Thin-section microscopy of decayed crystalline marble from the garden sculptures of Schoenbrunn Palace in Vienna

58 (2007) 1042–1051
Materials Characterization
Thin-section microscopy of decayed crystalline marble from thegarden sculptures of Schoenbrunn Palace in Vienna

J. Weber a,⁎, S. Beseler b, K. Sterflinger c

a Institute of Art and Technology, Conservation Sciences. University of Applied Arts Vienna. A-1013 Wien, Salzgries 14/1, Austriab Institute of Conservation and Restoration. University of Applied Arts Vienna A-1013 Wien, Salzgries 14/4, Austria

c Institute for Applied Microbiology, Department of Biotechnology. University of Natural Resources and Applied Life Sciences,Vienna A-1190 Wien, Muthgasse 18, Austria

Received 21 February 2007; accepted 13 April 2007

Abstract

Sterzing marble, a crystalline white marble used in the late-Baroque garden sculptures of Schoenbrunn Palace in Vienna, wasstudied by means of thin-section and scanning electron microscopy in order to obtain a better understanding of its surface decaycaused by atmospheric weathering. Following the classification of distinct phenomena of deterioration by visual on-site inspection,the microstructural features including surface erosion, micro-cracking, soiling, black crust formation, and microbiologicalinfestation are exemplified by microscopical images and are briefly discussed.

The results proved useful for evaluating and understanding the various types of marble decay for creating a safer basis forestablishing the procedural principles aimed at conservation and maintenance of the sculptures.© 2007 Elsevier Inc. All rights reserved.

Keywords: Marble; Surface decay; Microscopy; Conservation

1. Introduction

The UNESCO World Heritage Site of Schoenbrunn,the former imperial summer palace of the Habsburgdynasty, has a large French garden with a number ofarchitectural and sculptural elements. The late-Baroquestatues – most of them placed along the hedges flankingthe central axis of the Great Parterre – depictmythological figures of Graeco–Roman Antiquity. Thesite was designed and the sculptures produced in the1770's by Johann Christian Wilhelm Beyer [1].

⁎ Corresponding author. Tel.: +43 1 71133 4825; fax: +43 1 711334829.

E-mail address: [email protected] (J. Weber).

1044-5803/$ - see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2007.04.014

All but one of the sculptures are made from acrystalline white marble called Sterzing marble after theplace in Tyrol where it was quarried. Over 200 years ofoutdoor exposure have left their traces; virtuallyovergrown by hedges for many years, the immediateenvironment of most of the statuary has obviouslycontributed to the bad state of preservation of marblesurfaces in many places.

Within the framework of a student project carried outby the Stone Conservation Department of the Universityof Applied Arts Vienna, 40 statues were examined anddocumented for the materials involved, the state ofpreservation and conservation measures needed. Theresults were put in to a database and serve as a basis fordecisions on the future maintenance and conservation.

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http://dx.doi.org/10.1016/j.matchar.2007.04.014

Fig. 1. Thin-section images of Sterzing marble, X N. Fig. 2. Close-up view of a marble surface eroded by partial dissolutionof calcite.

1043J. Weber et al. / Materials Characterization 58 (2007) 1042–1051

Given the limited range of possible scientificmethods capable of in-situ assessment of the deteriora-tion of sculptural marble in a qualitative and quantitativeway, it was decided to employ all kinds of availablemicroscopic techniques on a number of samples taken.Not only can general guidelines for conservation beestablished through such analyses, but also much insightinto causes and mechanisms of decay can be gained.

Ultrasound transmission measurements were used asan additional investigative procedure, but they will bereferred to only occasionally in the present paper.

2. The stone material — Sterzing marble

2.1. History of use

Sterzing marble was quarried near the town ofSterzing (Vip iteno) in South Tyrol. For a long periodthis was merely at a local level for construction andcarving, probably because of its large grain size. Theheyday of its use was before the end of the 18th century,when the palace gardens of both Vienna's Schoenbrunnand Munich's Nymphenburg palaces were embellishedwith sculptures, vases and fountains of Sterzing marble.In the 19th century, its use declined in favour of finermarbles such as those from Laas and Carrara, eventhough the Sterzing stone was still used for a number ofdecorative elements of late 19th century Ringstrassen

Table 1Petrophysical parameters of unweathered Sterzing marble [4]

Typical grain size 4–6 mmSpecific weight 2.71 g/cm3

Bulk density 2.70 g/cm3

Total porosity 0.20% (v/v)Specific surface area 0.05 m2/gWater absorption 0.34% (w/w)

buildings in Vienna. Today it is quarried only to produceadditives and fillers for various purposes.

