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Article

A Novel Approach to Carbonate Stone Conservation: Induced Calcium Oxalate Formation Through the Application of Ammonium N-Ethyloxamate (AmEtOxam) on White Carrara Marble

by
Simone Murgia
1,
M. Carla Aragoni
1,
Gianfranco Carcangiu
2,
Laura Giacopetti
1,
Domingo Gimeno Torrente
3,
Vito Lippolis
1,
Eleonora Loi
1,
Paola Meloni
4,5,
Antonia Navarro Ezquerra
6,
Enrico Podda
1,7,
Anna Pintus
1,
Riccardo Serra
1 and
Massimiliano Arca
1,*
1
Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, S.S. 554 bivio per Sestu, 09042 Monserrato (Cagliari), Italy
2
Consiglio Nazionale delle Ricerche (CNR), Istituto di Scienze dell’Atmosfera e del Clima (ISAC), UOS di Cagliari c/o Dipartimento di Fisica, Università degli Studi di Cagliari, S.S. 554 bivio per Sestu, 09042 Monserrato (Cagliari), Italy
3
Facultat de Ciències de la Terra, Universitat de Barcelona, c/ Martí i Franquès s/n, 08028 Barcelona, Spain
4
Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, via Marengo 2, 09123 Cagliari, Italy
5
Laboratorio Colle di Bonaria, Università degli Studi di Cagliari, via Ravenna snc, 09125 Cagliari, Italy
6
Departamento de Tecnología de la Arquitectura, EPSEB-UPC, Avda. Doctor Marañón, 44-50, 08028 Barcelona, Spain
7
Centro Servizi di Ateneo per la Ricerca (CeSAR), Università degli Studi di Cagliari, S.S. 554 bivio per Sestu, 09042 Monserrato (Cagliari), Italy
*
Author to whom correspondence should be addressed.
Molecules 2026, 31(5), 776; https://doi.org/10.3390/molecules31050776
Submission received: 28 January 2026 / Revised: 18 February 2026 / Accepted: 20 February 2026 / Published: 25 February 2026
(This article belongs to the Section Inorganic Chemistry)
Browse Figures
Versions Notes

Abstract

Ammonium N-ethyloxamate (AmEtOxam) was synthesized, fully characterized by microanalytical and spectroscopic means, and assayed as a precursor of calcium oxalate, acting as a protecting agent for white Carrara marble. The monohydrate form of AmEtOxam shows a water solubility of 1.5 mol·L−1 (~23% w/w), significantly higher than that of common calcium oxalate precursors (CaOx), such as ammonium oxalate (0.4 mol·L−1, ~5% w/w). While AmEtOxam is stable in water solution and in the solid state in its monohydrate form, during the application on carbonate stone it undergoes a complete hydrolysis resulting in the formation of a uniform weddellite layer (CaC2O4·2H2O) on carbonate stone surfaces. Application of 5% w/w aqueous solutions by spraying, brushing, and immersion resulted in different effects. Spraying yielded the most balanced performance, improving mechanical strength, reducing water absorption, recovering superficial tension, and limiting chromatic alteration. Brushing achieved significant gain in surface hardness with minimal esthetic impact. Immersion most effectively reduced porosity and increased surface tension. These results, coupled with the negligible chromatic changes induced in all cases, make AmEtOxam a promising candidate for the conservation of stone cultural heritage.

