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WEATHERING AND CONSERVATION OF APSARA RELIEFS AT ANGKOR VAT, CAMBODIA -

PROJECT ANGKOR

LEISEN, HANS Fachhochschule Kain, Fachbereich Restaurierung und Konservierung von Kunst- und Kulturgut, KOln, Germany

VON PLEHWE-LEISEN, ESTHER

untersuchungslabor tor Fragen der Natursteinerhaltung, Koln, Germany

SATILER, LUDWIG Labor filr Erforschung und Begutachtung umweltbedingter GebaudescMden, Miinchen, Germany

SUMMARY

The temples of Angkor (Cambodia) and their reliefs belong to the world 's most outstanding cultural

monuments. These reliefs are subject to intensive degradation. The Project Angkor combines

scientific research with the transfer oft the results into practical conservation at site and the training of

local conservators. The paper deals with the preparative investigations into the material parameters,

the weathering behaviour and damage situation at site and their relevance for a conservation

operation. A synopsis of all informations yielded allows the compilation of a funded conservation plan

and is the base of tests with conservation agents. It can be shown that the degradation forms and

grade of the individual reliefs change extremely, thus asking for a flexible conservation system, that

may be combined individually to compete the varying demands.

1. INTRODUCTION

The temples of Angkor (Cambodia), UNESCO World Heritage Site since 1992, belong to the world 's

most outstanding cultural monuments. Among the over a hundred temple sites, Angkor Vat is the

most prominent example of Khmer architecture, built in the 12th century by god king Suryavarman II

(Fig. 1 & 2).

Fig. 1: The Temple of Angkor Vat from west.

ANGKOR VAT 1st, 2nd & 3rd enclosure N

s

I E j

I

I I

I

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Fig. 2: The Temple of Angkor Vat has been erected as a .temple-mountain", a symbolic imitation of the

mountain .Meru" from the Hindu mythology . The sanctuary is surrounded by four enclosures (mountain ranges) and a moat (protosea). Plan modified after [1 ].

Angkor Vat is superior by its multitude of art-historically important bas relief carvings showing mythological scenes, divinities (Devatas) or celestial dancers (Apsaras). The freely exposed reliefs are intensively threatened because of degradation by contour scaling and desquamation. The Project Angkor of the Fachhochschule Koln aims at the conservation of the most endangered Apsara reliefs at Angkor Vat temple. It includes scientific investigations into the material properties and the damaging factors as preparation for a conservation measure as well as a training of local conservators from the Angkor Conservation Office (ACO). The program combines scientific and applied research with practical execution and training. The investigations presented deal with the first phase of the project e.g. the scientific preparation of the conservation measure: description of and investigations into damages, estimation of degradation velocity, environmental climatic measurements as well as a detailed portrayal of stone parameters of the sandstone varieties used. Thus, they provide the basis for the preservation conception and conservation tests.

2. APSARA RELIEFS

The outer and inner surfaces of the Angkor Vat temple are covered by more than thousand Devata and Apsara reliefs [2]. The devinities and celestial dancers decorate the walls in frontal presentation either isolated or in groups (Fig. 3) .

Fig. 3: Apsara reliefs from the Cruciform Gallery of Angkor Vat temple. The reliefs are in a very good condition because they are not exposed to rain and sun.

Building material is a . grey-greenish sandstone quarried from the Kulen Hills some 40 kilometres to the north-east of Angkor and transported by ship down the Siem Reap River. The reliefs were carved as bas reliefs in situ directly into the smoothed ashlars of the masonry.

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3. CONSERVATION HISTORY

After the discovery of the ruins of Angkor by H. MAHOUT in 1860, the Ecole Fran<;aise d' Extreme-Orient (EFEO) established the Angkor Conservation Office in 1908 and started restoration operations mainly applying anastylosis. During the sixties detailed studies were carried out by HYVERT [3] and FUSEY [4]. They installed several testing areas for cleaning and conservation treatments all over the Angkor Vat temple. Between 1986 and 1993 the Archeological Survey of India performed structural stabilization, stone cleaning by use of water, ammonia solution and non-ionic detergents as well as impregnation with biocidal agents and PMMA for strengthening [5].

4. RESULTS

Investigations at site yielded information on the weathering state of the figures as well as their climatic environment, while laboratory work provided knowledge on material properties and behaviour. Combining these results, degradation factors and risk potential are detectable and a scientifically based conservation plan can be formulated. 4.1. Investigations into preservation state and environmental conditions of the reliefs

4.1.1. Documentation of weathering state

The observed damages are manifold. Besides extreme static problems that may even lead to complete breakdown of parts of the temple, there exists a multitude of damages to the decorative stone material itself. The decay mechanisms are exceedingly active and lead to a quick destruction of the artificially worked and decorated temple walls.

