Effectiveness of oxalic acid treatments for the protection of marble surfaces

Abstract

Naturally formed rims of calcium oxalates developed on calcareous stones have been recognized as effective protective coatings. Inspired in nature, it has been recently proposed the use of oxalate salts for the protection of stone surfaces via dissolution of the calcitic substrate and the subsequent precipitation of oxalate phases. In contrast, the application of an oxalic acid solution on carbonate stones has been generally avoided due to assumed hazards associated with enhanced substrate dissolution. Nonetheless, it has been reported that coherent oxalate layers and preservation of textural features only occurs at low pH, which could be beneficial from a conservation point of view. Here, the application of oxalic acid treatments on two calcitic and dolomitic Spanish marbles from Macael area has been studied as a means to develop effective oxalate protective coatings. Morphological and compositional analyses show that reacted marble surfaces develop μm-thick calcium or calcium and magnesium oxalate rims on calcitic and dolomitic marble, respectively. The presence of such oxalate layers strongly reduces chemical weathering due to acid dissolution and sulfation, without altering the color of the marble substrates. This protection methodology overcomes the limitations of previous oxalate treatments and may represent a highly efficient conservation methodology.