Graphical Abstract

1. Introduction

White Carrara marble, due to its textural features, is known for its brilliant appearance and fascinating shades, which have made it one of the materials most used in architecture and figurative art [1,2,3]. Calcite constitutes more than 95% of this particular marble variety [4] and most of the physical weathering phenomena are due to spontaneous weakening of this phase, [5,6] that, combined with the anisotropic thermal behaviour of this material, develops a network of microcracks during day/night and heating/cooling cycles [7,8]. The water dissolution of calcite [9,10] (Ksp = 3.27 × 10−9 at 25 °C) [11] is dramatically accelerated by acid rain, resulting from air-pollution due to anthropic activities, with the conversion of calcite into soluble phases, such as calcium nitrate or gypsum [12,13]. Finally, the dissolution can be promoted by the acidic metabolites typical of biological colonies of lichens, fungi, algae, and lithoautotrophic bacteria [14,15,16]. These phenomena contribute to the weathering process, which poses a substantial challenge in the preservation of architectural elements in buildings and the mortars used in their construction, as well as artistically carved stone details.
Stone consolidants typically enter the cracks and re-establish adhesion between detached grains [17], thus limiting the stone deterioration, avoiding loss of material, and the roughening of the surfaces [18]. To date, only a restricted number of inorganic compounds have been assessed as conservation agents for carbonate stone substrates [19]. Examples include alkaline earth hydroxides [20,21,22,23,24], CEGC (calcium ethylene glycol complex) [25], which assist in the remineralization of calcium carbonate [26,27,28], and salts such as diammonium hydrogen phosphate (NH4)2(HPO4) [12,29,30,31] and ammonium hydrogen phenylphosphonate [32]. Over the past few decades, there has been a renewed interest in the use of ammonium oxalate (NH4)2(C2O4) (AmOx) as a passivation and protection agent [33,34]. AmOx reacts with calcium carbonate to produce calcium oxalate (CaOx) hydrate Ca(C2O4)·nH2O (n = 1, whewellite, Ksp = 2.0 × 10−9 at 25 °C; n = 2, weddellite, Ksp = 3.8 × 10−9 at 25 °C), as shown in Equation (1) [35,36]:
CaCO 3 ( s )   +   ( NH 4 ) 2 ( C 2 O 4 ) ( aq )   +   ( n   −   1 )   H 2 O ( l )   ⟶   Ca ( C 2 O 4 ) · n H 2 O ( s )   +   2   NH 3 ( g )   +   CO 2 ( g )
The newly formed hydrated calcium oxalate layer not only provides consolidation but also acts as a protective barrier against acid rain and polluted atmospheric conditions due to its resistance to hydrolysis in acidic environments. The formation of a hydrate CaOx patina ultimately resembles the action of natural microorganisms [37,38], which produce biologically induced constructive carbonate-oxalate patinas that contain both whewellite and weddellite [39] (in both natural rock and architectural stained-glass windows) [40,41,42,43]. Furthermore, AmOx turns the gypsum deposits (resulting from acidic rains) into whewellite, Equation (2) [19]:
CaSO 4 · 2 H 2 O ( s )   +   ( NH 4 ) 2 ( C 2 O 4 ) ( aq )   ⟶   Ca ( C 2 O 4 ) · H 2 O ( s )   +   ( NH 4 ) 2 SO 4 ( sq )   +   H 2 O ( l )
Unfortunately, the water solubility of AmOx at room temperature (maximum 0.4 mol·L−1) limits the concentration of the solutions used for the treatment of marble. Consequently, the treatments are implemented using diluted solutions or dispersions (e.g., in poultice applications) [44].
These disadvantages can be overcome by following two different strategies, i.e., (i) obtaining the oxalate anion in situ from suitable precursors or (ii) substituting the oxalate anion with related derivatives whose ammonium salts display a higher water solubility than AmOx and can thus precipitate as insoluble calcium salts on the stone material [44,45,46,47,48,49,50]. In this context, dimethyloxalate and diethyloxalate were evaluated as precursors of the oxalate anion (Scheme 1) [45,51]. Recently, approaches involving sequential treatment with mixtures of oxalates were proposed, including AmOx mixed with calcium acetate or dimethyloxalate, with the latter being sensibly more soluble in water than the ethyl homologue [45]. Since the ammonium salts of oxalic acid monoesters (NH4)(ROC(O)COO) (R = Me and Et; AmMeOx and AmEtOx, respectively, featuring the MeOx− and EtOx− anions, Scheme 1), showing a higher solubility than AmOx (1.22 mol·L−1 (12.3% w/w) and 1.49 mol·L−1 (20.1% w/w), respectively), are prone to hydrolysis, they have been exploited as precursors for the formation of whewellite protective layers with remarkable penetration capabilities [46,48]. The thickness of the CaOx coating depends on the characteristics of the stone samples, with the deposition layer on marble samples varying in depth from 10 to 50 μm. However, a larger variability was observed upon treatment on more porous limestones [44,46,47].
The oxamate NH2C(O)COO− and aryl monooxamate ArNHC(O)COO− anions (Oxam− and PhOxam−, respectively, in Scheme 1) demonstrated enhanced water stability [48] and the capacity to generate calcium oxamate coatings, despite their susceptibility to hydrolysis over an extended period of time [49]. Conservative treatments on white Carrara marble were successfully tested using ammonium oxamate (NH4)(NH2C(O)COO) [37] (AmOxam) and N-phenyloxamate (NH4)(PhNHC(O)COO) [51] (AmPhOxam), although the introduction of a phenyl ring at the nitrogen atom of the oxamate anion in AmPhOxam resulted in a dramatic decrease in water solubility [49]. Alkyl oxamate derivatives show a greater tendency to hydrolysis. Some of the authors recently reported the case of ammonium pyridin-2-yloxamate (AmPicOxam, N-(pyridin-2-ylmethyl)oxamate, PicOxam−, in Scheme 1), which exhibits a high water solubility (1.01 mol·L−1) and acts as a precursor of CaOx [37,50].
The possibility of designing oxalate anion derivatives to fine-tune solubility of the consolidant and the nature of the coating on treated marble samples encourages the testing of different CaOx precursors to establish robust relationships between the chemical structure of the consolidant and its consolidation and protective performance. In this work, the investigation on the consolidation ability of ammonium N-ethyloxamate (AmEtOxam, Scheme 2) on white Carrara marble stone mock-ups is described.

2. Results and Discussion

2.1. Synthesis and Characterization

The synthesis of ammonium N-ethyloxamate (AmEtOxam, Scheme 2) was carried out following a two-step synthetic methodology [49,50]. Diethyloxalate was made to react with ethylamine in a 1:1 molar ratio and the resulting monoester (O-ethyl-N-ethyloxamate, Scheme 2) was subsequently converted into AmEtOxam by reaction with (NH4)(HCO3) in water.
The former reaction involves the formation of small amounts of the N,N’-diethyloxalamide as a secondary product. This compound can be easily separated by simple filtration thanks to its low solubility, and 1H-NMR (Figure 1) spectroscopy measurements confirmed its almost complete removal with a very low intensity for the singlet signal corresponding to the two NH protons (7.46 ppm).
The characterization of O-ethyl-N-ethyloxamate and AmEtOxam was carried out by microanalytical (m.p. determination and elemental analysis) and spectroscopic (1H-NMR, UV-Vis, and FT-IR) methods (Table S1, Figure 1 and Figures S1–S4). Based on elemental analysis and thermogravimetric analysis (TGA; Figure 2), AmEtOxam was isolated as a pure product in its monohydrate form. The solubility (~23% w/w, corresponding to a 1.5 mol·L−1 concentration of a saturated aqueous solution) was determined spectrophotometrically after recording a calibration curve (Figure S5).

2.2. Reaction of AmEtOxam with CaCO3

The reactivity of AmEtOxam·H2O with calcite was evaluated by reaction with a stoichiometric amount of CaCO3 powder in an aqueous suspension for 24 h at room temperature (Scheme 3). A white solid was collected and analyzed using powder X-ray diffraction (PXRD), which demonstrated that the calcium salt underwent hydrolysis during precipitation, resulting in the formation of calcium oxalate hydrate in the form of both weddellite and whewellite (Figure S6), confirming the tendency of alkyloxamates to hydrolyze in the presence of solid calcium carbonate [48].

