Innovative consolidating products for stone materials: field exposure tests as a valid approach for assessing durability
© Natali et al.; licensee Springer. 2015
Received: 30 September 2014
Accepted: 10 February 2015
Published: 2 March 2015
The impact of climate on cultural heritage surfaces leads to several damage processes and the protection and the preservation of works of art is a challenge for conservation scientists and restorers. Traditional and innovative products are used in consolidating treatments in order to reduce the effects of the interaction environment-materials. The EC NANOMATCH Project aims at the development of innovative consolidating agents for carbonate matrices, wood and glass whose features should result in high compatibility, efficiency and long-lasting effect. In this project, metal alkoxides, molecular precursors for the deposition of metal carbonate are synthesized, characterized, tested and proposed as an alternative to traditional consolidating agents as well as to calcium hydroxide nanoparticles. This paper gives an overall description of the methodological approach adopted for the in field evaluation of durability taking into account the environmental impact. Preliminary results of the analyses carried out on carbonate stones aimed at investigating the features of the consolidating treatment are here presented and discussed.
Climate is predicted to change in the near and far future. The EC NOAH’S ARK Project [1-3] has contributed to the production of scenarios and maps representing the European situation concerning climatic events and related damage processes affecting outdoor built cultural heritage. The foreseen impact of climate change on carbonate stone, marble and limestone, for the 21st century indicates that they will undergo an increase of surface recession, resulting mainly from the yearly precipitation amount and the rise in carbon dioxide concentration; in addition, thermal stress will be experienced by marble, and increased salt crystallization by porous stone [4-7]. Although knowledge on deterioration processes of building materials have been greatly improved, no really effective solutions to preserve and protect these materials have been found yet.
Both organic and inorganic consolidating products have been traditionally used in order to recover the mechanical properties of damaged materials of built heritage. A wide range of synthetic polymers has been extensively used in stone conservation treatments, but their long lasting efficacy and performance in preventing further deterioration has been recently drastically reconsidered as they frequently undergo chemical modifications induced both by environmental conditions and irreversibility of the treatment [8-10]. Traditional inorganic treatments, though usually more compatible with the carbonate matrices, suffer from low solubility and therefore a resulting low amount of applied product as well as from a scarcely cohesive effect. Penetration depth, physico-chemical compatibility, stability, absence of by-products and partial filling of porosity are those characteristics to be evaluated in choosing the best consolidating material for a valid conservation treatment.
In the last decade nanomaterials have been specifically developed for restoration procedures. In particular calcium, magnesium, barium hydroxide nanoparticles have been largely studied and their potentiality as surface consolidating agents has been demonstrated [11-13]. Although these innovative materials have been extensively characterized, little is known about their behaviour in outdoor environment and their durability.
The EC NANOMATCH Project is aimed at developing innovative consolidating products for carbonate matrices, wood and glass. In particular, metal alkoxides have been identified and then synthesized as molecular precursors for the deposition of metal carbonate as consolidating agent for carbonate stones, the same product as alkaline reservoir to prevent acidity of wooden materials and metal oxide as a glass consolidant. Alkaline earth and semimetal alkoxides are suitable molecular precursors because the corresponding solutions or nano-particulate inorganic sols, upon evaporation of the solvent based carrier, undergo hydrolysis and condensation or carbonation inside the porous structure of the substrate. Particularly, calcium alkoxides, acting as precursors of corresponding carbonate [14,15], can be considered for stone strengthening as an indispensable and essential preliminary action for its conservation, as demonstrated within the bilateral Italian-French GALILEO Project . The same material has been used for wood as these materials, besides strengthening effects, also ensure an alkaline supply to mitigate acidic deterioration processes of cellulose structures.
The expected features making them competitive respect to conventional conservation products are their (i) compatibility with the main materials used in built heritage like stone and wood -even painted- and glass, ensuring enhanced durability, re-treatability, sustainability and efficiency, (ii) easy and safe handling during applications, (iii) satisfactory selling prices.
