An Assessment of the Role of an Open Shelter in Reducing Soiling and Microbial Growth on the Archaeological Site of the Bishop’s Palace, Witney, England

Abstract

Two sets of twelve Portland limestone tablets were attached to carousels outside and under the open, lightweight shelter at Bishop’s Palace (Witney, England) for 18 months to assess the influence of shelters on soiling and microbiological growth on stone. At the start, and at 6-month intervals, tablets were analysed using microscopy (optical and SEM), colour measurement (spectrophotometry), weight change measurement and salt content determination (ion chromatography). Concentrations of NO₂ and SO₂ were also monitored for a month using diffusion tubes. In addition, results were compared with those obtained by the UK National Materials Exposure Programme (NMEP) on chemical weathering rates. Differences in soiling and biological growth on sheltered and unsheltered samples, and their influence on limestone decay were established. The shelter is likely to reduce biological growth by modifying the micro-environmental conditions. However, it may also exacerbate dust deposition, which might increase decay rates in the long-term.

Keywords:

• Shelters for archaeological sites
• Bishop’s Palace
• soiling
• microbial growth
• Portland limestone tablets
• decay rates

Acknowledgments

We would like to thank Christiane Jeuckens (Oxfordshire County Council) and Chris Welch (Historic England, formerly English Heritage) for allowing us to study Bishop’s Palace.