Tag: alloys

Creeping bronzes

Evidence of journalistic creep in 'surprising' Benin bronzes claim

Keith S. Taber

How certain can we be about the origin of metals used in historic artefacts? (Image by Monika from Pixabay)

Science offers reliable knowledge of the natural world – but not absolutely certain knowledge. Conclusions from scientific studies follow from the results, but no research can offer absolutely certain conclusions as there are always provisos.

Read about critical reading of research

Scientists tend to know this, something emphasised for example by Albert Einstein (1940), who described scientific theories (used to interpret research results) as "hypothetical, never completely final, always subject to question and doubt".

When scientists talk to one another within some research programme they may used a shared linguistic code where they can omit the various conditionals ('likely', 'it seems', 'according to our best estimates', 'assuming the underlying theory', 'within experimental error', and the rest) as these are understood, and so may be left unspoken, thus increasing economy of language.

When scientists explain their work to a wider public such conditionals may also be left out to keep the account simple, but really should be mentioned. A particular trope that annoyed me when I was younger was the high frequency of links in science documentaries that told me "this could only mean…" (Taber, 2007) when honest science is always framed more along the lines "this would seem to mean…", "this could possibly mean…", "this suggested the possibility"…

Read about scientific certainty in the media

Journalistic creep

By journalistic creep I mean the tendency for some journalists who act as intermediates between research scientists and the public to keep the story simple by omitting important provisos. Science teachers will appreciate this, as they often have to decide which details can be included in a presentation without loosing or confusing the audience. A useful mantra may be:

Simplification may be necessary – but oversimplification can be misleading

A slightly different type of journalist creep occurs within stories themselves, Sometimes the banner headline and the introduction to a piece report definitive, certain scientific results – but reading on (for those that do!) reveals nuances not acknowledged at the start. Teachers will again appreciate this tactic: offer the overview with the main point, before going back to fill in the more subtle aspects. But then, teachers have (somewhat) more control over whether the audience engages with the full account.

I am not intending to criticise journalists in general here, as scientists themselves have a tendency to do something similar when it comes to finding titles for papers that will attract attention by perhaps suggesting something more certain (or, sometimes, poetic or even controversial) than can be supported by the full report.

An example of a Benin Bronze (a brass artefact from what is now Nigeria) in the British [sic] Museum

(British Museum, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons)

Where did the Benin bronzes metal come from?

The title of a recent article in the RSC's magazine for teachers, Education in Chemistry, proclaimed a "Surprise origin for Benin bronzes".1 The article started with the claim:

"Geochemists have confirmed that most of the Benin bronzes – sculptured heads, plaques and figurines made by the Edo people in West Africa between the 16th and 19th centuries – are made from brass that originated thousands of miles away in the German Rhineland."

So, this was something that scientists had apparently confirmed as being the case.

Reading on, one finds that

  • it has been "long suspected that metal used for the artworks was melted-down manillas that the Portuguese brought to West Africa"
  • scientists "analysed 67 manillas known to have been used in early Portuguese trade. The manillas were recovered from five shipwrecks in the Atlantic and three land sites in Europe and Africa"
  • they "found strong similarities between the manillas studied and the metal used in more than 700 Benin bronzes with previously published chemical compositions"
  • and "the chemical composition of the copper in the manillas matched copper ores mined in northern Europe"
  • and "suggests that modern-day Germany, specifically the German Rhineland, was the main source of the metal".

So, there is a chain of argument here which seems quite persuasive, but to move from this to it being "confirmed that most of the Benin bronzes…are made from brass that originated …in the German Rhineland" seems an example of journalistic creep.

The reference to "the chemical composition of the copper [sic] in the manillas" is unclear, as according to the original research paper the sample of manilla analysed were:

"chemically different from each other. Although most manillas analysed here …are brasses or leaded brasses, sometimes with small amounts of tin, a few specimens are leaded copper with little or no zinc."

Skowronek, et al., 2023

The key data presented in the paper concerned the ratios of different lead isotopes (205Pb:204Pb; 206Pb:204Pb; 207Pb:204Pb; 208Pb:204Pb {see the reproduced figure below}) in

  • ore from different European locations (according to published sources)
  • sampled Benin bronze (as reported from earlier research), and
  • sampled recovered manillas

and the ratios of different elements (Ni:AS; Sb:As; Bi:As) in previously sampled Benin bronzes and sampled manillas.

