STONE CONSOLIDATION: RESEARCH AND PRACTICE

Transcription

1 STONE CONSOLIDATION: RESEARCH AND PRACTICE J. Delgado Rodrigues Geologist, Principal Res. Officer (ret.), LNEC, Lisbon, Portugal; ABSTRACT Stone consolidation has raised the interest of scientists and conservator-restorers since long ago, and, among the treatments applied to stone elements, it is the one that raises more engaging debates and larger controversy. This happens at the practical level, due to the frequent unsatisfactory and even disastrous results, but also at the research level, for the lack of common understandings, for the very personal approaches to the research protocols, and for the difficult exchange of results and viewpoints. Reported casestudies on past treatments are scarce and little of well documented cases is available to complement our own knowledge. This fact strongly affects the possibility to validate the research results and therefore it also limits the capacity to transpose each one s knowledge to the real world. On the other hand, most persons involved in this field know (documented or undocumented) a certain number of cases where consolidation has shown poor performances, possibly with fatal implications for the treated object. Sometimes this may be a consequence of insufficient preparatory studies, but it may also be due to inadequate transposition of the research results to the real practical world. These shortcomings may seem incomprehensible in face of the so huge amount of papers that have been published in the last 4 decades, and yet, the actual situation still encompasses cases that do not have proved solutions, results that cannot be transposed to the real practice, novel products that do not correspond to the expected attributes, and positive performances with treatments that would be expected to fail. 1. INTRODUCTION Stone consolidation is a conservation action currently carried out by conservator-restorers and is a research theme that interests the entire community of researchers in stone conservation. Some consolidation procedures are known for centuries and evidences of past treatments are widespread. And yet, consolidation is still a poorly known conservation action, in spite of the extensive research that has been carried out in the last 4 decades. Some critical insufficiencies were certainly present during the process to justify the scarce outcome that resulted from the extensive efforts carried out in research. It is not the scope of this paper to analyse this process in depth, but some highlights can be listed as contributions for this process. Many research results appear in contexts quite distant from the real world and it is hard to evaluate how they could be transposed to it. On the other hand, many consolidation works were carried out with little scientific support and with even scarcer diffusion of information, as a result of a lack of appropriate communication channels between science and practitioners communities. Some products have been taken as susceptible to be applied to any substrate, even when scarce arguments can be added to support such an approach. Similar approaches have been followed irrespective of the stone properties in question, indicating that most practical actions are justified more in the basis of personal skills and experience than in scientifically justified arguments. Stone composition, dimension and type of the porous space, type and properties of the consolidation products, application protocols, curing conditions, etc. are some relevant items to be considered when a consolidation action is to be carried out. The assessment of adequacy of stone consolidants should start at the laboratory level, but in general the information gathered does not answer all the relevant questions, and inwork performance is also a necessary step to be fulfilled. The appropriate exchange of knowledge between researchers and practitioners, and a good deal of information on long term performance of consolidation case-studies are requisites to move forward this important conservation theme.

