Repair of sugaring marble by ammonium phosphate: Comparison with ethyl silicate and ammonium oxalate and pilot application to historic artifact

No fully satisfactory consolidant for sugaring marble currently exists, hence the use of ammonium phosphate to form hydroxyapatite (HAP) inside marble micro-cracks has recently been proposed. This study was aimed at: (i) investigating different treatment formulations (application of a 3 M aqueous solution of diammonium hydrogen phosphate (DAP), with/without addition of 1 mM or 3 mM CaCl2, with/without subsequent limewater poultice application) and characterizing the new Ca-P phases; (ii) systematically testing mechanical effectiveness and compatibility with the substrate of the most promising formulations, in comparison with ethyl silicate and ammonium oxalate; (iii) performing a pilot application of the most promising formulation to a real marble artwork affected by sugaring. The results of the study indicate that application of a 3 M DAP solution, followed by limewater poultice application, produces remarkable consolidation of weathered marble, with only slight alterations in pore size distribution and color change, thus providing much better results than both ethyl silicate and ammonium oxalate. Notably, while the above-described treatment conditions caused formation of tricalcium phosphate and octacalcium phosphate in artificially weathered samples, the same conditions caused formation of HAP in the historic artwork, presumably because some gypsum residues were present and calcite grains had high surface roughness.