PAPER TITLE: “Preliminary results on biomimetic methods based on soluble ammonium phosphate precursors for the consolidation of archaeological wall paintings”

Magdalena Balonis-Sant, Xiao Ma, Ioanna Kakoulli

245th ACS National Meeting and Exposition April 7-11, 2013
DIVISION: HIST: Division of the History of Chemistry
SESSION: 12th Archaeological Chemistry Symposium

Abstract: This research develops hydroxyapatite (HAP)-based, inorganic mineral systems with improved properties for the consolidation of powdery wall paintings of archaeological significance. The scientific approach exploits biomimetic (biologically inspired design) principles to induce the formation of protective HAP crystals by triggering reactions  between the calcium carbonate-rich layers in wall paintings and ammonium phosphate precursors. The high solubility and absence of toxicity of ammonium phosphates (precursors) and the stability of the calcium hydroxyapatite (reaction product) at varying pH, renders this treatment extremely promising for consolidation and protection of weathered wall paintings. Tests were carried out on experimental wall painting panels (representing the most common typologies across space and time) applying cellulose compresses of 1M and 2M solutions of diammonium hydrogen phosphate for 3 to 6 hours contact time. The consolidating effect, influence of the solution and conditions (composition, pH, contact time, application method) on hydroxyapatite formation (rate, extent) was evaluated through a series of structurally and compositionally sensitive analytics including: VPSEM-EDS, sorption studies, optical and mechanical analyses. Preliminary results indicated the formation of a porous hydroxyapatite network at the subsurface of the wall painting test panels, reduction of water absorption and dissolution at low pH and insignificant color change. These data show the potential of this treatment for the consolidation of powdery multi-layered wall paintings and their protection from weathering and deterioration induced by passage-of-time and environmental action linked effects.