2.3. Treatment of White Carrara Marble Samples

The marble lithotype adopted for this study came from the Michelangelo quarry in the Apuan Alps, and the typology “Statuario” was selected, thanks to its historical importance, to prepare stone mock-ups that were as similar as possible to the materials found in actual monuments. With a grain size of ~100 µm, this material is very uniform and features a typical granuloblastic texture that makes a mosaic or a non-oriented polygonal fabric. Subhedral CaCO3 crystals often had 120° junctions and clear, straight edges. PXRD measurements showed that the marble texture contains only small amounts of muscovite/illite and other phyllosilicates, as well as opaque minerals, in addition to calcite, which amounts to over 99% of the stone substrate (Figure S7). Stone samples were either cut into prisms of approximately 2.0 × 2.0 × 8.0 cm or small fragments with a volume of ~1 cm3 (Table S1). Samples in a high state of degradation could be obtained by means of a mild thermal degradation (Table 1) that includes a rapid heating (from room temperature to 300 °C at 2.50 °C·min−1), a static heating (300 °C for 5 h), and a gradual cooling to room temperature (1.66 °C·min−1; Figure S8). In comparison to the pristine specimens, a general deterioration of dynamic, structural bulk and surface properties (Table S1) was observed for weathered samples. This mild treatment confirms the effectiveness of thermal weathering achieving the Köhler classification of high degradation for historical marble [52,53]. Thermally weathered samples were characterized, before and after the treatments, by colorimetry, tensile strength determination, ultrasonic pulse velocity measurements, helium pycnometry, contact angle, surface roughness, and hardness (Shore Scale D). Based on previous results obtained for AmPicOxam, which indicated that the application of CaOx precursors at higher concentrations (12% w/w) leads to less uniform coatings [50], a 5% w/w aqueous solution of AmEtOxam·H2O (0.33 mol·L−1) was applied to stone samples using the static immersion method, brushing, or air spraying. Table S2 summarizes the conductivity and pH of the solutions before and after the immersion. Notably, the partial hydrolysis of N-ethyloxamate (Scheme 2) does not significantly affect the pH of the solution (pH ~5.5).

2.4. Characterization of Treated White Carrara Marble Mock-Ups

Macroscopic observation of the treated marble samples with optical microscopy showed no evident chromatic alterations (Figure S9). For all the treatments, a clear deposition was visually recognizable thanks to the newly obtained superficial homogeneity. The study of cross-sections of all the kinds of treated specimens allowed us to recognize the formation of a coating with a homogenous width (~30 µm for immersion and up to ~50 µm for spraying) and a significant penetration depth (in all cases higher than 120 µm, up to 210 µm for spraying; Figure S10).
SEM images of all the treated marble specimens showed a homogeneous deposition of assembled crystals (average crystal size 2.5 µm) distributed over the surfaces (Figure 3).
The chemical nature of the crystal deposition on the stone surface was determined by XRD analyses, which were carried out directly on the surface of bulk samples. In all cases, the diffractograms allowed us to clearly identify weddellite as the only component of the coating (Figure S11). The comparison between pristine (PR), thermally weathered (TW), and treated marble samples showed clear differences in how the three treatment methods—immersion, brushing, and spraying—affected the macroscopic properties of the material (Table 1; Figures S12 and S13). Weathering led to a strong deterioration, resulting in a major increase in ultrasonic propagation time (tuts, +138%) reflected in a 60% decrease in velocity (vuts), along with a substantial reduction in Young modulus (Ed, −82.9%) and tensile strength (fh, −39%). Treatments with AmEtOxam improved these values, but spraying and brushing were more effective than immersion in restoring mechanical performance (Figure S12). Spraying reduced tuts more than 70% compared to TW and increased vuts and Ed to values even higher than those of the original PR samples, showing a significant improvement in structural cohesion. Brushing also gave excellent results, while immersion produced smaller improvements. These outcomes suggest that surface-targeted applications, such as spraying and brushing, may reinforce the material more efficiently than immersion. In terms of surface hardness, all treatments improved values beyond PR levels. Tensile strength fh also increased significantly with treatment, especially in specimens treated by brushing and spraying, and reached a value as high as 1.00 N·mm−2, i.e., much higher than those measured for both TW and PR samples. Notably, this value is the highest ever reported in comparison with treatments using other ammonium oxamate derivatives such as AmEtOx and AmPicOxam [49,50]. No significant changes are evident in skeletal density, while compactness index (Ic) and water open porosity ΦH2O% showed more noticeable trends. Open porosity increased by 200% due to weathering, decreased by over 80%, 70% and around 63% after immersion, brushing, and spraying, respectively. However, spraying was the only method that slightly increased porosity compared to PR. This result could be related to a major effect of the porosity of the newly formed coating, which, in the case of spraying, had the highest covering width.
Colorimetric measurements showed minor changes: all treatments caused a slight colour difference (ΔE), with spraying and brushing remaining around or below the threshold of visual perception (ΔECIELAB1976 ~3.0). Immersion caused a slightly larger difference (ΔECIELAB1976 = 3.19; ΔECIELAB2000 = 2.88), still acceptable for conservation. Accordingly, immersion also changed the white index and chromatic coordinates more than the other methods.
Surface roughness, which increased after weathering, was reduced by immersion and spraying. Brushing slightly raised roughness levels compared to PR, possibly due to mechanical abrasion caused by the brush, while spraying preserved surface texture close to the original state. One of the most significant differences between treatments appears in contact angle measurements (Figure 4). TW samples showed extremely low contact angles (~10°), but after treatment all samples significantly recovered their superficial tension. Immersion performed best, with contact angles close to hydrophobicity (over 80° at 2 s) and relatively stable over time (still above 55° after 60 s), indicating a strong and lasting water-repellent layer. Brushing and spraying also improved the superficial tension of the material, returning to values close to the pristine material.
In terms of water transport properties, spraying was more effective. Capillary water uptake (CA) was very high after TW (+1900% vs. PR), but spraying reduced it by 97%, more than brushing or immersion. The drying index (DI) also improved most with spraying, indicating better moisture release properties (Figure S14).
Finally, the consolidation performance of AmEtOxam can be compared to that of the traditional benchmark, ammonium oxalate [33], and several other oxalic acid derivatives previously published (Table S3) [44,46,49,50]. This comparison demonstrates how AmEtOxam successfully addresses the three primary limitations of the conventional treatments: solubility, penetration, and chemical compatibility. In fact, AmEtOxam achieves a solubility of 1.50 mol·L−1, a four-fold increase over AmOx (0.36 mol·L−1), allowing for higher mineral deposition in fewer application cycles. Moreover, AmEtOxam exhibits a superior penetration depth of up to 210 µm, exceeding the sub-micron performance of other derivatives by several orders of magnitude. Furthermore, its moderate pH of 5.46 ensures greater substrate safety compared to the more acidic reference.

