The degradation of the 19 th century Udelfanger sculptures at the Sint-Jan Cathedral in Den Bosch and the possible relation to a former cleaning treatment in 2010

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The degradation of the 19

th

_{century Udelfanger sculptures at the Sint-Jan}

Cathedral in Den Bosch and the possible relation to a former cleaning

treatment in 2010

Master thesis

Master programme in the Conservation and Restoration of Cultural Heritage.

Specialisation: Glass, Ceramic and Stone

Anna Maria Rupert, 6135005

University of Amsterdam, Amsterdam

Supervisors: Mandy Slager and Hilde De Clercq

Second assessor: Evelyne Snijders

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Acknowledgements

For the realisation of this master thesis I would like to thank the following people:

My supervisors, Mandy Slager (UvA) and Hilde De Clercq (KIK-IRPA) for their feedback and

inspiration. Special thanks to Kate van Lookeren Campagne, for helping with the English and the text structure. Rene Peschar for the critical evaluation of the technical analysis.

Specialists from the RCE: Hendrik- Jan Tolboom, Rutger Morelissen, Ineke Joosten and Bertil van Os.

Those involved with the Sint- Jan, Maikel Niël of BBM restauratiearchitectuur, Joan van de Hurk and Leo Peeters.

Harry Boekwijt and Ronald Glaudemans of the community of Den Bosch.

Henry van Oversteeg, from the company Superlook, who was very generous in providing information and allowing me to observe their method of working.

Wim Dubelaar (TNO) for his research on Udelfanger stone and his help with sources.

Matthias Boon from the company Meestersin, for providing me with a sample of new Udelfanger stone.

My fellow students at the UVA: Caitlin Southwick, Roy van der Wielen, Corinna de Regt, Magdalena Pilko and ex-student Nikè Haverkamp.

My employer Rene Hoppenbrouwers (SRAL) for creating the possibility for me to pursuing this study.

Manon and Duco Boer and Karin Bruil for support with the text. And I thank my daughter, Hazel van Dijk.

Anne Rupert 10 juli 2017

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Summary

This research relates to the Udelfanger stone sculptures at the Northern portal of the Sint-Jan Cathedral in Den Bosch. The sculptures show severe degradation phenomena which seems to have worsened since a cleaning treatment in 2010. Although this type of lime-sandstone is known for its poor resistance to outside conditions, there has been a question as to whether the “steam cleaning” treatment at that time may have aggravated the existing situation and accelerated the decay phenomena. This lead to the main question of this research ‘what type of degradation is found on the Udelfanger sculptures of the Sint-Jan and is it related to the cleaning treatment undertaken in 2010?’

This research revealed that the Udelfanger stone is poorly resistant to weathering conditions and that it might also have different characteristics than previously thought. The Udelfanger sandstone is a heterogeneous stone with a layered structure. The cementing or binding material seems to vary, giving the Udelfanger stone its specific characteristics and behaviour with relation to degradation processes.

Weathering processes such as the sulphatation process, salt and frost damage can all contribute

to the degradation of Udelfanger stone which can then be accelerated by the layered structure. The effect of weathering conditions is mainly determined by the pore structure of the stone, which

can further be influenced by the cementing material and the presence of clay and silt. It was found that damage is not only caused by the swelling of clay minerals, but certain types of clay material can cause damage by the creation of micro pores.

It was discovered that the cleaning treatment that was carried out in 2010, was in fact a high-pressure, hot-water, cleaning process. High pressure and high temperature can pose a risk for Udelfanger stone and cause mechanical damage, especially when the stone is already degraded. The use of large amounts of water should also be taken into consideration, as it can induce salt or frost damage and the transportation of impurities.

The main conclusion of this research is that it is very difficult to know what has influenced the current damage situation due to the many different factors involved. All the degradation

phenomena that were found could be related to the weathering processes, although the cleaning treatment can also have played an important role. The technical analysis that was performed on

samples of some of the sculptures did not result in clearer answers and further research is needed.

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Samenvatting

Dit onderzoek gaat over natuurstenen sculpturen aan het noorderportaal van de Sint-Jan kathedraal in Den Bosch. Op de sculpturen zijn ernstige degradatie fenomenen aanwezig, die mogelijk zijn verergerd door een reinigingsbehandeling in 2010. Ondanks dat deze kalkzandsteen bekend staat om zijn slechte weervastheid, is er de vraag ontstaan of de stoomreiniging de degradatie heeft versneld. Dit bracht de hoofdvraag van dit onderzoek: ‘wat voor type degradatie kan er worden gevonden en wat kan de mogelijke relatie zijn met de reinigingsbehandeling?’

Onderzoek wijst uit dat de Udelfanger steen slecht bestand is tegen verwering en dat het ook andere karakteristieken heeft dan in eerdere onderzoek werd bevonden. The Udelfanger steen is een heterogene steen met een gelaagde structuur. Het cementmateriaal kan variëren van

samenstelling en geeft de Udelfanger steen zijn specifieke karakteristieken en gedrag bij degradatie processen.

Verweringsprocessen, zoals vergipsing en zout- en vorstschade kunnen allemaal bijdragen aan de degradatie, die versneld wordt door deze gelaagde structuur.

Het gedrag in verwering wordt vooral bepaald door de poriënstructuur. Dit kan worden beïnvloed door het cementmateriaal en de aanwezigheid van klei of leem. Daarbij is onderzocht dat niet alleen zwelbare klei mineralen schade kunnen veroorzaken door uitzetting, maar ook andere kleiachtige materialen door het creëren van microporiën en het vasthouden van vocht.

De reinigingsbehandeling van 2010 was geen stoomreiniging, maar in feite een hogedruk en heet water reiniging systeem. De hogedruk en hoge temperaturen kunnen riskant zijn voor de Udelfanger steen en mechanische schade veroorzaken, vooral als de steen al gedegradeerd is. Daarbij moet er rekening worden gehouden met het feit dat er veel water werd gebruikt bij deze manier van

reinigen, waardoor zout- of vorstschade kan ontstaan en het transporteren van onzuiverheden in de steen.

De belangrijkste conclusie die kan worden genomen na het onderzoek, is dat de degradatie kan zijn beïnvloed door verschillende factoren. Alle degradatie fenomenen die gevonden zijn zouden aan verweringsprocessen gerelateerd kunnen worden, maar daarbij kan de reinigingsbehandeling ook een belangrijke rol hebben gespeeld. De technische analyses, die waren uitgevoerd op enkele monsters van de beelden, gaven daarin ook geen duidelijk uitsluitsel en verder onderzoek zal daarom nodig zijn.

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Introduction

The Sint-Jan cathedral in Den Bosch is one of the most famous cathedrals in the Netherlands, in particular for its sculptural detail. The cathedral originates from the 11th_{century with its tower in}

Roman style being extended through the centuries. Around 1860 the northern transept was restored for the first time and the sculptures on both sides of the northern portal were replaced by newly carved ones. These sculptures were designed following a new iconographic scheme, while still being based on the original severely-decayed medieval sculptures. The sculptures were carved from a newly-introduced type of stone, Udelfanger sandstone. This particular stone, quarried in the village of Udelfangen, close to Trier in Germany, was recommended at the time and found to be aesthetically pleasing, easily workable and a weather-resistant type of stone. However, very soon the Udelfanger sandstone showed signs of degradation from weathering.1

During the restoration carried-out between 2007 and 2010, the building stones of the northern façade were replaced and the sculptures of the northern portal dismantled. Some of the sculptures, specifically the ones from the upper level, were in such a poor state that they were replaced by reconstructions in Bentheimer sandstone.2_{According to a damage-report of BBM}

Restauratiearchitectuur, all the sculptures were cleaned with a ‘steam cleaner’ in 2010.3

Unfortunately, due to the absence of relevant documentation, both the degree of pollution and the reason for cleaning are uncertain. It can be assumed that the sculptures were contaminated with algae growth and black discoloration that was considered to disfigure the sculptures.

