The kinetics of salt weathering of porous materials : stone monuments and wall paintings
The aim of this research is to improve ways of reducing the damage caused by salts to
cultural property. A specific focus of attention is the use of environmental control as a
passive measure. Environmental control attempts to specify optimum ranges of relative
humidity (RH) and temperature (T), to control salt phase transitions.
To undertake environmental control a range of suitable climatic conditions are
selected on the basis of thermodynamic calculations. These calculations are used to
determine a range of RH and T in which salt phase transitions are minimised, and require
very accurate analytical data of the salt content of the object. Unfortunately, in practice it is
rarely possible to maintain this close range of RH and T. It therefore becomes necessary to
know the speed of salt deterioration at levels of RH and T outside the optimum range, and
establish the time it takes for damage to occur. Consequently, for environmental stabilisation
measures to be successful, there are two critical areas where further research is mandatory.
Environmental control is dependent on reliable information of the object's salt content, and
better methods for determining this are needed. Moreover, an understanding of the rate of
salt phase transitions is essential, so that the degree of control achieved is sufficient to limit
the damage. These two issues are addressed by the present research.
The work comprised both ex situ and in situ investigations. A key feature throughout
was the use of statistical methods for the design of each component of the project. This
approach provided a means of unravelling complex multi factor interactions, and gave clear
unequivocal results. Laboratory experiments were undertaken to assess the rate of water
vapour sorption by salt-contaminated stone and limeplaster. Experimental design and
analysis of variance techniques were used to determine the relative significance of the
following kinetic factors: RH, T, airspeed, salt mixture composition, salt concentration, and
support type. In situ investigations were carried out at Cleeve Abbey, Somerset, to study the
Cl3th wall paintings in the Sacristy over one year. The work included documentation,
sampling and analysis of the paintings, and environmental monitoring. The results were
subjected to statistical analysis to assess changes in the salt distribution, spatially and over
time, in relation to the environmental conditions.
The outcome of the ex situ and in situ investigations collectively provide important
new information about the kinetics and mechanisms of salt damage, and reveal better
practical methods for assessing and ameliorating these problems