2.2. Geologic and petrographical data

In geological terms, the marbles of Sterzing belong tothe same unit as, e.g., the well-known marbles of Laaswhich have found more widespread use than the formerin European stone carving. Their formation by meta-morphism at about 750 °C and 5 kb is ascribed to theVariscian Orogenesis [2]. Sterzing marble is a white,coarse-crystalline metamorphic marble almost entirelycomposed of calcite, CaCO3. Its polished surface re-veals a characteristic lacklustre to light-grey appearancecaused by the coarse, slightly translucent crystals ofirregular crystallographic orientation.

As the only main constituent of Sterzing marble,calcite forms a compact seriate texture, its crystal sizecan reach about 4–6 mm and even more. The irregularlyshaped grains are strongly interlocking along theirboundaries. Twinning and well developed cleavage arevery common features of the calcite crystals, Fig. 1.

Fig. 3. SEM–SE micrograph of eroded marble surface; calcitecleavage and grain boundaries are accentuated by solution.

Fig. 4. Thin-section image of a marble surface eroded by selectivechemical dissolution; X N.

Fig. 6. Thin-section image of a marble surface with total graindisintegration at the surface, leading to surface erosion by mechanicalmeans, // N.

1044 J. Weber et al. / Materials Characterization 58 (2007) 1042–1051

Sigificantly bent twin lamellae, undulating extinctionand the formation of sub-grains indicate a late-meta-morphic deformation of the rock [3].

Accessory minerals include occasional quartz, mica,actinolite, apatite, pyrite, etc. Some decisive parametersfor Sterzing marble are given in Table 1.

2.3. Weathering

Generally, Sterzing marbles appear to be moreresistant to weathering than fine- or medium-grainedtypes as, e.g., Carrara and Laas marbles [5]. Neverthe-less, due to the anisotropy of thermal dilatation of calcite[6], Sterzing marbles are also known to suffer stressesinduced by changes in temperature. The resulting strainscause increasing losses of cohesion linked to the for-mation of a pore system which is predominantly locatedalong the grain boundaries. A number of petrographicalparameters determine the extent to which such thermalstresses may act, namely the size of the single crystalsand the grade of their orientation. Thus, fine-grained

Fig. 5. Thin-section image of a serrated marble surface; dissolution atthe inter-section of cleavage planes; X N.

marbles without textural orientation are known to suffersignificant deformation [7].

The above mechanism opens the way for water toenter the subsurface zones of the marble, with all theposible consequences such as frost–thaw cycles, disso-lution of calcium carbonate, chemical reaction withmoisture and air pollutants resulting, e.g., in the crys-tallisation of gypsum, and microbiological growth. Thetotal porosity of a marble can thus increase to valuesabove 10 vol.% [8], although Ruedrich [3] measuredonly 0.50% as a typical value for weathered Sterzingmarble. Since all the above factors are connectedwith theaccess of liquid water which, for the unweathered bulk ofa marble object is very low anyhow, the resulting decayphenomena would be normally limited to the near-sur-face zones. The microclimate of a sculpture as well as theparticular exposure situation for each of its details isdecisive for the type and degree of deterioration that mayoccur. In-depth disintegration, a decay mechanism ob-served especially for Carrara marble statues, and which

Fig. 7. Close-up view of a detail with cracks probably caused byenhanced numbers of moistening–drying cycles.

Fig. 10. Thin-section image of a crack with small amounts of gypsum(undyed areas), // N.

Fig. 8. Thin-section image of marble from Fig. 7 with numerous inter-and intra-crystalline cracks, // N.


is at present still lacking satisfactory explanation, has notbeen reported for Sterzing marbles [5].

2.4. Conservational approach

Disintegrated marbles may need consolidation byimpregnation in order to prevent the surface from furtherlosses. The choice of appropriate products and methodsof application is based on the key questions of theabsorption capacity of the decayed marble portions andof the width of the gaps to bridge. This makes it neces-sary to study the frequency, nature and approximatediameters of cracks and pores in the marble structure,and the depth to which such failures occur.