A few years after the cleaning treatment, severe surface degradation of the treated sculptures was highlighted by the building committee Sint-Jan. The degradation seemed especially evident on the lower areas of the sculptures. Although surface degradation had already been seen on some of the sculptures before the cleaning treatment, the degradation seemed to have worsened and to have developed faster after the cleaning treatment. 4

For this reason, the Cultural Heritage Agency of the Netherlands (RCE) got involved and research was undertaken. 5_{At the same time, the sculptures were photographed by the company BBM}

Restauratiearchitectuur, during two periods in 2013, in order to monitor the increase in degradation.6

Specialists at the Rijksdienst voor het Cultureel Erfgoed (RCE) had concluded that the protective gypsum layer that had formed as a skin on the surface, had been removed by the cleaning

treatment, leaving a disintegrated surface behind.7_{However, there were still questions as to}

whether the cleaning was really the cause of the deterioration process which was why I was approached by the RCE to investigate this issue further.

1_{Peeters, C. De Sint Janskathedraal te ´s-Hertogenbosch, Den Haag, 1985: 72}

2_{The reconstructions were made by Ton Mooy in Amersfoort. The originals are kept in Museum de}

bouwloods, next to the Sint-Jan Cathedral.

3_{Niël, M (BBM restauratiearchitectuur). Schade rapportage: Udelfanger beeldhouwwerken, portaal}

steunberen Noord transept Sint-Janskathedraal te ’s-Hertogenbosch. August, 14. 2013

4_Idem

5_{XRF analyis performed by RCE, 2013. Will be further discussed in chapter 1} 6_{The sculptures were monitored two times: January and August 2013.} 7_{BBM, Schade rapportage, 2013}

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This lead to the formulation of the main question of this research project, namely: ‘what type of degradation is found on these Udelfanger sculptures of the Sint-Jan and can it be related to the cleaning treatment undertaken in 2010?’

The aim of this research was to obtain a better understanding of the degradation phenomena and processes and the potential risks of this type of cleaning treatment. Can the degradation process be considered to have been ‘natural’, or was it in fact related to the steam-cleaning treatment, as was assumed?

The first stage of this research was to carry out detailed mapping of the degradation pattern on the sculptures using the ICOMOS glossary of stone deterioration patterns as a reference.8_{As will be}

discussed in chapter 2, two sculptures were chosen for the mapping which were considered to be as representative of the group. Another sculpture that did not undergo the cleaning treatment, was included enabling comparisons to be made. Samples of the sculptures were taken and analysed using XRF and SEM-EDS in order to observe the degradation features and processes in detail.

In addition, the cleaning treatment was investigated both through archival investigation and by contacting the company that had carried out the cleaning. Knowledge of the technique and the exact protocol that was applied, were of major importance for the diagnosis, as well as a better

understanding of the potential risks of the technique. Literature research was undertaken to better understand the nature of Udelfanger stone and the

factors that could have played a role in its degradation.

8_{Verges-Belmin, Veronique, et al. Illustrated glossary on stone deterioration pattern. ICOMOS-ISCS: Paris,}

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1. Current scientific knowledge

The first phase of the thesis involved research into the current scientific knowledge on the subject. In an attempt to find answers to the main question, three main areas of research have been followed. The first relates to Udelfanger sandstone and its susceptibility to degradation, the second considers the treatment biography of the sculptures and the third focusses on the cleaning

treatment that was carried out in 2010.

As not many sources were found describing the characteristics and behaviour of the Udelfanger stone, one of the most important sources for this research is a TNO (Toegepast

Natuurwetenschappelijk Onderzoek) report by Wim Dubelaar from 2003.9_{In this report, Udelfanger}

stone was investigated for its characteristics and potential decay processes. The elemental composition and the physical characteristics such as porosity, water permeability and pore-size distribution were determined and presented in tables.10

Dubelaar concluded that Udelfanger has a high porosity and permeability and therefore did not appear to be very susceptible to freeze-thaw cycles. However, he did mention that these cycles could result in physical pressure if silt layers are present inside the stone. 11_{He also concluded that}

the percentage of clay particles was very small (less than 1%) in all the samples and that stone degradation due to clay-swelling minerals was found to be unlikely. 12_{The process of the swelling of}

clay minerals in the pores causing damage to the substrate is not further discussed, but has been elaborated on in other studies. 13_{According to research by Ruedrich. J. et al. the moisture expansion}

of natural stone cannot be attributed to only clay-swelling minerals. Instead, a relation was found between the stone fabric and its pore space, and it was suggested that the micro pores and the differences in pressure during wet and dry cycles also played an important role in expansion due to moisture.14

Dubelaar concluded that a major reason for the degradation of Udelfanger was found in the weak calcite cementation between the quartz particles. This calcite cementation could vary per quarry seam and be weakened by exposure to outside conditions.15_{The idea that the binding}

material could play a crucial part in the durability and decay of sandstone is confirmed by Colin

9_{Dubelaar, W. Geologisch onderzoek van een aantal monsters uit een tweetal groeven bij Udelfangen}

(Duitsland) en van monsters afkomstig van het exterieur van de Remonstrantse Kerk in Rotterdam. Nederlands

Instituut voor Toegepaste Geowetenschappen: Utrecht, 2003

A follow- up on the report is the article by Dubelaar, W and Hendrik- Jan Tolboom. ‘Udelfanger zandsteen Mineralogie, fysische eigenschappen, verweringsverschijnselen en duurzaamheid van een historisch bouwmateriaal’. Grondboor & Hamer. nr. 5 (2004)

10_{The report was commissioned by the Cultural heritage Agency of the Netherlands (Rijksdienst Cultureel}

Erfgoed), the objective was to compose a geologic characterization of the selected samples and to indicate the suitability and applicability of the Udelfanger as a restoration stone.

