2A - Colloidal Science and Nanotechnology for Cultural Heritage Conservation

 

P. Baglioni, G. Bitossi, E. Carretti, L. Dei, V. Errico, R. Giorgi, A. Macherelli, M. Mauro, B. Salvadori

 

Aims

Nanomaterials for wall paintings and stones conservation.

Nanocontainer solutions for wall painting and stone surfaces.

Innovative gels for easel paintings conservation.

Nanotechnology for paper deacidification.

Anti-scaling molecules for inhibition of salt crystallization in porous media.

Stone investigation by Nuclear Magnetic Resonance techniques.

 

Results

CSGI is involved in several projects aimed to improve materials and techniques for the conservation of cultural heritage. The city of Florence is one of the most appropriate  "environments" for these studies. After the 1966 Florence flood, the research group directed by the CSGI co-founder Prof. Enzo Ferroni was one of the first Academic Institutions that applied a rigorous scientific approach to the investigation of Cultural Heritage conservation. The peculiarity of the research in the Science for Cultural Heritage resides in its multidiscipinarity, where basic studies are usually associated to technological researches.

Several conservation workshops have been carried out with CSGI scientific consultancy, and using innovative methodologies developed in the CSGI Laboratories (Masaccio's wall paintings in Cappella Brancacci, and Beato Angelico's wall paintings in San Marco Abbey, in Florence, Piero della Francesca's wall paintings in Arezzo, etc.).

CSGI contribution mainly consists of a co-operation with conservators and private and public institutions for experimentation of the innovative methodologies; this approach provides a continuous improvement of the conservation procedures. Moreover, CSGI provides also physicochemical diagnostics of materials: in particular, CSGI expertise is related to the characterization of pigments, dyes, and binders used for wall paintings or easel paintings, and also of the degradation products, as salts, varnishes and aged adhesives, in stones and wall paintings.

 

 

Fig. 1 San Marco Abbey (Florence), wall paintings by Beato Angelico. Conservation carried out with the Ferroni-Dini method (ammonium carbonate plus barium hydroxide)

 

Fig. 2 San Marco Abbey (Florence), wall paintings by G. D. Sogliani: Dino Dini at work

 

Nanomaterials  for wall paintings and stones conservation

Calcium hydroxide is probably the best solution for reinforcement of wall paintings, since it is physicochemical compatible and it is the 'original' binder used by the artists. Unfortunately the poor solubility of calcium hydroxide in water has prevented a large use of lime water. More concentrated systems, based on lime as binder, could be prepared as a dispersion, but lime dispersions in water are not stable, actually sedimentation rate is very fast. The formation of white glazing over painted surfaces is the main consequence of this behaviour.

Ca(OH)2 nanoparticles and stable dispersions, in nonaqueous media, have been obtained by several synthetic approaches. Scanning and Transmission Electron Microscopy coupled with X-Rays Diffractometry (SEM/TEM, XRD) showed crystalline, hexagonally shaped, nanoparticles with hexagon sides in the range 50-300nm. Atomic Force Microscopy (AFM) evidenced that the thickness were in the range 2-40 nm.

Calcium hydroxide stable dispersions were successfully applied, instead of organic glues, as fixatives to re-adhere lifted paint layers during several restoration workshops: wall paintings by Santi di Tito in the Cathedral of Florence, wall paintings by Filippo Lippi in the Cathedral of Prato, wall paintings by Conrad Albrizio in the State Exhibit Building Museum of Louisiana, stone materials of the Santa Prisca in Aventino Apse Church in Rome and of the Bell Tower at Impruneta near Florence.

 

 

 

Cathedral of Florence,  Santi di Tito’s wall paintings: conservation carried out by means of  lime nanodispersions

 

   

 

Fig. 3 Santa Maria Novella Basilica in Florence, wall paintings by Andrea da Firenze: conservation carried out by means of  lime/alcohol dispersions

 

Nanocontainer solutions for wall painting and stone surfaces

Oil-in-water microemulsions have been formulated for the application to wall paintings, easel paintings and stones, in order to remove hydrophobic materials (acrylic resins, wax, fats, etc.) from the works of art surfaces. Past treatments, based on the use of hydrophobic synthetic materials, failed since of their degradation. Actually, they lose its efficacy as protective and/or binder, and change in color producing dramatic side effects. The only way for removing them is, at the moment, limited to the usage of toxic mixtures of solvents.

