The mechanism of acid attack on marble and a comparison of surface treatments for improved acid attack resistance

Abstract

Acid rain damage is one of the primary threats to marble monuments and sculpture around the world. Therefore, research in conservation science has begun to focus on surface treatments to confer lasting acid resistance onto marble by reacting the CaCO3 mineral with a variety of salt solutions. The goal of this study was to evaluate the effectiveness of three of these treatments: ammonium oxalate, ammonium hydrogen tartrate, and diammonium hydrogen phosphate (DAP). Additional experiments were conducted to further clarify the mechanism of acid attack on marble and to determine whether triammonium phosphate could be used in place of DAP to create coatings of hydroxyapatite. A combination of nitric acid exposure tests and scanning electron microscopy were to used determine the relative merits of each treatment. The results indicated that the ammonium oxalate solution produced the most acid resistant coating, followed by the solution of DAP and CaCl2. The tartrate treatment was not effective at increasing acid resistance over the baseline for untreated marble. The supplemental experiments showed that acid attack on marble was concentrated at grain and twinning boundaries and produced surface etching topographies dependent on grain orientation. Finally, triammonium phosphate was not viable for replacing DAP as a component of surface treatments due to its precipitation characteristics in solutions with CaCl2 and unsatisfactory film deposition.