2 This presentation aims at briefly illustrating some of the abovementioned aspects. It shows some results from lab experiments as well as from real objects, and comments on some controversial aspects of stone consolidation. 2. STONE CONSOLIDATION AS A TANTALIZING AND CONTROVERSIAL TREATMENT The first assertive conclusion that one may extract from the past and actual panorama on stone consolidation is the lack of treatments that completely satisfy the requirements commonly asked to be fulfilled. The ideal stone consolidation treatment is still far ahead of us. This assertion can be taken as a driving force at the lab research level, but it constitutes a permanent headache for those in charge of implementing consolidation on real stone objects. To be effective, a consolidant has to be fluid enough to penetrate deep inside the stone and to provide the required strengthening effect. Therefore, to know how accurately as possible the penetration capacity of a product is one of the first steps to be searched. This is a simple and straightforward criterion to be asked to a consolidant, and yet it is one of the most difficult to be fulfilled. Current stone consolidants, such as ethyl silicates, acrylic, and epoxy resins have distinct performances in the same stone material. Fig 1 illustrates the typical behaviour of these three common products when applied in a porous limestone. The graphs were obtained with a DRMS (Drilling Resistance Measuring System) equipment in specimens with 25mm thickness treated by full immersion [1]. Figure 1 - Drilling resistance graphs in specimens consolidated by total immersion. The drilled depth corresponds to the full width of the specimens (25mm). Drilling conditions: 400rpm, 15mm/min (From [1]). In this figure we can identify clearly that ethyl silicate (TG) reaches a fairly high penetration depth, but with a very minor strengthening effect. On the other hand, both resins - the acrylic B72 and the epoxy EP - have a much lower penetration capacity, but have a very strong strengthening effect. Furthermore, it is worth noticing the very high peak strength developed in the upper 2-3mm, particularly with the acrylic resin, raising serious doubts on the performance of such treatments when exposed to the outdoor transient temperature and humidity conditions. Not only the type of product influences the depth that can be reached, but also the stone characteristics, particularly its pore space, are relevant for it. Fig. 2 shows the result of the ultrasound velocity measured along a profile in a prism of granite treated with the same epoxy resin, EP, used in Fig. 1. The method used to identify the presence of the consolidant was different in both cases since DRMS is not appropriate to test granite stones due to the high hardness and abrasiveness of its constituents. The typical values for the nottreated specimen stay below 2.0 km/s, while the treated zones show values that reach 4.0 km/s. Interestingly, the graph shows that the granite prism was consolidated until approximately 7 cm in depth, thus contrasting

3 with the 3-4 mm reached in the limestone specimen. And this distinct behaviour is much more positive in the granite specimen, regardless of the extreme disparity in their porosity: 3 % for the granite and 28 % for the limestone. The difficulty to consolidate this high porosity limestone can be extended to other limestones and it is particularly critical for the low porosity varieties. Fig. 3 displays the amount of product applied and absorbed (the difference is mostly lost by evaporation) in several carbonate stones with porosity ranging from 10 to 28%. It is interesting to see that the amount of product absorbed by stones with porosity lower than 15% is extremely low and therefore very little hypotheses exist that such stones might be consolidated. When such types of limestones are decayed and need to be consolidated, the process may turn highly problematic. In fact, the consolidation action risks staying exclusively in the degraded stone, stopping before a suitable anchoring to the substrate is reached. This situation has a high probability to lead to fast detachment of a consolidated layer poorly attached to the substrate. The scarce penetration depth that most consolidants are able to reach even in porous limestones is a significant drawback, and unsuccessful cases can be attributed to this fact. Fig. 4 shows a limestone specimen treated with Paraloid B72 that was subsequently submitted to a salt crystallization test. Its upper surface suffered a sudden detachment in a thickness of about 1 mm, a thickness that perfectly mimics the consolidated layer so well identified in Fig. 1. Figure 2 - Ultrasound velocity profiles along two granite specimens treated with epoxy resin EP. The consolidant was applied for about 14 h by direct capillarity (Adapted from [2]). Notice the progressive decrease in velocity (an indirect measurement of strength) from the surface down to about 7 cm deep. Figure 3 - Amount of product (ethyl silicate TG) consumed and absorbed by carbonate specimens. Application by brush (Adapted from [3]). Notice the scarce absorption for porosities below 15%. This case illustrates the risks that may arise to treated stones when some inconvenient combinations of properties are met, namely highly contrasting thermal expansion coefficients of the treated and not-treated stone. In limestones, the treated zones tend to attain a significantly larger coefficient, reaching values some 70% larger, as published elsewhere [2]. Although reported cases are not abundant, it is not difficult to find situations where consolidation has left a negative imprint. Fig. 5 illustrates one of those situations. However, for this specific case, it must be said that the absence of consolidation might have lead to a faster loss of the sculpture surface and this may also apply to many other similar cases. We don t know how the object was prepared for consolidation and what was the protocol used to do it, but we are quite certain that little, if any, monitoring and maintenance has been provided to the object since then.