Three (water, oil, surfactant), four (water, oil, surfactant, and co-surfactant), and five components (water, oil A, oil B, surfactant, and co-surfactant) systems were set up and checked in laboratory as 'cleaning agents'. The best formulation was tested in some case studies in the framework of the restoration workshops of the wall paintings by Filippo Lippi in Prato Cathedral and Spinello Aretino in the Guasconi chapel in San Francesco Church, Arezzo. The removal of hydrophobic polymeric resins from the fresco surfaces was complete and any side effects come out. Recently 250 m2 of the Conegliano (near Venice) Cathedral Façade mural paintings have been conserved using a micellar solution developed at the CSGI Center (and baptized ‘Conegliano’s potion’) to clean the wall painting from vynil and acrylic polymer used in past interventions in 1950 and 1989.

 

Fig. 4 Particular of the fresco by Pozzoserrato (XVI century) in the Conegliano’s Cathedral after the cleaning with the micellar solution developed ad hoc for this workshop.

 

Innovative gels for easel paintings conservation

In the last two years the Florence unit of CSGI has developed new gel systems with potential application for eesel paintings conservation. The basic idea is to overcome the problems associated to gelators with high molecular weight (very high viscosity creates diffuculties during the removal from the painted surfaces) by developing system with a gelator precursor (polyallylamine) that becomes a true gelator after CO2 bubbling with consequent formation of the polyallylammoniunm carbamate which behaves as a very good gelator for many solvents commonly used in easel paintings cleaning. Then the gelator can be easily removed disrupting the gel network by simple weak acid hydrolysis that gives back the polyallylamine precursor. This approach seems to have very stimulating outputs for potential application in cultural heritage conservation: important tests are in progress in cooperation with Museums and Galleries in Florence and Siena.

 

Nanotechnology for paper deacidification

Acidity is the main source of degradation in paper. Many different techniques and products have been studied or developed in order to eliminate acidity from paper. Unfortunately, up to now the problem is far from to be satisfactorily solved, and the specialists are looking for new products against the acidity present in paper.

Despite its scarce usage, calcium hydroxide is an excellent deacidifying agent. Calcium hydroxide is physico-chemically friendly to paper and, once converted to calcium carbonate, works efficiently as alkaline reservoir. The best way to apply is as a suspension shape.

Unfortunately, commercially available calcium hydroxide powders have a size distribution was quite broad and larger than several micrometers. Smaller particles were around 0.5 microns, 80% were greater than 1 micron. This originated two problems: 1)kinetic stability is good but not appropriate for some application procedures, for example spraying; 2)because of particle sizes, a white glaze (or spots) on the treated surfaces could form. Large calcium hydroxide particles showed a poor adhesions and penetrability into the cellulose fibers. Deacidifying agents must ensure good adhesion with fibers, so that the alkaline reservoir acts efficiently, and excellent penetrability through cellulose fibers. These requirements are ensured by the use of particles smaller than micron size.

Calcium hydroxide nanoparticles dispersed in alcohol offer an innovative solution for paper deacidification. The used solvents are environmentally friendly. Moreover, nanoparticles can be applied by different techniques, as brushing or spraying, providing excellent results.

Nanosized magnesium hydroxides and oxides have been recently made. Their application in paper deacidification is currently under investigation.

 

  

 

Fig. 5 Deacidification of paper by sprayed lime nanodispersions

 

Innovative methodologies for inhibition of salt crystallization in porous media

The growth of crystal phases within a porous matrix is one of the main sources of the degradation affecting the historical building materials. The formation of salts is connected to the environmental conditions and, therefore, in many cases they are not completely removable. Less disrupting effect, inside the porous matrix, would be obtained if we were able to minimise the growth of crystals (and the volume). Inhibitor molecules or habitus modifiers, adsorbing on crystalline growing faces or over the crystallization nuclei, provide the right way to do it.

The research project is devoted to the study of inhibition or modification effects by some classes of molecules. The research project is planned as follows:

1 - preparation of laboratory samples impregnated by saturated salt solution;

2 - physicochemical characterization of the crystalline phases, and the porous materials;

3 - monitoring of the crystallization process and analysis of the inhibition activity of additives after crystallization-dissolution cycles carried out in climatic chamber.