In this paper we will briefly describe the methodological approach adopted for the evaluation in field of the performances of newly developed products compared to commercial ones applied on carbonate stones. Furthermore, the study here presented is part of the experimental work carried out within the project to achieve an exhaustive performance evaluation of the developed material by investigating its efficacy, compatibility and durability. As the latter aspect is intimately linked to climate impact, environmental parameters having a crucial role in damage processes have been prioritized for a correct evaluation of the durability of the NANOMATCH product during the test in field. Climate parameters have been considered more important than pollution parameters in addressing the objectives of the NANOMATCH project. The main parameters linked to climate change and trigging future damage on materials constituting immovable and movable heritage have been identified from the results of previous EU projects [17-21] and taken into account for the definition of methodologies to assess long-term behaviour of treatments. Rain, relative humidity, air temperature, wind, solar radiation contribute to trigger decohesion processes affecting carbonate stones and therefore believed to have priority for the evaluation of the compatibility, efficiency and performance of nano-structured materials in field. In addition, surface temperature has been also taken into account as an important parameter linked to the material constituent the work of art. The aim of this paper is to highlight how field exposure tests can constitute a valid tool to investigate compatibility and durability of innovative nanostructured consolidating agents, specifically developed to be applied on outdoor built heritage surfaces. The methodological approach adopted to achieve the project objectives is described and part of the results obtained by analysing samples before/after treatment and after exposure by means of different analytical techniques are presented and discussed.
Materials and methods
Synthesis of calcium alkoxides
Different synthetic routes have been tested and more than 20 different alkoxides have been obtained in order to identify the right products and the suitable methodology for its production at industrial scale. In first instance, calcium and magnesium alkoxides were believed adequate to be used in the field of conservation of cultural heritage.
Synthesis of calcium alkoxides, involving ammonia gas-assisted reactions with the corresponding alcohol, have been carried out in nitrogen-filled gloves-boxes with the exclusion of moisture and oxygen according to procedures already described in the literature . The reaction mixture can arise as a white suspension or as a clear solution depending on the alkoxide: in the case of a white suspension, the white precipitate is recovered by centrifugation while in the case of a clear solution, the product is recovered by solvent evaporation.
Then, two alkoxides have been selected on the basis of their properties (solubility in common organic solvents, volatility of the corresponding alcohol, low toxicity): Ca(OCH2CH3)2 (NANOMATCH2) and a Ca(OTHF)2, where THF = tetrahydrofurfuryl moiety (NANOMATCH1). These have been up-scaled to produce about 3Kg of each compound.
Although several synthetic methodologies can be used  to synthesize magnesium alkoxides, preliminary carbonation tests performed with available commercial products demonstrated that they are unsuitable for conservation of built heritage [24,25].
The effects of solvents and different relative humidity conditions (50 and 90%) on carbonation rate and phase formation as well as the effects of outdoor environment and liquid water have been investigated. The results evidenced the complexity of carbonation process, where the most kinetically favoured vaterite is always formed but it can evolve to the thermodynamically stable calcite by addition of water and with high relative humidity. The choice of the solvent is extremely important to control carbonation rate: higher vapour pressure (fast evaporation rate) leads to faster carbonation. Slow carbonation is also related to the formation of the thermodynamically favoured phase calcite.
Lithotypes selection and application trials
The selection of stones has been restricted to carbonate ones since the Ca-alkoxide products developed in the NANOMATCH project are mainly meant for the consolidation of calcite-based matrices by deposition of calcium carbonate. Stone materials have been selected on the basis of their Water Absorption Coefficient (WAC) and total porosity. In particular, lithotypes characterized by having very different values of these two properties have been chosen; then, the final choice took into account stones presenting extreme, completely different values of total porosity and WAC.
Moreover, the Ca-alkoxide consolidants developed during the NANOMATCH project have been compared to a commercial consolidants currently on the market, CaLoSiL® manufactured by IBZ-Salzchemie (Freiberg, Germany).
A number of application trials (Figure 2b) have been carried out on all stones in order to optimize the application parameters of both compounds. The trials enabled to select the application solvent, the product concentration and the application conditions that gave the best results. Four different solvents (2 alcohols and 2 hydrocarbon based) or mixture of them at different concentration have been applied on the different lithotypes.
The best results have been obtained with solution of alkoxides with 20 g/L of Ca in ethanol:ligroin 1:1 applied by brush for two times (different application methods/condition gave worse results), on the basis of visual appearance, amount of consolidant introduced as well as scratch test.