11_{Dubelaar, 2003: 25} 12_idem

13_{See chapter three}

14_{Ruedrich, J. et al. ‘Moisture expansion as a deterioration factor for sandstone used in buildings’. Environ}

Earth Sci, 63 (2011): 1545

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Muir.16_{He writes that the weathering and decay of sandstone is affected by the stones internal}

structure together with its binding agent saying, ‘Indeed, sandstone is only as strong as its binder, since the individual silica crystals are so structurally strong as to be effectively indivisible”.17

Another important phenomenon that could influence the degradation of Udelfanger stone, is salt crystallization. Soluble salts that enter the substrate with moisture, or that are formed by a chemical reaction within the stone mineral, can crystallize in the pores and exert pressure on the stone substrate.18_{According to a dissertation by Kraus, Udelfanger stone showed signs of damage}

form salt crystallisation relatively quickly (after 14 cycles).19_{Specific delamination of layers parallel}

to the surface was also found to be typical with salt damage.20_{Besides delamination, other decay}

phenomena can also be caused by salt crystallization.21

A typical characteristic of Udelfanger stone is poor weathering resistance. Quist comments that in some cases, Udelfanger stone already showed signs of decay after 10 to 20 years. 22_{This poor}

reputation is also confirmed by an anecdote found in the publication Utrecht in steen.23_{During a}

meeting in 1906, between the quarry boss and the intendent of the community Utrecht about the poor durability of the stone, it was said that the quarry provided such poor quality stone that it had been abandoned, the stone then being excavated in another area. 24_{Although Udelfanger stone is}

naturally not weather resistant, there is evidence that more durable examples do exist.2526

In a publication by Kramer and Feenstra, the difference in quality of Udelfanger is mentioned. According to the authors, this difference is related to the layers in the quarry seams. Layers that are rich in silt should not be used, layers from the core proving to be stable. 27_The_{different layers in}

the quarry seams were also investigated by Dubelaar who compared a green and a yellow grey layer. The mineralogical composition was found to be the same, but a difference was found in the amount of silt present.2829

Regarding the objects under study and their biography, three time periods should be taken into account. The first two periods are described in a publication by C. Peeters, more in general than

16_{Muir, Colin. ‘Sandstone’ In: Henry, Alison. Stone conservation. Principles and practise. Shaftesbury:}

Donhead, 2006: 190

17_{Muir, 2006:192}

18_{Dusar, M, R, Dreesen, A, De Naeyer. Natuursteen in Vlaanderen, versteend verleden. Mechelen: Wolters}

Kluwer, 2009: 68

19_{Kraus, K. Experimente zur immisionsbedingten Verwitterung der Naturbausteine de Kolner Doms im}

Vergleich zu deren Verhalten am Bauwerk. Diss. University Cologne, 1985. Cologne: Hansen, 1985: 134

20_{Dubelaar, 2003: 26} 21_{Muir, 2006: 192-204}

22_{Quist, W. Vervanging van witte Belgische steen. Materiaalkeuze bij restauratie. Diss. Technische Universiteit}

Delft, 2011. Delftdigitalpress, 2011: 162

23_{Dubelaar, W, Timo G. Nijland, Hendrik Jan Tolboom. Utrecht in steen. Historische bouwstenen in de}

binnenstad. Utrecht: Matrijs, 2007: 93

24_{This is also suggesting that different types of stone were exported as Udelfanger.} 25_{Dubelaar, 2004: 106. Example given: Sculpture of Piet Heyn (1870) Delfshaven, Rotterdam}

26_{Quist, 2010: 163. Example given: Onze lieve vrouwe koor, Grote kerk, Dordrecht} 27_{Kramer and Feenstra, 1988: 59}

28_{Dubelaar, 2003: 26}

29_{In the report Dubelaar mentions the Dutch terms “leem” and “silt” at the same time. In this research only}

the term “silt” will be used, this is the English, as well as the Dutch term for the fraction of the particles between 63 and 2 micron (between the sand and clay fraction).

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specifically related to the sculptures.30_{In the first period, during the first extensive restoration of}

the Sint-Jan (1858-1870) the northern portal was renewed and the sculptures were carved. In the second restoration period (1961- 1984) structural parts of the Northern transept were repaired and the sculptures were cleaned. During the third restoration period (2000-2010) the sculptures were dismantled, cleaned and some were replaced with reconstructions. A report by Ronald Glaudemans from 2010 is a very valuable source with relation to this last restoration.3132_{This report that is the}

twelfth of a series, covering the last restoration. It contains many images (historic and

contemporary) and provides a lot of information on the handling and condition of the sculptures in that period. Most of the images show the situation after the cleaning treatment (December 2010 and January 2011) except for one image that shows the face of the sculpture Mattheus before cleaning. It can be seen that the black crusts that existed before the cleaning had been removed (see figure 1 and 2).33

As mentioned before, chemical analyses had been carried out in 2013 on some of the sculptures. The analysis was undertaken with X-ray fluorescence (pXRF, thermos scientific Niton XL 3T, handheld on a standard) and was a first attempt to try and find causes for the worsening of the condition that seemed to appear a few years after the cleaning treatment in 2010. The analysis was performed by RCE researchers on fragments of the sculpture of the angel and the wise virgin, situated next to each other.34_{The results showed that there was more sulphur in the deteriorated sculptures}

compared with the sample of un-deteriorated Udelfanger.35_{The conclusion was that a gypsum crust}

had been formed as a result of exposure to high levels of SO₂ in the air (as will be explained in chapter 3). The gypsum crust seems to have held the stone surface together. During the steam cleaning the gypsum crust dissolved leading to the loss of the outer layer.36_{An explanation for this}

specific process has not been found in written sources, although it is known that the formation of gypsum crusts results in the development of a friable layer under the surface which can make the surface vulnerable to mechanical damage.37

Research into steam/ high pressure water cleaning systems and its danger for natural stone is documented in the stone conservation literature. A publication by Slaton and Normandin discusses the effects of water and steam cleaning systems. 38_{One important factor is the pressure of the}

water, high pressure being known to erode fragile stone surfaces. Other issues that should be taken into account are water infiltration and the effect of trapped water and potential staining by

impurities.

30_{Peeters, 1985: 124}

31_{Glaudemans, R. Het Noordtransept, Bouwhistorische waarnemeningen 2001-2010. Verslagen van het}

bouwhistorisch onderzoek van de Sint-janskathedraal. Nr.11, december 2010

32_{Ronald Glaudemans is a building historian at the municipality of Den Bosch. He was closely involved with}

the last restoration of the Sint-Jan and has written several publications about the cathedral. He has just finished his PhD about the medieval sculptures in the Sin-Jan.

33_{Glaudemans, verslagen van het bouwhistorisch onderzoek van de Sint-Janskathedraal, 2010: 95}

34_{The pXRF analysis was undertaken by the RCE without a specific research request. The aim was to gather}

additional information to compare with the pXRF database on Udelfanger samples (samples Dubelaar, 2003)

35_{The un-deteriorated sample was from the RCE natural stone collection.} 36_{Niël, 2013}

37_{Muir, 2006: 193}

38_{Slaton, Deborah, Kyle C.Normandin. ‘Masonry Cleaning Technologies. Overview of Current Practice and}

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According to Yves Vanhellemont, another risk is the consequence of using high temperatures, which could lead to mechanical damage by thermal stresses. 39

Fig. 1: Image of Mattheus before cleaning in 2010. Fig.2: condition Mattheus in depot Sint-Jan, (photo in: Glaudemans, 2010: 94) February, 2017

39_{Vanhellemont, Yves, Willem van Peer, Nathalie Verminne. Module Gevelreiniging. Onroerend erfgoed,}

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2. The sculptures

The objects that are the focus of this research are part of a group of 28 sculptures, all of Udelfanger stone. 40_{They are situated against the buttresses on both sides of the northern portal of the Sint-Jan}

Cathedral (see figure 3 -5).The group of sculptures consists of twelve apostles, ten virgins, a devil, an angel and four heralds with coat of arms. The sculptures were dismantled and cleaned during the restoration of the Northern portal from January 2007 until December 2010.41

Two of these sculptures were chosen to be further investigated as to be representative for the group, The Angel and Jacobus de Meerdere (Saint James). An important selection criterion was the fact that former research was already carried out on those two sculptures, as described in chapter 1. A sample of the Angel was taken and analysed with X-Ray Fluorescence at the Cultural Heritage Agency in February 2013 (see appendix I). In relation to the sculpture of Jacobus de Meerdere, in 2013 loosened scales from the lower zone were removed. The reason for this was to form a

reference point from which further degradation could be detected (if and how changes in condition will take place in time) and monitored.42

In order to compare the condition of the group on the Northern portal to that of other

sculptures that had not undergone the cleaning treatment in 2010, the sculpture of Franciscus was chosen for further investigation and analysis. This is situated on the same side of the Cathedral, in the Northern side aisle, meaning that it is exposed to similar weathering conditions as the

sculptures under investigation (see figure 6).

