Preservation of murals on salt loaded masonry vaults by electromigration

Abstract

Salt induced deterioration of murals in churches has increased within the last decades in Denmark. However, no lasting preservation method has been found. Either murals are repainted by conservators or placed in climate chambers which give the deterioration a break. Ion migration in a solution in an applied electric DC field is termed electromigration and is a well known and accepted transport process. In wet porous clay brick masonry, which most Danish church vaults consist of electro migration is expected to be an efficient transport mechanism for ion extraction out of the vaults. This idea was tested in the present work. Electromigration experiments in bricks with low NaCl concentration (spiked into the brick by soaking) showed that it is possible to clean a brick to its initial ion content new from the factory. An experimental setup which is applicable under real conditions and with ion contents corresponding to heavily polluted church vaults (1.4 weight chloride) showed significantly decreasing ion contents as a function of time. Experiments are ongoing to reach a harmless ion content which is thought to be a matter of duration and optimization of experimental conditions. Simultaneously with the electromigration process varying extends of acid and base is produced due to electrode reactions. Clarifying experiments showed extreme pH changes and pointed out the need for developments of electrode setups to avoid these changes. New experiments with the developed electrode units reduced the pH changes remarkably, which make the method usable in connection with cultural heritage. On the basis of successful laboratory results electrokinetics for extraction of ions from masonry vaults might be the missing lasting method to extract the damaging salts and thereby preserve the murals.