Eidgenössische Technische Hochschule Zürich Institut für Denkmalpflege

Transcription

1 Eidgenössische Technische Hochschule Zürich Institut für Denkmalpflege Andreas Arnold, Oskar Emmenegger, Andreas Küng, Franz Mairinger, Manfred Schreiner und Konrad Zehnder Deterioration and Preservation of Carolingian and Mediaeval Mural Paintings in the Müstair Convent (Switzerland) Reprints from: "Case Studies in the Conservation of Stone and Wall Paintings" Preprints of the Bologna Congress. -Published by the International Institute for Conservation of Historic and Artistic Works IIC London, 1986.

2 DETERIORATION AND PRESERVATION OF CAROLINGIAN AND MEDIAEVAL MURAL PAINTINGS IN THE MÜSTAIR CONVENT (SWITZERLAND) PART 1: DECAY MECHANISMS AND PRESERVATION A. Arnold, A. Küng and K. Zehnder 1 THE PROBLEM Since the wall paintings were restored in accelerated decay has been observed. Three types of deterioration occur: I deposition of dirt, 2 detachment of the Romanesque intonaco from its sup port, 3 decay produced by soluble salts. In order to understand the entire problem, some general features and the main events which have affected the paintings will be mentioned. The Müstair Convent is situated on an alluvial cone at 1250 meters above sea level, in the Münstertal, a valley issuing to northern Italy at the southeastern border of Switzerland. The alluvial cone is used as farming land. The church is integrated into the building complex of the convent and its southern and eastern side lie next to a public cemetery. The walls, composed of local rubble stones (schists, gneisses and rauhwackes) and lime mortars, still carry the original mediaeval lime plaster on their outside. Only in the basal zones, where they were affected by ground moisture, the plaster has been replaced by mortars containing Portland cement. The walls of the convent show the normal aspect of rising damp up to some three-four meters above the soil. The inner surface of the church is covered by the Carolingian plasters and frescoes, on which the Romanesque ones with their intonaco are superposed. Actually the preserved portions of the Carolingian frescoes are visible on all walls with the exception of the three apses, where the well-preserved Romanesque paintings are partially superposed. The main events that have affected the mural paintings are given in Table 1. The most remarkable events are two fires in the eleventh and fifteenth centuries, and the last restoration of which has drastically changed the Table I Major events concerning the mural paintings in the Church of the Müstair Convent Date Event ~ 800 Origin of the Carolingian frescoes ~ 1080 Rededication of the church after fire ~ 1200 Romanesque paintings are superposed on the Carolingian ones Gothic additions. The mural paintings are covered by whitewash 1492 Fire in the roof New Gothic modifications. The whole interior is repainted with new decorations Rediscovery of the Carolingian paintings Complete restoration of the Carolingian and Romanesque paintings including: - drying of the walls, through the installation of a ventilated drain to stop rising damp - installation of a central heating system with radiators in the church Since then Removal of some Romanesque parts of the paintings threatening to scale off from the Carolingian ones Considerable deposition of dust Scaling off of Romanesque paintings with their intonaco Accelerated decay connected with soluble salts moisture balance of the walls and the room climate due to the installation of exterior drainage and central heating. The actual speed of deterioration is now so great that we estimate that the paintings would no longer exist in a zone up to four meters above the floor if it had progressed as quickly during the past seven centuries. This is even true when we allow that the applied Gothic limewash had to deteriorate before the underlying paint layers could have been affected. This case is not the only one that we know. Many wall paintings all over Switzerland and Central Europe undergo the same accelerated decay after having been restored in recent decades. Under the actual conditions, even though the dirt deposition is important, and the scaling off seems to progress considerably, the major decay of mural paintings is that caused by the salt concentrations and crystallizations, which depend in their turn on the room climate. Hence we have to concentrate our attention on the decay processes due to salt concentrations present in the walls. The effect of pollution can be neglected; it cannot be a major cause of decay in this case and it will not be discussed further in this paper. 2 DETERIORATION CAUSED BY SALT CONCENTRATION If we aim to understand the processes of decay comprehensively, we have to consider the whole salt concentration, including the nature and distribution of the forms of decay, the crystalline salt species and the solutes in the aqueous solution, and then we have to observe and study the conditions of crystallization. The studies have to be treated extremely concisely within the present paper. The deterioration observed on the mural paintings in the apses occurs in the zone of rising damp, affecting both the Carolingian and the Romanesque wall paintings, up to a level of about three-four meters above the floor. Within this zone all the visible deterioration has been accurately Table 2 Identified salt species Name Sulfates Thenardite Epsomite Gypsum Nitrates Nitronatrite Nitrokalite Nitromagnesite Carbonates Thermonatrite Hydromagnesite Nesquehonite Chlorides Halite* Composition Na 2SO 4 MgSO 4.7H 2O CaSO 4.2H 2O NaNO 3 KNO 3 Mg(NO 3) 2.6H 2O Na 2CO 3.H2O Mg 5[OH(CO 3) 2] 2.4H 2O MgCO 3.3H 2O NaC1 * Halite has not been found in the church but on other walls of the convent. 190