40_{In the beginning of the research Hendrik-Jan Tolboom (natural stone specialist RCE) questioned whether}

some of the sculptures were made of a Luxemburg sandstone instead of Udelfanger. However, after microscopic examination of samples of the three sculptures, Bertil van Os (geologist RCE) found similar petrographic features in all the samples identification then all as Udelfanger stone.

41_{Glaudemans, R, Wim Hagemans. De Sint-Janskathedraal van ’s-Hertogenbosch. De restauratie 1999-2010.}

Veerhuis: Alphen aan de Maas, 2011: 117

42_{The removing of the scales was done with a house hold sponge and was carried out by BBM,}

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Fig.3: Group of 14 sculptures on west side of

Northern portal

Fig.4: On the far right: the group of 7 sculptures on Fig.5: group of 7 sculptures on the west side of the

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Fig.6: The northern side aisle on the west side of the northern portal. The sculpture of Franciscus is located behind the scaffolding (February, 2017)

2.1 Description of the sculptures

The sculptures are situated in ornamented alcoves with baldachin, on both sides of the northern portal. The Angel (N56) is situated on the east side while the Jacobus de Meerdere (N50) on the head-end of the west side (figure 7 and 8). 43_{The sculpture of Franciscus is located on the side aisle}

more to the west of the portal (figure 9). The three sculptures are of approximately the same size, being +/- 150 cm tall and have a width of 60 cm. The Angel is barefooted and wears a long robe and a tiara on her head. In her right hand, she is holding a sceptre. The apostle Jacobus de Meerdere, is also depicted barefoot with a long coat. He is wearing a beard and a pilgrim’s hat. In his right hand he is holding a stick with a knapsack and in the left hand a book. The Franciscus is presenting an open book with an engraved text and is also holding a feather in his hand on which a pigeon is sitting.44

The stone objects are sculpted in an ochre- white Udelfanger sandstone, which is in some areas discoloured grey to black in colour. The Franciscus has in general more grey and black areas, while the Angel and Jacobus have warmer ochre to orange coloured areas.

43_{In Koldeweij, A.M, W.J.J. Adriaanse, W. van Rosmalen. Transeptportalen, de St Janskathedraal}

’s-Hertogenbosch, Den Bosch, 1982. A numbering system was made for the group of sculptures at the northern

portal. This numbering system was also used in de bouwverslagen, Glaudemans, 2010

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Fig. 7: The Angel Fig.8: Jacobus de Meerdere Fig.9: Franciscus (April, 2017) (May, 2017) (R. Morelissen. April, 2017)

2.2 Origin and date

The sculptures are a later addition to the iconography of the Sint-Jan Cathedral and originate from the period of the first restoration, from 1858 until 1945. This restoration was led by the architect Louis Veneman (1812-1888) who, due to sickness, was replaced in 1863 by his assistant Lambert Hezenmans (1841-1909). 45

The Northern transept was totally renewed in the period from 1858 until 1870. Copies were based on the remains of the original 15th_{- century sculptures of the northern portal and the}

iconographic scheme was extended with new figures. It is uncertain who made the design for this new ensemble. According to Peeters it might have been Jan Hezenmans, the brother of Lambert Hezenmans. 46

The Angel and the Jacobus de Meerdere were carved around 1864, in the workplace of

Hezenmans, which was situated in the building lodge next to Sint-Jan. The sculptors were probably students of De Bosche Koninklijke school voor Nuttige en Beeldende Kunsten, as was the sculptor Hendrik van der Geld. 4748_{The Franciscus is presumably from a slightly later date. As the restoration}

of the Northern side- aisle took place from 1874 until 1878, it can be dated from that period. 4950

45_{Peeters, 1985: 157} 46_{Peeters, 1985: 253} 47_{Peeters, 1985: 71}

48_{Hendrik van der Geld (1838-1914), sculptor in Den Bosch who had his own studio since 1872.} 49_{Peeters, 1985: 139}

50_{On a photograph dated around 1878, it can be seen that the alcove is still empty and that the Franciscus}

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The style of the figures is typically neo-Gothic with a smooth finish and rather slick appearance as when compared to other sculptures of the Sint-Jan. 51_{Already at the end of the 19}th_{century the}

sculptures were not positively received. In ‘Holland op zijn smalst’ Victor de Stuers wrote about the mediocre artistic quality, which was not surprising to him and a consequence of the budget that was too low to hire qualified sculptors. 52

2.3 Historical context of the use of Udelfanger stone

The use of Udelfanger stone covers only a short period in the history of the Sint-Jan Cathedral, as it proved not to be durable very soon after its first applications. Although the stone was already used in the 13th_{century in the Liebfrauenkirche in Trier, its use in the Netherlands was primarily in the}

late 19th_century.53_{The introduction of the stone to the Sint-Jan Cathedral, was initiated by the}

governmental engineer Leendert Rijsterborgh in 1864 and, because he had high expectations of the stone, it was widely used in the Northern side of the church during the first restoration. 54_The

decision for its use is probably related to the construction of a rail link to the quarries of Udelfangen in 1863 which resulted in easy access to the stone. In addition, the stone was considered easy to sculpt and was appreciated for its fine-grained structure which allowed sculptors to achieve a smooth finish. 55_{However, its bad durability soon became apparent and the use of Udelfanger stone}

on the Sint-Jan stopped after 1888.56

Nevertheless, in the first few years during the regular inspections the stone was approved. Among those that approved it was the governmental advisor and architect: P.J.H.Cuypers.57_He

quite often used the stone for his neo-gothic church designs at the end of the 19th_century,

explaining why the stone is sometimes called “the stone of Cuypers”. 58

2.4 The current condition of the sculptures

The overall condition of the objects seems to be quite instable, by the different damage phenomena that can be seen. The main degradation phenomena are delamination or scaling of the surface layers, differential erosion, and a granular disintegration of the sandstone. 59_{These degradation}

phenomena were considered to have worsened after the cleaning treatment in 2010. Another feature is the diverse range of colours that the sculptures have, the more orange colour on the lower areas being especially significant.

