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Article

Comparative Study of Ethyl Silicate and Nanolimes with Fluorescent Properties as Consolidants for Mural Paintings and Mortars

by
Patricia Rebollo
1,
Javier Becerra
2,*,
Ioannis Karapanagiotis
3 and
Panagiotis Spathis
3
1
Independent Researcher, 08009 Barcelona, Spain
2
Department of Painting, Faculty of Fine Arts, University of Seville, 41003 Seville, Spain
3
School of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
*
Author to whom correspondence should be addressed.
Heritage 2025, 8(12), 529; https://doi.org/10.3390/heritage8120529
Submission received: 17 October 2025 / Revised: 8 December 2025 / Accepted: 9 December 2025 / Published: 12 December 2025
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Versions Notes

Abstract

The consolidation of mural paintings presents a significant challenge for conservators, as the treatments applied must not only be effective but also preserve the aesthetic qualities of the artwork. Ongoing research focuses on developing new products that are more efficient, durable, and compatible with the physicochemical and aesthetic characteristics of the original materials, thereby addressing the limitations of existing consolidants. This study examines two consolidants for mural painting restoration: Estel 1200® (C.T.S., Madrid, Spain), a commercially available and widely used ethyl silicate-based product, and Nanorepair UV® (Patent: ES-2766074-B2, Pablo de Olavide University, Seville, Spain), a nanocomposite composed of calcium hydroxide nanoparticles doped with zinc quantum dots. On mortar specimens, prepared according to the Roman fresco technique, the application method for the proposed treatments was optimized. The applicability of the treatments for mural painting conservation was studied by colorimetric measurements and SEM imaging to detect and characterize the formation of surface layers. The effectiveness of the treatments was quantitatively evaluated with tape-peeling cycles. The results show that, although both treatments enhance the consolidation state of mural paintings, Nanorepair UV® proved to be a more effective consolidant, without altering the aesthetic or physicochemical properties of the artwork. Additionally, this treatment allows for straightforward evaluation of its penetration and enables distinction between treated and untreated areas through the fluorescence of the zinc oxide quantum dots.

1. Introduction

One of the primary types of degradation found in mural paintings is the disintegration of both the pictorial layer and the substrate or support. To address this issue, restorers use consolidating materials designed to restore the degree of cohesion in the treated materials. However, these treatments often have the disadvantage of being irreversible, due to their inherent characteristics, which contradicts one of the main principles of restoration practice [1]. Thus, when discussing consolidants, it is essential to consider their retreatability a key requirement, ensuring that their application does not hinder future treatment applications [2,3]. Additionally, consolidants used in cultural heritage conservation should meet several additional criteria, including physicochemical compatibility with the original material, homogeneous and deep action and penetration without affecting the porous system, preservation of aesthetic properties, and stability, effectiveness, and durability [3,4,5].
In general, because the composition of mural painting substrates is similar to that of certain types of stones, consolidation treatments originally designed for stone have been applied on paintings. Since the 1960s, acrylic-based consolidants have been widely used for the consolidation of mural paintings, mortars, and degraded stones. The main advantage of these materials is their high adhesion capacity and stability [6]. However, they have several drawbacks, as they may change the material’s porosity, which in turn affects properties such as capillarity and permeability, and furthermore they are susceptible to color changes as they age [7,8,9,10]. Another commonly used consolidants are alkoxysilanes, particularly ethyl silicate (TEOS) [11]. This material has the advantage of low viscosity, allowing for deep penetration and high chemical stability. However, it tends to form surface films that are prone to cracking and shows limited effectiveness on carbonated rocks [9,12,13].
One of the historical treatments that has demonstrated great chemical compatibility with carbonated substrates is limewater, which is an aqueous solution of calcium hydroxide (Ca(OH)2). However, its application has some drawbacks, including its reliance on water as a dispersing medium and the need for repeated applications to achieve effective results [14]. This can promote the migration of soluble salts or cause the collapse of pores during freeze–thaw cycles. Another limitation is its low penetration capacity, which often results in the formation of a whitish veil on the surface [8,15].
To overcome these drawbacks, advances in nanotechnology have enabled the development of new consolidating products based on colloidal dispersions of inorganic nanoparticles [16,17]. The main feature of these nanomaterials is their high reactivity, derived from their large specific surface area. In combination with other factors such as particle size and chemical composition, this results in materials with multiple properties [18,19]. Most research on these materials has focused on their use as consolidants for stones [20,21,22,23]. Although mural paintings are mainly composed of carbonated materials, similar to carbonated stones, their diverse execution techniques and polychromy present certain characteristics that need to be analyzed individually.
Ca(OH)2 nanoparticles have been extensively studied for the consolidation of mural paintings is. These nanoparticles exhibit excellent chemical compatibility with carbonated substrates, such as mural paintings and mortars [19,24]. They have a typical range of size from 50 to 600 nm and are dispersed in an alcoholic medium [25,26]. Their small size enhances the penetrability of the consolidant into the porous system, making them highly effective for pre-consolidation treatments [9,24,27]. However, their application can lead to the formation of a whitish veil on the treated surface, which is particularly noticeable in mural paintings. This effect is believed to arise from the migration of nanoparticles to the surface during solvent evaporation [28,29]. Several studies [30,31,32] have analyzed the influence of the dispersing medium on colloidal stability and product penetration, which also depends on the substrate’s porosity [29]. However, other researchers suggested that veil formation can be avoided by adding a certain amount of water to the alcoholic dispersing medium [30,33] or by applying a coating that prevents rapid solvent evaporation, such as a wet dressing [31,34,35].
This study aims to evaluate two consolidated products (Estel 1200® and Nanorepair UV®) for mural paintings and to determine their optimal application methods. Estel 1200® is an ethyl silicate-based consolidant, widely used for the consolidation of inorganic heritage materials. Nanorepair UV® is a nanocomposite based on Ca(OH)2 nanoparticles doped with zinc oxide (ZnO) quantum dots (Patent: ES-2766074-B2 [36]), which has been previously studied as a consolidant for stone [29,37]. In addition to its effectiveness in consolidated carbonated stones, Nanorepair UV® has the advantage of fluorescence due to the presence of ZnO quantum dots, which exhibit high luminescent efficiency, stability, non-toxicity, and low cost [38,39]. This fluorescence allows the analysis of the consolidant’s penetration into the porous structure, as it is easy to distinguish the treated areas from the untreated ones. Notably, this is the only treatment that meets this important restoration criterion [1] for consolidation. The suitability of the treatments for mural paintings is evaluated through colorimetric measurements to detect aesthetic changes and SEM images to identify the formation of surface layers. The effectiveness of the treatments is quantitatively assessed by means of peeling tests.