4 It is clear that consolidation is a risky operation that is to be carried out when no other solution is safe, therefore, when it is decided to consolidate it is of a major importance that the object is monitored and assistance provided whenever degradation signs start showing up. Figure 4 - Limestone specimen (28% porosity) treated with Paraloid B72. After consolidation, the specimen was exposed to salt crystallization that triggered the detachment of the upper crust (approximetly 1mm thick) Figure 5 - Sculptured surface treated at an unknown past. Notice the indurated layer and the scars left by the detachment of this layer. The progressive loss of the surface and the noticeable lack of conservation care may lead to the complete loss of the sculpture in a not distant future. An interesting case-study recently published [4] reports the evaluation of a consolidation treatment carried out in a first stage in1979, and completed in The monument is built of a porous limestone (porosity around 20%) and the consolidant was a methyl phenyl resin Rhodorsil 11309, for the first part and the

5 Rhodorsil RC-90, for the last treated area. The authors state that the penetration capacity was restricted to a very thin surface layer, not larger than 100 micrometers. Although not reported, we may reasonably presume that consolidation turned the upper thin layer quite hard and contrasting with the under layer, in a manner that seems to be similar to the situations discussed here above. The detachment of scales illustrated in the paper fits well in the behaviour that could be expected from a very thin indurated layer. Thus, it is not surprising to read that problems start occurring in 2003, leading to the need of a new retreatment and to the conclusion that retreatment may be required every 6 years. From those results and case-studies, it is not very risky to extrapolate that consolidation of very decayed stones (particularly limestones) is still a very delicate problem [5]. It raises difficulties at the level of the lab research, but it is at the practical level that most doubts arise. In a paper recently published [6], we have reported the intervention of a stone portal made of porous limestones, during which some serious conservation issues were raised. In some areas, the stone blocks contain a fairly high amount of clay minerals that were responsible for severe decay features that ranged from powdery surfaces to deep delamination of blocks. Fig. 6 shows the aspect of one block and displays a few profiles of DRMS resistance taken from it. The figure clearly shows that consolidation would be justified to delay the scaling degradation and even a deep consolidation would be beneficial to reach the multiple fractures that occur deep inside, at depths beyond 4 cm. Based on the available information and taking the reasoning made above as a picture of the problems that a consolidation treatment had to face, the advisory team considered that surface consolidation was too risky and that deep consolidation was out of reach. The conservation concept then adopted addressed the fixing of the scales and sealing of fractures and voids with light mortars and the elimination of the income water as far as it could be done. No consolidation was applied. 40 Block - 3WI hole 1 Force [N] hole 2 hole Depth [mm] Figure 6 This and some other blocks showed intense superficial scaling and mass loss and the DRMS profiles demonstrated that this problematic superficial condition was continued inwards through an intense system of multiple fractures adding a substantial increment to the complexity of the problems. From [6] The selection of a consolidant should start in the lab and shall take into consideration the stone to be treated, its properties and the problems affecting it, it shall define the objectives to be reached and shall consider the potential effectiveness, harmfulness and stability in time of the envisaged treatment. To have a good impregnation depth has been recognised since long ago (see for instance [7, 8, 9] and even theoretical analyses have been made on the optimal thickness to be required for a satisfactory behaviour [10]. Modifications of the swelling behaviour [11], thermal dilatation [12], water vapour permeability [12], and colour parameters [12] are frequently mentioned as warning signs, but the progress to establish a decision algorithm to accept or reject a consolidation product at the laboratory level has been relatively slow. Some papers address this theme either by dealing with parameters independently [12, 13, 14], or by resorting to multivariate analysis of sets of parameters [15, 16], and a review on this subject can be found in [17]. In spite of the fact that the first references to the theme happened more than three decades ago, the subject has rarely been object of any systematic approach, and therefore we may reasonably consider that this subject is still open to further research efforts.