51_{Glaudemans, R, W. Hagemans, H. Boekwijt. De Sint-Janskathedraal van ’s-Hertogenbosch, geschiedenis van}

de bouw. Alphen aan de Maas: Veerhuis, 2010: 268-276

52_{Stuers, Victor de. ‘Holland op zijn smalst’. In: De gids. Jaargang 37 (1873): 48} 53_{Dubelaar et al, 2007: 91} 54_{Peeters, 1985: 72} 55_{Dubelaar et al, 2007: 92} 56_{Peeters, 1985: 140} 57_{Peeters, 1985: 72} 58_{Dubelaar et al, 2007: 90}

59_{Granular disintegration: occurs in granular sedimentary and produces debris which can be seen}

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Regarding the sculpture of the Angel, the body seems to be in a moderate condition, although the wings are suffering from severe degradation and are in a very unstable state (figure 10, 13). The wings show clear signs of granular disintegration; sand grains and fragments being found below her on the plinth. This disintegration is accompanied by delamination, a pattern of loosened flakes that are falling off, sometimes with an air-filled space behind (figure 14). In some cases the disintegration and delamination show signs of alveolization, which is the formation of cavities (figure 15). 60_{On the}

back of the wings the surface seems to be intact although this might be a later repair material (figure 15).

The dress appears to be in a more stable condition. The main degradation is differential erosion, an irregular deterioration pattern that is induced by selective material loss (figure 16) 61_{The erosion}

can mostly be seen on the higher parts of the contour, for instance on the protruding part of the folds (figure 17). The erosion has affected almost the whole figure. Only the surface of the area around her chest and shoulders seems to be intact (figure 18). In a few places a kind of deposit can be seen which seemed to be pink in colour. This could be the remains of a painting material (figure 19). On the right hand a cementitious filling is visible and the whole of the left hand is a

reconstruction made from a new piece of stone.

The overall condition of Jacobus de Meerdere appears to be more stable than that of the Angel and the degradation phenomena are mainly found in the lower area or on the top (figure 8, 11). The stone in these areas is suffering from detachment by contour scaling where in some areas this is progressing into delamination (figure 20, 21, 22). Blistering can also be found on the hat, where an air-filled space can be found behind the flake (figure 23). 62_{Where the detachment was already}

active, the loss of material induced differential erosion, leaving an eroded surface and showing the natural sedimentation of the stone’s structure (figure 24). These areas have a warmer ochre, more orange colour. Besides these main degradation phenomena, some minor degradation patterns include pitting on his right hand and, similar to the Angel, deposits of pink coloured remains in some areas.

On visual inspection, it seems that the condition of Franciscus is somewhat different compared to that of the two other sculptures (figure 9, 13). The first difference is the colour. The Franciscus is less ochre in colour and more grey, some areas being dark grey to black which is not seen on the other two. Encrustations can be found on these dark areas, which are mainly found on the upper part of the figure (figure 25). In some cases, these encrustation have fallen off leaving an eroded surface.

Other significant deterioration patterns include the extensive loss of several layers through delamination. This has left cavities of variable shapes and sizes which will be referred to as

differential erosion (see figure 26, 27, 28). Near these areas cracks of variable size and direction can

60_{Delamination is a detachment process, affecting laminated stones (Udelfanger) and corresponding to a}

physical separation of several layers following the stones laminae. ICOMOS- ISCS

61_{Differential erosion: A feature that is induced by material loss. This occurs when erosion does not proceed}

at the same rate from one area of the stone to the other. ICOMOS- ISCS: 30

62_{Contour scaling: detachment of stone as a scale (or stack of scales) not following any stones structure and}

detaching like fish scales or parallel to the surface. Contour scaling is scaling in which the interface with the sound part of the stone is parallel to the stones surface

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be seen (figure 29). 63_{The large losses can also be identified as missing parts; for instance on the}

face, foot, pigeon and, at its most extreme, the lower folds of the coat (figure 25, 30). 64

As was mentioned in the introduction, the condition of the sculptures was monitored twice in 2013, by BBM Restauratiearchitectuur. When comparing the condition on the images from that time with the current condition in 2017, no great difference was noticed. It could therefore be assumed that the degradation has not worsened dramatically since then.

Fig.10: Damage mapping of the sculpture of the Angel

63_{Crack: Individual fissure, clearly visible by the naked eye, resulting from separation one part from another.}

ICOMOS- ISCS: 10

64_{Missing parts: Empty space, obviously located in the place of some formerly existing stone part. ICOMOS-}

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Fig.11: Damage mapping of the sculpture of Jacobus de Meerdere

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Fig.13: Severe degradation of the wings Fig.14: Severe delamination on the wings (R.Morelissen. April, 2017) (R.Morelissen. April, 2017)

Fig.15: The Angel: Cavities and grey material Fig.16: Angel: Differential erosion on the dress on the back of wing (R.Morelissen. April, 2017) (R.Morelissen. April, 2017)

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Fig. 17: The Angel. Differential erosion on protruding parts Fig. 18: The Angel. Differential erosion on the cheek. of the folds. (R.Morelissen. April, 2017) (R.Morelissen. April, 2017)

Fig.19 : The Angel. Pink remnants on the sleeve Fig.20: Jacobus de Meerdere. Delamination and

(R.Morelissen. April, 2017) contour scaling (R.Morelissen. May, 2017)

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Fig. 21: Jacobus de Meerdere. Delamination Fig.22: Jacobus de Meerdere. Contour scaling (R. Morelissen. May, 2017) (R. Morelissen. May, 2017)

Fig. 23: Jacobus de Meerdere. Contour scaling and blistering (left) Fig.24: Jacobus de Meerdere. Differential- (R.Morelissen. May, 2017) erosion (R.Morelissen. May, 2017)

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Fig.25: Franciscus. Dark discolorations and missing parts Fig.26: Fransicus. Delamination (R. Morelissen. April, 2017) (R. Morelissen. April, 2017)

Fig.27: Franciscus. Delamination Fig.28:Fransiscus. Differential erosion by delamination (R.Morelissen. April, 2017) (R.Morelissen. April, 2017)

Fig.29: Franciscus. Delamination and Cracks Fig.30: Franciscus. Severe loss of material underzone (R. Morelissen. April, 2017) (R.Morelissen. April, 2017)

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3. Udelfanger stone

3.1 Lithologic information

Udelfanger stone is a sandstone that belongs to the large group of sedimentary rocks. 65_{All stones}

can be classified into three main geological groups which are named after the way they were formed. The igneous rocks and metamorphic rocks are the other two classifications. 66_Certain

sedimentary stones are formed by the weathering and erosion of igneous rock, such as granite. The mechanical and chemical breakdown of such stone, produces organic and mineral source material, which is then mainly transported by running water and deposited as a layer or bed in a certain area, mostly in the ocean. During transport the particles are reduced in size and rounded- off by

abrasion.67

The group of sedimentary stones can be divided into smaller sub groups, the Udelfanger stone belonging to the group of clastic sedimentary rocks. These clastic rocks consist of weathered and ground rock that is transported by water, deposited and cemented together by a binding material to form new rock.68_{The cementing material originates in time and under increasing pressure, of}

precipitated minerals from the pore’s water, which will be mainly carbonates and silicates.69

The clastic group is further subdivided, according to the dominant particle size. The group of sand-stones mainly consist of the mineral quartz. 70_{The different types of cementing material and}

the other minerals besides quartz can give a further classification. As Udelfanger stone consists of 6-9 % feldspar, it belongs to the group of sub-arkosic sandstones.71_{Because the sand grains are}

cemented by carbonates, Udelfanger stone is also classified as calcareous sandstone.72

Udelfanger stone is excavated on the north-west side of Trier (Rhineland- palatinate) Germany, from two quarries close to the village of Udelfangen. 73_{In the geological time scale, the Udelfanger}

stone was formed during the Mesozoic era, around 242-247 million years ago, in the middle Triassic: Anisian period. Stratigraphically it belongs to the under- Triassic limestone (Muschelkalk), the older formation Bunter (Buntsandstein) lying beneath it. 74_{It is assumed that the sediments were}

deposited in a sea, which would explain the shelves in the stone that sometimes occur and the

65_{A rock is any mass of mineral matter, which forms part of the earth. The word stone is admissible only in}

combinations, such as sandstone, or where it is used as the name for the extracted material. In: The Penguin

Dictionary of Geology by D. G. A Whitten & J. R. V. Brooks, published in 1979.