This approach aims to control the growth of crystalline phases, by reducing the mechanical stresses of the pore walls. The project contribution to the conservation field is remarkable because it offers new tools for prevention of the degradation processes, instead of mechanical cleaning of surfaces affected by salt crystallization.

 

Stone investigation by Nuclear Magnetic Resonance techniques

Deterioration of the porous building materials can be considered as a sort of "stress-corrosion process" in which chemical and physical attacks are combined, giving a mechanical alteration of the stone. Water amount inside porous stones plays a crucial role in  most of the decay processes. Water is involved in many chemical processes, such as pollutants chemical reactions (i.e. sulphur dioxide), frost attacks, salt crystallization cycles (in particular sulphates and nitrates), and physical attacks, like pore gelivity, or biological degradation due to the action of fungi, bacteria, and so on.

Experiments have been performed, by using innovative experimental approach based on nuclear magnetic resonance (NMR) techniques. NMR exploits the magnetic properties of nuclei (hydrogen in water in our case) under an external high magnetic field. By studying the time length needed for the spin system magnetization to reach the equilibrium value, after a selected perturbation obtained by a radio-frequency pulses sequence, a plenty information on the energy exchanges between the system and its surroundings is obtained. By proper pulse sequences, high-resolution 2D-images of water distribution inside low porous materials (1-3% open porosity) were obtained. Analysis of water proton relaxation rates provided direct information on the porous structure.

This point is meaningful since we allowed the investigation of the stone features after a restorative treatment, in not invasive way. The importance of the physicochemical compatibility among chemicals used by restorers and the original materials was completely neglected in the past. By this reason, wrong conservative restorations have been performed. NMR techniques probe in real time the water filling of stone and provide a direct correlation with the pore structure in restored materials.

 

 

 

Fig. 6 Salt crystallization effect on Italian limestone

 

 

References

 

Carretti, E., Dei, L., Baglioni, P., Solubilization of Acrylic and Vinyl Polymers in Nanocontainer Solutions. Application of Microemulsions and Micelles to Cultural Heritage, Langmuir 19 (2003) 7867. This paper was cited by Nature: Restoring the conserved, by Jane Morris, Nature-Materials Update – August 21, 2003.

 

Nanni, A., Dei, L., Ca(OH)2 Nanoparticles from W/O Microemulsions, Langmuir 19 (2003) 933.

 

Baglioni, P., Ceccato, M., Dei, L., Giorgi, R., Schettino, C. V., Nanotechnologies for Conservation of Cultural Heritage: Paper and Canvas Deacidification, Langmuir 18  (2002) 8198. This paper was cited by Nature: Nanoparticles save paper - A sprinkling of slaked lime conserves old documents, by Philip Ball, Nature, Science Update - 22 October 2002.

 

Giorgi, R., Dei, L., Schettino, C., Baglioni, P., A new method for paper deacidification based on calcium hydroxide dispersed in non-aqueous media, Preprint of the IIC Baltimore Congress 2002, Works of Art on Paper, Books, Documents and Photographs: Techniques and Conservation, P. Smith Ed., (2002) 69.

 

Alesiani, M., Baglioni, P., Capuani, S., Giorgi, R., Maraviglia, Effects induced on marbles, by water repellent compounds: the NMR contribute, Applied Magnetic Resonance 23 (2002) 1.

 

Ambrosi, M., Baglioni, P., Dei, L., Giorgi, R., Neto, C., Nanosized crystals of Ca(OH)2: properties and application, Langmuir 17 (2001) 4251. This paper was cited by Nature: Nanotechnology restores flaking frescos - An off-the-wall application of tiny particles re-unites paint and plaster, by Philip Ball, Nature, Science update - 11 July 2001.

 

Ambrosi, M., Baglioni, P., Dei, L., Giorgi, R., Neto, C., Dispersions of Ca(OH)2 in aliphatic alcohols: a new tool in Cultural Heritage conservation, Progress in Colloid and Polymer Science 118 (2001) 68.

