Frequently Asked Questions
What is slag?
Slag is a waste product from metalworking processes. In particular during the smelting process the unusable parts of the ore combine with either added flux, or material from the furnace lining, or some of the iron from the ore to produce a free flowing material which can be removed leaving the metal. Slag is also produced in forging (e.g. smithing hearth bottoms) and a wide range of other metallurgical processes.
Is slag the only metalworking debris?
In addition to true slags a wide range of debris may be present including fragments of the furnace/hearth lining, fuel, fuel ash, ores in various states of processing, hammerscale, metallic prills, spills and other fragments, crucible sherds, part finished artefacts and stock material such as bar ends as well as rarer debris from the purification and assaying of precious metals. One of the most characteristic indicators of smithing is hammerscale (see below) Better preserved sites may include the bases of hearths, furnaces, ore roasting pits (smelting),quenching pits (smithing), not to mention the structures that protected the working area.
Should I wash slag?
Most forms of slag are very robust and will not be harmed by washing. In fact it makes them easier to identify and cuts down of the weight of material for archiving. However other debris such as hearth lining may not be fully fired and vigorous scrubbing may damage it. More fragile still are fragments of (unfired) clay or loam moulds, Crucible should also be treated with care to avoid loss of any metallic residues. On balance, yes please, but proceed with care!
Should I save all debris from a site?
Ideally yes, at least until a specialist has seen it. However, where very large quantities of debris are present sampling may be unavoidable. In this case it is advisable to set up a site reference collection and to estimate the quantities (note whether by weight or volume) of material saved compared with that excavated. Also, where features are not full excavated an estimate of the likely proportion recovered. Knowing the amount of debris on site is likely to be the major indicator of the scale and importance of the metalworking site.
See McDonnell & Starley (2002) Excavation and Sampling Strategies for metalworking Sites
http://hist-met.org/hmsdatasheet09.pdf
What is hammerscale?
The forging of iron produces micro slag of two types Flake hammerscale consists of fish-scale like fragments of the oxide/silicate skin of the iron dislodged during working. Spheroidal hammerscale results from the solidification of small droplets of liquid slag expelled during hot working, particularly when two objects are being fire-welded together or when the slag-rich bloom of iron is first worked into a billet or bar. Hammerscale is considered important in interpreting a site not only because is highly diagnostic of smithing but, because it tends to build up in the immediate vicinity of the smithing hearth and anvil, it may give a more precise location of the activity than the bulk slags which may be transported elsewhere for disposal
See David Starley (1995) Hammerscale http://hist-met.org/hmsdatasheet10.pdf
How do I find hammerscale?
Hammerscale contains magnetite to the extent that when found in loose soil it can be attracted onto a bar magnet (keep one handy on site and check around possible hearths). However in heavy clays this may work less well. The presence of hammerscale may also be detected during post excavation processing. Commonly it is identified within the residues from wet sieving environmental samples (Most sinks but spheroidal hammerscale may float due to entrapped air). Hammerscale may also be identified adhering to unwashed artefacts and slag, or entrapped within the concretion around iron artefact to be revealed when the object is X-radiographed. When ironsmithing has been identified, or suspected, by the field worker 'industrial' soil samples should be saved for examination. These not only allow the presence to be determined, but for it to be quantified also.
See David Starley (1995) Hammerscale http://hist-met.org/hmsdatasheet10.pdf
Is Physico-Chemical Analysis always needed to identify the process from which metalworking debris derives?
No, much debris, particularly that from ferrous metallurgy is visually diagnostic. However, qualitative analysis would be recommended to identify more obscure processes or to determine the alloys used in non-ferrous metal working, such identification is likely to be carried out using a relatively rapid technique such as XRF (X-ray fluorescence) analysis on the surfaces of the debris.
Where metalworking is an important activity on the site and a more detailed picture of the processes undertaken on site is required, an assessment of the material might recommend a programme of quantitative analysis of prepared samples. Data from this may help to answer a range of questions about the source of the materials, the type of fuel used and the efficiency of the processes.
See Mortimer (1995) Chemical Analysis of Metalwork and Metalworking Debris
http://hist-met.org/hmsdatasheet12.pdf
What is metallography?