2. Materials and Methods

2.1. Materials

The mortars, used to produce the fresco mural samples (mock ups), were composed of lime (calcium hydroxide), as the main binder, and sand as the aggregate. On top of the final mortar layer, the following pigments were applied: Pozzuoli earth (PI226/0100), light green earth (PI228/0100), and Egyptian blue (10060), all purchased from Kremer Pigmente. The white section corresponded to the raw mortar, without any added pigment. These pigments were selected because they are commonly reported in studies of Roman frescoes [40] and, moreover, they produce different colors (red, green, blue, and white), which is important to evaluate the overall aesthetic impact of the applied consolidants. In addition, the selected pigments differ in their chemical compositions: Pozzuoli earth is rich in Fe; light green earth contains mainly Fe, Al, Si, and Mg; Egyptian blue is primarily composed of Cu, Ca, and Si; the white area (calcite) is mainly Ca-based.
Two consolidants were tested: Estel 1200® (ES) and Nanorepair UV® (NR-UV). Estel 1200® is an ethyl silicate-based consolidant dissolved in isopropyl alcohol with a dry residue content of 25%. Nanorepair UV® (Patent: ES-2766074-B2) is a nanocomposite consisting of Ca(OH)2 nanoparticles doped with ZnO quantum dots, with an average particle size of 100 nm, dispersed in isopropyl alcohol. This product was developed at the University Pablo de Olavide [36] and has a dry residue content of approximately 0.5%.
Finally, 9 g/m2 Japanese paper was used to apply the consolidants onto the mural samples.

2.2. Sample Preparation and Consolidation

Eighteen (18) circular fresco mural samples (mock up), each 7 cm in diameter, were prepared and divided into four sections painted with different colors: red, white, green, and blue. The samples were prepared following the recipe for Roman frescoes, consisting of three layers: arriccio (the thickest layer, ≈1.5 cm, composed of 1 part lime and 3 parts coarse aggregate), intonaco (a thinner layer, ≈0.8 cm, made of 1 part lime and 2 parts finer aggregate), and intonachino (the thinnest layer, ≈0.3 cm, composed of 1 part lime and 1 part very fine aggregate). The mortars were allowed to dry at ambient temperature and humidity for 24 h between successive applications. While the final layer was still fresh, the pre-moistened pigments were applied. Finally, the samples were left to cure at ambient conditions for six (6) months.
Some important information about the applications of the consolidants is summarized in Table 1. Following the manufacturer’s instructions, Estel 1200® was applied undiluted [41], in a single dose. Nanorepair UV® was synthesized at a concentration of 5 g/L and subsequently diluted (1:5) to prevent the formation of whitish veils and to enhance penetration. The resulting working concentration was 1 g/L. This product was applied in five successive doses to reach a total amount equivalent to the initial concentration (5 g/L), taking into account the 1:5 dilution. Each dose was applied at 24 h interval. Prior to its application, Nanorepair UV® was sonicated for 3 min.
Following the restorers’ recommendations, the consolidants were applied in equal volumes of the commercial products, without adjusting for differences in dry residue content. The amount of product used for each application was determined by surface saturation [15,42], amounting to 0.07 mL/cm2 per application of consolidant dose as described in the previous paragraph.
Two different application methods were employed. All tests were conducted in triplicate. In Method 1 (M1), the consolidants were applied on Japanese paper using a soft brush, and the samples were left to dry at ambient temperature and humidity. In Method 2 (M2), after the treatment was applied as in Method 1, a wet tissue paper poultice soaked in demineralized water was placed on the treated surface and left until complete evaporation occurred. The wet tissue paper poultice was applied, according to previously published studies, which showed that moisture influences the formation of silica gel during ethyl silicate treatments [43] and accelerate carbonation processes in nanolime applications [44].
All samples were left to cure at ambient temperature and humidity for twenty (20) days. These conditions were monitored using a USB data logger (WK057). As shown in previously published studies, the curing period of 20 days is sufficient to ensure the formation of silica gel [45] and to promote carbonation [29].