66_{For more information on classification of rocks: John Ashurst and Francis G.Dimes. The conservation of}

building and decorative stone. Volume 1. London: Butterworth-heineman, 1990

67_{Ashurst, John and Francis G.Dimes. The conservation of building and decorative stone. Volume 1. London:}

Butterworth-heineman, 1990: 23

68_{Slinger, A. H, Janse. G, Berends. Natuursteen in Monumenten. Zeist: Rijksdienst voor de Monumentenzorg,}

1980, 12

69_{Dusar et al, 2009: 35}

70_{Ashurst and Dimes, Vol.1 1990: 61}

71_{Dusar et al, 2009: 283 Source: Bron R.H.Dott, 1964. Atlas Natuursteen in Limburgse monumenten: 89} 72_{Dubelaar, 2003: 6}

73_{Dubelaar and Tolboom, 2004: 90}

74_{In: Hirschwald, J. Handbuch der bautechnischen gesteins einsprufung. Berlin, 1912. The Udelfanger belongs}

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precipitated calcareous components. For this reason, the stone is sometimes also called a Mussel sandstone (Muschelsandstein).75

3.2 Mineralogical and petrographic features

Udelfanger stone is a fine-grained sandstone which mainly consists of quartz particles, feldspar, micas and other smaller lithic fragments (pieces of stone), the degradation products of granite. The feldspar, the quartz and the micas, are all silicate minerals. The quartz (mainly SiO₂) is very hard and resistant to chemical break down. The feldspar will weather more easily and the softer micas will easily split into smaller plates. 76_{For this reason, feldspar and mica are sometimes also referred to}

as clay minerals. 77

The mineral content and cementing material can vary within a quarry resulting in a

heterogeneous material. The average mineral content of Udelfanger stone as given by Dubelaar is quartz (65-70%), feldspar (6-9%) and mica (2-5%). These minerals are cemented together by carbonates that are partly dolomitised (calcium- ions are replaced by magnesium ions).78_This

average was based on other written sources notably Kraus (1985) and Grimm (1990). 7980

When one compares the information in the sources, slight differences can be found. 81_Kraus

states that the main content of Udelfanger stone is quartz (70%), and gives a slightly higher percentage of feldspar (10%), 6 % micas (biotite and muscovite) and dolomite (10%). The dolomite is a part of the cementing material and considered to be ferruginous. Calcium carbonate is not mentioned in this research.82_{According to Grimm, the carbonates found are idiomorphic dolomite}

crystals and the binding material was considered as clayish, partly ferruginous-clayish and kaolinite.83

In the research by Dubelaar, the calcareous components can vary from 3- 6,5% up to 15%. The variation is being related to the presumed secondary calcareous precipitation of ground water, formed during the lithification of the stone and, on the other hand, the local, in-situ dissolution of the calcareous components.84_{The micas that are present in the Udelfanger stone are mainly}

muscovite (transparent white coloured), but also biotite (dark coloured) and chlorite (greenish) are found. 85_{Other minerals that were found are: ilmenite, a titanium oxide, and apatite, a phosphate}

75_{Dusar et al, 2009: 284. Dubelaar and Tolboom, 2004: 107}

76_{Lijdsman, P.M.E. Bouwmaterialen, natuursteen. Zaltbommel: Van de Garde& Co, 1944: 43} 77_{Ineke Joosten. May, 26. Personal conversation Studio building Amsterdam.}

78_{Dubelaar is referring to two other sources: Kraus, 1985 and Grimm, 1990} 79_{Kraus, 1985: 112}

80_{Grimm, W.D. Bildatlas Wichtiger Denkmalgesteine Bundesrepublik deutchland. Munchen, 1990} 81_{Kraus, 1985 : 112}

82_{As sample material, Kraus used a new broken fragment of stock material of the Cologne cathedral, Germany} 83_{Grimm, 1990}

84_{Dubelaar, 2003: 18} 85_{Dubelaar, 2003: 11}

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mineral.86_{The dark spots that are significant for Udelfanger stone, are presumed to be manganese}

or iron oxides. 87_{However these were also thought to be biotite.}88

With relation to the fractions, the average size of the quartz particles were found to be in the sand fraction (around 100 micron). Twenty percent of the particles were presumed to be in the silt fraction (˂ 63 micron) and only a small percentage of particles in the clay fraction (˂ 2 micro meter), which was less than 1 %.89_{It should be taken into account that these percentages can vary and that}

these were only averages taken from some specific samples, making them only an indication. A significant petrographic feature is the layered structure of the stone, which seems to be caused during sedimentation by the angular, sometimes elongated form of the quartz and feldspar particles and the concentration of mica plates. 90_{The form of the particles and the mica plates result in a}

preference orientation of deposition which develops the layered structure and makes the stone easily fissionable. 91_{The layered structure is further created by the existing silt layers. In addition,}

lumps of silt also occur.92

3.3 Heterogeneity of Udelfanger stone

As described in the previous paragraph, it can be concluded that the Udelfanger stone is very heterogeneous. Even though natural stones will always have slight mineral differences, the Udelfanger stone shows relatively large variations in mineralogy and petrographic features. These features will consequently determine its weathering behaviour and that could be the reason why the stone was experienced to be variable with both good and bad qualities being found. 93

On the one hand this quality difference can be related to the stratigraphy. The different bedding layers in the quarry provide various mineral components which have an effect on the amount of silt, clay and cementing material. On the other hand, one finds in the literature that the name

Udelfanger was given to different types of stone from quarries from around the Trier area. This could explain the large variations of stone carrying the same name.

An example of the different types of stone that were exported as Udelfanger, was given by van der Kloes. 94_{He mentions that all the quarries from both sides of the river Sauer that served as the}

border between Luxemburg and Germany, could have exported their stones as ‘Udelfanger stone’. Van der Kloes describes how the quarries on both side of the banks of the river Sauer (like Born and Girst) within a distance of 42 km and with a difference in height of 47 m, must have delivered many different types of material. This is because, due to the geological map of that area, all stratifications

86_{These minerals were detected with SEM-EDS. Ineke Joosten (Cultural Heritage Agency) May, 26. Studio}

building Amsterdam.

87_{Dubelaar, 2003:12. Friedrich Muller. Internationale naturstein kartei. Ebner verlag Ulm, 1989. Page/ cart}

6.2.32

88_{Personal conversation with Bertil van Os, April, 20. 2017. Cultural Heritage Agency, Amersfoort.} 89_{Dubelaar, 2003: 11-12. He refers hereby to the sources of Grimm, 1990 and Kraus, 1985}

90_{Angular particles are a typical characteristic for a “fresh” sediment, a sediment that is deposited nearby the}

source and whereby the particles are not rounded off during the transportation. Information by Bertil van Os, RCE, Amersfoort.

