A study of salt reduction of globigerina limestone surfaces

Abstract

The Nymphaeum, found at Argotti Botanic gardens in Floriana, is an 18th century monument constructed in Globigerina Limestone and partially decorated with a shell, coral and precious stone mosaic that is undergoing active deterioration due to salt weathering. The study reviews the characteristics and deterioration mechanisms of Globigerina Limestone, using possible salt reduction systems adopted both locally and internationally in the field of conservation. The study focuses on poultices for salt reduction, a commonly used method that can be applied on the Nymphaeum’s limestone surface to slow down its rate of deterioration and to serve as preparation for other conservation treatments. A condition assessment of the Nymphaeum indicates that this semi-open structure is actively deteriorating because of high chloride and sulfate ion concentrations just below the surface of the limestone. Long term environmental monitoring also suggests that the salt mixture present is undergoing dissolution and precipitation because of high and low peaks of relative humidity. A series of laboratory tests were designed to find an effective salt reduction poultice method, testing commonly-used materials like cellulose powder, clay minerals, and quartz sand. The poultices’ working properties, and salt reduction efficiency, were tested on halite-contaminated Globigerina Limestone surfaces. The tests showed that poultices containing cellulose, bentonite and quartz sand were the most effective in salt reduction, out of the tested series of materials. The poultice material however tended to leave residues on the surface, and to partially redistribute the salt solution deeper inside the substrate. These limitations should be resolved in further studies before the poultice method is applied at the Nymphaeum or other historical monuments in Malta.