For International Day of Clean Air (September 7), we spoke to Dr Katrin Wilhelm, Researcher and Departmental Lecturer at the School of Geography and the Environment. She revealed that the iconic Sheldonian’s heads tell a captivating 350-year story of urban architectural decay, conservation, environmental change, and changing attitudes and perceptions.

Did you know that the iconic 13 carved heads that ring the perimeter of the Sheldonian Theatre are the third generation to have sat there watching over passers-by? These heads have been continuously exposed to varying environments, both social and natural. Regrettably, the first two sets of heads were never thoroughly documented, and their whereabouts remained a mystery. That was until a dedicated team from the School of Geography and the Environment embarked on a ‘Head hunt’.

You can find out more about the hunt here.

Dr Katrin Wilhelm and her team delved into the history of these three generations of stone heads. They discovered that each generation was exposed during three distinct periods of atmospheric pollution: the early Industrial Revolution, the Victorian era, and the modern 20th century.

The surface stratigraphies (crust layers) of the sculptures' fabric silently archive this history, serving as a hidden intangible institutional memory or knowledge from the past. This phenomenon, which Dr Wilhelm refers to as “timing pollution”, is part of her research strand titled "Learning from the Past". Here, built structures become ‘material witnesses’ and agents collecting direct and circumstantial evidence of complex environmental interactions.

3rd generation Heads at the Sheldonian Theatre 2nd generation Head at Nuneham Courtenay, now in Wytham 1st generation Head in Worcester College

Photos, left to right: 3rd generation Head at the Sheldonian Theatre (©Wilhelm 2012); 2nd generation Head at Nuneham Courtenay, now in Wytham, which is on display at the Weston Library until July 2019 (©Wilhelm 2017); and 1st generation Head in Worcester College, which is on display at the Weston Library until July 2019 (©Fusade 2017).

When the first two generations of sculptures were relocated to less polluted areas, such as Oxford private and college gardens, their 'pollution clocks' effectively stopped. This presents a unique chance to establish empirical links between stone crust pollution signals (the silent memory) and changing sources of air pollution associated with transport and the industrial evolution.

The originals heads were carved from local weathering resistant Taynton limestone and lasted nearly 200 years. However, they were replaced in 1869 by Victorian replicas. Unfortunately, the second generation was crafted from the softer and more reactive Milton limestone due to some potentially economically-driven decisions.

Like many of Oxford’s buildings at the time, the heads were tarnished by soot and disfigured by acid rain. This was worsened by countless coal fires lit in hundreds of students’ college rooms. Oxford’s air was thick with smoke and its buildings were deteriorating heavily. Only 57 years after post-installation, John Betjeman described the second-generation heads as “mouldering busts”, “unknown bearded guards” and compared them to “illustrations of skin diseases in a medical textbook”. This period coincided with the Historic Buildings Fund (1957–1974), a substantial effort aimed at cleaning-up, repairing, and replacing the Oxford’s historic stone fabric on a large scale.

Due to a shift in conservation ethics and practices, attempts were made to preserve three of the heads using what was then considered cutting-edge technology. Unfortunately, these efforts proved fruitless as the sculptures were beyond saving. Consequently, the University commissioned local sculptor Michael Black to create the current set that now adorns the theatre.

animated gif of pore space from a microCT scan Martin Coombes sampling microbes to understand the biodiversity on a 2nd generation Head.

Left, an animated gif of pore space from a microCT scan (© de Kock 2018) and, above, Dr Martin Coombes samples microbes to understand the biodiversity on a 2nd generation Head (© Wilhelm).

Wondering how to decipher this 'pollution clock'?

Dr Katrin Wilhelm said: “Our research has uncovered fascinating insights into urban air pollution history. We studied the layers of black crust that accumulate on stone structures over time, serving pollution level records. By analysing the crusts’ composition, we traced lead pollution sources spanning centuries.”

She further emphasized the broader implications of their findings, "Our insights into the effects of air pollution can aid in long-term environmental impact assessment, inform mitigation strategies and offer a benchmark for evaluating the effectiveness of environmental policies. This, in turn, might inform future urban planning, pollution control, and risk management strategies." 

Overall, their research offers invaluable perspectives on air quality history and its impact on stone structures, bolstering essential conservation endeavours and enhancing our grasp on air pollution's long-term consequences.

Further information

  • 1 - Von Uffenbach, writing in 1710, says ‘The other busts and decorations on the outer wall are so badly and so coarsely fashioned that I was astounded' (Oxford in 1710, ed. W. H. and W.J . C. Quarrell, p. 10). So the ancient opinion or them seems little better than the modem, but they may have been worn even when Uffenbach saw them.” (Cole 1949 e.g. p67 footnote). Betjeman was even minded to liken them in 1960 to “illustrations in a medical textbook on skin diseases” (Betjeman 1960 p.15). 
  • The study is funded in part by the John Fell Fund.
  • Wilhelm, K., Longman, J., Standish, C.D. and De Kock, T. (2023) The Historic Built Environment as a Long-Term Geochemical Archive: Telling the Time on the Urban "Pollution Clock". Environmental Science and Technology.
  • The Pollution Clock work also featured on the first episode of Channel 4/More4's Cleaning Britain's Greatest Treasures (at 43 minutes, 30 seconds).