Metallography is the examination of prepared metal samples under the optical microscope. This long established technique provides information on the alloys used, the smiths’ techniques and skill and the quality of the product. The drawbacks of metallography are the time-consuming nature of the examination, but also the requirement for ‘destructive’ removal of a sample. Such damaging intervention must be balanced against the likelihood that any heavily corroded object with an iron core will continue to deteriorate in store. Under such circumstances a sound justification can be made for selective sampling and examination to retrieve information that may well otherwise be lost.
See Starley (1995) Metallographic Examination http://hist-met.org/hmsdatasheet11.pdf
When should an archaeometallurgical specialist become involved in a project?
As soon as the presence of significant quantities of debris, or any structural evidence becomes evident. Earlier if desk –based assessment suggests the site may include metallurgical features
See Starley (2002) Metalworking Evidence and the Management of Archaeological Sites http://hist-met.org/hmsdatasheet16.pdf
Which geophysics techniques are best for locating metalworking sites?
The most powerful techniques are magnetometry and magnetic susceptibility; beware of plots which have been filtered to remove the strongest ‘modern’ anomalies. These may be just what we’re looking for
See Geophysical Techniques Applied to Early metalworking Sites (McDonnell 1995) http://hist-met.org/hmsdatasheet04.pdf
Does slag always remain in the vicinity of the metalworking site?
As mentioned above some forms of debris, such as hammerscale may build up in the immediate vicinity of the hearth and anvil, whereas bulk slags may be removed off-site, particularly to be used as hardcore or track metalling. When blast furnaces came into use (from the end of the C15th in the UK), old ‘bloomery’ slag heaps were often re-used instead of, or in addition to, ore. Slag was not always seen as an entirely useless waste product. Here are a few other uses I’ve come across:
In ceramics, as temper in Iron Age pottery and grit in Roman mortaria
As building material, either cast into blocks, used as wall copings or, decoratively as in the grotto at Chiswick House, or in historical times as an ingredient of ‘metallic stucco’
Modern blast furnace slag is pulverised and used as fertiliser
As a colorant in Saxon glass
As a refractory material for lining new hearths
As ships’ ballast
As a material of ritual importance. In Nigerian slag shrines and probably in British Iron Age sites
How does one know whether a furnace or hearth has been used for oxidizing or reducing processes?
Knowing the redox conditions of any hearth or furnace is important in determining its function. Take careful note of any changes in colouration in the heat affected zones.
Are you available for guest lectures?
Yes, though some of those given recently, such as the following concern my work at the Royal Armouries and will require their permission:
The Knight and the Laboratory; The Greatest Discoveries of the Royal Armouries Science Lab
Saxon Weapons: Grave Misconceptions
Protect and Survive: The Scientific Investigation of Historic Armour and Weapons
Swords, The Cutting Edge of Technology
Arrowheads: Taking a Closer Look
The Scientific Study of Japanese Arms and Armour
More general talks on the wider aspects of metalworking in the past and studying its evidence can be tailored to any audience, from local interest groups to university modules at U/G or Masters level.
I am available to provide hands -on training workshops including an introduction to the examination and identification of metalworking debris. I can also provide an introduction to the technique of metallography.
Can you recommend an introductory text on archaeometallurgy?
A good introduction, from the field archaeologists’ point of view is provided by English Heritage’s Centre for Archaeology Guidelines; ‘Archaeometallurgy’(2001) although it should be noted that the ‘where to get help’ listings are in need of updating.
http://www.english-heritage.org.uk/upload/pdf/cfa_archaeometallurgy2.pdf
This is heavily based on the sound, but less glossy, individual data sheets produced by the Historical Metallurgy Society:
http://hist-met.org/datasheets.html
For more recent industrial remains English Heritage have produced a follow –up volume ‘Science for Historic Industries; Guidelines for the Investigation of 17th to 19th-Century Industries’
http://www.helm.org.uk/upload/pdf/Science-Historic-Industries.pdf?1255614467
Unfortunately few good up to date text books provide a reliable overview, but it is good to see new reprints of R F Tylecote’s (1992) 2nd edition A History of Metallurgy and his (1986) The Prehistory of Metallurgy in the British Isles.
V F Buchwald (2005) ‘Iron and Steel in Ancient Times’ is also highly recommended for the serious scholar with a ferrous bias, including much on Scandinavian material
P T Craddock (1995) ‘Early Metal Mining and Production’, covers much ground, though the early and the exotic are covered in more detail than most of the more commonly encountered processes from UK sites.
