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CONSERVATION OF MARBLE AT THE FACADE OF THE CATHEDRAL IN SALZBURG

KOLLER,M., NIMMRICHTER,J. and PASCHINGER,H.

Vienna

The mainfacade of the Salzburg Cathedral connects in a successful way the use of local stones with the monumental forms of postpalladian character. This is the first work of Italian early baroc style north of the Alps. The cathedral was planned by Vincenzo Scamozzi in 1610, completed by Santino Solari and inaugurated in 1628. The Towers we·re finished in 1675 and the arches in 16631. Scamozzi praised in his architectural treatise the quality of the local limestone, Untersberger marble. Especially he liked this fine white stone of big proportiones for its excellent suitability for statues, columns and ornaments. This stone can be carved in any way and polished as any fine Italian stone. Besides of the white stone, greenishblue, yellowish and reddish Untersberger marbles were used for the cathedral2. The facade of the Salzburg cathedral is the biggest historic marble monument of Austria. The height of the towers is 79 meters, the surface is 7900 m2 and consists of about 10.000 carved square stones.

Use and technics of the stones

The mainbuilding stone of the cathedral is a conglomerat from the glacial period (Salzburger Nagelfluh). This is used for the front of the nave, the trikonchos and the dome tambour. With it's rough, dark surface this stone is a concious contrast to the bright marble used for the window profiles and sills. The sizes of the square stones are very regular, height 60 cm, length 80 and 403 . The outside of the two-tower facade and the arches consists of massive blocks in bright Unterberger marble (reworked and rearranged detritic reef limestone4 ) and the inside consists of Salzburger Nagelfluh. Inside this double masonry there are quarry stones, probably from the former romanic church. The upper octogonal parts of the towers are exclusevely made of Untersberger marble. Inside the masonry of each octagonal part there are two iron anchor rings, as normal for baroc architecture5

Restoration history and conservation state

After the second world war the dome and choir were reconstructed6. The last restoration of the main facade was in 1898-1903. Photographs from 1906 show a clean facade without black crusts, whereas photographs from 1896 show it dark (fig.1 ). By then the

1Tietze, H.: Die kirchlichen Denkmale der Stadt Salzburg, Osterr. Kunsttopographie IX, Wien 1912, S. I ff. 2Scamozzi, V.: L'idea dell'architecttura unversale Parte II, Libr. VII, Cap. IX, Venedig 1615 (Zit. nach Tietze 1912, s.2) 3Hell,M.: Die Baustoffe des Salzburger Domes, in : Der Dom von salzburg. Zurn dreihundertjahrigen Jubilaum 1628 - 1928, Salzburg 1928, S. 139 ff. Kieslinger, A.:Die nutzbaren Gesteine Salzburgs, Salzburg 1964,S.96f.

4Snethlage, R.: Report on the conservation of the facade of Salzburg Cathedral, 4.11.94. 5Hart, F.: Kunst und Technik der Wolbung, Miinchen 1965. 66sterreichische Zeitschrift fiir Kunst und Denkmalpflege 1950, S.114, 1957, S. 141 f.

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architectural part of the gable was renewed but the original decorative parts and sculptures were kept. The major part of the rest of the facade was recarved by push hammer. This was the way of cleaning at this time. Also a big part of the profiles were reduced with 2-4 cm. In this way not only the form was changed but also the joints were opened and water could enter into the quarry stones inside the masonry work (fig. 2). Ti1e protectiva cubber roofs above the windows and the big cornices were mounted. As mortar for the joints grey cement was used. A concequence of the work with the push hammer are microfractures parallel to the surface due to the concussion. The same conservation problem exsisted at the pest column in Vienna7 .

Deterioration and analyses

In recent time cracks, loss of pieces and black crusts has developed. Due to these problems a scaffolding was installed for the research on deterioration and conservation problematics in 1994. The Laboratories and workshops of the Austrian Bundesdenkmalamt started with serious of examinations and tests for consolitation and cleaning. The results could be discussed with a group of invited experts from Austria, Germany and ltalys.

Decay problems of the marble were:

- Original use of bad quality stones. - Original use of bad quality mortars for joints. - Use of push hammer in the beginning of this century. - Dirt and sulfatation coming from air pollution. - Water and salt problems inside the masonry work. - Frost decay. - Sagging of the walls. - Micro and macro biological growth, especially in open joints (fig. 3). - Corrosion of iron with consequently bursting of stones. - Thermal and hygric dilatation - Antipigion paste.