The tendency to consider a chain of argument where each link seems reasonably persuasive as supporting fairly certain conclusions is logically flawed (it is like concluding from knowledge that one's chance of dying on any particular day is very low, that one must be immortal) but seems reflected in something I have noticed with some research students: that often their overall confidence in the conclusions of a research paper they have scrutinised is higher than their confidence in some of the distinct component parts of that study.

An example of a student's evaluation of a research study

This is like being told by a mechanic that your cycle brakes have a 20% of failing in the next year; the tyres 30%; the chain 20%; and the frame 10%; and concluding from this that there is only about a 20% chance of having any kind of failure in that time!

A definite identification?

The peer reviewed research paper which reports the study discussed in the Education in Chemistry article informs readers that

"In the current study, documentary sources and geochemical analyses are used to demonstrate that the source of the early Portuguese "tacoais" manillas and, ultimately, the Benin Bronzes was the German Rhineland."

"…this study definitively identifies the Rhineland as the principal source of manillas at the opening of the Portuguese trade…"

Skowronek, et al.,2023

which sounds pretty definitive, but interestingly the study did not rely on chemical analysis alone, but also 'documentary' evidence. In effect, historical evidence provided another link in the argument, by suggesting the range of possible sources of the alloy that should be considered in any chemical comparisons. This assumes there were no mining and smelting operations providing metal for the trade with Africa which have not been well-documented by historians. That seems a reasonable assumption, but adds another proviso to the conclusions.

The researchers reported that

Pre-18th century manillas share strong isotopic similarities with Benin's famous artworks. Trace elements such as antimony, arsenic, nickel and bismuth are not as similar as the lead isotope data…. The greater data derivation suggests that manillas were added to older brass or bronze scrap pieces to produce the Benin works, an idea proposed earlier.

and acknowledges that

Millions of these artifacts were sent to West Africa where they likely provided the major, virtually the only, source of brass for West African casters between the 15th and the 18th centuries, including serving as the principal metal source of the Benin Bronzes. However, the difference in trace elemental patterns between manillas and Benin Bronzes does not allow postulating that they have been the only source.

The figure below is taken from the research report.

Part of Figure 2 from the open access paper (© 2023 Skowronek et al. – distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The chart shows results from sampled examples of Benin bronzes (blue circles); compared with the values of the same isotope ratios from different copper ore site (squares) and manillas sampled from different archaeological sties (triangles).

The researchers feel that the pattern of clustering of results (in this, and other similar comparisons between lead isotope ratios) from the Benin bronzes, compared with those from the sampled manillas, and the ore sites, allows them to identify the source of metal re-purposed by the Edo craftspeople to make the bronzes.

It is certainly the case that the blue circles (which refer to the artworks) and the green squares (which refer to copper ore samples from Rhineland) do seem to generally cluster in a similar region of the graph – and that some of the samples taken from the manillas also seem to fit this pattern.

I can see why this might strongly suggest the Rhineland (certainly more so than Wales) as the source of the copper believed to be used in manillas which were traded in Africa and are thought to have been later melted down as part of the composition of alloy used to make the Benin bronzes.

Whether that makes for either

  • definitive identification of the Rhineland as the principal source of manillas (Skowronek paper), or
  • confirmation that most of the Benin bronze are made from brass that originated thousands of miles away in the German Rhineland (EiC)

seems somewhat less certain. Just as scientific claims should be.

A conclusion for science education

It is both human nature, and often good journalistic or pedagogic practice to begin with a clear, uncomplicated statement of what is to be communicated. But we also know that what is heard or read first may be better retained in memory than what follows. It also seems that people in general tend to apply the wrong kind of calculus when there are multiple source of doubt – being more likely to estimate overall doubt as being the mean or modal level of the several discrete sources of doubt, rather than something that accumulates step-on-step.

It seems there is a major issue here for science education in training young people in critically questioning claims, looking for the relevant provisos, and understanding how to integrate levels of doubt (or, similarly, risk) that are distributed over a sequence of phases in a process.