 

Giorgi, R., L. Dei, and P. Baglioni, A New Method for consolidating Wall Paintings based on Dispersions of Lime in Alcohol, Studies in Conservation 45 (2000) 154.  This paper was translated in Spanish: Giorgi, R., Dei, L., Baglioni, P., Nuevo metodo para la consolidacion de pinturas murales basado en dispersiones de cal viva en alcohol, Boletin del Instituto Andaluz del Patrimonio Historico, PH/34, March 2001, 57.

 

Dei, L., E. Carretti and P. Baglioni, Microemulsions in Cultural Heritage Conservation to Solubilise Hydrophobic Materials, in Proceedings of the 5th World Surfactants Congress - Cesio 2000, Florence, May 29 - June 3, (2000), pp 517-524.

 

Alesiani, M., Baglioni, P., Capuani, S., Giorgi, R., Mancini, L., Maraviglia, B., New results in the application of innovative experimental techniques for investigations of stone decay's processes, in Proceedings of the XI International Congress on Deterioration and Conservation of Stone, ed. Elsevier, June (2000), Venice, 587.

 

Dei, L., P. Baglioni, M. Mauro, C. Manganelli Del Fa' and F. Fratini, Growth of Crystal Phases in Porous Media, Langmuir 15 (1999) 8915. This paper was cited by Chemical & Engineering News January 24, 2000, The Right Chemistry for Fragile Frescoes by Rebecca Rawls.

 

Dei, L., A. Ahle, P. Baglioni, D. Dini and E. Ferroni, Wall Paintings Conservation and Green Degradation Products of Azzurite, Studies in Conservation 43 (1998) 80.

 

Baglioni, P., L. Dei, F. Pique', G. Sarti and E. Ferroni, New Autogenous Lime-Based Grouts used in the Conservation of Lime-Based Wall Paintings, Studies in Conservation 42 (1997) 43.

 

Dei, L., P. Baglioni, G. Sarti and E. Ferroni, Aging Effects on Ammonium Carbonate/Acetone Solutions and Cleaning of Works of Art, Studies in Conservation 41 (1996) 9.

 

Pique', F., L. Dei and E. Ferroni, Physicochemical Aspects of the Deliquescence of Calcium Nitrate and its Implications in Wall Painting Conservation, Studies in Conservation 37 (1992) 217.

 

Carretti, E., L. Dei and P. Baglioni, Aqueous Polyacrylic Acid Based Gels: Physicochemical Properties and Applications in Cultural Heritage Conservation, Prog. Coll. & Polym. Sci. 119 (2002) in press.

 

Baglioni, P., E. Carretti, L. Dei and R. G. Weiss, Physicochemical Properties of Polyallylamine Based Gels with CO2 as Gellant, J. Am. Chem. Soc. 125 (2003) 5121.

 

Carretti, E. and L. Dei, Physicochemical Characterization of Acrylic Polymeric Resins coating Porous Materials of Artistic Interest, Progr. Org. Coat., (2004) in press.

 

Salvadori, B. and L. Dei, Synthesis of Ca(OH)2 Nanoparticles from Diols, Langmuir 17 (2001) 2371.

 

Baglioni, P., G. Bitossi, L. Dei, F. Fratini, S. Rescic, Salt Crystallization in Porous Media: Physicochemical Aspects and Effects of Anti-Scaling Additives, J. Cultural Heritage (2004) in press.

 

Dei, L., P. Baglioni and M. Mauro, Materials for Wall Paintings Conservation: Changes of Physicochemical Properties, Aging Effects, and Reversibility, In Preprints of the Conference Reversibility: Does it Exist ?, Oddy, A. & Carroll S. Eds., London, 8-10 September 1999, pp. 73-80. This paper has been translated in German: Dei, L., P. Baglioni, M. Mauro, Materialen für die Konservierung von Wandmalereien: Änderung der physikochemischen Eigenschaften, Alterungseffekte und Reversibilität, Konservierung von Wandmalerei, Arbeitshefte des Bayerischen Landesamtes für Denkmalpflege, Band 104, München, 2001, pp. 250-254.

 

Salvadori, B., V. Errico, M. Mauro, E. Melnik and L. Dei, Evaluation of Gypsum and Calcium Oxalates in Deteriorated Mural Paintings by FTIR Spectroscopy, Spectrosc. Letters 36 (2003) 497.