نبذه عن الكتاب: York Minster is over 500 years old and requires very expensive maintenance to ensure its existence for future generations. It is composed of a wide variety of materials, which have been subjected to erosion. The external fabric of stone has been eroded both by natural weathering and by chemicals put into the atmosphere by man’s actions (Figure 1.1). Winning the battle between the Minster and its environment lies not only in reducing atmospheric pollution but also in the replacement of damaged stone. This replacement is not a simple matter because, although there is fresh stone in abundance, the continual replacement of stone can undermine the integrity of a building by causing the structure to become unstable. At York Minster the stonemasons try, as far as possible, to incorporate all old stone into any repair work because there is the danger that if too much fabric is replaced there could be a serious loss of authenticity (Brimblecombe and Bowler 1990). What chemicals then have caused the erosion of the stone? The pollutants sulphur dioxide (SO2), nitrogen oxides (NOx) and ozone (O3) are believed to be the main culprits. Although York Minster has suffered from anthropogenic effects, there is no method by which these effects can be readily separated from other contributions such as poor construction techniques, wrongly chosen materials, natural weathering and biological attack. The rate of destruction can be significantly increased during heavy rainfall if vast quantities of water containing pollutants percolate through the stone carrying away any reaction products in the run off (Baer and Snethlage 1997). The reaction products are mainly salts which are formed by the reaction of negatively charged anions (e.g. SO4 2
) from pollutant gases or acids, with positively charged cations in the stone (e.g. Ca2).