2.3. Evaluation Methods

The aesthetic changes, which were induced by the application of the treatments, were evaluated by documenting the sample surfaces before, during, and after product application, using both visible and ultraviolet light. The surfaces of the samples were examined using a portable digital microscope, MiScope® model Megapixel 2 (Zarbeco, Succasunna, NJ, USA). Additionally, fragments of the samples were studied using scanning electron microscopy (SEM) to assess possible surface alterations. For this purpose, a JEOL JSM-5400 SEM (JEOL, Tokyo, Japan) equipped with an Oxford INCA Energy 200 microanalyser (Oxford Instruments, Abingdon, UK) was employed. The samples were pre-coated with carbon to ensure conductivity.
Chromatic changes were measured using a PCE-CSM 2 colorimeter (PCE Iberica, Tobarra, Spain), featuring a 45°/0° geometry and a measurement aperture of Ø8 mm. Nine measurements were taken for each color sector of each sample before and after treatment. The measurements were conducted using the CIELab system. To calculate the color differences (ΔE*), Equation (1) was used, where ΔL* represents changes in lightness, Δa* represents changes along the red-green axis, and Δb* represents changes along the yellow-blue axis [46].
∆ E ∗ = ( ∆ L ∗ ) 2 + ( ∆ a ∗ ) 2 + ( ∆ b ∗ ) 2
Although there is no standardized criterion for the acceptable color changes induced in heritage objects by the application of a consolidating product, the threshold of ΔE* < 5 is commonly adopted [29,47]. However, some researchers propose a less stringent approach, suggesting that ΔE* < 10 may be acceptable, particularly for areas with low visual impact [48,49].
The effectiveness of the consolidants was evaluated using the tape peeling test, conducted on the color sectors of each sample both before and after the application of the treatments. Scotch® Crystal Tape (3M, Maplewood, MN, USA) was applied, with 10 repetitions conducted on the same surface, as outlined in the literature [50,51]. The consolidation percentage (%C) was calculated using Equation (2), where TRMuntreated (Total Removed Material) represents the average amount of material removed (mg/cm2) before consolidant application and TRMtreated represents the total amount of material removed (mg/cm2) after treatment [29].
% C = TRM untreated − TRM treated TRM untreated ∗ 100
Finally, the depth of penetration of Nanorepair UV® was measured, as follows. Cross-sections of the treated samples were examined under ultraviolet light at 254 nm using a Spectroline® E-Series UV lamp (6 W output, 230 V AC input, Spectronics Corporation, Melville, NY, USA) taking advantage of the fluorescence of the ZnO quantum dots. The penetration depth of the consolidant was measured using ImageJ 8 software. The image scale was calibrated based on the known length of the scale bar (Set Scale function). After calibration, the penetration depth was measured along the cross-sections using the straight-line tool. Results are reported in micrometres (µm).

2.4. Statistical Analysis

Statistical analyses were performed to evaluate differences between treatments. For each consolidant, a Student’s t-test was used to compare the two application methods, based on three independent replicates per treatment. Data were assessed for normality and homogeneity of variances, and depending on variance equality, either a standard two-sample t-test or Welch’s correction was applied. A significance level of p < 0.05 was used.
Additionally, a one-way ANOVA was performed to compare the four treatments (NR-UV Method 1, NR-UV Method 2, SE Method 1, and SE Method 2). Assumptions of normality and homogeneity of variances were verified prior to the analysis. When the ANOVA revealed significant differences between group means, a post hoc Tukey HSD test was applied to identify which specific pairs of groups differed, allowing for a robust comparison while controlling for type I error.

3. Results and Discussion

This study investigates the optimal method for applying the consolidants Estel 1200® and Nanorepair UV® to mural paint samples, assessing both their effectiveness and suitability as consolidating agents. The selection of the most appropriate treatment and application technique was based on several criteria: physicochemical compatibility with the original material, preservation of the treated surface’s appearance, effectiveness, and the ability to penetrate deeply into the porous structure [3,4,5].
The physicochemical compatibility of the proposed treatments has been highly studied [23,52,53]. In the case of ethyl silicate, the silica gel interacts with hydroxyl groups which are present on the pore surfaces of materials that contain silicate minerals. This reaction helps to partially restore the natural binding material and reconnect detached mineral grains. Nevertheless, when the material has a high carbonate content, the silica gel merely precipitates within the pores and the consolidation relies only on physical forces [52,53]. In the case of nanolimes, the consolidants have the same composition as the mortar binder (Ca(OH)2), and, therefore, they have good compatibility with calcareous and carbonated substrates [23].
Interactions between nanolime suspensions and specific pigments have the potential to induce chromatic shifts while improper application protocols may cause significant conservation issues in mural paintings. Existing studies indicate that alkaline conditions and elevated humidity can trigger chemical transformations and color changes in Cu- and Pb-bearing pigments [54]. Future research will investigate these reactions in detail, both with and without binding media, particularly in the case of Egyptian blue (CaCuSi4O10).