All research conclusions (in any empirical study in any discipline) rely on a network of assumptions and interpretations, any one of which could be a weak link in the chain of logic. This is my take on some of the most critical links and assumptions in the Benin bronzes study. One could easily further complicate this scheme (for example, I have ignored the assumptions about the validity of the techniques and calibration of the instrumentation used to find the isotopic composition of metal samples).

Work cited:
Note:

1 It is not clear to me what the surprise was – but perhaps this is meant to suggest the claim may be surprising to readers of the article. The study discussed was premised on the assumption that the Benin Bronzes were made from metal largely re-purposed from manillas traded from Europe, which had originally been cast in one of the known areas in Europe with metal working traditions. The researchers included the Rhineland as one of the potential regional sites they were considering. So, it was surely a surprise only in a similar sense to rolling a die and it landing on 4, rather than say 2 or 5, would be a surprise.

But then, would you be just as likely to read an article entitled "Benin bronzes found to have anticipated origin"?

Can phosphorus prevent rusting?

Is phosphorus the alumina of the ancient world?

Keith S. Taber

An ancient iron column: Did "a very thin layer of phosphorus formed, between the rust and the fresh metal and basically stop… it from rusting any more"

What do you need to build a skyscraper?

I was listening to a podcast from the Royal Institution (where Humphrey Davy and Michael Faraday were based). I must confess I had downloaded the 'Recipe for a Skyscraper' episode some time ago but it had been passed over for other titles.

Royal Institution podcast: Recipe for a Skyscraper – with Roma Agrawal

My mistake. In this talk "structural engineer Roma Agrawal delves into the history of the materials that enable immense construction and the developments that have made our structures what they are today. All while noting the accomplishments of key visionary engineers of the past". This proved to be an engaging and fascinating talk.

A 'mega badass engineer'

On her website, Roma Agrawal , "a structural engineer, author and broadcaster, with a physics degree" describes herself as a "mega badass engineer". She is not above being a little mischievous.

The crumbly ages

For example, she has her own take on what historians used to call the 'dark ages', 1

"So, oddly enough, once the Roman empire fell, the use of concrete basically ended for nearly a thousand years, so that we call it the dark ages, or the crumbly ages as I like to call it, because they went back to using slightly older [construction materials], you know, mud and brick and things like that."

Roma Agrawal talking at the Royal Institution

But while the Romans may have championed the use of concrete, the Indians were outperforming them in the production of high quality iron: "The Romans actually used to import Indian steel at the time and they never knew how to make it because that secret was closely guarded…"

Iron is too reactive to be found 'native' but has to be produced by roasting its ores (that contain compounds of iron) with materials that will reduce the iron compounds to iron, and produce, as a by-product, slag – a complex mixtures of substances. The iron produced will contain some slag mixed into the metal unless this is carefully removed. 2

The Delhi column

As an example of the Indian expertise, Roma Agrawal referred to an old iron column near Delhi which "had not rusted" despite having been erected 1500 years ago.3 The column had originally been a stand for a statue of Garuda, the divine winged creature/demigod who acted as the vehicle for Vishnu. Garuda seems to have flown, but the iron column remains.

The (not quite 4) 'rustless wonder' (Srinivasan & Ranganathan, 2013): the Qtub Iron Pillar

(Photograph taken by Mark A. Wilson, available at https://en.wikipedia.org/wiki/Iron_pillar_of_Delhi#/media/File:QtubIronPillar.JPG)

Lord Vishnu on his mount Garuda (wood carving). It is thought the iron pillar near Delhi once supported a statue of Garuda.

(Image by waradet from Pixabay)

Iron is the main constituent of alloys known as steels, and by mixing other elements (principally, but not only, carbon) with iron it is possible to create steels with various properties, including corrosion resistance. 2 But iron itself readily rusts. The rust formed when iron corrodes is permeable and crumbly, exposing the unreacted metal beneath, which in turn forms rust that again fails to protect the iron beneath it. So, over time, a piece of iron can simply 'rust away' as the reacted material will simply fall off, or be eroded by weather.

