The Wall paintings at the Kaiserdom in Königslutter (Imperial Cathedral at Königslutter)

Author: Hans-Jürgen Schwarz, Erwin Stadlbauer, Rolf Niemeyer
English version edited by Sandra Leithäuser
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Abstract

This case study deals with the salt damage to the wall paintings at the Königslutter Imperial Cathedral (German: Kaiserdom) describing the methods used to analyze and evaluate the problem as well as the measures taken to extract the salts present in the wall paintings.

Introduction

The contamination of walls with salts and salt solutions is often one of the main problems for the conservation of wall paintings. The Imperial Cathedral at Königslutter (Northern Germany) is a good example of this problem [Schwarz.etal:2008]Title: Diagnostic investigations to define the possibility and the effectiveness of desalination with the poultice-technique - a case study
Author: Hans-Jürgen Schwarz; Erwin Stadlbauer; Rolf Niemeyer
.

The 13th century wall paintings of the Imperial Cathedral at Königslutter were over-painted at the end of the 19th century. In the second half of the 20th century the structure of the building was stabilized by injection of cement grouts. This initiated the salt crystallization deterioration process at the interior surfaces of the cathedral walls. In 2006, following the repairs to the exterior of the cathedral, the conservation of the interior, i.e., wall paintings, plasters etc., was begun. The analyses of surface samples and drill-samples showed a high salt content consisting of sulfate and calcium ions, nitrates (mostly in combination with potassium, but also with calcium) and rarely chloride and sodium. Salt efflorescence often consisted of gypsum, partially with calcite as a sinter crust. Fluffy efflorescence consisted of niter. Only in some cases did the total amount of salt decrease with the depth of sampling.

In order to extract the salts in the wall painting and the plaster, the poultice method was chosen. However, before applying this method, the surfaces were checked to ensured that they complied with the requirements for a successful application, e.g., appropriate moisture transport characteristics. The results clearly showed that the first plaster layer, which was slightly hydrophobic in some areas, transported moisture very slowly. Nonetheless, the poultice method was tested, because salt action was considered to be more damaging than the treatment. These tests showed that it was only possible to extract a small amount of salt.

This case study focused on diagnostic investigations in order to characterize selected areas and estimate the effectiveness of the desalination via the poultice-technique. For this purpose it was necessary to determine the exact amount of salts present, the distribution of the different ions and the salt minerals which could form as a function of climatic parameters. Of equal importance were the hygric properties of the materials regarding the capillary transport of moisture, as well as the diffusion of water vapor [Niemeyer.etal:1996]Title: Die Wasserdampfdurchlässigkeit der Wandmalereioberflächen im Innenraum der Stiftskirche Königslutter
Author: Niemeyer, R.; Stadlbauer, E.
.

Case History

The wall paintings of Königslutter Imperial Cathedral date back to the 13th century. Like many other wall paintings of that time they were altered and underwent changing conditions regarding physical, chemical and biological influences. Presumably, a whitewash covered them from the 16th until the end of the 19th century. In the years 1886-1894 the wall paintings were uncovered and restored by Essenwein and Quensen [Koenigfeld:1996]Title: Die Raumausmalung
Author: Königfeld, P.
; [Grote.etal:2001]Title: Wandmalereien in Niedersachsen, Bremen und im Gronigerland
Author: Grote, R.; van der Ploeg, K.
. Reference was made to the original medieval surface, but in fact it was nearly completely over painted.

• Salt damage of the wall paintings in Königslutter Imperial Cathedral
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In 1974, to stabilize the building, large amounts of cement grouts were injected into the walls. The wall paintings were previously treated with an acrylic resin in order to protect the surface. However, while the structural consolidation succeeded, the protection of the wall paintings failed. Around 1990, comprehensive diagnostic investigations were started and it was evident that the masonry had a high moisture content as a result of the grout injections and the resulting salt mobilization and crystallization [Roesch:1996]Title: Salz- und Fuchtebelastung
Author: Rösch, Heinrich
. On the basis of these results, a long-term project for the preservation of the cathedral, including the conservation and restoration of the wall paintings, was developed by an interdisciplinary group of experts. As a first step, the urgent repairs to the exterior of the cathedral were completed in 2006. This was combined with the stabilization of the climate, in particular relative humidity, in the interior of the building. Furthermore, the effectiveness of these measures was controlled from 2003 onwards, by monitoring the representative areas of the surface [Behrens.etal:2005]Title: Wandmalerei-Referenzflächen-Monitoring am Beispiel der Stiftskirche Königslutter – Methoden und Erkenntnisse für die praktische Denkmalpflege
Author: Behrens, Elke; Berling, Helmut; Hornschuch, A.; Niemeyer, Rolf; Recker, Bernhard; Schwarz, Hans-Jürgen; Stadlbauer, Erwin
. The photographic monitoring was combined with a variety of non-destructive diagnostic investigations and with a limited number of samples for chemical analysis.