Slag is a waste product from metalworking processes. In particular during the smelting process the unusable parts of the ore combine with either added flux, or material from the furnace lining, or some of the iron from the ore to produce a free flowing material which can be removed leaving the metal. Slag is also produced in forging (e.g. smithing hearth bottoms) and a wide range of other metallurgical processes.
Is slag the only metalworking debris?
In addition to true slags a wide range of debris may be present including fragments of the furnace/hearth lining, fuel, fuel ash, ores in various states of processing, hammerscale, metallic prills, spills and other fragments, crucible sherds, part finished artefacts and stock material such as bar ends as well as rarer debris from the purification and assaying of precious metals. One of the most characteristic indicators of smithing is hammerscale (see below) Better preserved sites may include the bases of hearths, furnaces, ore roasting pits (smelting),quenching pits (smithing), not to mention the structures that protected the working area.
Should I wash slag?
Most forms of slag are very robust and will not be harmed by washing. In fact it makes them easier to identify and cuts down of the weight of material for archiving. However other debris such as hearth lining may not be fully fired and vigorous scrubbing may damage it. More fragile still are fragments of (unfired) clay or loam moulds, Crucible should also be treated with care to avoid loss of any metallic residues. On balance, yes please, but proceed with care!
Should I save all debris from a site?
Ideally yes, at least until a specialist has seen it. However, where very large quantities of debris are present sampling may be unavoidable. In this case it is advisable to set up a site reference collection and to estimate the quantities (note whether by weight or volume) of material saved compared with that excavated. Also, where features are not full excavated an estimate of the likely proportion recovered. Knowing the amount of debris on site is likely to be the major indicator of the scale and importance of the metalworking site.
See McDonnell & Starley (2002) Excavation and Sampling Strategies for metalworking Sites
http://hist-met.org/hmsdatasheet09.pdf
What is hammerscale?
The forging of iron produces micro slag of two types Flake hammerscale consists of fish-scale like fragments of the oxide/silicate skin of the iron dislodged during working. Spheroidal hammerscale results from the solidification of small droplets of liquid slag expelled during hot working, particularly when two objects are being fire-welded together or when the slag-rich bloom of iron is first worked into a billet or bar. Hammerscale is considered important in interpreting a site not only because is highly diagnostic of smithing but, because it tends to build up in the immediate vicinity of the smithing hearth and anvil, it may give a more precise location of the activity than the bulk slags which may be transported elsewhere for disposal
See David Starley (1995) Hammerscale http://hist-met.org/hmsdatasheet10.pdf
How do I find hammerscale?
Hammerscale contains magnetite to the extent that when found in loose soil it can be attracted onto a bar magnet (keep one handy on site and check around possible hearths). However in heavy clays this may work less well. The presence of hammerscale may also be detected during post excavation processing. Commonly it is identified within the residues from wet sieving environmental samples (Most sinks but spheroidal hammerscale may float due to entrapped air). Hammerscale may also be identified adhering to unwashed artefacts and slag, or entrapped within the concretion around iron artefact to be revealed when the object is X-radiographed. When ironsmithing has been identified, or suspected, by the field worker 'industrial' soil samples should be saved for examination. These not only allow the presence to be determined, but for it to be quantified also.
See David Starley (1995) Hammerscale http://hist-met.org/hmsdatasheet10.pdf
Is Physico-Chemical Analysis always needed to identify the process from which metalworking debris derives?
No, much debris, particularly that from ferrous metallurgy is visually diagnostic. However, qualitative analysis would be recommended to identify more obscure processes or to determine the alloys used in non-ferrous metal working, such identification is likely to be carried out using a relatively rapid technique such as XRF (X-ray fluorescence) analysis on the surfaces of the debris.
Where metalworking is an important activity on the site and a more detailed picture of the processes undertaken on site is required, an assessment of the material might recommend a programme of quantitative analysis of prepared samples. Data from this may help to answer a range of questions about the source of the materials, the type of fuel used and the efficiency of the processes.
See Mortimer (1995) Chemical Analysis of Metalwork and Metalworking Debris
http://hist-met.org/hmsdatasheet12.pdf
What is metallography?