2.5. DFT Calculations

Quantum-mechanical calculations were performed at the DFT level to investigate the tendency of the N-ethyloxamate anion to hydrolyze during the treatment on powder calcium carbonate and marble mock-ups [54]. Based on the results reported previously [48], the hybrid PBE0 functional [55] was adopted, which was paralleled by a Def2-TZVP [56] triple-ζ split-valence basis set, including polarization functions for all atomic species. All calculations were referred to an aqueous media by implicitly modelling the solvent by means of the integral equation formalism of the polarizable continuous model (IEF-PCM) within the self-consistent reaction field (SCRF) approach [57]. The potential energy surface (PES) corresponding to the rotation of the carboxylate moiety around the C–C bond was explored (Figure 5), revealing a very low rotational barrier (3.40 kcal·mol−1), the planar conformation (torsion angle τ = 0°) being more stable than the staggered one (τ = 90°), similar to what was found in the case of differently substituted oxamate RNHC(O)COO− derivatives (R = H, Me, and Ph) [48]. The staggered conformation shows the Kohn-Sham LUMO mostly localized on the amide carbon atom (Figure S15), which features a remarkably positive natural charge (QC = +0.552 |e|) [58,59]. This increases the tendency to undergo a nucleophilic attack, and, in particular, makes it prone to hydrolysis in aqueous media. Accordingly, previous DFT calculations carried out on the closely related N-methyloxamate anion showed that the interaction with the crystal lattice of calcite induces a remarkable twisting of the anion (τ = 44.09°), very close to that calculated for the methyl oxalate anion (τ = 44.00°) [48], which is in fact another precursor of calcium carbonate on carbonate rocks [46]. Hence, DFT calculations justify the stability of the EtOxam− anion in water solution and in the solid state, where it possibly assumes a planar configuration as proved by the crystal structures of all the previously characterized N-substituted oxamate anions (R = H, Ph), while the interaction with the CaCO3 surface induces a twist of the oxamate anion, thus rendering it prone to hydrolysis to give the observed deposition of CaOx on the surface.

3. Materials and Methods

Reagents and solvents were used without further purification. The white marble variety “Statuario Michelangelo” from the Apuan Alps, was quarried from Cava Franchi (Carrara, Italy). Prism-shaped specimens (2.0 × 2.0 × 8.0 cm) and small irregular fragments (with volume ~1 cm3) were prepared. All marble specimens selected for the application of AmEtOxam underwent thermal weathering treatment in a Carbolite CWF 1200 muffle furnace, where the temperature was ramped to 300 °C (2 h), held at that temperature for 5 h and cooled down to 25 °C over a period of 3 h (Figure S10).

3.1. Synthesis

3.1.1. Synthesis of O-Ethyl-N-Ethyloxamate

An ethylamine solution prepared by dissolving 5.5 mL of 70% v/v solution of ethylamine in 25 mL of ethanol was added dropwise to a diethyloxalate (10.0 g, 6.80·10−2 mol) ethanol solution (40 mL). The reaction mixture was refluxed for 20 h and then concentrated at reduced pressure. The remaining oil was kept at 8 ± 3 °C overnight, and the crystalline precipitate, identified as N,N’-diethyloxalamide, was removed by filtration, obtaining O-ethyl-N-ethyloxamate as a clear colourless liquid. Yield 9.77 g, 98.4%. UV-Vis (CH3CN): λmax (ε) = 212 (6600 dm3 mol−1 cm−1) nm. 1H-NMR (600 MHz, CDCl3) δ = 7.11 (br s, 1H, NH), 4.34 (m, 2H, CH2); 3.38 (m, 2H, CH2); 1.37 (m, 3H, CH3); 1.21 (t, 3H, CH3) ppm.

3.1.2. Synthesis of Ammonium N-Ethyloxamate (AmEtOxam) Monohydrate

A solution of O-ethyl-N-ethyloxamate (10.0 g, 3.44·10−2 mol in 20 mL of distilled water) was slowly added to a solution of ammonium bicarbonate (6.74 g, 4.26·10−2 mol in 20 mL of distilled water). The resulting solution was refluxed for 4 h and subsequently poured onto a glass plate where the solvent was slowly evaporated to dryness. AmEtOxam·H2O was obtained as a white crystalline solid that was washed with dichloromethane. Yield: 8.42 g (80.3%). M.p. 160 °C. UV-Vis (H2O): λmax (ɛ) = 194 (8320 dm3 mol−1 ·cm−1) nm. FT-IR (KBr pellet): ṽ = 3290 (vw), 2983 (vw), 2937 (vw), 2875 (vw), 1654 (vs), 1529 (m), 1400 (s), 1232 (m), 1145 (w), 1056 (w), 1016 (w), 896 (w), 802 (m), 773 (w), 640 (w), 520 (w), 455 (w) cm−1. 1H-NMR (600 MHz, DMSO-d6) δ = 8.12 (s, 1H, NH), 3.11 (p, 2H, CH2), 1.06 (t, 3H, CH3) ppm. Elemental analysis (calcd. for C4H12N2O4): C 31.8 (31.6), H 7.1 (7.9), N 19.1 (18.4)%.

3.2. Coating Preparation

3.2.1. Treatment with AmEtOxam Solution by Immersion

The mass of the mock-up was kept steady in a thermostat heater at 60 ± 2 °C. Subsequently, the specimens were immersed in a 5% w/w water solution of AmEtOxam·H2O and maintained in a static bath for 24 h. The pH of the solution was measured before and after the treatment, and the unreacted salts were washed with distilled water, and pH and conductivity of the washing solutions were monitored (Table S1). The specimens were air-dried, then kept in a thermostatic heater at 60 °C for 24 h.

