron from its oxides. The bloomery was the earliest form of smelter capable of smelting iron. Bloomeries produce a porous mass of iron and slag called a ''bloom''. ...

furnace and the

blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric ...

. 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 The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...

of iron metal.

Carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

from the incomplete combustion of charcoal slowly diffused through the hot

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of which ...

ore, converting it to iron metal and carbon dioxide.

Blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric ...

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 Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...

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 Charcoal is a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) in minimal oxygen to remove all water and volatile constituents. In the traditional version of this pyrolysis process, cal ...

was added to the furnace. It reacted with

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well ...

, and generated

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

, 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."Archaeometallurgy" in ''Centre for Archaeology Guidelines'' rochure English Heritage, Wiltshire, 2001. In addition, the

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...

(purifying agent), the charcoal

ash Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non-gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...

and the furnace lining contributed to the composition of the slag. Slag may also form during

and

refining {{Unreferenced, date=December 2009 Refining (also perhaps called by the mathematical term affining) is the process of purification of a (1) substance or a (2) form. The term is usually used of a natural resource that is almost in a usable form, ...

. The product of the bloomery process is

heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, size, ...

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 An open-hearth furnace or open hearth furnace is any of several kinds of industrial furnace in which excess carbon and other impurities are burnt out of pig iron to produce steel. Because steel is difficult to manufacture owing to its high me ...

, 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 The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsc ...

, scanning electron microscope (

SEM SEM can refer to: Computing * Search engine marketing, promoting websites by increasing their visibility in search engine results * Security event manager, a security log tool used on data networks Economics and management * Stock Exchang ...

), X-ray Fluorescence ( XRF), X-ray diffraction ( XRD) and inductively coupled plasma-mass spectrometry (

ICP-MS Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It is ...

) 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. 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.Hauptmann A. ''The archaeo-metallurgy of copper: evidence from Faynan, Jordan'' Springer, New York, 2007. Through chemical analysis, the composition of the charge, the firing temperature, the gas atmosphere and the reaction kinetics can be determined. Ancient

composition is usually a quaternary eutectic system CaO-SiO₂-FeO-Al₂O₃ simplified to CaO-SiO₂-FeO₂, giving a low and uniform melting point. In some circumstances, the eutectic system was created according to the proportion of silicates to metal oxides in the

, together with the type of ore and the furnace lining. In other instances, a

was required to achieve the correct system.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 The melting temperature of slag can be determined by plotting its chemical composition in a

ternary plot A ternary plot, ternary graph, triangle plot, simplex plot, Gibbs triangle or de Finetti diagram is a barycentric plot on three variables which sum to a constant. It graphically depicts the ratios of the three variables as positions in an equi ...

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

K_ = \frac \,

where

K_

is the index of viscosity. With recent advances in rotational viscometry techniques, viscosities of iron oxide slags are also widely undertaken. 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. 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.

Mineralogical analysis

The

optical microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsc ...

, scanning electron microscope,

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angle ...

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,

oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the ...

s and

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...

s. Bachmann classified the main silicates in slag according to the ratio between metal

s and

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

. :::::Ratio MeO : SiO₂ silicate examples :::::::2 : 1

fayalite Fayalite (, commonly abbreviated to Fa) is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system (space group ''Pbnm'') with cell param ...

:::::::2 : 1

monticellite Monticellite and kirschsteinite (commonly also spelled kirschteinite Klein and Hurlbut ''Manual of Mineralogy'' 20th ed., p. 373) are gray silicate minerals of the olivine group with compositions Ca Mg Si O4 and Ca FeSiO4, respectively. Most mo ...

:::::::1.5 : 1

melilite Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is ( Ca, Na)2( Al, Mg, F ...

:::::::1 : 1

pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

Fayalite Fayalite (, commonly abbreviated to Fa) is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system (space group ''Pbnm'') with cell param ...

(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.

reacts with

to form

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...

: :3Fe₂SiO₄ + O₂= 2FeO·Fe₂O₃ + 3SiO₂ Therefore, the gas atmosphere in the furnace can be calculated from the ratio of

in the slag. The presence of metal

s suggests that a sulfidic ore has been used. Metal

s survive the oxidizing stage before smelting and therefore may also indicate a multi-stage smelting process. When

is replete with CaO,

and

form. They are an indicator of a high

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

content in the ore.

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. The amount of each of the four stable

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass numbers ...

s of

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, l ...

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

s are all products of the

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly ...

and thorium. When ore is deposited,

and thorium are separated from the ore. Thus, deposits formed in different

geological periods The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochron ...

will have different

signatures. :::::::²³⁸U →²⁰⁶Pb :::::::²³⁵U →²⁰⁷Pb :::::::²³²Th→²⁰⁸Pb For example, Hauptmann performed lead isotope analysis on slags from Faynan,

Jordan Jordan ( ar, الأردن; tr. ' ), officially the Hashemite Kingdom of Jordan,; tr. ' is a country in Western Asia. It is situated at the crossroads of Asia, Africa, and Europe, within the Levant region, on the East Bank of the Jordan Rive ...

. The resulting signature was the same as that from ores from the

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms when ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especially ...

deposits in the Wadi Khalid and Wadi Dana areas of

Physical dating

Ancient

is difficult to date. It has no organic material with which to perform

radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was dev ...

. There are no cultural artifacts like pottery shards in the slag with which to date it. Direct physical dating of slag through

thermoluminescence Thermoluminescence is a form of luminescence that is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon hea ...

dating could be a good method to solve this problem.

Thermoluminescence Thermoluminescence is a form of luminescence that is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon hea ...

dating is possible if the slag contains crystal elements such as

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical fo ...

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspa ...

. However, the complex composition of slag can make this technique difficult unless the crystal elements can be isolated.Haustein M. et al "Dating archaeometallurgical slags using thermoluminescence" in ''Archaeometry'' 2003, 45:3 p519-530.

References

{{DEFAULTSORT:Archaeometallurgy-Slag Analysis-Non-Ferrous Metallurgy Archaeometallurgy History of metallurgy