3. Decay assessment and sampling on-site

Prior to any sampling, a thorough visual inspection ofall sculptures was undertaken, and decay categorieswere established and graphically documented. All maincategories were subdivided into several degrees of

Fig. 9. Thin-section image ofmicro-cracking ofmarble at the surface,XN.

intensity, resulting in a total of 27 classes. The graphicand photo documentation was filed in a data bank. Thefollowing classification of decay was established:

▪ Surface erosion in three steps, ranging fromincreased roughness to the loss of more than onelayer of crystals.

▪ Cracks in different forms and shapes, ranging fromsystems of irregular micro-cracks to much larger sub-parallel cracks, and cracks obviously caused by staticproblems or rusty iron clamps.

▪ Soiling in three steps, ranging from light patch-wisesoiling to black crusts.

▪ Lacunae connected to scale formation beneath.▪ Microbiological growth, including epi- and endolithiclichens, as well as fungi, cyanobacteria and algaelocated in fissures and cracks in the interior of thestone.

Sampling was aimed at studying all important decayphenomenamore in detail. Thus, sampleswere taken from

Fig. 11. Patch-wise surfaces soiling, frequent in semi-sheltered zones.

Fig. 14. Thin black crust on apparently smooth marble surface.Fig. 12. SEM–SEmicrograph of Fig. 11 showingmicro-crusts of gypsum.


places where the above described classes of deteriorationoccurred in a significant and representative way.

4. Methods of analysis

The samples taken were first studied under a stereo-microscope and eventually split into several pieces.

Thin-sections were then produced from a number ofrepresentative samples by vacuum-embedding them in

Fig. 13. a, b: Thin-section image of a marble with patch-wise soiling;thin gypsum crust interrupted by cavities due to selective dissolution ofcalcite, // N (a), X N (b).

epoxy resin to which a blue dyestuff was admixed. Thisprocedure, frequently applied in petrographic samplepreparation, allows the identification of all kinds ofpores by their colour. The sections were studied under apolarising microscope with transmitted light, at magni-fications up to 1000 times.

Remaining pieces were used to produce polishedsections; the procedure of embedding was the same asabove, but with the exception that no dye was used. Thesections were first studied under incident light, thencoated with carbon and analysed by a scanning electronmicroscope (SEM) at high vacuum employing back-scattered image (BSE) facilities and energy-dispersiveX-ray analysis (EDX).

In a parallel examination, selected sample pieces weresputtered with gold; their irregular surfaces were studiedby SEM using the secondary image (SE) detector.

5. Results

In general, it can be stated that the observations andanalyses by microscopical and electron microscopical

Fig. 15. SEM–BSE micrograph showing gypsum (dark) oncracked marble.

Fig. 16. Thin-section image of a thin gypsum crust on marble, // N. Fig. 18. Thick black crust on marble surface.


means confirmed the visual diagnoses of phenomenaand proved useful for understanding the mechanismsinvolved as well as to evaluate the extents of decay andthe conservation requirements.

5.1. Surface erosion

Two different forms of erosion have been identifiedby the scientific analyses, both occurring in specificsituations and following different mechanisms.

In rain-exposed areas, chemical dissolution of calciteproceeding from marble surfaces inwards appears to bethe most frequent mechanism. The process results inincreasingly rough surfaces, and microscopy clearlyshows that such dissolution occurs preferentially alonggrain boundaries and cleavage planes. The surface reliefappears strongly serrated, and microorganisms frequent-ly colonise the indentations. The phenomenon can beseen on the macro- and microscale in Figs. 2, 3, 4 and 5.

In areas receiving less moisture but more exposed tothe sun, mechanically-induced disintegration of thegrain fabric, probably due to thermal dilatation, prevails.This process leads to the loss of single crystals or groups

Fig. 17. Thin-section image, thin gypsum crust on etched marblesurface, // N.

of several grains along fissures which are easily visibleunder the microscope (Fig. 6). The resulting surfaceappears less serrated but comparably higher rates ofmaterial loss occur.