3.2.2. Treatment with AmEtOxam Solution by Brushing

The mass of the mock-up was kept steady at 60 ± 2 °C in a thermostatic heater. A 5% w/w water solution of AmEtOxam·H2O was subsequently applied by means of a synthetic brush of suitable dimension. The specimen was rotated by 90° between each application, and a one-minute pause was allowed between each application to let the solution absorb. In total, each surface was treated with 10 brushstrokes. The mock-up was permitted to rest until the treated surface appeared dry before passing to a subsequent surface treatment on a different face. The samples were covered with parafilm and left to rest for 24 h. Any unreacted salts were subsequently washed off with distilled water, and the pH and conductivity of the washing solutions were monitored. The specimens were air-dried and then kept in a thermostatic heater (60 °C) for 24 h.

3.2.3. Treatment with AmEtOxam Solution by Spraying

The mass of the mock-up was kept steady at 60 ± 2 °C in a thermostatic heater. A pressure nebulizer was employed to apply a 5% w/w water solution of AmEtOxam·H2O until it was rejected (flow rate was 16.4(8) mL·min−1, distance 20 cm). This procedure was repeated six times for each face, with a five-minute interval between repetitions to allow the solution to absorb. After the treatment, the samples were covered with parafilm and left to rest for 24 h. Any unreacted salts were subsequently washed off with distilled water, and the pH and conductivity of the washing solutions were monitored. The specimens were air-dried and then kept in a thermostatic heater (60 °C) for 24 h.

3.3. Physical-Chemical Characterization

The FT-IR characterization of the samples (as KBr pellet) was performed by means of a Thermo-Nicolet 5700 spectrometer, Thermo Fisher Scientific, Waltham, MA, USA (room temperature, KBr beam-splitter, KBr windows, 4000–400 cm−1, resolution 4 cm−1). Electronic absorption spectra were recorded in a quartz cell on a Thermo Evolution 300 (10.00 mm optical length, 190–600 nm, 25 °C). Compositional features were investigated using a 2400 series II CHNS/O elemental analyzer (925°) and a FALC mod. C melting point apparatus (25–300 °C). Nuclear magnetic resonance (1H-NMR) measurements were performed using a Bruker Advance III HD 600 MHz spectrometer, Bruker, Fällanden, Switzerland (14.1 T, 25 °C, operating frequence: 600.15 MHz) in CDCl3 or DMSO-d6. The chemical shifts reported were calibrated to the solvent residue in ppm (δ). Conductivity and pH of the solution used for the coating treatments were monitored using a Crison GLP instrument and a Hanna 112 pH-metre, calibrated with 4, 7, and 10 pH buffers and KCl standard solutions, respectively. Ksp and solubility were evaluated with a quantitative spectrophotometry measure on a filtered saturated solution after recording a calibration curve.

3.3.1. Powder X-Ray Diffraction

Powder X-ray diffraction (PXRD) analyses were performed on AmEtOxam·H2O and the solid obtained from the preliminary test with calcium carbonate by means of a Bruker D8 venture diffractometer, Bruker, Karlsruhe, Germany (CuKα source, operating at 40 kV and 40 mA) equipped with a LynxEye XE-T position sensitive detector, using a PTFE specimen holder. Marble mock-ups that were placed onto a UMC xyz motorized stage, and measurements were performed directly on the stone surface with the same instrumentation after proper alignment. Data collection was performed in both cases in a 2θ 5–50° range, in the first case, with a step size of 0.03° and counting time of 2 s for each step, while the latter had a fixed sample illumination of 18 mm by means of motorized slits.

3.3.2. Thermogravimetric Analysis (TGA)

Thermal analyses were conducted with a Netzsch STA 449 F3 Jupiter instrument, NETZSCH-Gerätebau GmbH, Selb, Germany. Approximately 50 mg of sample were heated from 25 to 900 °C at a rate of 10 °C·min−1 under dried air (40 mL·min−1 flow), and weight loss was continuously monitored.

3.3.3. Microscopical Examinations

The surfaces of the stone mock-ups were examined using a Zeiss Axioskop 40-AxioCam HR optical microscope (Carl Zeiss AG, Oberkochen, Germany) and with the use of scanning electron microscopy (SEM) with a Zeiss Evo LS15 microscope, Carl Zeiss AG, Oberkochen, Germany (LaB6 filament as electron source, operating at 20.00 kV) equipped with a solid-state detector (50 mm2 window). Electric conductivity of the samples was enhanced before analysis with a homogenous gold coating obtained with a Leica EM SCD005 sputter coater, Leica Microsystems, Wetzlar, Germany (sputter set-up: 80 s, with a current of 50 mA and a pressure of 0.05 mbar).

3.3.4. Stone Mock-Ups General Characterization

Ultrasonic pulse velocity (vuts) tests were performed before and after each treatment using a CNS Electronics Pundit device (±0.1 ms accuracy). Transducers (150 MHz, 11.82 mm diameter) were applied directly to the stone surface using a carboxymethyl cellulose-based paste (Henkel Sichozell Kleister) to improve acoustic coupling. Three consecutive measurements were taken along the horizontal axis and averaged. The vuts values were then used to calculate the dynamic Young modulus (Ed) [32], using skeletal density (ρ) determined via helium pycnometry and a fixed Poisson’s ratio (δ = 0.29 for marble) [60]. In particular, skeletal density used in the elastic modulus calculation was obtained using a Micromeritics AccuPyc II 1340 helium pycnometer (Micromeritics Instrument Corporation, Norcross, GA, USA) with a 3.5 cm3 chamber. Fragments with volume between 1 and 2 cm3 were selected, and five consecutive measurements were performed on three pieces per treatment.
Colour measurements were conducted with a Konica Minolta CM-700d spectrophotometer (Konica Minolta Sensing Americas, Inc., Ramsey, NJ, USA) under D65 illumination. For each surface, six measurements were taken and averaged. Colour parameters were expressed in CIELAB coordinates (L*, a*, b*), and total colour difference (ΔE) was calculated using both the CIELAB1976 and CIELAB2000 standards [61].
Water absorption tests were carried out to determine skeletal density (ρskeletal), bulk density (ρbulk), compactness index (Ic), and open water porosity (ΦH2O%) as previously reported [32]. Samples were oven-dried at 60 ± 2 °C until constant mass and then immersed in distilled water for 24 h. Masses were recorded in dry state, submerged (hydrostatic balance), and after saturation.
Drying tests were based on NorMal 29/88 [62]. After water saturation, mass loss was monitored at defined time intervals.
Capillary absorption behaviour was assessed following UNI EN 15801:2010 [63]. Samples were pre-dried (60 °C, 24 h) and placed in contact with distilled water. Water uptake (Q) was recorded over time and plotted against the square root of time. The initial slope of the curve was used to calculate the capillary absorption coefficient (CA) [32].