91_{Dusar et al. 2009: 283} 92_{Dubelaar, 2003: 12}

93_{Dubelaar et al, 2007: 92-93}

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of the Triassic period can be found there: Muschelkalk (middle Triassic)/ Bunt Sandstein (early Triassic) and the Keuper (late Triassic). 95

Other significant issues that confirm the heterogeneous character of the Udelfanger stone, are the two types of cementing material that are described in the different written sources. Van der Kloes writes that the Udelfanger stone contains silt as a binding material, which he concluded from test with hydrochloric acid, the Udelfanger sample not being seen to effervesce.9697_However,

according to Quist, this interpretation was considered not to conform with the current terminology. Quist claims that the Udelfanger stone contains a high amount of silt particles, and is cemented by carbonate.98

Anyway, Lijdsman claims that two types of Udelfanger stone exist: a strong and soft variety. The stronger variety was considered to be weather resistant and not to effervesce with hydrochloric acid. 99_{This suggests that the stronger variety does not contain calcareous cementing material}

which would react with acid. However, this is in contradiction with the Dubelaar’s conclusions in 2003. He claimed that the differences in the stratigraphy were found in the large variation in the amount of carbonate between two banks: a green and a more yellow-coloured type. When investigating the upper green-coloured bank, a higher percentage of silt was found, showing the existence of more finely grained particles and a lower amount of carbonate. The yellow layer consisted of more carbonate and coarser grains. Although the amount of carbonate also varied in the yellow bank, this layer was assumed to be more durable and appropriate as a building stone than the upper layers containing lots of silt. 100

The claim that a clayish material is the cementing material, was found in a more recent article about water repellence and the humidity of sandstones. 101_{Here it is stated that Udelfanger}

sandstone consists of fine quartz grains with a “clay type” binding material that was assumed to be partly kaolinite. It can therefore be concluded that there are differences between, or combinations

of, cementing (binding) material in Udelfanger stone.

3.4 The degradation of Udelfanger stone, weathering processes in relation to the

characteristics of the stone.

When trying to understand the influence of a former cleaning treatment on the degradation of the sculptures, it is important to consider natural degradation of Udelfanger. By natural degradation it is meant a decline in condition, quality or functional capacity that is caused by the process of

95_Idem

96_{By testing a stone with hydrochloric acid, the stone will effervesce if Calcium carbonate is present.} 97_{The reliability of the test could be questioned. Dolomite is much more resistant to acid attack and will not}

effervesce immediately.

98_{Quist, 2011:43}

99_{Lijdsman, 1944: 140. It is mentioned that when calcium sulphate is present, effervescence will not occur.} 100_{Dubelaar et al, 2007: 90-91}

101_{Behre, S, K. Littmann. ‘Measurement of Humidity in Sandstones Treated with Water Repellent Agents’. In:}

De Clercq, H, A, Elena Charola. Hydrophobe V: 5th International Conference on Water Repellent Treatment of Building Materials. Aedificatio Publishers: Freiburg, 2008: 75

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weathering, this being the process by which a stone that is exposed to weather undergoes structural change.102

The main three weathering processes are: physical damage due to salt crystallisation, due to frost, and chemical/ physical damage due to the effects of acid rain. Salt and frost damage both involve porous materials and moisture. Frost damage occurs when water enters the pores of a stone and the water transforms into ice. Water expands by 10% when it forms ice, resulting in pressure on the internal stone structure. When there are cycles of freezing and thawing of water, the repeated pressure will lead to increased damage of the pore structure.103

The process of salt damage is similar to frost damage. 104_{Salts in solution, originating from the}

material itself or an external source, will enter the pores of the stone and crystallize once the relative humidity is below the point of deliquescence. This crystallization can occur at the outside surface, which is called efflorescence, or inside the stone structure, which is called crypto

florescence or sub florescence. It is this last category that evokes pressure due to increase of the crystal volume, depending on the type of salt. Frequent cycles or further growing of these crystals will damage the inner structure.105

The third main weathering process is the effect of acid rain on calcareous stone. Sulphur dioxide in the air reacts with water to form sulphuric acid, which enters the pores of the stone in rain. This can then dissolve the calcareous component of the carbonates to form calcium sulphates. Calcium sulphate has a higher molecular volume than calcium carbonate with the result that this

sulphatation process can cause damage to the pore structure. When the calcium sulphates migrate towards the surface and the water evaporates, di-hydrated calcium sulphate (gypsum) can

precipitate. This process can be outlined by the chemical formula: CaCO₃+ H₂SO₄= CaSO₄. 2H₂O. 106

The process occurs partly internally in the pores, but mostly externally by forming crusts on the surface. Because the gypsum is slightly soluble, the gypsum will be washed away from the parts of the building where the rain lands, leading to erosion of the stone. In sheltered areas, the gypsum will grow into a thick and rough crust that will incorporate airborne particulates and gaseous pollutants in its porous network which will eventually transform into black crusts. 107_{The gypsum}

crusts are in a process of continuous transformation. With repeated wet and dry cycles the gypsum can slightly dissolve. Damaging salts can infiltrate the stone substrate, recrystallize and cause micro fissures. These deterioration processes are accelerated by the physical and chemical differences between the crust and the stone, causing stresses to the stone surface layer. Additionally, a crust can grow so thick that it will fall off at some point and leave an eroded surface where the process will start all over again.108

It should be kept in mind that the Udelfanger stone may contain dolomite (containing magnesium ions) and therefore the sulphatation process can also form magnesium sulphate

102_{Illustrated glossary on stone deterioration pattern. ICOMOS: 9} 103_{Dusar et al, 2009: 68}

104_{According to Hilde Declercq, there is a difference and the salt damaging processes are still not fully}

understood. Oral conversation. June, 28. 2017

105_{Shahidzadeh-Bonn, Noushine. et al. ‘Damage in porous media due to salt crystallization’. In: Physical}

review E, June (2010)

106_{Dusar et al, 2009: 69}

107_{Amoroso, G. G, V, Fassina. Stone decay and conservation. Amsterdam: Elsevier, 1983: 256} 108_{Amaroso and Fassina, 1983: 256}

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hexahydrate as a dry deposition. 109_{This salt is very destructive, since it occurs in different hydration}

forms. It is highly soluble and only a slight difference in the relative humidity can cause cycles of hydration and mechanically damage the stone surface. 110

3.5 Porosity

In this section the main features of the Udelfanger stone that may have an influence on the weathering phenomena, as discussed in chapter 3.4, will be described. Important features are the pore structure, the cementing material and the presence of clay and/or silt in the structure. The pore structure and permeability is one of the most important physical factors that influences the weathering processes. To quote Hunt in his book Chemical and Geological Essays: “Other things being equal, it may properly be said that the value of a stone for building purposes is inverse to its porosity or absorbing power.”111

By ‘permeability’ one means the property that permits any substance to penetrate, or pass through another substance, or the rate at which water will flow through. 112_{This is determined by}

the effective or open porosity of a stone, which will define the amount of moisture uptake as the pore size distribution and the capillaries; the connectivity between pores, influence permeability and will determine how fast the water will be absorbed and ability to evaporate. 113