5.2. Cracks and fissures

Out of the different types of micro-cracks formed inthe marble as a result of several reasons, only thoseobviously connected to atmospheric weathering werestudied in the laboratory. They usually appear in the formof systems composed of numerous cracks of irregularshape and short length. Typical zones where such cracksystems would appear are thin sculptural details in placeswell exposed to run-off water (Fig. 7). Such areas aresubject to an increased number of moistening–dryingcycles. Similar to the other decay phenomena, cracks runpreferentially along grain boundaries or through thecleavage of single crystals (Fig. 8).

In the interior of such cracks, limited amounts ofgypsum can be found (Fig. 9). Their width can exceedseveral hundreds of microns (Fig. 10), and they canreach to a depth of up to 1 cm or more, in which case the

Fig. 19. SEM–SE micrograph showing crust with cauliflower-like surface.

Fig. 20. SEM–SE micrograph showing tabular gypsum crystals withinthe crust (detail of Fig. 19).

Fig. 22. a, b: Thin-section image of a calcite crystal partly replaced bygypsum (pseudomorphosis), // N (a), X N (b).


loss of significant amounts of material is just a matter oftime. Cracks of that kind are therefore considered as aspecial case of surface erosion. The marble consolida-tion in affected zones is a difficult task since a range ofproducts must be selected capable of penetrating andstrengthening a wide range of crack widths.

5.3. Soiling

For all degrees of soiling, gypsum was revealed to bethe major constituent of dirt layers or crusts. The habit ofgypsum with in those layers can vary from tiny prisms tolarge tabular crystals.

A type of soiling characteristic for semi-shelteredzones consists in a patch-wise darkening of the marble(Fig. 11). Light microscopy and SEM show that the darkspots are formed by small-scaled crusts sitting on top ofthe calcite crystals (Figs. 12 and 13); due to local dis-solution along the crystal boundaries, micro-erosiondown to a depth of approx. 0.5 mm occurs in a way thata bright network within a darkened area is visible. Typ-ically, no subsurface decay can be detected in suchplaces.

Fig. 21. SEM–SE micrograph showing layered internal structure of thegypsum crust.

Another type of discolouration, frequent in well-protected places, is characterised by uniform, thin layersof grey to black colour (Fig. 14).Microscopically (Figs. 15,16 and 17), such layers are revealed to be compact thoughmany times very thin, e.g., in the range of 50 μm. Theytend to be well-adhered to the surface which, however,frequently shows evidences of dissolution at low rates(Fig. 17). It thus seems likely that water accumulating insmall amounts with in or beneath such thin crusts has a

Fig. 23. Thin-section image showing a crack parallel to the surfacewith algal and cyanobacterial colonisation, // N.

Fig. 24. Thin-section image (detail of Fig. 23); photosynthetic cells in asub-surface crack, // N.

Fig. 26. Thin-section image showing fungi cells on the marblesurface, // N.


limited capacity to dissolve the stone material without,however, completely dissolving the gypsum in the over-lying layer.

Thick black crusts of irregular shape are just anextreme case of the above, forming in protected areaswhere deposited material was leached out from neigh-bouring places (Fig. 18). Since such crusts are brittle andtend to detach from the marble, no continuous samplecontaining both marble and the crust could be obtained.Under the microscope, these crusts reveal their cauliflow-er-like surface appearance (Fig. 19) which is frequentlyfound with many types of stone and is thus not dependenton the specific grain fabric of the marble. The internalstructure of the crusts is characterised by tabular gypsumcrystals resembling a house of cards (Fig. 20); a sequenceof more or less compact layers within such crusts can befrequently observed, obviously a consequence of complexprocesses of dissolution and precipitation (Fig. 21).

5.4. Replacement of calcite by gypsum

A phenomenon not recognisable by visual inspectionwith the naked eye is the partial replacement of surface

Fig. 25. Microscopical image of a eroded marble surface with fungi.

calcites by limpid gypsum, Fig. 22. It can be best ob-served on thin-sections and is characterised by aggre-gates of extremely tiny gypsum crystals of no distinctorientation, so that with crossed polarisers no birefrin-gence can be observed even at high magnifications.Such a pseudomorphic growth of gypsum in marblecalcite, which has been previously reported by Vergès-Belmin et al. [8], indicates slow and steady conditions ofchemical reaction by the action of sulphurous airpollutants.

