Sticking stones: rediscovering medieval wood tar adhesives for stone conservation

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 historic materials and techniques may provide the key to saving our architectural stone heritage. How and why was wood tar used by craftspeople in the repair of stone buildings during the Middle Ages? To answer this question, the Sticking Stones project will undertake pioneering research into the use of wood tar as stone adhesive in cathedral construction and repair. 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 tar repairs can be of help in developing new adhesive types for the conservation of stone architecture. In uncovering conclusive evidence of widespread medieval European use of tar repairs, long-forgotten craft practices can be relearnt. The concepts of tacit knowledge and the intangible cultural heritage of craft skills will be explored in depth to shed light on the decisions made by medieval craftspeople. Fieldwork and building archaeology will be combined with a community science project to map evidence of adhesive repairs in Europe. 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 will address through material characterization and adhesive reengineering. This ambitious and novel project aims to revolutionise conservation practice by adding historic methods to the contemporary sustainable conservation toolkit.

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.