3.1. Visual Condition Under Vis and UV Light

Photographs of the samples before and after the application of the consolidants are shown in Figure 1. Focusing on the red sector, it is evident that the consolidant with the most significant visual impact is Nanorepair UV® applied without a dressing (NR-UV-M1). The microphotographs of Figure 2 shows that a homogeneous whitish veil is clearly visible on the sample surface which was treated with the NR-UV-M1 method (Figure 2B). However, only small scattered surface aggregates were formed on the surface of the sample treated with the NR-UV-M2 method (Figure 2D). These small accumulations tend to form in pores, grain boundaries, and cracks, as previously described by Lanzón et al. [55]. Finally, according to the photographs of Figure 1, Estel 1200® did not induce any significant change to the treated samples (Figure 1D,H), likely due to the colorless nature of the material.
UV photographs were used to evaluate the application methods of only the NR-UV treatments, because the ES treatments do not exhibit fluorescence under UV light. Examination of the Japanese papers used during the treatments (Figure 3) made it possible to assess whether the nanoparticles migrated toward the sample surface during solvent evaporation. Samples treated with NR-UV-M1 method displayed significant fluorescence under UV light (Figure 3A). This fluorescence was attributed to the presence of ZnO quantum dots, which are associated with the nanocomposite used as the consolidant. In contrast, the Japanese papers used in the NR-UV-M2 application method did not exhibit fluorescence (Figure 3B). The fluorescence observed in the dressings, in this case, was due to the intrinsic composition of the paper itself.
Additionally, UV photographs (Figure 4) show the areas where the treatment has been deposited, which can be easily distinguished from missing areas in the polychrome areas due to the presence of ZnO quantum dots and their characteristic yellowish fluorescence.

3.2. Chromatic Changes

The aesthetic effects observed by visual inspection were confirmed by ΔE* measurements (Figure 5). For the samples treated with the NR-UV-M1 method (Figure 2B), the ΔE* value is 31 (Figure 5), significantly higher than the recommended threshold, which indicates that the treatment is overall unsuitable for application. However, in the blue and green sections, the ΔE* values decrease to approximately 11, suggesting that the treatment may be acceptable in areas where the visual impact is less critical
The ΔE* values measured on pigmented surfaces treated with the NR-UV-M1 method are associated with the formation of whitish veils, which cause a substantial increase in the L* value (Table 2). Borsoi et al. [32] suggested that these veils result from the surface deposition of the consolidant, which may be triggered by the migration of nanoparticles during solvent evaporation. Consequently, several studies have investigated the stability of these colloidal products to mitigate the veil formation [23,37]. Moreover, it was suggested that in order to improve the penetration depths of the consolidants without causing nanoparticle migration to the surface, it is essential to slow down the solvent evaporation rate [23]. Notably, these surface deposits can be removed using an aqueous medium [56].
To avoid the veil formation, a second application method (NR-UV-M2) was developed, involving a wet coating that prevents rapid solvent evaporation [31,34,35]. As shown in Figure 1D, the whitish veil formation is almost entirely eliminated in NR-UV-M2 method compared to NR-UV-M1 method (Figure 1B), which is confirmed by a ΔE* value below 5 (Table 2). Statistical analysis using Student’s t-test indicated that the differences in the ΔE* values between the two NR-UV treatments are significant, except in the white areas (Table 2). These results suggest that, for the nanoparticles-based treatment, a slower solvent evaporation rate is critical to mitigate the formation of whitish veil.
Samples treated with Estel 1200® correspond to ΔE* values which remained below 5 (Figure 5). The highest ΔE* values were observed in the blue sectors, while the red sectors, in contrast to the results obtained with Nanorepair UV®, exhibited the lowest ΔE* values. Nevertheless, no statistically significant differences were observed between the ES-M1 and ES-M2 application methods (Table 2).
A one-way ANOVA was performed to compare the four treatments. The analysis revealed a significant effect of treatment on the measured variable (p < 0.05), except in the white area (p = 0.231). Post hoc Tukey HSD tests indicated that the NR-UV-M1 method differed significantly from the other three methods, which did not differ significantly from each other (Table 2).

3.3. SEM-EDX Analysis

SEM images (Figure 6) of the samples treated with Estel 1200® reveal the formation of a uniform surface layer when treatment is applied using method 1 (ES-M1). In some cases, these layers appear fragmented, a phenomenon which was attributed to the stresses induced on the treated surface [57,58]. When Method 2 was applied, some aggregates were observed in the SEM images, appearing larger in the blue sectors (Figure 6).
For the NR-UV treatments, dispersed aggregates were observed and confirmed by EDX analysis. Zinc (Zn) was detected in the microanalysis of the NR-UV treated surfaces (Figure 7). Notably, Zn was absent in both the untreated samples and those treated with Estel 1200®.

