Development of a poultice for electrochemical desalination of porous building materials: desalination effect and pH changes

Salt induced deterioration of structures and stones are generally recognized as a frequent and serious problem. The deterioration is especially undesired in relation to cultural heritage as it is impossible to recreate original material (e.g. original murals). By presence of salts in decorated vaults two different techniques are applied: poultices or establishment of climate chambers. Both techniques can result in ion transport away from the valuable surfaces with murals, but satisfying desalination has not been obtained according to conservators from the Danish National Museums mural preservation section in consistence with the present available literature. In the present paper the possibility for salt removal by utilizing a well known and accepted transport process, electromigration, is investigated, i.e. movement of ions in a solution in an applied electric DC field. An experimental laboratory setup was designed to approximate real conditions in vaults and with ion contents corresponding to normal heavily polluted church vaults (1.0 wt% Chloride, added as NaCl). During the electromigration process acid and base is produced at the electrodes due to electrode reactions and in a clarifying experiment with a traditional poultice significant pH changes and an absence of satisfying high desalination effect was measured. The new idea in the present paper was to introduce a calculated amount of buffer components corresponding to the productions during the electrode processes to a poultice (a solid) to minimize the adverse effects and to optimize on the effects. The results showed good ability to retain neutral pH values in the substrate which is of major importance when the method should be applied on existing structures. Also the desalination process continued until a very low and harmless salt content was reached after introduction of the buffer components.