Investigation of Calcium and Magnesium Phosphate Crystals in Stones Treated with Diammonium Hydrogen Phosphate Conservation Product: Potential of Micro-Raman Spectroscopy

Abstract

This study is aimed at investigating crystals (calcium and magnesium phosphates) formed due the interaction of an inorganic conservation treatment (diammonium hydrogen phosphate—DAP) with carbonatic (calcitic and dolomitic) stones through micro-Raman Spectroscopy. The addressed questions concern (i) the identification of magnesium and calcium phosphate minerals crystallized within dolomitic stone samples with a different degree of conservation state and treated with two different DAP solution molarities and (ii) the distinction of complex calcium phosphate mixtures (hydroxyapatite—HAP and octa calcium phosphate—OCP) crystallized within a calcarenite stone treated with DAP. A statistically relevant number of Raman spectra have been acquired in sample cross sections and curve fitting analysis has been performed for the in-depth interpretation of data. The outcomes indicate that Raman Spectroscopy is an effective alternative method for the identification of poorly crystalline calcium phosphates (not easily detectable with X-ray diffraction), even when scarcely present in mixture with magnesium phosphates. Evidence of the Raman analytical capability and high potential to distinguish HAP and OCP in calcitic stones are also presented and discussed.

Synchrotron radiation X-ray diffraction computed tomography (XRDCT): a new tool in cultural heritage and stone conservation for 3D non-destructive probing and phase analysis of inorganic re-treatments

“…Recent studies have investigated the crystal-chemistry and 2D distribution of newly formed calcium oxalates and calcium phosphates formed by AmOx and DAP with lab-scale instruments and large-scale sources 13,15,17,20,22,23,48 and have shown that a lot of variables (e.g., the minerals, microstructure and grain size of the substrate; pH; free calcium ion availability, etc.) affect the reaction of inorganic treatments with stone materials.…”