3.4. Consolidation Percentage

Regarding the degree of consolidation achieved (Table 3), Nanorepair UV® proved most effective when applied with Method 2. Specifically, the blue sector achieved 95% consolidation, while the other sectors exceeded 80%, except for the green sector (64%). In contrast, Method 1 caused an overall increase in surface decohesion, resulting in negative consolidation values in all sectors, except the blue sector (64%). The negative values resulted from the adhesive tape which lifted the whitish veil that had formed on the surface. Examination of the adhesive peeling tapes under UV light confirmed the removal of applied treatment, as evidenced by ZnO fluorescence (Figure 8). The formation of these veils contrasts with previous studies which investigated the application of this treatment for the consolidation of stone [29,37], where no such alteration was observed despite the absence of a dressing. López-Arce [59] noted that the degree of consolidation improves at relative humidity levels between 75 and 90%. Accordingly, the use of wet dressings enhanced the degree of consolidation achieved (Table 3) by promoting the formation of more stable mineral forms of calcium carbonate [44]. Student’s t-test indicated that the differences in %C values between the application methods for the NR-UV treatments were significant in the red and blue areas (Table 3).
For the samples treated with Estel 1200®, it was observed that the presence of a wet dressing enhances the product’s consolidation power (Table 3), although the improvement was not statistically significant. The presence of moisture is beneficial for the consolidation process, as it enhances the hydrolysis of ethyl silicate and promotes the subsequent condensation of silanol groups into a coherent silica gel network. This process facilitates a more effective polymerization within the substrate, thereby improving the consolidating performance of the treatment [43,60].
In the white sector, where no wet dressing was applied the lowest consolidation value was recorded (%C ≈ −68). This outcome may be attributed to the incompatibility between the consolidant and the lime-rich surface layer [61] and it is consistent with the findings of Becerra et al. [37], who showed that the consolidating performance of ethyl silicate is enhanced in stones with high silicon content, indicating that the treatment is more effective in silica-rich substrates. In contrast, the application of wet dressing led to an increase in %C to 34, although this value remained lower than those recorded in the other color quadrants. Moreover, this method (M2) favors the formation of large aggregates on the surface, as shown in Figure 6E.
An additional noteworthy observation is that the standard deviations of %C values increase considerably in treatments applied without wet dressing (Table 3). This increased variation may be associated with the removal of the superficial aggregates that occurs during the peeling test. Consequently, the elevated standard deviations reflect substantial variability in the results, and therefore they reduced the statistical power of the analysis and limited the detection of significant differences. Notably, the lowest standard deviations are observed in the Nanorepair UV® treatment when applied using Method 2, suggesting a more uniform application of the consolidant across the analyzed samples.
A one-way ANOVA revealed a significant effect of treatment on the measured variable (p < 0.05), except for the blue area (p = 0.068). Post hoc Tukey HSD tests showed that most treatments differed significantly from each other (Table 3).

3.5. Depth of Penetration

The use of Nanorepair UV® presents an advantage for restorers, as it enables easy assessment of its penetration depth by examining a cross-section under UV light. This is possible due to the luminescence of ZnO nanocrystals [62]. ZnO quantum dots have low toxicity and are biodegradable [38,63], although as nanomaterials, they have also been studied for their biocidal properties [64,65]. Consequently, regulatory bodies have introduced specific guidelines and exposure thresholds to reduce the risks associated with their use [66]. Future studies will evaluate the long-term stability of ZnO fluorescence under UV irradiation, as well as under varying humidity and temperature conditions.
As shown in Figure 9, the treatment’s penetration is deeper when a wet dressing is applied, with values exceeding 200 µm (Table 4), and maximum values of 544 ± 33 µm and 628 ± 237 µm in the white and blue sectors, respectively. In this study, and likely due to a smaller pore size, the penetration depths did not reach the values reported by Becerra et al. [29,37] for carbonate stones, although they are higher than those obtained by other authors who used Ca(OH)2 nanoparticles for stone treatment [9]. In the context of mortar consolidation, Otero et al. [15] reported that nanolime treatments exhibit greater penetration depth, as evidenced by measurements obtained with the drilling-resistance technique.
Anupama et al. [67] reported that variations in both pore-size distribution and porosity fundamentally regulate the accessible diffusion pathways for treatment penetration within the substrate. In mortars, particle size plays a more significant role in defining pore structure [68]. Similarly, the particle size of pigments applied to surface mortars affects their color characteristics [69]. This influence was also evidenced by the differences in penetration depth between color areas treated with the same consolidant (Table 4). As shown in Figure 7, blue areas (Egyptian blue pigment) exhibit higher open porosity compared to red areas (iron oxide red pigment) prior to consolidant application. The cross-section of blue-area sample treated with the NR-UV-M2 method (Figure 9B) shows an irregular penetration depth of the consolidant, which is related to the area’s higher open porosity. Nevertheless, when the consolidant remains on the surface and exhibits minimal penetration (Figure 9A), the influence of open porosity is not evident in the cross-section. Therefore, and in agreement with the statistical analysis presented in Table 4, the application method has a significant impact on the penetration depth of the nanolimes.