The systematical research of conservation methods of this kind of Alpine marble (Adneter, Untersberger marble, Veronese etc.) started in the beginning of the 70ties at the facade of San Petronio in Bologna9 , 10. Based on these results the Trinity column in Vienna and furtheron many other monuments from this material in Austria have been treated si nee 1981 11 •

7Koller, M., Paschinger. H., Pnmdtstetten. R..: Die Wiener Pestsaule. Restauratorenblatter 6, Wien 1982, S. 15 f. 8Invited experts: Dr. Raffaela Rossi - Manaresi, Fondazione Cesare Gnudi, Bologna, Prof. Dr. Rudolf Sneethlage Zentrallabor Bayer. Landesaint fi.ir Denkmalpflege, Munchen, Prof. P. Mirwald, Institut ftir Minerlogie, Universitiit Innsbruck, Dr. Andreas Rohatsch, Institut ftir Geologie, Technische Universitat Wien, Dr. Johannes Weber und Dr. Karol Bayer, Institut ftir Silikatchemie. Hochschule ftir angewandte Kunst Wien 9Rossi-Manaresi, R., Torraca. G., The treatment of stone, Bologna 1972. Rapporti della Sopraintendenza alle Gallerie di Bologna 14, 1972. 10Gnudi, S., Rossi-Manaresi. R .. Nonfarmale, 0., Notizie sul restauro della facciata di San Petronio, Bologna 1979 11Sneethlage, R. ; Grimm, W.D.,: Adneter Rotrnarmor. Vorkommen und Konservierung. Arbeitsheft 25, Bayer. Landesamt fur Denkmalpflege, Miinchen 1984.

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The Untersberger marble has a compact texture (0, 12 Vol. % wateruptake). This gives the stone a high strength and ductility. The compressive strength from fresh quarried stones is between 140 and 170 N/mm2 • Because of the lack of open pores and the relative homogenity of the mechanical properties, the stone has a high resistance against weathering. This means that erosion takes place on the surface and not in depth. But hygric and thermal dilatation makes the components of the stone loosen, because of incomplete connection between these components. The formed micro-fractures allowe an easier water penetration. Salt and frost actions lead to expansion of fractures. In the advanced state of decay parallel crack systems are formed below the surface and scales (from micro to 6 dm2) are produced (fig. 4), especially near the joints. The decay pattern was accelerated due to the use of push hammer in the beginning of this century.

The average value of grain size is between 0, 125 - 2,6 mm. The coarse grains (2 - 1 oo mm) prevail. The smaller aggregates are micrit - respectively sparit grains. At the healthy stones clay is very seldom. The compressive strength of the marble of the cathedral is 56,04 N/mm2 , which is much lower as for the healthy stone.

In the areas protected from rain, the stone is very dirty and has gypsum crusts in different formations on the surface and in the microfractures. In the unprotected areas there is no dirt. But there is a washout of CaC03 and a formation of CaS04.

The bad quality stones have a high frequency of layers in all three directions and contain clay. The mortar consists of a mixture between marble sand, dolomitlime and gypsum (fig. 5). This kind of mortar has been used in other places in Austria on buildings from the 17th to the 19th century12. When the mortar gets humid a chemical reaction occurs and magnesiumsulphate is formed. Magnesiumsulphate and calciumsulphate penetrate into the micorfractures of the stone and the action of these salts causes material loss (fig. 6a and 6b), especially near the joints. Nitrates and chlorides from pigeon excrements were detected near the decorative parts. Ultrasonic measurements show that the external parts of the decorative stones are weak untill a depth of 5 cm. After the conservation treatment the strength of these stones was increased 13(fig. ?a and 7b) .

Conservation and analyses

Because of the bad condition of the stone a consolidation treatment was necessary. The aim of the consolidation is to connect the microfractures and to reduce the water penetration. Different analyses were carried out in order to find the most adequate product (Table 1 ).

