What can we learn from medieval craftspeople to help preserve our cultural heritage in an uncertain future? Medieval northern European stone churches are at increased risk of damage due to climate change, and forgotten historical materials and techniques may provide the key to saving our architectural stone heritage. How and why were stone adhesives used by craftspeople during the Middle Ages? To answer this question, the Sticking Stones project is undertaking pioneering research into the use of wood tar as stone adhesive in cathedral construction. We aim to rediscover the lost medieval art of traditional stone repair using wood tar adhesives, recreate the historic adhesive mixture and subsequently repurpose it for modern conservation practice and the built environment.
The knowledge and technology behind 700-year-old adhesives can help in developing new adhesive types for the conservation of stone architecture. In uncovering conclusive evidence of widespread medieval European use of stone adhesives, long-forgotten craft practices can be relearnt. The concepts of tacit knowledge and the intangible cultural heritage of craft skills are being explored in depth to shed light on the decisions made by medieval craftspeople. Fieldwork and building archaeology are being combined to map evidence of adhesive repairs in European medieval stone cathedrals. What is the composition of historic tar adhesive mixtures? Can we successfully replicate these historic adhesives for contemporary use? How does the performance of a sustainable wood tar adhesive compare to modern synthetic adhesives currently used in conservation? These are just some of the many questions that our highly interdisciplinary team is addressing through material characterisation and adhesive reengineering. This ambitious and novel project aims to revolutionise conservation practice by adding historic methods to the contemporary sustainable conservation toolkit.
A PhD candidate is investigating craft theories, tacit knowledge, and technological complexity of adhesives, focussing on wood tar adhesives and their place within wider theoretical perspectives. The PhD candidate is studying written sources on historical stone adhesives, compiling art-technological sources from the medieval period to modern times, and covering three geographical regions. An open access database of this dataset will be published as part of this work. A database of medieval adhesive finds is being established, with the aim of sampling for analysis. Collaboration with Nidaros Cathedral Restoration Workshop has been established, and several visits to European cathedrals have been completed.
We have documented almost 500 adhesive use cases on Stavanger Cathedral. Information about each case has been systematised and categorised and will be used for statistical analyses. Where possible, miniscule samples were collected after approval from the Directorate for Cultural Heritage. Of these, 44 were selected for inorganic and organic analyses. The analyses of the composition of the historical tar samples are now in progress. The organic composition of 42 samples have been analysed so far, while the inorganic composition has been analysed in four of these . The organic materials have been analysed by Gas Chromatography Mass Spectrometry (GC/MS) at the Getty Conservation Institute (USA), while inorganic material is examined by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) at the University of Bergen, Norway. We are working on the interpretation of the results. Statistical analyses will be performed when we have collected more data, but descriptive statistics of the different tar samples from Stavanger cathedral is ongoing. We have undertaken radiocarbon dating of adhesive, mortar and scaffolding samples to refine dates of construction for Stavanger Cathedral and contextualise the adhesive samples.
Pine tar production has been undertaken and documented, in collaboration with craftspeople in Norway. This material will be analysed and tested subsequently. Adhesive and substrate sample preparations are underway for shear and tensile testing of adhesives on different soapstone samples at Delft University of Technology in the Netherlands.
Within the first year, project researchers have been very actively disseminating to the general public and specialist audiences. We have published two peer-reviewed scientific articles, and held a wide range of talks and workshops nationally and internationally. We have begun collaboration with an artist for a final transdisciplinary exhibition at the end of the project period. The project has already been visible in two museum exhibitions over the past year. Project members are also involved in the EU COST Action network CA22155 (POTARCH Network for forest by-products charcoal, resin, tar, potash). A Masters thesis affiliated with the project has been submitted in the field of traditional crafts.
Medieval northern European stone churches are at increased risk of damage due to climate change. The STICKING STONES project will rediscover forgotten medieval techniques of stone repair and construction using wood tar adhesives, repurposing this sustainable adhesive material for modern heritage conservation practice. The recent, hitherto uncharted discovery of hundreds of medieval wood tar repairs on Stavanger cathedral (Norway) provides significant potential for shedding light on an understudied historic craft tradition, advancing the state of the art in building archaeology, stone conservation and adhesives science. Forgotten medieval techniques of construction and repair which have successfully lasted 700 years can be drawn on to address the limited selection of suitable adhesives for stone conservation, contributing to the preservation of medieval stone architecture. The project uses one case study combined with extensive fieldwork and transnational perspectives to examine the broader medieval European context of architectural adhesive use. Intangible heritage and embodied knowledge are key to understanding the craft skills of wood tar repairs across medieval northern Europe, in conjunction with comprehensive material characterization of historic samples.
This tar discovery presents a novel solution for modern conservation in the form of alternative sustainable materials based on historic techniques. A multidisciplinary project team consisting of specialists from conservation, archaeology, history, materials science and geology will undertake pioneering research into an understudied topic, the historic use of tar as stone adhesive, subsequently testing its potential application in architectural conservation. In this manner, we will rediscover lost medieval knowledge and repurpose it for modern conservation practice by creating applied knowledge on the performance of reengineered wood tar adhesives for contemporary stone conservation and the built environment.