Yet this iron column, erected around the time of the final collapse of the Roman Empire, seems to have survived throughout 'the crumbly ages' and through to the present day. Although, it is not that it never started rusting 4, but rather,

"it did initially rust, but then because of the climate in Delhi, the phosphorus, a very thin layer of phosphorus formed, between the rust and the fresh metal and basically stopped it from rusting any more…"

Roma Agrawal talking at the Royal Institution

Corrosion (as with tarnishing) is a generic term. Corrosion leads to structural damage to metal objects (whereas tarnishing is a surface effect).

Rusting is specific to iron as it refers to the material produced when iron corrodes – i.e., rust.

Unreactive phosphorus?: An alternative conception

Roma Agrawal's claim seems incredible to a chemist or science teacher because phsophorus is a very reactive element, and a very reactive element does not seem a good choice of material to protect iron from reacting! Even if the phosphorus did not itself react with the iron and so corrode it, it would soon react with air. In the laboratory, some forms of phosphorus can burst into flames spontaneously, suggesting it is very unlikely to remain intact very long exposed to the elements in India. Certainly not many centuries.

Sacrificial elements

Now, sometimes a more valuable metal is protected by connecting it physically to a more reactive but less valuable metal which preferentially corrodes. As the metals are in electrical contact, the one that loses electrons and releases cations more readily reacts first. The metal allowed to corrode is called a 'sacrificial' metal. For example, bars of sacrificial metal may be dangled from piers or oil rigs to protect the structural metal. The sacrificial metal will slowly 'dissolve' away into the sea 5 – but not that slowly that it would not need replacing for over a millennium. In any case, phosphorus is a non-metal, where the sacrificial element of the pair needs to be the more electropositive. So, there is no helpful explanation there.

Alumina – when tarnishing prevents corrosion

Aluminium is a more reactive metal than iron, yet does not readily undergo substantive corrosion. This is because the surface of an aluminium object readily reacts with oxygen from the air to form a layer of aluminium oxide (alumina). This then protects the aluminium because the alumina formed is a fairly inert substance (unlike the highly reactive phosphorus), and it forms an impermeable layer (preventing oxygen from the air reaching the metal beneath).

Any layer that were to form on iron protect it from rusting also needs to be impermeable and relatively inert. Unlike reactive phosphorus.

Phosphorus would not protect iron

Phosphorus is a fire hazard that burns to produce toxic fumes. In the laboratory, the direct reaction of iron and phosphorus usually requires heating to initiate reaction. Without active heating, the rate of reaction would be too low for a useful laboratory process. However, a very low rate of reaction would not prevent reaction over the centuries since the iron column was erected.

Even if phosphorus was able to form a layer that coated over the iron, using it as a means to prevent corrosion would be like fireproofing a wooden building by coating it with petroleum jelly (e.g., Vaseline). [A correspondent to the British Dental Journal (Brewer, 2017) warned of "the death of a bedbound patient who smoked following application of E45 cream…a paraffin-based product, the residue of which can act as an accelerant when ignited". Smoking kills. And even more rapidly if you smother yourself in flammable oil products prior to lighting up.]

So, it seems we have a mystery.

Or, Roma Agrawal simply got it wrong.

Or, perhaps, more likely, when Roma Agrawal refers to a 'layer of phosphorus' she is using the term loosely, and is actually referring to something else. That is, the protective layer may contain one or more phosphorus compounds, but not phosphorus – just as a layer of the unreactive aluminium compound alumina stops corrosion, although aluminium itself is reactive. Is this distinction just being pedantic? Not to a science educator.

An elementary misconception

The claim that a layer of phosphorus could protect iron from corrosion is therefore not credible to the scientifically literate, but might seem perfectly reasonable to a person with limited science background. One of the great challenges of learning chemistry is making sense of the set of ideas that:

  • the compound of an element is a completely different substance to the element itself
  • the properties of compounds are often quite different (sometimes contrastingly so) to those of the elements the compound was formed from
  • although the compound does not behave like the elements, and does not 'contain' the elements in any straightforward way, there is a sense in which something of the elements persists in (and so the element may be recovered from) the compound.

So, sodium is a reactive metal that burns in air, and chlorine is a green, toxic, choking gas; and both should be avoided unless taking very careful precautions; yet they react, very energetically, to give the relatively unreactive compound sodium chloride – which people readily use in cooking, and to season their food, and to dissolve in water to gargle with, or to soak tired feet. Chlorine would destroy the lining of your throat. Yet sodium chloride solution (despite its chlorine 'constituent') will help ease a sore throat! Still, the sodium chloride has the potential to be 'separated' into the elements with their dangerous properties intact.

