Time-Resolved ATR-FTIR Spectroscopy and Macro ATR-FTIR Spectroscopic Imaging of Inorganic Treatments for Stone Conservation

Abstract

In this study, the novel application of ATR-FTIR spectroscopy and macro ATR-FTIR spectroscopic imaging overcame an analytical challenge in conservation science: the time-resolved, chemical, and spatial investigation of the reaction of inorganic treatments for stone conservation (ammonium oxalate, AmOx; ammonium phosphate, DAP) occurring in water-based solutions. The aim was to (1) assess the composition and localization of reaction products and their phase variation during the reaction in real time and directly in an aqueous environment and (2) investigate the reaction of AmOx and DAP with calcite and the transformations induced to the substrate with a time-resolved approach. The new analytical results showed that for both treatments, the formation of new crystalline phases initiated at the early stages of the reaction. Their composition changed during the treatment and led to more stable phases. The reactivity of the stone substrate to the treatments varied as a function of the stone material features, such as the specific surface area. A clear influence of post-treatment rinsing on the final composition of reaction phases was observed. Above all, our research demonstrates the actual feasibility, practicality, and high potential of an advanced ATR-FTIR spectroscopic approach to investigate the behavior of conservation treatments and provided new analytical tools to address the choices of conservation in pilot worksites. Lastly, this study opens novel analytical perspectives based on the new possible applications of ATR-FTIR spectroscopic imaging in the field of conservation science, materials science, and analytical chemistry.

Figure 1

AmOx treatment. ATR–FTIR spectra of the following: deionized water (a), AmOx solution (b), AmOx solution applied to Carrara marble (c), untreated Carrara marble (d), and Carrara marble slab after 2′30″ (e) and 142′30″ (f) from the beginning of the AmOx treatment. The ATR–FTIR spectra (e,f) are obtained by spectral subtraction of the AmOx solution.

Figure 2

Integrated absorbance of the ν_as(CO) (▲) and δ_in-plane(OCO) (●) marker bands measured on powders (“Pwd”, red lines) and slab (“Slab”, blue lines) of the Carrara marble plotted versus the treatment duration. Integrated range for the ν_as(CO) band: 1345.9–1247.5 cm^–1. Integrated range for the δ_in-plane(OCO) band: 810.05–725.19 cm^–1.

Figure 3

DAP treatment. ATR–FTIR spectra of water, of the DAP solution, and of the Carrara marble treated in real time at the beginning of the reaction (t₁), during the reaction (t₂–t₁₄), and at the end of the reaction (t₁₅).

Figure 4

Integrated absorbance of the ν₃(CO₃^2–), ν₂(CO₃^2–), and ν₄(CO₃^2–) vibrational bands measured during the DAP treatment on the surface of the Carrara marble slab plotted versus the treatment duration. Integrated ranges are as follows: 1519.0–1336.1 cm^–1 for the ν₃(CO₃^2–); 891.1–783.87 cm^–1 for the ν₂(CO₃^2–); and 724.08–686.53 cm^–1 for the ν₄(CO₃^2–).

Figure 5

Macro ATR–FTIR spectroscopic images of Carrara marble powders treated by DAP in real time. The chemical images show the spatial distribution of the ν₃(CO₃^2–) of calcite (1400 cm^–1), ν₃(HPO₄^2–) of DCPD (1125 cm^–1), ν₃(PO₄^3–) of HAP and C-HAP (1023 cm^–1), and ν₃ (H₂O) of the water-based DAP solution (1636 cm^–1) in relation to the different treatment time (∼20 min, ∼30 min, ∼1 h, and ∼2 h). The white arrows in HAP chemical images indicate the clear formation of the apatite crystal network after ∼1 h of the DAP treatment. The imaging area is ∼0.6 mm (vertical) × 0.55 mm (horizontal).