Semi‐quantitative analysis of the formation of a calcium oxalate protective layer for monumental limestone using combined micro‐XRF and micro‐XRPD

Abstract

A current method for the protection of cretaceous limestone present in various monuments consists of performing a passivating treatment with ammonium oxalate (AmOx). A calcium oxalate protective layer is formed on the surface and enhances the acid resistance of the stone. The in-depth formation of the calcium oxalate layer was investigated on cross sections by using combined micro X-ray fluorescence and micro X-ray powder diffraction (mXRF/mXRPD). XRPD showed the presence of both whewellite and weddellite in the calcite stone matrix. A correction was made for sample misalignment, which was visible in both the fluorescence and the diffraction line measurements. A semi-quantitative analysis was performed on the basis of Klug's equation for a two-phase mixture (the presence of weddellite was neglected) without the need for a known reference sample. By assuming two extreme compositions for a reference weight fraction (1 and 99 wt%), it was possible to obtain whewellite concentration profiles, which can be used for comparing the effectiveness of different methods for the application of AmOx to the stone surface and the effect of treatment time and AmOx concentration used. It is shown that for the relative amounts of whewellite formed, the differences due to the assumed weight fractions are smaller than the errors due to sample heterogeneity and preferred orientation.

Protection of Stone Monuments Using a Brushing Treatment with Ammonium Oxalate

“…In our previous research [13,14], we developed a procedure for applying ammonium oxalate (AmOx) by a brushing method with the aim of protecting monumental limestone objects. The efficiency of the brushing procedure for creating the protective CaOx layer was compared with the typical poultice treatment and with an immersion method.…”

“…For the second part of this research, the treated stones were exposed to 2% sulfuric acid for a period of 1 min. While SR-FTIR micro-scanning was used in our previous study to map the oxalate distribution on the surface of the treated stones [14], the quality of the obtained spectral datasets was limited because of large Reststrahlen bands which resulted in a high background noise. Therefore, in the present study, the formation of gypsum, which is the result of the reaction between calcite and the acid, is monitored using scanning SR-µ-XRF, exploiting the fact that sulfur is only present in trace quantities in marble and limestone building materials.…”

Synchrotron radiation μ X-ray diffraction in transmission geometry for investigating the penetration depth of conservation treatments on cultural heritage stone materials

“…A diffractometric approach (X-ray diffraction, XRD) is expected to be the most suitable strategy to characterize the crystalline reaction products and their diffusion below the surface. 30 Focusing on DAP treatments, recent studies carried out using X-ray powder diffraction (XRPD) 4,[11][12][13]17,31 investigated the phosphate phases formed on different carbonatic stone substrates by selecting sample powders at different depths from the DAP-treated surface. However, problems arise with conventional XRPD due to: (i) the prevalence of strong reection peaks of calcite with respect to calcium phosphate phases, which are minor phases; (ii) the presence, in the bulk analysis, of a complex mixture of crystalline phases which have similar lattice parameters, as in the case of calcium phosphates, with the overlapping of their marker reection peaks; (iii) the formation of non-stoichiometric, ionic substituted, calcium-decient or poorly crystalline phases; (iv) the destructive nature of the investigation (scratching and grinding) coupled with the poor selectivity of the sampling procedure (conventional XRPD requires a relatively high amount of material, which involves a coarse sampling of treated stone).…”

Synchrotron radiation Ca K-edge 2D-XANES spectroscopy for studying the stratigraphic distribution of calcium-based consolidants applied in limestones

“…Nevertheless, extended overlapping of the vibrational bands in these techniques may hamper clear distinction of the newly formed phases in the microstructure of the stone matrix. In such scenario, synchrotron radiation (SR)-based μ-XRD technique complements the range of the micro-analytical tools available for these applications, providing highly specific identification of the crystalline consolidant phases and their localization at a comparable lateral resolution 11 – 13 .…”