Tag: chemical composition

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"?