6 Another aspect that is usually considered as having strong impact in the consolidation action is the presence of soluble salts in the areas to be consolidated. In a study carried out with salt laden granite specimens, we could demonstrate that salts do not necessarily inhibit consolidation, but they do influence its subsequent behaviour, particularly when water is allowed to percolate across the consolidated zones [18]. Fig. 7 summarises the results with two common stone consolidants: one ethyl silicate (graphs on the left side) and one cycloaliphatic epoxy resin (graphs on the right). The graphs represent profiles of ultrasound velocity taken along the specimens at 5mm interval for the following successive testing steps: 1 - Initial conditions before treatment; 2 - after a deep impregnation with a sodium sulphate solution; 3 - after the application of the consolidants; 4 - after the first cycle of water percolation; 5 - after the second cycle of water percolation. Salt solutions, consolidants and water were applied from the bottom surface of the specimens. Specimens were oven dried at 60ºC before the ultrasound measurements were taken. The results here summarised show some interesting features that help to understand the role of salts in the consolidation action: i) the presence of salts induces a slight increase in the ultrasound velocity (profile 2), suggesting that salt crystals are acting as bridges across fissures; ii) in spite of the presence of salts, consolidants are able to penetrate inside the granite specimens and induce a significant consolidation action (profile 3); iii) salts do not become encapsulated by the consolidants and therefore when water is allowed to percolate, salts are put in motion again and the consolidation action is concomitantly depreciated (profile 4); iv) the second cycle of water percolation induces further depreciation of the consolidation action (profile 5) that may almost reach its complete elimination (v.g. ethylsilicate); v) consolidants may show very distinct behaviours as here illustrated by these two products. The ethyl silicate is very sensitive to the presence of salts since the two cycles of water percolation have almost completely eliminated its consolidation action. On the other hand, the epoxy resin could withstand the water percolation in much better conditions and, after the second percolation cycle, the consolidation action still exhibits very significant values. Drying surface salt consolidant salt consolidant st cycle 2nd cycle st cycle 2nd cycle Height (cm) salt / treatment / water V P (m/s) AI Sulph. SE W1 W 2 V P (m/s) AI Sulph. EP W 1 W 2 Figure 7 - Synoptic interpretation of tests carried out on granite specimens. Graphs show profiles of ultrasound velocity. Specimens were, in sequence, impregnated with sodium sulphate solutions, consolidated with ethyl silicate (left) and with epoxy resin (right), and percolated in two successive cycles. Sequential graphs were taken: 1 - before any action; 2 - after salt crystallisation; 3 - after consolidation; 4 - after the first cycle of water percolation; 5 - after the second cycle of water percolation (adapted from [18]). The need to carry out a good desalination condition has been advised since long [19], and these results prove that it may be very risky to implement consolidation in salt laden objects before an appropriate desalination

