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
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
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.
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.