The impressive appearance of the second largest Gothic cathedral in Northern Europe, York Minster, has dominated the townscape since medieval times (Figure 1). Representing the architectural expression of developing Christianity during that period, its international historical significance is unquestioned, and the Minster has provided a source of inspiration for scholars through the ages. However, recent centuries have seen conservationists battling to preserve the authenticity of this unique monument for future generations, against weathering of the magnesian limestone exterior stonework by atmospheric pollutants.
Scientists from Cardiff University, the University of Iowa (USA), and Diamond Light Source, have recently investigated the potential of hydrophobic surface coatings as protection for the cathedral walls and grotesques. Their findings, published in this week’s Scientific Reports, a new open access journal from the Nature Publishing Group, may now pave the way to improving masonry resilience to acid rain, and conserving cultural heritage.
Figure 1. The South Transept of York Minster and circular Rose Window. (Courtesy of R. Walker and A. Holton.)
The Minster was constructed between 1220 and 1470 using magnesian limestone (CaxMgy(CO3)2) amongst other building materials, much of it sourced from local quarries. These sedimentary rocks possess a fine grained structure, and proved a perfect material for medieval builders finding widespread application in historical buildings across the UK, Europe and North America.
Unfortunately, magnesian limestone, and its simpler building counterpart limestone (CaCO3), is prone to attack by common environmental pollutants, such sulfur containing oxides in acid rain, or particulate sulfate matter, predominantly released into the atmosphere during the combustion of fossil fuels in power stations or vehicles. The resulting sulfate-induced limestone decay, via formation of sparingly soluble Ca2+ salts from the reaction of limestone with SO2 or acid rain, is visible at many points around the cathedral exterior, causing stonework to blacken and slowly erode, destroying finer details created by stone cutters in the middle-ages.
Hydrophobic surface coatings offer one solution to protect vulnerable stonework. Alternative coatings are disadvantageous, as they may seal the masonry microstructure, reducing ‘breathing’ and causing mould growth and salt efflorescence. The present research utilised powerful laboratory and synchrotron X-ray methods to investigate the impact of a new conformal surface coating, derived from naturally-sourced free fatty acids, combined with trace amounts of fluorinated alkylsilanes, which impart super hydrophobicity
to pure limestone; super hydrophobic surfaces exhibit the so-called ‘lotus effect’, repelling water and snow and enabling easy removal of dirt. The benefits of this super hydrophobic coating on retarding the sulphate-induced decay of limestone were quantified by X-ray Absorption Spectroscopy measurements on beamline B18 of the Diamond Light Source. These studies led the Cardiff team to apply the super hydrophobic formulation to 19th century stonework from York Minster, protecting it against sulphuric acid permeation (Figure 2).
Figure 2. Mechanism of sulphate-induced limestone decay, and protection by super hydrophobic coating. (Image courtesy of Scientific Reports.)
This work is helping to elucidate the mechanisms of limestone weathering and decay, and guiding the development of conservation techniques for the ongoing restoration of York Minster, helping to protect the future of historic architectures.