Stone Conservation: An Overview of Current Research (2010)

Stone Decay 9has shown that if both layers are fully saturated with water at the start ofa drying experiment, the stone will dry after the plaster and soluble saltsin the stone will tend to be retained.All the Information We Need?With such sophisticated forms of investigation being pursued, one mightbe forgiven for thinking that no problems remain in the measurement ofstone decay. There is, however, a long way to go. Stone decay is a complexphenomenon, and no single technique can disentangle and quantifyits causes and effects. Advances in experimental work, field measurements,and theory—each building on the other—are still needed. Thetechniques that we have looked at thus far are certainly useful, butthe methodical measurement of decay and our understanding of decayprocesses over time have not yet met the goal set forth earlier of conservationdecisions being based on measurements instead of assumptions.CAUSES OF DECAYBefore we can take any action to prevent or to remedy the deteriorationof stone, we must understand what is causing that deterioration.Sometimes the cause is obvious; sometimes there may be several differentcauses acting at once. In an attempt to clarify the relative importanceand interdependency of individual causes, Verdel and Chambon(1994) have introduced the principles of system dynamics. 1 Stone decaymechanisms and rates are reviewed in the proceedings of two Dahlemmeetings (Doehne and Drever 1994; Viles 1997), and both reports pointout areas where additional research is needed, essentially providing usefulroad maps for research. An interesting example of quantifying therelative importance of a range of factors—in this case for absorptionand desorption of moisture—is the careful research by Sawdy (1995;2002). She found, for example, that for environmental control of saltdecay in wall paintings, relative humidity (RH), airflow, substrate type,and temperature are important factors, while earlier research hademphasized only RH.Some of the causes of stone decay are sudden and rapid in theireffect. Those toward the latter part of the following list are slow andmore insidious: earthquake, fire, flood, terrorism, vandalism, neglect,tourism, previous treatments, wind, rain, frost, temperature fluctuations,chemical attack, salt growth, pollution, biodeterioration, intrinsic factors,and so on.The literature includes many papers dealing with the causes ofdecay and some reviews are available, e.g., Ashurst and Dimes 1998;Honeyborne 1998; Grassegger 1999; Feilden 2003; Smith, Gómez-Heras,and McCabe 2008. Goudie and Viles (2008) trace the remarkable historyof the study of physical, chemical, and biological weathering. Recent literatureis dominated by three topics: air pollution, salts, and biodeterioration.These are considered in the following sections.