4. Conclusions

This study evaluated the effectiveness of two treatments for the consolidation of mural paintings and mortars: Nanorepair UV®, based on Ca(OH)2 nanoparticles, and Estel 1200®, an ethyl silicate, both dispersed in isopropanol. The results demonstrated that the application of both treatments with wet dressings significantly improved their effectiveness and applicability, yielding better results than the applications without dressings.
In particular, the treatment with Nanorepair UV® applied using wet dressings reduced the ΔE* value to below 5, a threshold commonly used to indicate that a treatment causes only slight, and therefore acceptable, aesthetic changes which are hardly perceived by the human eye [47,70]. This result suggests that Nanorepair UV® is suitable for application on surfaces of different colors, as it does not cause significant visual alterations, an essential requirement in heritage conservation. The ΔE* values obtained for Estel 1200® were also satisfactory, remaining below 5, indicating that this treatment is also aesthetically acceptable.
Regarding consolidation, Nanorepair UV® proved to be more effective than Estel 1200®, achieving up to 95% consolidation in areas treated with Method 2 (wet dressings), particularly in the blue sectors. This enhanced performance may be attributed to the high compatibility of Nanorepair UV® with carbonate materials, resulting in deeper penetration depth and more effective consolidation of the porous substrate. Notably, this treatment showed an average penetration depth of over 200 µm.
Additionally, the ability to easily assess the penetration of Nanorepair UV® due to the fluorescence of the ZnO quantum dots provides an additional advantage. This property allows for precise evaluation of the treatment’s distribution on the surface and enables clear differentiation between treated and untreated areas.
In conclusion, the application of treatments with wet dressings (Method 2) significantly enhanced their effectiveness, and Nanorepair UV® emerged as the preferred option for the consolidation mural paintings and mortars. Nanorepair UV® offers excellent chemical compatibility, minimal aesthetic changes, and superior consolidation. Moreover, it allows for deeper penetration and precise assessment of its effectiveness. Future research should focus on evaluating the stability of this consolidant through aging tests, which will include wetting–drying cycles, salt crystallization cycles, and water crystallization cycles, among others. Although Becerra et al. [37] conducted some preliminary studies on the aging of Nanorepair UV®, the long-term stability of ZnO fluorescence must be further investigated, focusing on the effects of surface modification and surrounding environment.

Author Contributions

Conceptualization, P.R. and J.B.; methodology, P.R. and J.B.; validation, J.B., I.K. and P.S.; formal analysis, P.R. and J.B.; investigation, P.R., J.B., I.K. and P.S.; writing—original draft preparation, P.R. and J.B.; writing—review and editing, J.B., I.K. and P.S.; funding acquisition, J.B. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially funded by the Institutional Plan B.3—VI PPIT 2023–2026 for the Development of Independent Research Lines (PPI2304), Pablo de Olavide University, Seville.

Data Availability Statement

Data available on request from article authors.

