How Can Heavy Metal Fly?

Scientists found a surprising amount of lead in a glacier. They were studying atmospheric pollution by sampling ice cores taken from Alpine glaciers. The surprising part is that they found more lead in strata from the late 13th century than they had in those deposited at the height of the Industrial Revolution. Surely mediaeval times were supposed to be more about knights in shining armour than dark satanic mills, what on earth was going on? Why was the lead industry in overdrive in an age when a wooden water wheel represented high technology?

The answer lies in the lead smelting methods used a thousand miles away from that glacier, and in the martyrdom of a mediaeval saint.

A Sort of Cathedralic Arms Race

The magnificent late-13th-century priory church at Chetwode, Buckinghamshire.
The magnificent late-13th-century priory church at Chetwode, Buckinghamshire.

In 1170 the Archbishop of Canterbury, Thomas Becket, was assassinated in his cathedral by a group of knights said to be acting on an exhortation by King Henry the Second of England. This crime resulted in Becket’s canonisation, and ultimately in Henry’s embarking on an unprecedented programme of building and extending cathedrals, monasteries, and other religious institutions in penance for Beckett’s death. The effect can still be seen today in the ecclesiastical architecture across his former kingdom, and in the glacial deposits of the time.

It seems that lead production in the English uplands for all those church roofs caused a spike in lead pollution that exceeded that seen during the Industrial Revolution, and that this is clearly visible to the scientists in the ice all these years later. This is an interesting journey into the murky politics of the 13th century, but the question for Hackaday was just how can so much lead have been released by mining at a mediaeval scale, and then how can so much of it have been carried all the way across a continent by the weather?

From Cathedral Roof To Alpine Glacier

A sample of galena ore mined in Missouri, USA. Didier Descouens (CC BY-SA 4.0)
A sample of galena ore mined in Missouri, USA. Didier Descouens (CC BY-SA 4.0)

The most abundant lead ore is galena, lead sulphide, which can be readily smelted to retrieve metallic lead by heating in a fire or furnace. It’s a two-stage reaction in which the sulphur is first removed to leave lead oxide and sulphur dioxide, and then the resulting lead oxide is reduced in the presence of carbon from the fuel to form molten lead and carbon dioxide.

This ease of extraction means that it has been used by humans for millennia, anywhere with a ready supply of fuel and galena could easily support lead production. The mediaeval lead extraction was performed in a crude furnace referred to as a bole smelter, an open fire consisting of alternate layers of wood and ore, usually placed on high ground to take advantage of the wind, and surrounded by stone walls to funnel the wind into the fire. These smelters were extremely inefficient to the extent that their slag contained enough left-over lead to be profitably recovered in later centuries, and in that a significant quantity of lead vapour and dust escaped with their exhaust gasses. Their sites are often still polluted with lead residues after hundreds of years, and those exhaust gasses were toxic enough to poison local farm animals and the people working the smelters.

Part of the several-mile-long complex of horizontal lead smelting flues, Allendale, Northumberland. Oliver Dixon (CC-BY-SA/2.0)
Part of the several-mile-long complex of horizontal lead smelting flues, Allendale, Northumberland. Oliver Dixon (CC-BY-SA/2.0)

In later centuries lead smelters with more efficient forced-air furnaces recovered much of the lead vapour and dust by building very long horizontal flues like the one shown here in which they could condense and be recovered. Their accounts contained estimates of the quantities on the walls of these flues at any one time, and could run into the hundreds of tons. Passing directly into the atmosphere though, the emissions from the 13th century smelters would have been carried into the upper atmosphere by the weather systems, and thus moved the thousand miles or so across Continental Europe to the Alpine glaciers. A modern parallel comes in sand from the Sahara in North Africa appearing in raindrops in Northern Europe, as any car owner from those regions will tell you when their vehicle is covered in dust by a rainstorm.

Our cultural imagination of mediaeval England from movie depictions is of castles, knights in shining armour, and probably a jolly peasantry in a bucolic countryside. The reality was one of grinding poverty and disease in a country much of whose landscape was still forested, so should it be so much of a shock to discover that in places it was exposed to pollution every bit as toxic and widespread as in later centuries? This industry didn’t just leave a trace on a scientist’s analysis of ice cores centuries later, it stripped away thousands of trees used for fuel and contributed to a profound change in the landscape. Something to bear in mind, should your tourism take you into the towering splendour of a lead-roofed mediaeval cathedral.

18 thoughts on “How Can Heavy Metal Fly?

  1. A metallurgist writing in The Register once described his currentl location as “Hunting slags in the Ore Mountains”. Apparently there’s a lot of useful stuff in the spoil heaps: elements that weren’t even known when the lead, silver and so on were extracted.


  2. That is an interesting fact to counter the community of fools that propagates nonsense like “cancer (or allergies, or autism) is a 20th century disease and it is caused by environmental pollution”.

    1. It may still be. Back in the day allergies “didn’t exist” because the people simply didn’t believe in allergies. They had other explanations. If 13th century pollution was causing people to turn autistic because of lead poisoning, they wouldn’t have attributed it to the smelters.