Methods

Quantitative chemical analyses
The total amount of salt was estimated by conductivity of the aqueous extract of the samples. The quantitative determination of Na⁺ , K⁺, Ca²⁺, Mg²⁺, NO₃ ^-, Cl^- and SO₄ ^2- was carried out by photometry and a Na - selective electrode.

X-Ray Diffraction Analyses (XRD) of mineral phases
Powder samples were analyzed by X-ray diffraction (XRD) for the mineral phases present with a Bruker D4 Endeavor instrument.

Salt screening with the Polarising Microscope (PM)
A polarizing microscope (Zeiss Axioplan 50) was used to analyze the salt phases and hygroscopic behavior of the salt system of aqueous extracts of a sample after drying on a microscope slide.

Water uptake with the Mirowski method
The Mirowski method was used to measure the capillary uptake of water on surfaces, i.e., the hydrophobic / hydrophilic properties. In the test, water is introduced to the surface by a glass tube with a sponge on the end facing the wall. The greater the suction value of a surface the more water is taken from the cylinder. Surface points, which are observed to be hydrophobic with the droplet method, are not further studied with the Mirowski method.

Figure 2: Application of the Mirowski method

Figure 3: The droplet method

Determination of the absorption of water with the droplet method
The method places about 10 μl micro-drops of water upon the surface to be tested. The time required for the total absorption of the droplet is a quantitative measure of the water absorption velocity, i.e., the hydrophobic / hydrophilic properties.

Samples, Measuring points

Two model areas were chosen for testing the poultice method. Before and after applying the poultices, samples of up to 10 cm in depth were taken with a spiral drill (1cm Ø) and analyzed for salt ions. Before the application of the poultices in the model areas, 22 samples were quantitatively analyzed for their ion contents. Another 18 samples taken from the surfaces and efflorescence at various locations were analyzed for the salt phases and the hygroscopic behavior of the salt system. After the application of the poultices 12 samples from these areas were taken to estimate the effectiveness of the desalination process. To characterize the moisture transport on the surfaces on the points in each model area, measurements with the Mirowski-tube were carried out. The droplet method was applied at more than 40 places for each model area.

Poultices

Three recipes were used:

• Recipe 1 "wet": 20g poultice mixture 1:1 (Arbocel® 200 und Arbocel® 1000) and 168 ml deionized water.
  
• Recipe 2 "let dry I: 20g poultice mixture 1:1 (Arbocel® 200 und Arbocel® 1000) and 129 ml deionized water with an intermediate layer of Japanese paper.
  
• Recipe 3 "let dry II": 20g poultice mixture 1:1 (Arbocel® 200 und Arbocel® 1000) and 127 ml deionized water without an intermediate layer of Japanese paper.
  

Results

The results of the analyses of salts and salt forming ions were the following: In all sampled areas on or near the surface, the amount of salt found was very high. The salt composition consisted mostly of sulfate and calcium (fig. 4), in some areas nitrate with potassium, but also calcium, as the counter ion were identified. Only in very few cases was chloride and sodium found. The white powdery efflorescence were mainly [Gypsum|gypsum]] and sometimes calcite as a (sinter crust). The fluffy whiskers consisted of niter (KNO₃).

Figure 4: Results of the salt analyses after desalination procedure

Water uptake by the surfaces of the wall paintings

The uptake of water by capillary sorption was low to very low. With the Mirowsky method values of about 0.8 ml/h were measured. The average time it took for a 10μl droplet to be absorbed was 1–10 minutes. The most rapid water absorption occurred on surface defects and gilded areas, but only rarely was the micro-drop taken up in a few seconds. In some areas, such as the green regions in the “SE Fluss” the droplets were visible even after 40 minutes (hydrophobic surface).

Studies with the droplet method were also carried out on samples of the plaster layers, which are divided in a first and a second layer. The first layer always showed very low water absorption, whereas the second plaster layer immediately incorporated the droplet into the structure. The first layer showed the effect of low water suction on both sides of a sample.

Testing the salt reducing method

The salt reducing methods were applied and verified in two areas of the cathedral vault, which suffered from damage due to different salt systems. The rhree recipes mentioned above were tested using a step by step application of the cellulose poultice. The first poultice was applied for a very short time (2-3 minutes) followed by a second poultice layer, which was left to dry out and take up any dissolved salts from below. However, only the poultices without a protective layer of Japanese paper did not fall off and dried out in situ.

Analyses of salts in the poultices

Samples for analysis were chosen to assess the results from the poultices applied in the model areas:

• The poultices after drying,
• A neutral poultice (with just the starting materials) as well as again
• Drill dust samples from the areas which had been sampled before so as to reflect the salt content after the application of the poultices,

Overall, only 5% of the salts could be extracted by using the applied methods. The poultices did not reach depths of more than 1 cm.

Simulation calculation of the sequence of crystallization

Using the software RUNSALT/ECOS [Price:2000]Title: An Expert Chemical Model for Determining the Environmental Conditions Needed to Prevent Salt Damage in Porous Materials, European Commission Research Report No 11, (Protection and Conservation of European Cultural Heritage)
starting with the quantitative results of our samples, the possible crystallization sequences were calculated depending on relative humidity and air temperature. After the salt reducing tests two samples were taken at the “NW-Fluss”. Both show a calculated start of salt crystallization at about 80% RH with KNO₃ as the most important salt besides the omnipresent gypsum. The test areas at the S-transept after the desalting show by calculation a salt system starting to crystallize just below 40% RH, which means that in addition to gypsum no other salt will crystallize in the actual climate of the cathedral.