Tests in field
Lithotypes and sites of exposure selected for the field test within NANOMATCH Project
Site of exposure
Santa Croce Basilica Florence, Cologne Cathedral, Oviedo Cathedral, Stavropoleos Monastery Bucharest
Stavropoleos Monastery Bucharest
Artificial deterioration and consolidating treatments carried out on the four lithotypes
Model sample (10 × 10 × 5) cm 3
NANOMATCH1 solution in 1:1 ethanol:ligroin at 20 g/L of Ca
Contaminated with a 5% w/w Na2SO4 solution
COMMERCIAL: CaLoSiL® – 20 g/L of Ca in ethanol
It is well known that pollutants in synergy with environmental factors trigger many damage processes affecting surfaces. According to the methodological approach adopted in the NANOMATCH Project, samples have been placed outdoor, in an unsheltered area and so exposed to the rain-wash out. However, all samples were placed in urban environments therefore affected by problems related to the vehicular traffic, domestic heating and industry. Moreover, Cologne, Oviedo and Bucharest have also to face pollution caused by power plants (some of them also fuelled by coal) located within the cities or in their periphery. Oviedo is also placed in a coal mining area, implying a high concentration of SO2, NOX, CO2 and traces of heavy metals in the atmosphere. However in Florence, Oviedo and Bucharest samples were exposed in pedestrian areas and therefore not directly exposed to vehicular traffic impact. On the contrary, Cologne cathedral is aside the railway station and in traffic congested area. The yearly reports about the air quality status for the year 2013 of the four cities confirm the main pollution problems are the still high concentration of nitrogen oxides and particulate matter, both connected with vehicular traffic emissions. Examining the yearly report about the air quality status of Tuscany for the year 2013 , some events of pollutants concentration exceeding suggested thresholds were observed in the congested monitoring station of Gramsci Avenue, located near Santa Croce Basilica. For example, the daily mean limit value of PM10 regulated by the European Directive 2008/50/EC and the Italian D.Lgs 155/2010 and D.Lgs 250/2012 was not respected as well as the concentration of ozone (O3). Moreover, the concentration of NO2 exceeded the annual limit value of 40 μgm−3. During the 2013 there was also a reduction of the yearly mean concentration of carbon monoxide (CO) and sulfur dioxide (SO2). Concerning Cologne, the annual report on air quality of the North Rhine-Westphalia relative to 2013  highlighted that the amount of particulate matter slightly increased while the hourly limit value of nitrogen dioxide was respected in all the monitoring stations, although some excesses of the annual limit value occurred in congested areas such as the one located near the Cologne Cathedral. An improvement in the concentration of sulfur dioxide and ozone was observed, the latter facilitated by few long sunshine periods and by the decrease in emissions of precursors such as NOX and volatile hydrocarbons. During the eleven months of stone samples exposition in Oviedo no excesses in sulfur dioxide concentration were recorded by the monitoring station nearest to the Cathedral, both of the hourly and the daily limits (respectively 350 μgm−3 and 125 μgm−3) . Moreover, there was an improvement in the emission of ozone in respect to the previous years while the concentration of nitrogen dioxide and particulate overtook the limits, remaining, however, below the number of possible excesses. The Rumanian annual report on air quality relative to 2013  highlighted that the concentration of nitrogen oxides and of the particulate matter were still high in Bucharest while the sulfur dioxide emissions did not manifest any excess of the hourly and daily limit values.
Optical Microscopy (OM) both in transmitted and reflected light on thin and polished cross sections of each lithotype using an Olympus BX51 microscope;
Scanning Electron Microscopy - Energy Dispersive X-ray spectroscopy (SEM-EDX) using a ZEISS 1530 instrument, equipped with two different Secondary Electrons (SE) detectors, the InLens (IL) and the Everhart-Thornley detectors (ETD);
Color measurements (spectrophotometry) carried out according to the CIE L*a*b* chromaticity diagram and to the UNI 8941 Standard Colored surfaces, using a KONICA MINOLTA CM700d Spectrophotometer, performing measurements with a spot size of 8 mm diameter in the 400–700 nm spectral range.
Scotch Tape Test (STT)
Capillarity water absorption test.
Analyses performed on the four lithotypes samples and objectives
Cohesion, surface properties, penetration depth, interaction
Carrara marble, Savonniéres limestone
Cohesion, penetration depth, interaction
Carrara marble, Savonniéres limestone
capillarity water absorption
Before treatment/after exposure
Carrara marble, Savonniéres limestone (on selected samples purposely prepared)
Cohesion, surface properties, penetration depth, interactions with stone, porosity and water absorption have been therefore evaluated and compared.