3 Fig. I Map showing forms of deterioration and distribution of saline deposits and salt minerals on a portion of the Carolingian and Romanesque mural paintings in the apses of the church at the Müstair Convent (Switzerland) [1]. observed and documented. Figure 1 shows a portion of the map documenting the forms of deterioration, the habits and aggregates of efflorescent salts and the different crystalline salt species found on the wall. All the identified salts are listed in Table 2. The general forms of deterioration have been described [1,2] and will only be mentioned here. The efflorescences are: bristly efflorescences composed of very loosely coherent aggregates of hair-like crystals, pulverulent efflorescences of loose aggregates of very fine-grained salts, salt crusts which are rather compact aggregates, salt pustules which are compact aggregates restricted to small areas. The forms of deterioration connected with efflorescences are pits, detachment of small paint particles, granular disintegration (including alveolar forms), scaling off of intonaco and repairs, and dark spots due to thin incrustations, The distribution of crystalline salts in Figure 1 shows the following pattern. Gypsum is distributed all over the area under consideration. This is due to the numerous gypsum repairs spread over the paintings. The distribution of the other salts shows that nitrates (KNO 3, NaNO 3 ) are concentrated in the higher parts, while the soluble sulfates are concentrated in the lower parts. The ionic distribution within the salt concentration is given in Figure 2. The data have been obtained from chemical analysis of water extracts from absorbent papers us Fig. 2 Distribution of the water-soluble ions along a vertical profile through the zone of rising damp, analyzed on water extracts from papers applied on the mural painting for desalination [4]. 191