11~ ANGKORVAT

I \ I ( I~

LEGEND

Fig. 4: Mapping of weathering forms on a selected area, 2nd enclosure east. The mapping area is freely exposed to weathering.

Some of the reliefs are badly damaged. A very prominent type of deterioration is contour scaling. Thus, total parts of the precious surfaces are de­stroyed. Additionally there exists a great variety of further decay phenomena, such as spalling, flaking, cleaving, sanding, alveolation, splitting, salt efflorescences and iron crusts as well as biological growth and microbiological contamination (Fig.4).

Photomonitoring (Fig. 5) illustrates very clearly the dynamics of weathering. The photograph on the left taken around 1963 by LEBON-NGUYEN VAN [6] still shows two nearly complete Apsaras while on the picture at the right from 1995 the two figures are

almost vanished.

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Fig. 5: Preservation state of Apsara reliefs, 2nd enclosure, north-east tower. Left: photograph taken in the

sixties, right: photograph taken in 1995.

4.1.2 Investigations at site into weathering state

In-field measurements are applied directly at the reliefs and their surrounding. Thus, only non-destructive or at least little affecting methods may be used. Information on variations of water transport behaviour of a material caused by weathering processes or former conservation treatments is easily received by means of Karsten tube measurements [7] and is of big importance for further impregnation. By this method densified outer zones and areas with still effective hydrophobation are detected just as well as deconsolidated areas with a higher absorbency and frequently surface parallel water propagation. The existence of contour scaling is not always visually detectable. Areas with an intact outer appearence already may have formed scales in various stages. These scales, if not treated adequately, impede impregnation measures to a high degree and may even cause additional endangering by conservation. The drilling resistance method provides knowledge on presence and extention of scales with only very little harm to the object [8]. Non-destructive ultra sonic pulse measurements lead to a better understanding of structural degradation or consolidation caused by weathering and conservation [9].

Measurements of capillary water uptake by means of the Karsten tube at Angkor Vat on a multitude of Apsara reliefs and their surrounding confirmed the presence of scales in different stages of development (Fig. 6). Crusts (1) led to a significant reduction of capillarity , while brittle zones under lost scales (2,4) showed w-values around 2.5 kg/m2 --./h . Water uptake droped again by renewed densification of the new surface (3).

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ANGKOR VAT Capillary Water Absorption by Karsten Tube Measurement

Fig. 6: Measurement of capillary water absorption by Karsten tube method. The differences in water uptake are caused by alteration processes due to weathering.

drilling resist. (-)

ANGKOR VAT - APSARA PROJECT 2nd enclosure NE (E)

sandstone decay types

10 - - - - - - ._ - - - - - -(~--~-~ - - - - - - - - -

. . . . . . 5 - - - - - - ._ - - { - - - ....---~

cont.scale 1 in contact (1)

1 cont.scale I lost (3)

I •••-' - - • - • -..... . -,. : flaking spalling (2.4) . .

o'--~~~--~~~~~~~~~~~ ..... 0 10

FH KOln/Hl 3/96

20 depth (mm)

30 40

Fig. 7: Diagram of drilling resistance measure­ments. Area near to and comparable with the situation of Fig. 6.

Drilling resistance measurements provided additional infonnation on corresponding situations of the same relief (Fig. 7). Measuring point 1 lies on a scale still in contact with the stone interior and shows a consoli­dated outer zone up to 5 mm depth followed by a zone of reduced strength. The depth of the weathering zone is determined as 15 mm, where the values of unweathered material are reached. The strength of a brittle zones under lost scales (2,4) is characterized by low values at the surface and a slow stepped increase of drilling resistance, while renewed consolidation of the second surface under a lost scale is reflected by a higher strength in the uppennost 2 mm of point 3. Measurements on areas under lost scales reach values of unweathered material only in a depth of 25-27 mm, thus, reflecting the progressive weathering development.

Differences of structural degradation have been detected by ultrasonic pulse measurements (Fig. 8). The ultrasonic velocities are distinctly higher in the head and body of the Apsara figure than in the more delicate extremities, reflecting the better preservation state of massive parts. Equally a higher degree of dete­rioration in the lower parts of the figure can clearly be detected demonstrating the progressed weathering state of this aslar.

ANGKOR VAT Ultrasonic Pulse Measurement

2nd enclosure i NE

4.1.3 Soluble salts

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Fig. 8: Ultrasonic pulse measurements demonstra­ting variations of structural alteration.

Salts in the pore space of stones are able not only to destruct the stone structure by migrating and cristallization processes but also to impede conservation treatments. Isolated analysis of soluble salts have been carried out by AAS and ICP normally showing values of 500 µval/g for samples of the investigated Apsara reliefs. Brownish or white crusts that sometimes cover the stone surfaces, however, contain extremely high contents of soluble salts up to 3300 µval/g. These crusts have extraordinary high amounts of phosphate. most probably caused by bats. Besides, all samples contain calcium and sulfate and considerable contents of fluor.