Results and discussion
Analyses on collected samples have been carried out with the aim to verify the cohesive properties conferred to the surface by the treatments, the possible textural and structural modifications induced by the application of the nanomaterials to the stone surface, the penetration depth of the consolidating product, as well as possible interactions with the stone.
In this section the most important contributes obtained from each analytical technique, with exception of capillarity water absorption test, are shown in order to highlight the potentialities of the adopted methodological approach.
On the contrary, CaLoSiL® treatment is still clearly visible on Carrara Marble surface after exposure and it appears as a discontinuous brownish surface layer in plane polarized light, and as a bright greenish microcrystalline layer in cross polarized light (Figure 6c, d).
In general, surface treatments adopted in conservation procedure should not strongly modify the aesthetic aspect of the work of art. Spectrophotometric measurements have been performed according to the CIE L*a*b* chromaticity diagram in order to verify if possible differences in colour of surfaces induced by NANOMATCH1 treatment would be noticed and if they might be considered negligible or not. Taking into account the indication given by García and Malaga  with regard to the threshold value of ΔE* accepted as reference in the evaluation of a conservation treatment, the ΔE* values, calculated using colour coordinates referred to the surface of the samples before and after treatment, cannot be seen by a human eye when they are < 5 units.
In this paper the methodological approach adopted by field exposure tests in the EC NANOMATCH project for the evaluation of climate impact on the performances of newly developed products was described. The presented data should be read as part of the preliminary results of the analyses carried out before and after the application of the product on stone samples.
In terms of aesthetic compatibility NANOMATCH1 product can be considered suitable to be used as consolidant for the selected lithotypes as it does not affect the appearance of the stone when appropriately applied. Results showed that it leads to the formation of microcrystalline aggregates on porous limestones’ surfaces, somewhere penetrating the stone matrix; on stones characterized by low porosity or microporosity (as Carrara Marble) it forms a discontinuous microcrystalline surface layer. After exposure, morphological observations performed on stone surfaces and sections of samples treated with NANOMATCH1 and CaLoSiL® products, demonstrated that the calcium carbonate coating deriving NANOMATCH1 product is barely observable over the surface and is randomly distributed on wider pores and cracks below the stone surfaces, while the consolidant obtained from carbonation of CaLoSiL® is exclusively located on the outmost surface and it is affected by fissures as the substrate itself due to outdoor weathering.
A good surface cohesive effect is achievable in a short time after its application on stones, due to the fast reactions of alkoxides with atmospheric H2O and CO2. Moreover, greater cohesion of the surface has been detected after the exposure period, even though the NANOMATCH product is almost absent on the stone surface, as confirmed by optical and electron scanning morphological observations. Comparing and combining OM and SEM results with those obtained by the peeling test (STT) it has been possible to argue for the calcium carbonate deriving from the calcium alkoxide a partial dissolution, penetration and recrystallization of the product within the porous matrix as a consequence of the exposure.
The comparison and the crosschecking of all the results obtained by the analyses of samples before/after treatment and after one year of exposure will allow to evaluate the durability of the nanostructured material developed within the NANOMATCH project and the climatic impact on the innovative treatment and the treated surfaces related to different geographical areas. The preliminary analyses carried out on Carrara Marble, the only lithotype exposed in all the sites, indicate that the differences in the environmental factors present at each site do not translate in significant differences between the marble samples exposed at the different sites.
The NANOMATCH Project (Nano-systems for the conservation of immoveable and moveable polymaterial Cultural Heritage in a changing environment) has received funding from the European Union’s Seventh Programme for research, technological development and demonstration under grant agreement No . Authors express their gratitude to Elsa Bourguignon and Vincent Detalle (CPP- LRMH, France), Barbara Lubelli and Rob Van Hees (TNO, Netherlands), Ulrike Brinkmann and Michael Hauck (Dombauverwaltung Köln, Germany), Martin Labouré (Eschlimann Atelier, France), Marco Pancani (Opera di Santa Croce, Firenze, Italy) and Luis Valdeón (Gea asesoría geológica, Spain) for the fruitful discussion and support during the experimental work.
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