Conclusions

The results of the analyses of surface- and drill-samples up to 10cm depth, confirmed a high salt content consisting of sulfate and calcium ions, considerable amounts of nitrate (mostly with potassium but also with calcium) and rarely chloride and sodium ions. Most of the salt efflorescence was gypsum, in part with calcite forming a sinter crust. The fluffy efflorescence consisted of niter. The total amount of salt decreased with the depth of sampling only in some cases. Salt efflorescence was easily removed using dry methods. The poultice method was chosen for extracting the more soluble salt forming ions from inside the wall painting and the plaster. Before this method can be applied it is important to ensure that the surfaces have the appropriate properties for a successful application, particularly with regard to moisture transport characteristics. The results showed clearly that the first plaster layer transported moisture very slowly, which means that it is slightly hydrophobic. These were not a good conditions for applying the poultice method. The results of test applications showed that only a small amount of salts could be extracted. Altogether less than 5% of the total amount of salt could be removed, which meant that the method as tested did not promise to be very successful.

Based on the low efficiency of the tested method it should not be applied since because of the low capillary water suction characteristics of the first plaster layer, it will not succeed in attaining an in depth effectivenes. Apart of the dry removal of the visible salts, the recommendation for desalination- as the result of the tests carried out- is that the salts near the surface should be removed by application of one thin poultice in combination with a controlled climate. By application of a single, not excessively wet poultice, a relatively high amount of these salts can be removed. After drying, the near surface salts can be taken off. Although not a great amount of salts will be removed, the surface of the wall paintings will be better prepared for all further applications, e.g., retouching. Moreover any salt damage will probably be delayed for some time. Once more, the investigations have clearly shown that before salt removal poultices are used, the water transport rate and hydrophobic/hydrophilic properties of the underlying plasters should be determined.

Literature

[Behrens.etal:2005]	Behrens, Elke; Berling, Helmut; Hornschuch, A.; Niemeyer, Rolf; Recker, Bernhard; Schwarz, Hans-Jürgen; Stadlbauer, Erwin (2005): Wandmalerei-Referenzflächen-Monitoring am Beispiel der Stiftskirche Königslutter – Methoden und Erkenntnisse für die praktische Denkmalpflege. In: Berichte zur Denkmalpflege in Niedersachsen, (1), 9-12
[Grote.etal:2001]	Grote, R.; van der Ploeg, K. (2001): Wandmalereien in Niedersachsen, Bremen und im Gronigerland, Deutscher Kunstverlag
[Koenigfeld:1996]	Königfeld, P. (1996): Die Raumausmalung. In: Braunschweigischer Kloster- und Studienfond, Institut für Denkmalpflege (eds.): Der Kaiserdom in Königslutter. Ein Kulturdenkmal auf dem Prüfstand, Niedersächsisches Landesverwaltungsamt, Hannover, 10-13.
[Niemeyer.etal:1996]	Niemeyer, R.; Stadlbauer, E. (1996): Die Wasserdampfdurchlässigkeit der Wandmalereioberflächen im Innenraum der Stiftskirche Königslutter. In: raunschweigischer Kloster- und Studienfond, Institut für Denkmalpflege (eds.): Der Kaiserdom in Königslutter. Ein Kulturdenkmal auf dem Prüfstand, Niedersächsisches Landesverwaltungsamt, Hannover, 54-58.
[Price:2000]	Price, Clifford A. (eds.) (2000): An Expert Chemical Model for Determining the Environmental Conditions Needed to Prevent Salt Damage in Porous Materials, European Commission Research Report No 11, (Protection and Conservation of European Cultural Heritage), Archetype Publications Ltd, London
[Roesch:1996]	Rösch, Heinrich (1996): Salz- und Fuchtebelastung. In: Grote, Rolf Jürgen; Königfeld, Peter (eds.): Der Kaiserdom in Königslutter. Ein Kulturdenkmal auf dem Prüfstand, Braunschweigischer Kloster- und Studienfond, Institut für Denkmalpflege, Hannover, 59-63.
[Schwarz.etal:2008]	Hans-Jürgen Schwarz; Erwin Stadlbauer; Rolf Niemeyer (2008): Diagnostic investigations to define the possibility and the effectiveness of desalination with the poultice-technique - a case study. In: Ottosen, Lisbeth M.; Rörig-Dalgaard, Inge; Larsen , Poul Klenz; Brajer, Isabelle; Bøllingtoft, Peder; Marciniak, Mette; Svane, Maja (eds.): Salt Weathering on Buildings and Stone Sculptures, Technical University of Denmark, Lyngby, Denmark, 237-248.

crystallization of a dissolved salt

X-ray diffraction

relative humidity