7 step is carried out. A few review papers on salts and on desalination are excellent sources of additional references on this subject [20, 21, 22]. In spite of the low porosity that fissured stones may have, their consolidation is usually a much simpler and effective task. The excellent connectivity that fissures provide to these materials allows an easy impregnation with most common consolidation products, and significantly large depths can be properly consolidated. Marbles in the group of the metamorphic rocks, and granite as representative of the igneous group, are examples of fissured-type materials. The graphs shown in Fig. 2 correspond to granite specimens of about 3% porosity in which a 7 cm thickness could be consolidated. Examples with other products on granite materials can be found elsewhere [23]. Carbonate stones, especially the more porous varieties, have shown to be difficult to consolidate and much effort has been made to find alternatives to solve problems of decayed calcareous objects. Inorganic consolidants have been developed and applied (see [24] for a review], and modifications of current products have been tried aiming at solving some of their shortcomings [25, 26, 27]. Ancient approaches were revisited, such as the application of limewash on decayed surfaces, and the new technological developments on nanolimes may well enter in the pack box of the future conservator restorer [28, 29]. REFERENCES 1 Delgado Rodrigues, J.; Ferreira Pinto, A. & Costa, D. Tracing of decay profiles and evaluation of stone treatments by means of microdrilling techniques. Journal of Cultural Heritage 3, 2002, pp Delgado Rodrigues, J. & Costa, D. - Occurrence and behaviour of interfaces in consolidated stones. STREMA 95, Creta, Grécia, Ferreira Pinto, A.P. & Delgado Rodrigues, J. - Stone consolidation: the role of treatment procedures. Jour. Cultural Heritage, 9 (2008), pp Benchiarini, S.; Fassina, V. & Molin, G. - In situ evaluation of restoration treatment on the Loggia Cornaro in Padova, Italy. Proc. 11th Int. Cong. on Deterioration and Conservation of Stone, Torun, Edited by. J. Lukaszewicz & P. Niemcewicz, 2008, vol. II, pp Delgado Rodrigues, J. - Consolidation of decayed stones. A delicate problem with few practical solutions. Proc. Int. Seminar on Historical Constructions, Guimarães, Nov Delgado Rodrigues J., Ferreira Pinto A. P. (2007). - Sampling and characterization issues in the study of a stone portal with microdrilling. In Proceedings of the International Seminar Small Samples Big Objects, Munique, May Edited by Bayerisches Landesamt fur Denkmalpflege, pp Rossi-Manaresi, R. - Effectiveness of conservation treatments for the sandstone of monuments in Bologna. Proc. Int. Symp. The Conservation of Stone II, Bologna, 1981, Centro per la Conservazione delle Sculpture all'aperto, pp ,. 8 Rossi-Manaresi, R. and Tucci, A. - SEM examination of a biocal carenite treated with acrylic polymers, silane or silicone resins. Proc. 5th Int. Cong. on Deterioration and Conservation of Stone, Lausanne, Switzerland, Sept. 1985, Presses Polytechniques Romandes, pp De Witte, E., Charola, A.E. & Sherryl, R.P. - Preliminary tests on commercial available stone consolidants. Proc. 5th Int. Cong. on Deterioration and Conservation of Stone, Lausanne, Switzerland, Sept. 1985, Presses Polytechniques Romandes, pp Hosek, J. & Panek, J. - Depth of impregnation as the criterium for durability of consolidated stones. Proc. 5th Int. Cong. on Deterioration and Conservation of Stone, Lausanne, Switzerland, Sept. 1985, Presses Polytechniques Romandes, pp Felix, C. & Furlan, V. - Variations dimensionnelles de grès et calcaires liées à leur consolidation avec un silicate d'éthyle. pp , Proc. 3rd Int. Symp. on The Conservation of Monuments in the Mediterranean Basin, Venice, June, 1994, Soprintendenza ai Beni Artistici e Storici di Venezia. 12 Gauri, K.L.; Gwinn, J.A., & Popli, R.K. - Performance criteria for stone treatment. Proc. 2nd Int. Symp. on the Degradation of Building stones, Athens, Greece, 1976, pp Sleater, G.A. - Stone Preservatives: Methods of Laboratory Testing and Preliminary Performance Criteria. NBS Technical Note No. 941, National Bureau of Standards, Washington, D.C. (1977).