Although the distinction between elements and compounds is a lot easier to understand once students learn about molecules and atoms (at least, if avoiding the alternative conception that compounds comprise of molecules and elements comprise of atoms!) this topic is fraught with complications and hang-overs from historical ideas about atoms (Taber, 2003).

If not a layer of phosphorus?

The chemist or science teacher hearing about a protective 'layer of phosphorus' preventing rusting will immediately thinks this is not viable…but a compound of phosphorus might well have the necessary properties. Indeed, generally, the more reactive the elements, the more stable the compounds they form when reacting.

It seems that the layer that formed on the iron column contains the phosphorus compound iron hydrogen phosphate hydrate (FePO4·H3PO4·4H2O),

"Several theories have been postulated regarding corrosion resistance of the Delhi iron pillar. Some of those refer to the inherent nature of the construction material, such as the selection of pure iron, presence of slag particles and slag coatings, surface finishing using mechanical operation, phosphate film formation, or the Delhi's climate…

Earlier studies have delineated the formation of crystalline iron hydrogen phosphate hydrate (FePO4·H3PO4·4H2O), 𝛼-, 𝛾-, 𝛿-FeOOH and magnetite in the case of Delhi iron pillar"

Dwivedi, Mata, Salvemini, Rowles, Becker & Lepková, 2021
The claim re-interpreted

Yet this critical, and somewhat counter-intuitive, distinction between elements qua elements and elements as in some sense 'components' (or 'ingredients') of compounds needs to be acquired. Novices have to learn this. A common alternative conception is to assume that the properties of elements are carried over into their compounds.

So, if students hear that

  • phosphorus is essential in our diet, and that
  • phosphorus is important for healthy bones and teeth,

they can draw the obvious and reasonable conclusion – that phosphorus must be a pretty innocuous substance as it is part of our bodies and we eat it quite safely in our food. Actually, we need compounds of phosphorus in our food to allow our metabolisms to build and repair tissues that contain phosphorus compounds – and anyone misguided enough to try to eat any actual (elemental) phosphorus risks a nasty burn.

In conclusion, as a science graduate, Roma Agrawal presumably appreciates the key distinction between (i) elements as substances and (ii) elements as chemically combined components of other substances, and, as a structural engineer knowledgeable about different material properties, is using 'layer of phosphorus' as a shorthand for a layer of material that includes one or more phosphorus compounds.

That is fine as long as those hearing her talk appreciate that. Another scientist would likely automatically hear 'phosphorus layer' as meaning 'phosphorus compound containing layer'. A science teacher, however, might suspect that the reference to how "a very thin layer of phosphorus formed, between the rust and the fresh metal and basically stopped it from rusting" is likely to be misunderstood, and indeed to mislead, some listening to the podcast.

Minding your Ps…

One of the sources referred to reported how:

"P is found present in slag whereas the presence of P in iron was not detected within the limit of the analytical techniques used in this study. On the basis of this result, we speculate application of lime and other basic compounds during the iron making process which would have led to the transfer P to slag."

Dwivedi, Mata, Salvemini, Rowles, Becker & Lepková, 2021

P is the symbol for phosphorus, the element. However, someone with a sufficient scientific background appreciates from the context that references to

  • P found in slag
  • P in iron
  • transfer [of] P to slag

cannot refer to P as phosphorus the element, but rather some compound or compounds of phosphorus. As a reactive element, phosphorus is not found native and so would not be present (as an element) in the raw materials and, in any case, could certainly not survive (as an element) the high temperature conditions of the processes of iron smelting. Therefore the relevant 'context' for reinterpreting 'P' as not standing for the element itself would be any set of circumstances other than the special conditions where phosphorus can be safely stored without risk of reaction.

This is the prerequisite background knowledge that prevents an audience member misinterpreting what must be meant by a "thin layer of phosphorus [sic]" protecting an exposed iron column – as it cannot possibly refer to a thin layer of [actual, elemental] phosphorus.