Paschinger H:: Alpenmannore - Schliden und Konservierung, in Natursteinkonservierung. Arbeitsheft 31. Bayer. Landesarnt fiir Denkmalpflege, Miinchen 1985,S. 163 - 167 . 12Paschinger, H.: Achtung Zeitbombe Magnesiumsulfat, in : Restauratorenbliitter 14, Wien ~994 , S. 10. 13Bayer K.,Fimmel R., P.W.Mirwald, Ultrasonic measurements at Salzburg cathedral, unpuhc report 1995

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Table 1 Weig_ ht Na 1 Nb2 Nc3 Nd4 Ne5 Nf 6 N__g_ 7 Nh 8 Nhu Before treatment (g) 726 709 722 709 719 673 717 715 722 After treatment (g) 728 710 723 710 721 674 718 716 722 Soaking in water (g) 727 729 723 710 720 676 718 715 722 After klima cycles (g) 727 709 722 711 720 676 718 715 722

Na 1: Ethylsilicate + methyl-ethyl-keton (Wacher OH) + PMMA (Paraloid 872 7% in metoxypropylacetat, butylacetat, xylol, 1 :1 :1 ).1 :1.

Nb 2: Ethylsilicate + methyl-ethyl-keton (Wacher OH) + silicon resin (Wacher 190 F 7% in metoxypropylacetat, butylacetat, xylol, 1 :1 :1) 1 :1.

Ne 3: Ethylsilicate + polymetilfenil siloxan (Rhone- Poulenc 11309), Resin RC 90 Nd 4: PMMA Paraloid B 72 (7% in metoxypropylacetat, butylacetat, xylol, 1 :1 :1) (fig. 8). Ne 5: Polyethylsilicate in ethanol, Motema 30. Nf 6: Ethylsilicate + methyl-ethyl-keton (Wacher OH). Ng 7: PMMA (Paraloid B 72 3,5% in metoxypropylacetat, butylacetat, xylol, 1 :1 :1) +Silicon resin

(Wacher 190 F 3.5 % in metoxypropylacetat, butylacetat, xylol, 1 :1 :1 ), 1 :1 . Nh 8: Polyethylsilicate (Motema 30) + PMMA (Paraloid B 72 3,5% in metoxypropylacetat, butylacetat,

xylol, 1:1:1).1 : 1. Nhu : Untreated.

Soaking in water: Soaking for one hour in destilled water and dried with towels. Climate cycles: -30 °C 12 hours, afterwards soaking in warm water, + 65 °C 12 hours, afterwards soaking in cold water, 20 times. 802 chamber , 3 times for two days.

The mixture between the silicon resin and the acrylic resin (Ng 7) showed to have the best properties (fig . 9). Former treatments with this material have also given good results. The appearance of the stone after this treatment didn't change much. The only problem turned out to be, that the firm has stopped the production of this product. We had to use a similar product, Wacker VB 1321. This product has a good penetration, surface residues can be removed easily and the strength and hydrofobic properties of the stone are good. Besides it was also possible to reach a good result with for consolidating the joints.

Analog tests have been carried out for different artificial masses (E-modul, porosity, pore size distribution, tensile strength, hygric and thermal dilatation, density and compressive strength has been determined too14). The limebounded masses were much too weak for the purposes and the tensile strength between stone and mortar was insufficient. As artificial masses we used a mixture with sand, cement in a high quality (Dykerhof, which has a very small amount of salts, less than most hydraulic limes commonly used in conservation) and pozzolana earth 5:1 :1/10 in acrylic dispersion 1 :10 in water. Because of the different appearances of the stones of the facade, 7 masses of different colours were used. In a more liquid solution with less binder added, this mass was also used for grouting injections. For the joint mortars we used slaked lime, sand and trassmehl (pozzolanic powder) 2:8:1. For the exposed joints we added some quality cement.

14Report from Mag. W. Strasser, Technische Priifanstalt, 1220 Wien, PolgarstraBe 30, artificial masses for the Untersberger marble, June 1994

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Cleaning

For the cleaning we tested five different methods: Micro particle sandblasting (ibb), rotation wet sandblasting (JOS), wet sandblasting (SAPI), ion exchanging treatment and water (fig. 10 and 11 ) : The best cleaning results we obtained with the JOS and the ibb systems. It was remarkable to see how these two systems are dependant of the operator. With the same cleaning parameters - pressure, powder and nozzle - we obtained very good and very bad results dispending upon the ability of operators.

The realization

1993 - 94 1994

1995

Laboratory tests and analyses. In situ cleaning, restoration and consolidation tests1s. Drawing of detailed plan of the facade (fig. 12a,b,c,d) (each block was hand measured 16) . Start of the restoration work.