8 14 Clifton, J.R., and Frohnsdorf, G.J.C., Stone-consolidating materials: a status report, in Conservation of Historic Stone Buildings and Monuments, National Academy Press, Washington, D.C. (1982), pp Sasse, H. R. & Snethlage, R. - Evaluation of stone consolidation treatments. Science and Technology for Cultural Heritage, Journal of the Comitato Nazionale per la Scienza e la Tecnologia dei Beni Culturali, CNR. 5(I), 1996, pp Delgado Rodrigues, J. & Grossi, A. - Indicators and ratings for the compatibility assessment of conservation interventions. Journal of Cultural Heritage, Vol. 8, Issue 1 (2007), pp Tabasso, M.L. & Simon, S. - Testing methods and criteria for the selection/evaluation of products for the conservation of porous building materials. Reviews in Conservation, No. 7, 2006, pp Costa, D. & Delgado Rodrigues, J. Consolidation treatment of salt laden materials. Methodology for their laboratory study. Proc. 11th Int. Cong. on Deterioration and Conservation of Stone, Edited by J. W. Kukaszewicz & P. Niemcewicz, Torun, 2008, pp Hempel, K., & Moncrieff, A. - Summary of work on marble conservation at the Victoria and Albert Museum Conservation Department up to August Proc. Int. Symp. The Conservation of Stone, Bologna, 1972, Centro per la Conservazione delle Sculpture all'aperto, pp Charola, A.E. - Salts in the deterioration of porous materials: am overview. Journal of the American Institute for Conservation, vol. 39, 2000, pp Doehne, E. - Salt weathering: a selective review. In Natural Stone, Weathering Phenomena, Conservation Strategies and Case Studies, Edited by Siegesmund, S., Weiss, T. & Vollbrecht, A., Geological Society of London, Special Publications, 2002, vol. 205, pp Vergès-Belmin, V. & Siedel, H. - Desalination of Masonries and Monumental Sculptures by Poulticing: A Review. Restoration of Buildings and Monuments: An International Journal, 2005, vol. 11 pp Delgado Rodrigues, J.; Costa, D. & Schiavon, N. - Spatial distribution of consolidants in granite stones, Proc. EC Workshop, Santiago de Compostela (Spain), November , Published by the European Commission. Directorate-General XII. Science, Research and Development, 1996, pp Hansen, E., Doehne, E., Fidler, J., Larson, J., Martin, B., Matteini, M., Rodriguez-Navarro, C., Sebastian-Pardo, E., Price, C., Tagle, A., Teutonico, J.M., & Weiss, N. - A review of selected inorganic consolidants and protective treatments for porous calcareous materials. Reviews in Conservation, No. 4, 2003, pp Wheeler, G., Mendes-Vivar, J., Goins, E.S. & Brinker, C. J. - Evaluation of alkoxysilane coupling agents in the consolidation of limestone. Proc. of the 9th Int. Cong. on Deterioration and Conservation of Stone, Edited by Vasco Fassina, Elsevier Science, 2000, pp Engel, J. - New consolidation agent for limestone. Proc. of the Int. Symp. on Stone Consolidation in Cultural Heritage Research and Practice, Edited by J. Delgado Rodrigues and J.M. Mimoso, Lisbon, Laboratório Nacional de Engenharia Civil, 2008, pp De los Santos, D.M., Sanmartin, P., Rivas, T., Silva, B., & Mosquera, M.J. - A new material with both consolidating and water-repellent properties for treating stone. Proc. of the Int. Symp. on Stone Consolidation in Cultural Heritage Research and Practice, Edited by J. Delgado Rodrigues and J.M. Mimoso, Lisbon, Laboratório Nacional de Engenharia Civil, 2008, pp Favaro, M., Ossola, F., Toamsin, P., Vigato, P.A., Rossetto, G., El Habra, N., & Casarin, M. - A novel approach to compatible and durable consolidation of limestone. Proc. 11th Int. Cong. on Deterioration and Conservation of Stone, Edited by J. W. Kukaszewicz & P. Niemcewicz, Torun, 2008, pp Ziegenbalg, G. - Colloidal calcium hydroxide- a new material for consolidation and conservation of carbonate stones. Proc. 11th Int. Cong. on Deterioration and Conservation of Stone, Edited by J. W. Kukaszewicz & P. Niemcewicz, Torun, 2008, pp