4 ed to desalinate the surface. Sampling with the papers was made along vertical profiles through the entire zone of rising damp that is affected by salt concentrations. The data are only serniquantitative, of course, but they nevertheless show a true picture of the distribution of the ions within the zone of rising damp on the painted surface along vertical profiles. The sulfates do not rise as far as the nitrates and the chlorides do. The sulfates disappear at the level where nitrates and chlorides show their maximum concentrations. The distribution of Mg, Na and K corresponds roughly to that of nitrates and chlorides. Hence the distribution of the solutes reflects the distribution of crystallized salts. However, considering that chlorides are as concentrated as nitrates in the extracted solutions, it is astonishing that no crystalline NaCl and KCI could be found on the walls. That may be due to the fact that Mg and Cl give a very strongly hygroscopic solution (MgCI 2.6H 2 O having an equilibrium relative humidity of 33%). The distribution of the moisture on the surface (Fig. 3) along vertical profiles shows the highest amount of moisture where the highest concentrations of hygroscopic chlorides and nitrates are found. Thus the moisture distribution reflects that of hygroscopic salts and not what could be expected from rising damp without salt concentrations. But we do not yet understand the phenomenon completely. Fig. 3 Semiquantitative humidity distribution along vertical profiles through the zone of rising damp. Measured with the French 'Mesurix' instrument. (M = middle apse, N = north apse, S=south apse) [4]. The genesis and distribution of the saline concentration in the vertical profile throughout the zone of rising damp can be explained as follows. Dilute aqueous solutions of NO 3, SO 4, CO 3, Mg, Na, K and some Ca rise from the ground into the wall. Above the floor, evaporation leads to deposition of salts in a sequence according to their increasing solubilities [3]. By this process, the solutes are fractionated to form a sequence in which the salts rise higher the more soluble they are. The chlorides and nitrates of Mg and Ca are strongly hygroscopic [1]; they can only crystallize when the air becomes very dry. For example NaNO 3 can crystallize when the ambient relative humidity becomes lower than 74%, Mg(NO 3 ).6H 2 O when it is lower than 53%, and Mg(Cl) 2.6H 2 O when it is lower than 33% [1]. If the relative humidity in a large room exceeds the equilibrium humidity of a saturated salt solution, the air delivers enough water to dissolve the relatively small amount of that salt on the wall. Vice versa the air absorbs enough water to let the same salt crystallize when the humidity decreases to values below the equilibrium humidity. Therefore the room climate becomes very decisive, because the air humidity now controls whether or not a salt can crystallize. But before looking at that, we have to mention that the Portland cement used outside, and possibly some waterglass consolidant used inside, led to the formation of sodium carbonates (thermonatrite) and magnesium carbonate (hydromagnesite). These new alkaline materials [41 influence the old salt concentrations, not only by increasing the amount of salts, but also by reactions transforming less harmful salts into more harmful ones. 3 CONDITIONS FOR PERIODIC SALT CRYSTALLIZATION We have established that the dominant crystallizing salts (sodium and magnesium nitrate) effloresce in wintertime when the air becomes dry, and they may disappear in summertime when the air gets humid [7]. So the observed periodic salt crystallization is controlled by the variation of the air humidity, and therefore it is important to know more about the evolution of the room climate and its relationship to the salt efflorescences. The monthly average values, measured from 1982 to 1985 by means of thermohygrographs in the church and from data of a neighbouring meteorological station, are shown in Figure 4, as well as the observed periodic crystallizations of sodium nitrate and magnesium nitrate salts. The temperatures and the relative humidities considered in this time scale show two types of periodic variations. Seasonal fluctuations, outside the church, lead to the lowest temperatures in January and February. The relative humidity is low in winter and spring, and high in late summer and autumn, the range reaching a maximum of 75-80% and a minimum of around 65%. Inside the church, temperatures in wintertime fall not lower than about 10 C because of heating. -The relative humidities drop to average values between 40% and 50% at the beginning of the heating period, so the winter period is mainly dry because of the heating. According to this climate periodicity, the periods of sodium nitrate efflorescence correspond to strongly decreasing relative humidities down to values well below 60%. Magnesium nitrate on a column crystallizes when the relative humidity becomes significantly lower than 50% in the ambient air. As the relative humidity becomes more or less stable at values below 60% and 50% respectively, salts will not crystallize any more, but they tend to transform from acicular habits and loose aggregates into more isomorphic habits and compact aggregates such as crusts. and pustules [2,5]. During periods of high relative humidity, the salts disappear, which means that they are redissolved in the moisture they get from the humid air by hygroscopic adsorption. It is obvious, and has been confirmed, that deteriorations occur at each period of salt crystallization. Irregular meteorological fluctuations are caused by shorter subperiods of dryness. So the summer periods of 1983 and 1984 had several very dry subperiods, as can be seen in Figure 4. Those periods also caused crystallizations of sodium 192

5 Fig. 4 Temperatures and relative humidities measured in the church of the Mustair Convent. Curves A= measured values (means), Curves B = values from the neighbouring meteorological station of St. Maria, Curve C = average values from from the St. Maria station [6]. The average curve is traced over the whole measuring period. On the lower part of the diagrams, the observed efflorescence periods of Mg(NO 3) 2.6H 2O and NaNO 3 are indicated. nitrate; yet these efflorescences have been much less important, and therefore produced less damage, than those caused by the seasonal periodicity. The first type of humidity fluctuation is a consequence of the room heating installed in 1951, while the second type is due to 'natural' climatic evolution. So we can say that the room climate of the church is normally controlled by the outside climate during summer, when the church is not heated, and it is mainly controlled by the 'heating' superimposed on the outside climate in wintertime. We consider the latter type (the heating) to be responsible for the major part of the accelerated decay due to soluble salts occurring on the wall paintings. Thus the causes and processes of deterioration may be summarized as follows. The mural paintings on the walls are affected by a saline system originating from rising damp. The salts have been concentrated in the wall during the I I centuries since the church was built, and their composition was changed in by the addition of some alkaline salts from Portland cement and possibly from other alkaline materials. The former external conditions led to a certain decay successively accelerated by the increasing quantities of concentrated salts. But the speed of decay was not so high as to destroy completely the paintings in the zone of rising damp in 700 and 1000 years respectively. The decay was produced by soluble salts: sulfates in the lower parts and nitrates in the upper parts of the zone affected by rising damp. In the church, the crystallization was controlled by fluctuations of the relative humidity, which in their turn depended on the variations of the outside relative humidity caused by meteorological factors. In addition, fluctuations of the moisture in the walls and subsequent salt crystallization may have arisen from water percolation. Since the last restoration in the walls have been partially dried, and the heating, in addition to the normal climatic fluctuations, had caused stronger seasonal variations of the relative humidity. These more frequent and stronger variations have become the main cause of the increasing speed of deterioration. 4 PRESERVATION Rising damp, concentrated salts and variations of the air humidity together lead to the decay. The reduction of rising damp in and its complete elimination alone, would only stop the supply of new solutes from the ground, but because the environment would become drier too, the actual decay would be accelerated rather than mitigated. Moreover, it is actually impossible to desalinate completely walls that are covered by original plaster and wall paintings. And finally the stabilization of the room climate would be extremely difficult and risky. So, for example, humidified air in wintertime would cause inevitable condensation in cold areas of the church, producing damage due to water and organisms. For these reasons there is no simple way to preserve the paintings by just one action. The only way is to develop concerted actions limiting the decay to a minimum. The aim is to proceed in rather small, corrigible steps to reduce the frequency of the very harmful salt crystallizations as much as possible. That will be achieved by observing and cataloging the types and processes of deterioration. The following actions have already been taken: 193