5.5. Microbiological growth

Naturally, microscopical analysis of microbiologicalgrowth does not allow the identification of genera andspecies; however, it is an important tool to analyse theoccurrence and spatial distribution of different organismgroups in the interior of a material.

In the case of the marble investigated, crustose li-chens cover large areas of the surfaces, whereas folioselichens occur only in particular spots. Apothecia oflichens are clearly visible, they are preferentially located

Fig. 27. SEM–SEmicrograph showing a crack colonised by fungal cells.

Fig. 28. SEM–SE micrograph (detail of Fig. 27).


in small cavities along the crystal boundaries of themarble.

Using thin-section microscopy, on the other hand,clusters of darkly pigmented fungal cells and aggregatesof green photosynthetic organisms can be found incracks of disintegrated subsurface zones of the marblesamples (Figs. 23 and 24). Although these groups oforganisms are not in a close morphological association,a physiological interdependence as endolithic lichencommunity cannot be excluded. Approximately onecrystal layer beneath the surface, algae and cyanobac-teria find excellent environments since they areprotected from the direct impact of the atmosphere,yet receive enough sunlight thanks to the translucentproper ties of the calcite crystals. Such algal andespecially cyanobacterial cells undergo cycles ofvarying humidity in their environment, passivelyswelling and shrinking with changing moisture and,hence, exert pressures on the pore walls. In this way, thecracks induced by various mechanisms mentionedabove are enlarged by biogene mechanisms.

Melanised colonies of micro-fungi have been foundin many of the samples. Under the microscope, their cellaggregates could be localised preferentially in tiny cav-ities and cracks of the surface but never in the depth ofthe marble (Figs. 25, 26, 27 and 28). The brownish spotsformed by these cells can be easily recognised with thenaked eye; they are especially frequent in chemically-eroded or locally-discoloured areas of the sculptures(Figs. 25 and 26).

In this context, it is worthwhile mentioning that mostof the sculptures were, to a large extent, surrounded byover grown plants of the garden architecture, whichhave been removed recently. This caused a certainmicroclimate favouring UV-protection and wind pro-tection (i.e., it retarded drying), and nutrient in flux fromthe plant material. These conditions must be regarded as

the most important reason for the strong micro-biological development on the sculptures.

6. Conclusions

Microscopical analysis proved a very useful tool toassess the surface decay of sculptural marble, especiallyif thin-section techniques are combined with SEM. Verycareful on-site visual inspection and documentation is aprerequisite, however, in order to ascribe micro-scaledfeatures to macro-scaled phenomena of decay.

Based on the results, a much better understanding ofthe most important types of weathering of the Sterzingmarble sculptures could be achieved. In particular, it canbe stated that:

– Weathering of Sterzing marble acts from the surfaceof the sculptures, i.e., any kind of stone decay appearsat or close to the surface; no in-depth disintegrationwas found, and ultrasound measurements on thesculptures confirmed this. This in turn allowed eval-uation of the state of preservation of all objects byvisual means, a procedure which resulted in a relativequantification of decay categories for each sculpture.

– In the above procedure it was found that, out of atotal of 40 sculptures, 11 need conservation relativelysoon, while 15 should be treated with in the next fiveyears, and 14 sculptures can be left untreated at leastfor the next decade.

– The detailed study of microstructural failures allowsthe definition not only of the need for marble consol-idation, but also the properties of the consolidant.The most relevant parameters assessed in this contextwere the typical widths of fissures and cracks and thedepths of granular disintegration.

– The pseudomorphic conversion of calcite to gypsumhasproved a widespread phenomenon and must thereforebe considered when water-based cleaning is performed.

– Microbiological growth was found to have penetratedinto the sub-surface zones of the marble fabric. It wasthus found necessary to envisage a biocide impreg-nation treatment following surface cleaning, even ifone has to keep in mind that no such treatment wouldprevent mid-term re-colonisation of the marble.

Acknowledgements

This study was performed within a larger projectcarried out in collaboration with the students of theStone Conservation Course of the University of AppliedArts, Vienna, by order of the Schloss SchoenbrunnKultur- und Betriebs-GesmbH.


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Thin-section microscopy of decayed crystalline marble from the garden sculptures of Schoenbrunn Palace in Vienna