Acknowledgments

This study was partially supported by the research teams TEP-199 from Junta de Andalucía.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Photographs of samples before (A,C,E,G) and after (B,D,F,H) the application of consolidation treatments which were applied according to the following methods (Table 1): NR-UV-M1 (B), ES-M1 (D), NR-UV-M2 (F) and ES-M2 (H).
Figure 1. Photographs of samples before (A,C,E,G) and after (B,D,F,H) the application of consolidation treatments which were applied according to the following methods (Table 1): NR-UV-M1 (B), ES-M1 (D), NR-UV-M2 (F) and ES-M2 (H).
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Figure 2. Photographs of the red sector before (A) and after (B) the application of the NR-UV-M1 method and before (C) and after (D) the application of the NR-UV-M2 method.
Figure 2. Photographs of the red sector before (A) and after (B) the application of the NR-UV-M1 method and before (C) and after (D) the application of the NR-UV-M2 method.
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Figure 3. Photographs under UV light of: the Japanese paper used in NR-UV-M1 method (A) and of the Japanese paper and wet dressing used in NR-UV-M2 method (B).
Figure 3. Photographs under UV light of: the Japanese paper used in NR-UV-M1 method (A) and of the Japanese paper and wet dressing used in NR-UV-M2 method (B).
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Figure 4. Photographs taken under UV illumination before (A) and after (B) the application of the NR-UV-M1 method, and before (C) and after (D) the application of the NR-UV-M2 method.
Figure 4. Photographs taken under UV illumination before (A) and after (B) the application of the NR-UV-M1 method, and before (C) and after (D) the application of the NR-UV-M2 method.
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Figure 5. Color changes after treatments. The dashed line represents the accepted color change threshold for consolidation treatments in heritage conservation (ΔE* < 5 [30,48]).
Figure 5. Color changes after treatments. The dashed line represents the accepted color change threshold for consolidation treatments in heritage conservation (ΔE* < 5 [30,48]).
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Figure 6. SEM images showing the surfaces of red and blue sectors for: untreated sample (A) and samples treated with the NR-UV-M1 (B) NR-UV-M2 (C) ES-M1 (D) andES-M2 (E) methods. Red arrows point to the formation of aggregates, whereas yellow circles denote the presence of a uniform surface layer.
Figure 6. SEM images showing the surfaces of red and blue sectors for: untreated sample (A) and samples treated with the NR-UV-M1 (B) NR-UV-M2 (C) ES-M1 (D) andES-M2 (E) methods. Red arrows point to the formation of aggregates, whereas yellow circles denote the presence of a uniform surface layer.
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Figure 7. EDX spectra (SEM image inset in each spectrum shows the analysis point) taken from the blue (prepared with Egyptian blue, CaCuSiO10) (A) and red (prepared with iron oxide red, Fe2O3) (B) areas. The areas were treated with the NR-UV-M1 method. Consolidant is identified by the detection of Zn, which originates from the ZnO quantum dots.
Figure 7. EDX spectra (SEM image inset in each spectrum shows the analysis point) taken from the blue (prepared with Egyptian blue, CaCuSiO10) (A) and red (prepared with iron oxide red, Fe2O3) (B) areas. The areas were treated with the NR-UV-M1 method. Consolidant is identified by the detection of Zn, which originates from the ZnO quantum dots.
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Figure 8. Material loss after ten consecutive applications on the same spot of a green area treated with the NR-UV-M1 method (A) and a blue area treated with the NR-UV-M2 method (B). The fluorescence observed in (A) indicates that the removed material corresponds to the whitish veil caused by the treatment.
Figure 8. Material loss after ten consecutive applications on the same spot of a green area treated with the NR-UV-M1 method (A) and a blue area treated with the NR-UV-M2 method (B). The fluorescence observed in (A) indicates that the removed material corresponds to the whitish veil caused by the treatment.
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Figure 9. Cross-section of blue-area samples treated with the NR-UV-M1 (A) and NR-UV-M2 (B) methods under UV light illumination. Fluorescence of the treated samples can be observed.
Figure 9. Cross-section of blue-area samples treated with the NR-UV-M1 (A) and NR-UV-M2 (B) methods under UV light illumination. Fluorescence of the treated samples can be observed.
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Table 1. Characteristics of the treatments and methods application.
Table 1. Characteristics of the treatments and methods application.
Consolidant ProductNumber of Applications (Dilution)Application Method *
(Abbreviation)		Amounts of Dry Product (mg/cm2)Environmental Conditions
Nanorepair UV®
(NR-UV)		5 (1:5)Method 1 (NR-UV-M1)0.5158 ± 9.2% RH
20 ± 1.8 °C
Method 2 (NR-UV-M2)
Estel 1200®
(ES)		1 (1:1)Method 1 (ES-M1)16.8958 ± 9.2% RH
20 ± 1.8 °C
Method 2 (ES-M2)
Untreated *
* The experiment was performed in triplicate.
Table 2. Results and statistical analysis of the color changes.
Table 2. Results and statistical analysis of the color changes.
SectorTreatment∆L∆a*∆b*∆E*T-Student
(α = 0.05, 2-Tailed) 		ANOVA (α = 0.05)
Significant Comparisons
(Tukey HSD)
RedNR-UV-M122.68 ± 2.54−15.60 ± 2.06−15.10 ± 0.8531.38 ± 3.02t(2) = 14.91
p = 0.004		F3,8 = 253.86, p = 2.86 × 10−8
Tukey HSD:
NR-UV-M1 vs. NR-UV-M2, ES-M1, ES-M2
NR-UV-M22.02 ± 0.97−0.37 ± 3.32−0.56 ± 3.584.54 ± 0.79
ES-M1−0.08 ± 0.44−0.50 ± 0.54−0.03 ± 0.590.82 ± 0.47t(4) = –1.25
p = 0.28
ES-M2−0.68 ± 0.48−0.21 ± 1.070.61 ± 0.321.31 ± 0.48
WhiteNR-UV-M1−0.21 ± 0.76−0.24 ± 0.13−0.39 ± 0.550.90 ± 0.23t(2) = −1.80
p = 0.21		F3,8 = 1.77, p = 0.231
NR-UV-M2−1.01 ± 1.240.69 ± 0.462.07 ± 1.252.53 ± 1.54
ES-M1−0.88 ± 0.600.62 ± 0.231.09 ± 0.131.58 ± 0.47t(3) = −1.16
p = 0.33
ES-M2−1.23 ± 0.950.66 ± 0.321.88 ± 0.712.38 ± 1.10
GreenNR-UV-M18.36 ± 4.663.07 ± 1.01−6.62 ± 2.2411.15 ± 5.10t(3) = 2.64
p = 0.08		F3,8 = 7.18, p = 0.012
Tukey HSD:
NR-UV-M1 vs. NR-UV-M2, ES-M1, ES-M2
NR-UV-M2−2.65 ± 1.770.10 ± 0.361.07 ± 0.652.90 ± 1.82
ES-M1−1.52 ± 0.450.72 ± 1.131.63 ± 0.402.54 ± 0.43t(4) = −0.35
p = 0.74
ES-M2−1.91 ± 0.110.64 ± 0.861.52 ± 0.822.67 ± 0.51
BlueNR-UV-M18.78 ± 1.680.28 ± 0.108.10 ± 1.9211.95 ± 2.53t(3) = 5.05
p = 0.015		F3,8 = 3.69, p = 0.062
Tukey HSD:
NR-UV-M1 vs. NR-UV-M2, ES-M1, ES-M2
NR-UV-M2−0.74 ± 0.73−0.85 ± 0.220.22 ± 4.613.87 ± 1.14
ES-M10.91 ± 3.78−0.65 ± 1.033.12 ± 4.204.21 ± 4.78t(4) = 0.05
p = 0.96
ES-M21.30 ± 3.610.43 ± 0.553.09 ± 3.774.02 ± 4.52
Table 3. Results and statistical analysis of the consolidation percentage (%C).
Table 3. Results and statistical analysis of the consolidation percentage (%C).
SectorTreatment%CT-Student
(α = 0.05, 2-Tailed) 		ANOVA (α = 0.05)
Significant Comparisons
(Tukey HSD)
RedNR-UV-M1−17.5 ± 31.4t(2) = 5.47
p = 0.032		F3,8 = 12.86, p = 0.002
Tukey HSD:
NR-UV-M1 vs. NR-UV-M2, ES-M1, ES-M2
NR-UV-M2 vs. ES-M1
ES-M1 vs. ES-M2
NR-UV-M281.8 ± 1.0
ES-M141.7 ± 33.4t(2) = 2.22
p = 0.156
ES-M285.0 ± 4.5
WhiteNR-UV-M1−67.3 ± 101.2t(2) = 2.48
p = 0.131		F3,8 = 4,09, p = 0.049
Tukey HSD:
NR-UV-M1 vs. NR-UV-M2, ES-M2
NR-UV-M2 vs. ES-M1, ES-M2
ES-M1 vs. ES-M2
NR-UV-M280.8 ± 20.9
ES-M1−68.2 ± 64.0t(3) = 2.34
p = 0.101
ES-M233.7 ± 40.0
GreenNR-UV-M1−33.1 ± 63.1t(2) = 2.60
p = 0.121		F3,8 = 5.86, p = 0.020
Tukey HSD:
NR-UV-M1 vs. NR-UV-M2, ES-M1, ES-M2
NR-UV-M2 vs. ES-M2
ES-M1 vs. ES-M2
NR-UV-M264.1 ± 14.0
ES-M163.4 ± 13.5t(4) = −1.02
p = 0.367
ES-M252.9 ± 11.7
BlueNR-UV-M164.2 ± 11.5t(2) = 4.47
p = 0.045		F3,8 = 3.54, p = 0.068
Tukey HSD:
NR-UV-M1 vs. NR-UV-M2, ES-M1
NR-UV-M2 vs. ES-M1, ES-M2
ES-M1 vs. ES-M2
NR-UV-M294.5 ± 2.6
ES-M151.1 ± 26.6t(3) = 0.86
p = 0.454
ES-M266.7 ± 16.9
Table 4. Results and statistical analysis of the penetration depth.
Table 4. Results and statistical analysis of the penetration depth.
SectorTreatmentDepth of Penetration (µm)T-Student
(α = 0.05, 2-Tailed)
RedNR-UV-M1169 ± 25t(8) = −5.71
p = 0.0004
NR-UV-M2237 ± 15
WhiteNR-UV-M1123 ± 1t(5) = −31.66
p = 5.9 × 10−7
NR-UV-M2544 ± 33
GreenNR-UV-M1161 ± 28t(10) = −3.05
p = 0.012
NR-UV-M2214 ± 32
BlueNR-UV-M1260 ± 33t(10) = −18.24
p = 5.3 × 10−9
NR-UV-M2628 ± 237
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MDPI and ACS Style