3.3.5. Mechanical Testing

Pull-off strength was measured according to UNI EN 1015-12:2000 [64]. Steel stubs (20 mm Ø) were grit-blasted and bonded to the stone surface using epoxy resin. After curing, samples (8.0 × 8.0 × 2.0 cm) were pulled in tension using a universal testing machine at 2 mm·min−1 and 25 °C. The tensile strength (fh, N·mm−2) was calculated from the peak load at detachment. Surface hardness was evaluated using a Shore D durometer (ASTM D2240) [65]. At least three measurements per sample were collected using standardized indenters and loading force (44.5 N), with scales ranging from 0 to 100.

3.3.6. Surface Characterization

Contact angle measurements were carried out per UNI EN 828:2013 [66] using a Nikon D800 camera (Nikon, Sendai, Japan) and macro lens. Drops of 0.6 μL distilled water were placed from ~1 mm height and photographed after 2, 30, and 60 s. Angles on both sides of each drop were analyzed using BMS.pix3 software. Tests were conducted on three surfaces per treatment, and average values were reported. Surface roughness was measured with a Mitutoyo SJ-201 portable tester (Mitutoyo Corporation, Kawasaki, Japan) according to ISO JIS 01. Each of two surfaces per specimen was scanned ten times, with a 1 mm displacement between scans.

3.4. QM Calculations

QM calculations were performed at the density functional theory (DFT) [54] with the suite of software Gaussian 16 [67] on the N-ethyloxamate (EtOxam−) anion. The hybrid PBE0 [55] functional was adopted along with the triple-ζ Def2-TZVP [56] basis set for all atomic species. Harmonic frequency calculations were carried out to verify the nature of the minima of each optimization, by verifying the absence of imaginary negative frequencies. A potential energy surface (PES) study was carried out in order to evaluate the rotational barrier around the C–C bond. Natural charge distributions [58,59] were calculated at the optimized geometry at the same level of theory. Water was implicitly modelled by using the integral equation formalism of the polarizable continuous model (IEF-PCM) within the self-consistent reaction field (SCRF) approach [57]. The programmes GaussView 6.1.1 [68] and Molden 7.3 [69], were used to investigate the optimized structures and the shapes of Kohn-Sham molecular orbitals. All calculations were carried out using a CINECA Marconi A1 supercomputer (CINECA, Bologna, Italy).

4. Conclusions

The synthesis, characterization, and consolidating capabilities of the novel salt ammonium N-ethyloxamate as a protective agent for white Carrara marble were described in this paper. AmEtOxam is stable in the solid state in its monohydrate form (AmEtOxam·H2O) and in water solution, where it is fivefold more soluble than commonly applied ammonium oxalate (saturated solution concentrations of 0.4 and 1.5 mol·L−1, respectively). On the contrary, in the presence of solid calcium carbonate, it undergoes hydrolysis resulting in the precipitation of calcium oxalate. Hence, AmEtOxam·H2O represents an unconventional, economically affordable, and more soluble precursor of weddellite and whewellite than the prevailing ammonium oxalate. The quantitative hydrolysis of AmEtOxam during the application prevents the surface of the treated stone from reacting later with groundwater ions and releasing lingering pollutants that might harm the environment.
The application of a 5% w/w water solution of AmEtOxam·H2O on thermally weathered Carrara marble samples resulted in the deposition of a uniform protective coating of calcium oxalate hydrate for all treatments that were performed using the three methodologies under investigation (immersion, brushing, and spraying). Overall, although all three methods improved the condition of weathered marble, they performed differently depending on the property considered. Spraying gave the most balanced and consistent improvements: it strongly enhanced mechanical properties, reduced water absorption, preserved surface texture, and improved water repellence, while keeping colour changes within acceptable limits. Brushing was also effective, especially for hardness and strength, and offered good esthetic results with limited surface modification. Immersion achieved a high increase in the superficial tension, with values close to water repellence and a strong reduction in porosity, but showed weaker mechanical reinforcement and more visible chromatic changes. Among the three application methods, immersion afforded a lower penetration depth as compared to brushing and spraying. This can be ascribed to the rapid surface hydrolysis of the N-ethyloxamate anion upon contact with the calcareous substrate, leading to the immediate formation of a superficial uniform “sealing layer” of calcium oxalate. This precipitation, consistent with the observed increase in surface tension, is likely to act as a barrier to further capillary penetration. However, multi-pass treatments likely disrupt the superficial film, enabling deeper and more gradual penetration of the consolidant into the stone matrix.
Since calcium oxalate is inert to acidic substances, treatment with AmEtOxam provides exceptional protection for artefacts that are left in the open air.
The outcomes of the AmEtOxam application clearly illustrate the potential of oxalic acid monoamide derivatives as a very promising class of new materials for the conservation of carbonate stone materials of interest in the field of Cultural Heritage.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules31050776/s1, Table S1, methodology adopted for the synthesis and characterization, preliminary test on calcium carbonate, preparation, and treatments of white Carrara marble samples; Table S2: pH Values and conductivity (mS/cm) determined for the water solution of AmEtOxam before and after the treatment of Carrara marble stone mock-ups; Table S3: treatment results; Figures S1–S5: spectroscopic characterization; Figures S6, S7 and S11: PXRD analysis; Figure S8: thermal treatment ramp; Figures S9 and S10: images for the surfaces and sections of white Carrara marble mock-ups; Figures S12 and S13: radar plots for the evaluation of the treatments efficacy; Figure S14: water capillary absorption and desorption curves; Figure S15: KS-LUMO calculated for the N-ethyloxamate anion.