Metallography is the examination of prepared metal samples under the optical microscope. This long established technique provides information on the alloys used, the smiths’ techniques and skill and the quality of the product. The drawbacks of metallography are the time-consuming nature of the examination, but also the requirement for ‘destructive’ removal of a sample. Such damaging intervention must be balanced against the likelihood that any heavily corroded object with an iron core will continue to deteriorate in store. Under such circumstances a sound justification can be made for selective sampling and examination to retrieve information that may well otherwise be lost.
See Starley (1995) Metallographic Examination http://hist-met.org/hmsdatasheet11.pdf
When should an archaeometallurgical specialist become involved in a project?
As soon as the presence of significant quantities of debris, or any structural evidence becomes evident. Earlier if desk –based assessment suggests the site may include metallurgical features
See Starley (2002) Metalworking Evidence and the Management of Archaeological Sites http://hist-met.org/hmsdatasheet16.pdf
Which geophysics techniques are best for locating metalworking sites?
The most powerful techniques are magnetometry and magnetic susceptibility; beware of plots which have been filtered to remove the strongest ‘modern’ anomalies. These may be just what we’re looking for
See Geophysical Techniques Applied to Early metalworking Sites (McDonnell 1995) http://hist-met.org/hmsdatasheet04.pdf
Does slag always remain in the vicinity of the metalworking site?
As mentioned above some forms of debris, such as hammerscale may build up in the immediate vicinity of the hearth and anvil, whereas bulk slags may be removed off-site, particularly to be used as hardcore or track metalling. When blast furnaces came into use (from the end of the C15th in the UK), old ‘bloomery’ slag heaps were often re-used instead of, or in addition to, ore. Slag was not always seen as an entirely useless waste product. Here are a few other uses I’ve come across:
In ceramics, as temper in Iron Age pottery and grit in Roman mortaria
As building material, either cast into blocks, used as wall copings or, decoratively as in the grotto at Chiswick House, or in historical times as an ingredient of ‘metallic stucco’
Modern blast furnace slag is pulverised and used as fertiliser
As a colorant in Saxon glass
As a refractory material for lining new hearths
As ships’ ballast
As a material of ritual importance. In Nigerian slag shrines and probably in British Iron Age sites
How does one know whether a furnace or hearth has been used for oxidizing or reducing processes?
Knowing the redox conditions of any hearth or furnace is important in determining its function. Take careful note of any changes in colouration in the heat affected zones.
Are you available for guest lectures?
Yes, though some of those given recently, such as the following concern my work at the Royal Armouries and will require their permission:
The Knight and the Laboratory; The Greatest Discoveries of the Royal Armouries Science Lab
Saxon Weapons: Grave Misconceptions
Protect and Survive: The Scientific Investigation of Historic Armour and Weapons
Swords, The Cutting Edge of Technology
Arrowheads: Taking a Closer Look
The Scientific Study of Japanese Arms and Armour
More general talks on the wider aspects of metalworking in the past and studying its evidence can be tailored to any audience, from local interest groups to university modules at U/G or Masters level.
I am available to provide hands -on training workshops including an introduction to the examination and identification of metalworking debris. I can also provide an introduction to the technique of metallography.
Can you recommend an introductory text on archaeometallurgy?
A good introduction, from the field archaeologists’ point of view is provided by English Heritage’s Centre for Archaeology Guidelines; ‘Archaeometallurgy’(2001) although it should be noted that the ‘where to get help’ listings are in need of updating.
http://www.english-heritage.org.uk/upload/pdf/cfa_archaeometallurgy2.pdf
This is heavily based on the sound, but less glossy, individual data sheets produced by the Historical Metallurgy Society:
http://hist-met.org/datasheets.html
For more recent industrial remains English Heritage have produced a follow –up volume ‘Science for Historic Industries; Guidelines for the Investigation of 17th to 19th-Century Industries’
http://www.helm.org.uk/upload/pdf/Science-Historic-Industries.pdf?1255614467
Unfortunately few good up to date text books provide a reliable overview, but it is good to see new reprints of R F Tylecote’s (1992) 2nd edition A History of Metallurgy and his (1986) The Prehistory of Metallurgy in the British Isles.
V F Buchwald (2005) ‘Iron and Steel in Ancient Times’ is also highly recommended for the serious scholar with a ferrous bias, including much on Scandinavian material
P T Craddock (1995) ‘Early Metal Mining and Production’, covers much ground, though the early and the exotic are covered in more detail than most of the more commonly encountered processes from UK sites.