Rebollo, P.; Becerra, J.; Karapanagiotis, I.; Spathis, P. Comparative Study of Ethyl Silicate and Nanolimes with Fluorescent Properties as Consolidants for Mural Paintings and Mortars. Heritage 2025, 8, 529. https://doi.org/10.3390/heritage8120529

AMA Style

Rebollo P, Becerra J, Karapanagiotis I, Spathis P. Comparative Study of Ethyl Silicate and Nanolimes with Fluorescent Properties as Consolidants for Mural Paintings and Mortars. Heritage. 2025; 8(12):529. https://doi.org/10.3390/heritage8120529

Chicago/Turabian Style

Rebollo, Patricia, Javier Becerra, Ioannis Karapanagiotis, and Panagiotis Spathis. 2025. "Comparative Study of Ethyl Silicate and Nanolimes with Fluorescent Properties as Consolidants for Mural Paintings and Mortars" Heritage 8, no. 12: 529. https://doi.org/10.3390/heritage8120529

APA Style

Rebollo, P., Becerra, J., Karapanagiotis, I., & Spathis, P. (2025). Comparative Study of Ethyl Silicate and Nanolimes with Fluorescent Properties as Consolidants for Mural Paintings and Mortars. Heritage, 8(12), 529. https://doi.org/10.3390/heritage8120529

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