      For example, hay fever was known to Persian doctors in the 10th century, but because medieval European medicine was based on Galen’s four humors, which were based on Aristotle’s elements theory that was adopted by the Catholic Church as official dogma, medicine in the 13th century couldn’t distinguish allergies and cancers as caused by environmental factors but symptoms of the “imbalance of humors”. Going against the grain to speculate other causes like lead in the wind would get you labeled as ignorant, a fool or a heretic, and this wasn’t a small matter because it went against the Catholic Church’s authority and could get the inquisition on you.

      1. It’s important to remember that the Inquisition was a specific event in the history of the Catholic Church, it was not however a constant entity or event. This is a common misconception, but an important point. While, I am no particular fan of the Catholic Church, this idea that innovation, or deviation from established norms would somehow result in church sanctioned torture or harassment is just plain wrong. It did happen, it was wrong, but it wasn’t a constant fear most people had at the time. Most people had much more to fear from the nobility at the time, and it was that group that violently resisted change much more often than the clergy. When mercantilism eventually replaced that system, it was guilds that resisted the change, and so on. To be sure, the church did it’s fair share of horribleness, but modern tellings seem to glass over the much more reprehensible deeds of other power groups.

        1. That was a figure of speech. Regardless of whether they actually sent THE inquisition on you, you could still get yourself in hot water if you doubted the church orthodoxy on Galen, Aristotle and the whole deal up until a certain person nailed a few theses on a church door and started a bunch of religious wars between Catholics and Protestants which eventually eroded the intellectual hegemony of the church.

          It was the paradigm of the day, and anyone who disagreed – like Galileo for example – would be laughed at and ostracized if they persisted in trying to convince people otherwise. Because of this, it wasn’t until the 19th century when allergies like hay fever were properly recognized by western medicine, and their real causes weren’t discovered until the 20th.

          This created the illusion that allergies, asthma, autism, etc. are a modern phenomena, since they can’t be found in historical accounts. Recognition that this is the case then created the opposite claim: that they’ve always existed, and not caused by modern air pollution or exposure toxic materials – but that too is wrong because the same pollution and toxins have existed at times in the past as well and our situation is not new.

        2. >To be sure, the church did it’s fair share of horribleness, but modern tellings seem to glass over the much more reprehensible deeds of other power groups.

          Up until the Reformation, the catholic church was the de-facto ruler of Europe. They sanctioned marriages, divorces, and meddled in all politics between these other power groups. That was the reason of the formation of the Anglican Church in the 16th century – the King of England wanted a divorce and didn’t want to ask Rome for the permission to do that.

          The Catholic Church had its Canon Law, and if a medieval kingdom or fiefdom started showing “heretical tendencies” like hosting people dissenting from the Church authority, that would be a great excuse for a land grab or a coup by their neighbors. Since political authority was derived through religious authority, handed down by the Pope, medieval Europe was a pretty stuck-up place regardless of whether the church was directly intervening.

          1. Small point, but what Henry VIII wanted was an annulment, not a divorce, and it was refused because of political interference from Emperor Charles V. Because while the pope had divine authority, the Holy Roman Emperor – or the French King, once or twice – could exercise military authority if they so chose.

        3. >it was not however a constant entity

          The Inquisition still exists in Italy even today. It was simply renamed in 1908, and again in 1965. It’s purpose is still to combat heresy – although they’re not allowed to garrote people to death for teaching Deism, or to kidnap children from families of other faiths by performing a baptism on them and then claiming that the kid is Catholic. They stopped doing that in the 19th century after a lot of public outcry. Today the Inquisition is known as the Congregation for the Doctrine of the Faith.

        4. And in terms of science, the Catholic Church weren’t exactly anti-science as such. They largely enforced what the previous generation of scientists had said was “settled science”.

          1. Exactly, and the science they enforced was taken directly out of the antiquity. The point was that the Catholic faith is based on the concept of natural law – that morality can be derived from physical facts by human reason – so they needed to control what was being said about the state of those physical facts.

            The practice of scholasticism that was practiced by the Catholic priests and education system was based on using semantics and logic to argue that two sources – such as Aristotle and the Bible – were really talking the same thing. One supported the other and expanded on where the other was lacking. Therefore if Aristotle was wrong on some things, the Bible was wrong on the same count, or the scholastics were mistaken and had to eat their words. Thereby, if you went nosing about in physics or medicine, it would quickly undermine the earlier authors and by proxy cause a heap of trouble for the Catholic faith.

          2. The Catholic church is actually a lot more pro-science now than many other religious groups these days.
            They have no problem with evolution, or the age of the universe, and in fact, there is an official Vatican (astronomical) observatory (, (although like most astronomy these days, most of the actual observations are done using time on large telescopes on top of mountains around the world).

          3. That’s nice and everything, but they’d be doing science a bigger favour if they just fucked off. Give the money to the poor, stop meddling in politics, and let people fall back into a state of pagan idolatry, as much as they really give much of a shit about it. Which isn’t much, now we can actually DO things about weather and disease, rather then just placating whoever we’ve attributed them to.

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