Author Contributions

M.A., P.M., M.C.A. and D.G.T., conceptualization; M.A., P.M. and D.G.T., supervision; S.M., G.C., E.P. and A.P., methodology; S.M., G.C., A.P., E.P., A.N.E., L.G., R.S. and E.L., investigation; S.M., M.A. and E.P., data curation; S.M. and M.A., writing; all authors, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

We thank the Università degli Studi di Cagliari for financial support and CeSAR (Centro Servizi d’Ateneo per la Ricerca) of the University of Cagliari, Italy, for NMR and PXRD measurements. PhD grant statement for S. M.: “This article was produced while attending the PhD programme in Chemical Sciences and Technology at the University of Cagliari, Cycle XXXVIII, with the support of a scholarship financed by the Ministerial Decree no. 351 of 9th April 2022, based on the NRRP-funded by the European Union-NextGenerationEU-Mission 4 “Education and Research”, Investment 4.1 “Extension of the number of research doctorates and innovative doctorates for public administration and cultural heritage”. The authors kindly acknowledge CINECA for the computational resources on the Marconi A1 supercomputer accessed within the ISCRA project “A Computer Aided Molecular Design approach for inorganic salts for the restoration of marble and limestone substrates” (MarbComp; code HP10CDWYH6). Fondazione di Sardegna (FdS Progetti Biennali di Ateneo, annualità 2022, grant no. F73C23001580007) is kindly acknowledged for financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 31 00776 sch001
Scheme 1. Oxalate and oxamates anion derivatives.
Scheme 1. Oxalate and oxamates anion derivatives.
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Molecules 31 00776 sch002
Scheme 2. Synthesis of ammonium ammonium N-ethyloxamate (AmEtOxam).
Scheme 2. Synthesis of ammonium ammonium N-ethyloxamate (AmEtOxam).
Molecules 31 00776 sch002
Molecules 31 00776 g001
Figure 1. 1H-NMR spectrum recorded for O-ethyl-N-ethyloxamate in CDCl3 solution. The marked resonance corresponds to the solvent residual signal (*). In the inset a comparison between the N-H signal for N,N′-diethyloxalamide (7.46 ppm) and O-ethyl-N-ethyloxamate (7.11 ppm) is depicted.
Figure 1. 1H-NMR spectrum recorded for O-ethyl-N-ethyloxamate in CDCl3 solution. The marked resonance corresponds to the solvent residual signal (*). In the inset a comparison between the N-H signal for N,N′-diethyloxalamide (7.46 ppm) and O-ethyl-N-ethyloxamate (7.11 ppm) is depicted.
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Molecules 31 00776 g002
Figure 2. Thermogravimetric analysis (TGA) under N2 flow for AmEtOxam·H2O.
Figure 2. Thermogravimetric analysis (TGA) under N2 flow for AmEtOxam·H2O.
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Molecules 31 00776 sch003
Scheme 3. Reaction of AmEtOxam with CaCO3 in water solution.
Scheme 3. Reaction of AmEtOxam with CaCO3 in water solution.
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Figure 3. SEM images of weathered Carrara marble treated with a 5% w/w water solution of AmEtOxam·H2O.
Figure 3. SEM images of weathered Carrara marble treated with a 5% w/w water solution of AmEtOxam·H2O.
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Figure 4. Contact angle measurements (°) were performed on both untreated stone mock-ups and the same samples treated with AmEtOxam solutions by immersion, brushing, and spraying. For each treatment, three measurements were carried out 2, 30 and 60 s after the deposition of the water drop.
Figure 4. Contact angle measurements (°) were performed on both untreated stone mock-ups and the same samples treated with AmEtOxam solutions by immersion, brushing, and spraying. For each treatment, three measurements were carried out 2, 30 and 60 s after the deposition of the water drop.
Molecules 31 00776 g004
Molecules 31 00776 g005
Figure 5. Relative variation ΔEe of the total electronic energy as a function of torsion angle τ (O1–C1–C2–O3 dihedral for the N-substituted oxamate ROxam− anion in the inset) calculated for the N-ethyloxamate anion at the DFT level (PBE0//Def2-TZVP). Solvation was implicitly accounted for at the IEF-PCM SCRF level.
Figure 5. Relative variation ΔEe of the total electronic energy as a function of torsion angle τ (O1–C1–C2–O3 dihedral for the N-substituted oxamate ROxam− anion in the inset) calculated for the N-ethyloxamate anion at the DFT level (PBE0//Def2-TZVP). Solvation was implicitly accounted for at the IEF-PCM SCRF level.
Molecules 31 00776 g005
Table 1. Macroscopic properties of pristine (PR), weathered (TW), and treated (TR) marble samples and relative variations (%) of TR samples with regard to PR (Δ%Improve) and TW (Δ%Restore) samples. Characterizations: ultrasonic propagation time, tuts; ultrasonic propagation speed, vuts; Young elastic module, Ed; skeletal density from water saturation, ρskeletal H2O; bulk density from water saturation, ρbulk H2O; compact index, Ic%; water open porosity, ΦH2O%; Shore scale hardness D, HD; tensile strength, fh; CIELAB colour space coordinates, L*, a* and b*; chromatic index, C; white index, WI; colour difference ΔE; arithmetical mean roughness value, Ra; mean roughness depth, Rz; root-mean-square roughness, Rq; contact angle, ϑc, measured after 2, 30, or 60 s; drying index, DI; capillary uptake coefficient, CA.
Table 1. Macroscopic properties of pristine (PR), weathered (TW), and treated (TR) marble samples and relative variations (%) of TR samples with regard to PR (Δ%Improve) and TW (Δ%Restore) samples. Characterizations: ultrasonic propagation time, tuts; ultrasonic propagation speed, vuts; Young elastic module, Ed; skeletal density from water saturation, ρskeletal H2O; bulk density from water saturation, ρbulk H2O; compact index, Ic%; water open porosity, ΦH2O%; Shore scale hardness D, HD; tensile strength, fh; CIELAB colour space coordinates, L*, a* and b*; chromatic index, C; white index, WI; colour difference ΔE; arithmetical mean roughness value, Ra; mean roughness depth, Rz; root-mean-square roughness, Rq; contact angle, ϑc, measured after 2, 30, or 60 s; drying index, DI; capillary uptake coefficient, CA.
PropertyPRTWΔ%WeatherImmersionΔ%ImproveΔ%RestoreBrushingΔ%ImproveΔ%RestoreSprayingΔ%ImproveΔ%Restore
Dynamics
tuts (µs)29.0 ± 0.168.9 ± 0.1+13831.2 ± 0.1+7.59−54.7219.3 ± 0.1−33.45−71.9917.8 ± 0.1−38.62−74.17
vuts (km·s−1)2.8 ± 0.21.2 ± 0.1−602.6 ± 0.2−7.14+116.674.2 ± 0.3+50.00+250.004.5 ± 0.3+60.71+275.00
Ed (MN·m−2)14.1 ± 0.52.41 ± 0.04−82.911.6 ± 0.6−17.73+381.3329.8 ± 1.5+111.35+1136.5135.2 ± 2.0+149.65+1360.58
Structure
ρskeletal H2O (g·cm−3)2.724(2)2.729(6)+0.182.718(4)−0.22−0.402.74(1)+0.59+0.402.74(1)+0.59+0.40
ρbulk H2O (g·cm−3)2.705(3)2.724(2)+0.702.69(4)−0.55−1.252.67(3)−1.29−1.982.696(5)−0.33−1.03
Ic %99.398.2−1.0798.2−1.11099.45+0.15+1.2799.37+0.07+1.19
ΦH2O%0.54(4)1.6(1)+2000.28(2)−48.15−82.500.5(1)−7.41−68.750.6(1)+11.11−62.50
HD87(1)75(1)−1488(1)+1.15+17.3392(1)+5.75+22.6791(1)+4.60+21.33
fh (N·mm−2)0.76 ± 0.020.46 ± 0.02−390.90 ± 0.02+18.11+95.651.00 ± 0.02+31.23+117.390.77 ± 0.02+1.05+67.39
Colorimetry
L* 92.7294.50-94.17--91.94--90.69--
a* −0.31−0.13-−0.09--−0.14--−0.26--
b* −0.590.60-2.24--1.25--1.43--
C0.671.04-2.24--1.26--1.45--
WICIELAB7685.0579.41-72.65--74.74--71.05--
ΔECIE1976--2.15 a-3.19 a1.67 a-2.01 a2.64 a-2.86 a3.90 a
ΔECIE2000--1.62 a-2.88 a1.56 a-1.88 a1.68 a-2.34 a2.47 a
Roughness
Ra (µm)5.0(3)6.4(5)+284.0(6)−20.00−37.505.7(5)+14.00−10.945.1(4)+2.00−20.31
Rz (µm)45(5)61(6)+3645.7(9)+1.56−25.0851(4)+13.33−16.3944(2)−2.22−27.87
Rq (µm)6.3(7)8.6(6)+365.5(7)−12.70−36.057.4(5)+17.46−13.956.5(2)+3.17−24.42
Contact Angle
ϑc 2 s left (°)44(12)9(6)-84(3)--44(2)--33(8)--
ϑc 2 s right (°)39(5)11(7)-84(4)--34.8(3)--30(2)--
ϑc 30 s left (°)32(12)--68.6(4)--35(1)--16(5)--
ϑc 30 s right (°) 31(8)--70(1)--31(1)--18(6)--
ϑc 60 s left (°) 32(12)--55(7)--31(1)--15(6)--
ϑc 60 s right (°) 27(8)--57(3)--31(1)--16(6)--
Hydric
DI (%h−1)297(32)66(1)−78292(7)−1.68+342.42276(18)−7.07+318.18299(27)+0.67+353.03
CA (kg·m−2·h−1/2)0.14(1)2.81(4)+19000.23(3)+64.29−91.810.27(6)+92.86−90.390.08(1)−42.86−97.15
a Δ% Calculated according to the CIELAB1976 or CIELAB2000 colour space standards definition of colour difference (ΔE).
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Murgia, S.; Aragoni, M.C.; Carcangiu, G.; Giacopetti, L.; Gimeno Torrente, D.; Lippolis, V.; Loi, E.; Meloni, P.; Ezquerra, A.N.; Podda, E.; et al. A Novel Approach to Carbonate Stone Conservation: Induced Calcium Oxalate Formation Through the Application of Ammonium N-Ethyloxamate (AmEtOxam) on White Carrara Marble. Molecules 2026, 31, 776. https://doi.org/10.3390/molecules31050776