Sources cited
  • Anantharaman, T. R. (1997). The iron pillar at Delhi. In S. Ranganathan (Ed.), Iron and Steel Heritage of India (pp. 1-28). Indian Institute of Metals and Tata Steel.
  • Brewer, E. Patient safety: Paraffin-based products. British Dental Journal 223, 620 (2017). https://doi.org/10.1038/sj.bdj.2017.936
  • Dwivedi, D., Mata, J. P., Salvemini, F., Rowles, M. R., Becker, T., & Lepková, K. (2021). Uncovering the superior corrosion resistance of iron made via ancient Indian iron-making practice. Scientific Reports, 11(1), 4221. doi:10.1038/s41598-021-81918-w
  • Falk, S. (2020). The Light Ages. A Medieval journey of discovery. Allen Lane.
  • Srinivasan, S., & Ranganathan, S. (2013). Minerals and Metals Heritage of India. Bangalore: National Institute of Advanced Studies.
  • Taber, K. S. (2003). The atom in the chemistry curriculum: fundamental concept, teaching model or epistemological obstacle? Foundations of Chemistry, 5(1), 43-84. (The author's manuscript versions is available here.)

Notes:

1 A simplistic view was that advancing civilisation underwent something of a relapse during the middle ages, until the gains of the classical age (the Greeks, the Romans) were rediscovered in the Enlightenment. Thus, the term 'dark ages' applied to the 'middle ages'.

There were no dark ages:
as a matter of fact, they are all dark

with apologies to Pink Floyd

That is clearly a great simplification, and ignores many medieval achievements, as well as being a rather Eurocentric view. Some historians have been seeking to redress this impression: for example, Seb Falk (2020) has renamed this period 'the light ages'.

2 To suggest that steel deliberately contains impurities added to iron could give the impression that iron artefacts are made of purer materials than steel ones. This is misleading. Basic iron smelting produces iron that is impure (sometimes known as 'pig iron') and which can contain quite high levels of impurities. Pig iron typically has a high level of carbon – more than is usually used in steels.

Wrought iron is produced by physical working of pig iron which expels much of the slag content, giving purer iron. Wrought iron has long been widely used in structures, but still does not have a high level of purity.

Alloys are mixtures of different metals, or of metallic elements with other elements. 'Metal' here is ambiguous as it can refer to

  • an electropositive element (the usual meaning in chemistry) or
  • a material with certain properties (the usual meaning in engineering) – i.e., malleable, ductile, high electrical and thermal conductivities, lustre, sonorous.

Steels are metals in the 'materials' sense, but 'chemically' are mixtures of the metallic element iron with other elements.

As the properties of steels are sensitive to the levels of other elements, making steel requires using high quality iron that has been treated to remove most of the impurities. This is similar to doping a semiconductor such as silicon to produce electronic components. Very pure silicon is needed as a starting point, so that just the right amount of a specific dopant can be added.

The Indian iron manufacture of Roman times tended to produce iron with a significant phosphorus content.

3 The column was made of wrought iron,

"The forging of wrought iron seems to have reached its zenith in India in the first millennium AD. The earliest large forging is the famous iron pillar with a height of over 7 m and weight of about 6 tons at New Delhi ascribed to Chandragupta Vikramaditya 400- 450 CE… the absence of corrosion is linked to the composition, the high purity of the wrought iron and the phosphorus content and the distribution of slag."

Srinivasan & Ranganathan, 2013

4 The lack of rusting may have been exaggerated,

"The first impression in 1961 was that the portion of the Pillar below the earth was "superficially rusted". However, on detailed examination, the buried portion of the Pillar was found covered with thick crusts of rust and, in fact, copious rust scales could be collected, ranging in thickness from a few millimeters (mm) to no less than 15 mm in some portions. Further, the bulbous base of the Pillar was found riddled with numerous cavities and hollows caused by deep corrosion and mineralization of the iron.

Anantharaman, 1997

Even so, the survival of an iron column exposed to weathering for this length of time is still worthy of note.

5 I thought I should put 'dissolve' into 'scare quotes' here. Corrosion is a chemical change, whereas dissolving refers to what is generally considered a physical change. As the sacrificial metal reacts, it releases cations into solution in the sea, in much the same was as, say, dissolving salt releases sodium ions when common salt is added to water. The metal reacts and enters solution – dissolves, if you are comfortable with that word in this context.