With regard to frost damage, Dubelaar stated that the porosity and structure of the Udelfanger stone was considered to give good permeability and potentially good resistant to frost attack. 114

This conclusion was based on data from Grimm (1990) given the relatively high total porosity (19-25%) compared to other stone types and the small amount of micro pores (˂1 micron). Small pores make a stone more vulnerable to frost, the small capillaries retaining the moisture longer so that damage by crystallization occurs more quickly. 115

In her dissertation, Kraus provides a table of the pore size distribution of the Udelfanger stone.116

It shows the presence of micro pores (˂1 micron), macro pores (1-7,5 micron) as well as large pores (˃7,5 micron). The micro pores being no more than 1/5 part of the total amount. Kraus thereby classifies Udelfanger stone as a “very good absorbing stone”. This is according to an absorption coefficient that depends on the capillary capacity of the pores on the water-front and the

permeability of the stone. 117_{Udelfanger is able to absorb a high amount of water in a short time,}

but after a while the absorption continues very slowly. 118_{For this reason, the total water saturation}

109_{Kraus, 1985: 115. Udelfanger can form gypsum from calcite and Magnesium sulphate hexahydrate from}

Dolomite.

110_{Information given by Hilde De Clercq. June, 28. 2017}

111_{Anovitz, Lawrence M, David R. Cole. ‘Characterization and Analysis of Porosity and Pore Structures’.}

Reviews in Mineralogy & Geochemistry, Vol. 80 (2015): 61

112_{Kessler, D.W. Permeability of stone. Washington government printing office: Washington, 1926} 113_{Dusar et al, 2009: 55}

114_{Dubelaar, 2003: 21 Grimm, 1990}

115_{Graue, B. Stone deterioration and replacement of natural building stones at the Cologne cathedral, A}

contribution to the preservation of cultural heritage. Diss. George –August University School of Science,

Gottingen, 2013: 69

116_{Kraus, 1985: 57} 117_{Kraus, 1985: 53} 118_{Kraus, 1985: 55}

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of the Udelfanger seems to be low, which is probably due to the relative large size of the pores whereby air penetrates the pores leaving no more space for the water.119

According to Dubelaar, while Udelfanger stone is supposed to have good resistance to frost attack, the resistance to damage by salts was assumed to be poor.120_{This assumption was based on}

tests with sodium sulphate and the fact that Udelfanger stone showed signs of disintegration after fourteen cycles of salt crystallization. 121

Frost damage differs from salt damage, although the influence of the pore structure is similar and the size of the pores also plays an important role. Salt damage will mostly be visible due to the presence of powdered stone, as the crystal growth reduces the stone to a powder, while the effects of frost damage are more dramatic in the form of cracking and the loss of larger stone layers. 122_The

rate of decay due to salt crystallization depends on the type of salt, the frequency of the cycles and the pore space. A relatively large pore diameter will be resist damage from salts much better.123

The assumed vulnerability of Udelfanger stone to salt damage can probably not be related to the good permeability of the stone, other features such as the layered structure probably being of greater influence.

3.6 Clay and Silt

The permeability and pore structure is influenced by the presence of silt /clay in the stone structure. As was described, 20 % of the grains were fund to be in the silt fraction (˂ 63 micron) and a small amount in the clay fraction (˂2 micron). 124_{However, these percentages can vary per sample}

investigated and should just be seen as an indication. The percentages of clay and silt might as well be higher which could change the permeability and the resistance to weathering processes.

Silt and clay can act as a ‘moisture barrier’ by contributing to the creation of micro porosity. In addition, some clay minerals have the ability to absorb moisture and the capacity to swell, which will cause pressure on the inner structure.

Swelling clays in stone can generate damaging stresses during a wetting and drying cycle which leads to deterioration of the stone. There are two types of swelling identified: the short range: intra crystalline swelling and the long range: continuous osmotic swelling.125_{The short range applies to}

clays with interlayer swelling properties and the pressures are signiﬁcantly higher than those produced by osmotic swelling, resulting in more damage. 126_{However the long range osmotic}

swelling pressure that is related to “non-swelling” clay minerals, is also recognized as a plausible cause for damage at the microscale. 127128

119_{Kraus, 1985: 54} 120_{Dubelaar, 2003: 24} 121_{Kraus, 1985: 134}

122_{Ashurst and Dimes, volume 2, 1990: 159} 123_idem

124_{Dubelaar, 2003: 11}

125_{Wangler, Timothy, George W. Scherer. ‘Clay swelling mechanism in clay-bearing sandstones’. Environ Geol,}

56 (2008): 529

126_{Ruedrich, J. et.al, 2011: 1547}

127_{Fontaine, Laurent, Roel Hendrickx, Hilde De Clercq. ‘Deterioration mechanisms of the compact clay-bearing}

limestone of Tournai used in the Romanesque portals of the Tournai Cathedral (Belgium)’. Environ Earth Sci, 74 (2015): 3207

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This mechanism is probably related to the presence of liquid water within the porous material and the presence of the micro porosity of clay minerals, causing moisture expansion of the stone. 129

Besides the moisture expansion, water will be retained longer inside the stone which can then lead to frost or salt damage.130

Silt is the geological term for particles between 2 and 63 microns, the mineral origin being mainly Quartz and Feldspar. 131_{According to Kramer and Feenstra, silt is also able to take up moisture and}

swell, causing damage to the stone structure, especially silt that is spread throughout the structure of the stone is believed to weaken the stone severely. 132_{The uptake of moisture and subsequent}

swelling is probably similar to that of clay minerals. The occurrence of silt can create micro pores that will have an impact on the hygroscopic behaviour of the stone.

On the other hand, silt can also exist in layers or lumps which can be rinsed out and leave cracks and cavities, although this seems to be less damaging than when the silt is spread throughout the stone. 133

3.7 Resume

Udelfanger stone is a heterogeneous stone, consisting of quartz, feldspars, lithic fragments and micas. Especially the micas give the Udelfanger a layered structure that makes it easily fissionable in degradation processes. The cementing material can be calcite, dolomite or a clayish material, which give the Udelfanger its specific characteristics and could be different per investigated sample.

Udelfanger stone appears to be poorly resistant to damage from acid rain, soluble salts and even frost, although this contradicts some of the results presented by Dubelaar in his research. In many cases the processes causing damage are related to moisture and the degree of resistance is mainly determined by the cementing material, the porosity and the presence of clay or silt. Especially significant is the presence of micro pores that can be determined by the presence of clay and silt, making the stone more vulnerable to moisture.

128_{The method for measuring the swelling of clay minerals, or dilatometry, is described in this article. This}

might provide a further step in the investigation of the Udelfanger sculptures and the influence of clay- swelling features.

129_{Ruedrich, J. et al. 2011: 1563}

130_{Duffes, P, T, Wangler, G.W.Scherer. ‘Swelling mechanism for clay-bearing sandstones’. In: Lukaszewicz,}

Jadwiga W, Pjotr Niemcewicz (eds). 11th_{International congress on deterioration and conservation of stone.}

Torun, Poland, 2008: 65

131_{As discussed in chapter 3} 132_{Kramer and Feenstra, 1988: 59} 133_idem

The degradation of the 19 th century Udelfanger sculptures at the Sint-Jan Cathedral in Den Bosch and the possible relation to a former cleaning treatment in 2010