AMA Style

Murgia S, Aragoni MC, Carcangiu G, Giacopetti L, Gimeno Torrente D, Lippolis V, Loi E, Meloni P, Ezquerra AN, Podda E, et al. A Novel Approach to Carbonate Stone Conservation: Induced Calcium Oxalate Formation Through the Application of Ammonium N-Ethyloxamate (AmEtOxam) on White Carrara Marble. Molecules. 2026; 31(5):776. https://doi.org/10.3390/molecules31050776

Chicago/Turabian Style

Murgia, Simone, M. Carla Aragoni, Gianfranco Carcangiu, Laura Giacopetti, Domingo Gimeno Torrente, Vito Lippolis, Eleonora Loi, Paola Meloni, Antonia Navarro Ezquerra, Enrico Podda, and et al. 2026. "A Novel Approach to Carbonate Stone Conservation: Induced Calcium Oxalate Formation Through the Application of Ammonium N-Ethyloxamate (AmEtOxam) on White Carrara Marble" Molecules 31, no. 5: 776. https://doi.org/10.3390/molecules31050776

APA Style

Murgia, S., Aragoni, M. C., Carcangiu, G., Giacopetti, L., Gimeno Torrente, D., Lippolis, V., Loi, E., Meloni, P., Ezquerra, A. N., Podda, E., Pintus, A., Serra, R., & Arca, M. (2026). A Novel Approach to Carbonate Stone Conservation: Induced Calcium Oxalate Formation Through the Application of Ammonium N-Ethyloxamate (AmEtOxam) on White Carrara Marble. Molecules, 31(5), 776. https://doi.org/10.3390/molecules31050776

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