6 4.1 Basic documentation - Observation, identification and registration of the distribution of different forms of deterioration, of crystalline and dissolved salts and of the humidity. - Periodic observations of salt crystallization (forms, habits, aggregates and resultant decay). 4.2 Control of the humidity balance : installation of a ventilated trench along the outside foundations. - Since 1975: new gutters, to avoid water falling from the roof directly on the outside surface which will then percolate to the interior surface. - Wooden board fences on the outside in wintertime to avoid the penetration and percolation of water from melting snow. These actions have been successful and the base of the wall has now dried out. 4.3 Work on reducing the salt concentration - Basic desalination of the mural paintings in the zone of rising damp, by application of wet blotting-paper over the whole surface. Analyses of the water extracts. - Repeated application of wet blotting-paper on the areas where important new efflorescences have been observed. - Repeated dry-brushing away of crystalline salts during crystallization periods. 4.4 Monitoring the room climate - Continuous measuring of room temperature and relative humidity by means of thermohygrographs and, more recently, by means of semiconductor systems. - Repeated measurement of the temperature distribution on the walls. The results of these measurements are related to the periodic observations on decay phenomena and salts. Interpretation will allow us to decide whether or not heating can be tolerated, and which kind of heating can be used under which condi- tions. For this we need more information on exterior and interior climate as well as on the reaction time of the wall and the salt systems to variations of the outside and room climates. It is obvious that monitoring of humidity and the room climate will be necessary for a long time in the future. The aim of these reversible and controlled actions is to mitigate the complex interactive decay processes as much as possible. We would like to avoid short-sighted irreversible actions which may lead to major decay processes. We have observed the results of such actions in many places with mediaeval mural paintings which are now decaying very fast after restoration in recent decades. ACKNOWLEDGEMENTS This study is supported by the Swiss National Science Foundation in the National Research Program 16 'Methods for the Preservation of Cultural Properties'. Our thanks go to Prof. G. Mörsch for his encouraging support, and to Mrs E. Grimbühler for her assistance, as well as to Mrs Ch. Bläuer and Dr B. Mühlethaler for reading the manuscript. REFERENCES I Arnold, A., 'Salzmineralien in Mauerwerken', Schweiz. mineral. petrogr. Mitt. 61 (1981) Arnold, A., and Ming, A., 'Crystallization and habits of salt efflorescences on walls, 1. Methods of investigations and habits' in Vth International Congress on Deterioration and Preservation of Stone, Proceedings, Ecole Polytechnique Fédérale, Lausanne (1985) Arnold, A., 'Rising damp and saline minerals' in Fourth International Congress on Deterioration and Preservation of Stone, Proceedings, University of Louisville (1982) Arnold, A., 'Monitoring of salt concentrations', Environmental Monitoring for Architectural Conservation (in press). 5 Arnold, A., and Zehnder, K., 'Crystallization and habits of salt efflorescences on walls, 11. Conditions of crystallization' in Vth International Congress on Deterioration and Preservation of Stone, Proceedings, Ecole Polytechnique Fédérale, Lausanne (1985) Schüepp, M., and Urfer, Ch., 'Klimatologie der Schweiz, D. Luftfeuchtigkeit', Beiheft zu den Annalen der Schweizerischen meteorologischen Zentralanstalt (1970) D8. 7 Zehnder, K., Arnold, A., and Spirig, H., 'Zerfall von Wandmalerei durch lösliche Salze', Restauro/Maltechnik (in press). 194