Based on the investigations a three-year restoration program, starting at the north tower, was planned. With the scaffolding and the test areas, restoration firms and stone

. masonries had the possibility to give their offer for the restoration work. An architect and a well qualified stone restorer 17 have to lead and control! the work, as well as train the workmen in restoration and documentation.

Until now (first part) the work has been done perfectly by all the professionals involved. Time scedules, budget, quality and quantity of the work are kept. New problems that appeared during the work have been during supervision from the Bundesdenkmalamt experts.

The second part of the facade is running at the moment. The work steps are as follows:

1. Documentation of conditions 2. Securing of loose parts. 3. Preconsolidation near the joints. 4. Removal and desinfection of biological growth. 5. Removal of all old mortart until a depth of 1 O cm. 6. Salt extractions by compresses. 7. Changing of corroded iron armaments to carbon and stainless steel armaments. 8. Removal of anti pigeon pastes. 9. Glueing cracks with elastical epoxy resin. 10. Cleaning with JOS system. 11. Fillings and grouting injections 12. Mortars (1 mm under surface level) and making small slides on the horizontal

surfaces in order to make the water run off. 13. Consolidation and hydrophobistion. It was necessary to treat the whole surface in a

homogenious way and for a uniform appearance.

Maintenance

A program for maintenance will be setup in order to minimize future conservation interventions. With car hoist and other modern techniques it is now easier to maintain building facades.

15carried out by: G: Zowa, N. Vujasin, J. Nigisch, J. Nimmrichter 16n ... T ........ u ..... _ .............. A _ _ .: ..... 1'.T: ...... .-1 ..... - 1 .. : _ ,.... i.. ,...._ •• • a,, ,.. ~ "" th~ 'lt. T"f"'h ;t~rt nf thP u.1Arl>-ln o ~ltP

Fig 1 The north side of the north tower: The bright surface is over carved with a push hammer.

Fig. 3 Detail from the north tower: Push hammered surface and

joinst filled with earth.

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Fig. 2 Detail from the north tower: The dashed lines show where the original stone reached and prevented water entrance

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Fig. 4 Detail from the north facade: Fig. 5 Detail from the west facade: Black crusts and scale formations Stone decay near old mortars.

Fig. 6a/b SEM photographs: Parallel microfractures due to the use of push hammer and

gypsumformations

-(.) Cl> en -E ........ -·-Cl> -~ O> ·-"C c ·-== .c 0

"' Cl> CJ I

(/) :::>

-0 Cl> en -E ........ -·-Cl> -~ C> -"C c: ·-== .c 0 (/) Cl> CJ

I

(/)

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Data US, Dom Salzburg/unbehandelt 8000

0

7000

6000

5000

4000 0

0" 5

0

10 15

Geslms

Probe

Vol.A

20 25

Gesteinsdicke (cm)

0

0

. 0

Vol.B

Vol.C

30 35

Data US, Dom Salzburg /behandelt 8000

7000 - • • • I-··~ . •

6000 Tl • • __, I-5000

4000

i -

Sims _11.beh. t-i ..... T Kette re.bah. - t • Block E4 beh . I-• Block E2 beh • • Vol.C beh • :::> 3000 0 5 10 15 20 25 30 35

Gesteinsdicke (cm)

Fig. 7a/b Ultrasonic measurements at the north tower a) before treatment. b) After treatment. In relation to healthy Untersberger marble.

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Fig. 8 SEM photograph. Stone surface after treatment with acrylic resin. Showes perfect penetration.

Fig. 9 SEM photograph: Stone surface after treatment with a mixture between acrylic resin and silicon resin.

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Fig. 10 North tower front. Test areas, autumn 1994.

Fig_ 11 Kapitel detail from the north facade. After cleaning with ibb system.

D Condition

Treatet surfaces Black krusts Lichens Moss Higher plants Weatherd stone with clay Microcracks and scales Missing parts Anti pigeon paste

Fig. 12 Documentation a) Conditions ---'---

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Fig. 12 Documentation b) Black crusts of the east side of the upper south tower

Fig. 12 Documentation d) Consolidation tests.

Preconsolidation with Paraloid 872 Paraloid 872 Paraloid B 72 +Wacker 190 F, 1.: 1 Paraloid B 72 + Wacker OH . 1 : 1 Wacker 190 F +Wacker OH, 1 : 1 Wacker H Removai of ali old mortar