4.1.4 Surface temperature measurements

Under tropical climate conditions the thermal behaviour may be a deciding risk factor for the decay of heat susceptible stones. With heavy rains, as they are common here, an overlapping and enhancement of thermal and hygric load is possible. The investigations into the influence of thermal stress on the building stones of Angkor Vat are not yet completed. The evaluation of exposure related damage mappings will help to estimate the importance of thermal load for the material.

The surface temperatures of fixed measuring points have been monitored in the course of a sunny February day by means of a thermal sensor. Fig. 9 shows the development of surface temperatures in .winter" related to the exposure direction. While stone surfaces with northern exposure reflect the air temperature and only heat up to around 30 °C, south and west exposed measuring points reached maximum temperatures up to 45 °C.

ANGKOR VAT Surface tern erature - 2nd enclosure 50

...... 45

~40 ci. 35 E 30 (I)

I- 25

06.00 09.00 12.00 15.00 18.00 21 .00

Time

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-air temp.

-E (MP2)

---S (MP4)

--- W (MPS)

····· N (MP6)

FH KOln!HL 5196 .. 'l<lima\960304.wl<4

Fig. 9: Development of surface

temperatures measured in different

exposition directions during a daily cycle .

The temperature increase is high in the morning for east exposed surfaces, in contrast, west directed

measuring points delineate a steep temperature drop in the evening. South directed surfaces show steep

gradients as well for heating up as for cooling. The highest temperatures were detected on black horizontal

surfaces. It is expected that they suffer most from heating up and shock cooling by rain. Furtheron, on

bended surfaces or edges conditions of thermal impact vary significantly within very short space thus creating a surface temperature gradient between closely neighbouring parts.

4.2 Characterization of stone materials

The grade of affection by the physically, chemically and biologically induced deterioration processes depends highly on the properties of the building stone. Thus the material parameters substantially influence

the weathering forms. Therefore, their detailed examination is absolute precondition for every scientific

conservation conception. Stone material for laboratory investigations consisted of fallen blocks without any decoration. Samples were

selected under the aspect of similarity to the materials of the reliefs.

4.2.1 Petrographic and structural properties

Building materials used are fine-grained layered sandstones (greywakes, arcoses) of Middle Jurassic -

Middle Cretaceous age in different varieties. Main minerals are quartz, feldpars, biotite, chlorite, muscovite,

iron minerals, sometimes calcite. Binding materials are clay minerals and iron hydroxides (Fig. 10). Both

minerals influence to a high degree the weathering behaviour of sandstones and are of high importance

concerning a conservation plan. The layering is caused by orientation of mica minerals. The detrital grains are subangular, reflecting a

littoral origin. Total porosity of the green variety is around 18 Vol.-% with maximum of pore radii distribution in 0.1-4 µm. Due to weathering the amount of pores > 4 µm enlarges. The less frequently used yellow-brown variety

shows very high porosities of more than 20 Vol.-% with increasing tendency for all pore radii classes during

degradation [1 OJ.

Fig. 10: Sandstone of Angkor Vat: feldspars

with clay mineral cement (SEM photograph)

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4.2.2 Water transport parameters

The measurement of capillary water uptake gives useful information for a conservation treatment by

impregnation. All samples showed a good capillarity ranging from 1 to 2.5 kg/m2-Vh (Fig. 11). These values correlate well with the Karsten measurements carried out at the temple walls. The total water absorption under atmospheric conditions is very similar for all samples and amounts to 5 - 6 m.-%. Concerning future conservation operations these results show, that, if not densified by crust or by hydrophobation, the stone has sufficient capillarity for an impregnation.

4.2.3 Hygric dilatation

Hygric dilatation is very important for the erosional behaviour of stone. Repeated swelling and shrinking is an extremely complex process that may lead to a structural alteration of the surface near zones. High dilatation of a stone may also involve a high risk caused by inconsiderate conservation treatments [11, 12]. During wetting all samples from Angkor Vat showed intensive swelling (Fig. 12). Maximum values are extremely high. The dilatation rates are higher for measurements vertical to the petrographic layering. This fact reflects the orientation of the clay minerals already obvious from the petrographic study.

Surfactants reducing hygric dilatation are used successfully since several years. Their influence on microbiological contamination, however, is still in discussion [12, 13].

Capillary Water Absorption ANGKOR VAT

Sample No.

Ip: vertical/parallel to layering ~ K61n/HL 5196

Fig. 11 : Capillary water absorption of sandstone samples from Angkor Vat

4.2.4 Water vapour diffusion coefficient

'E

Hygric Dilatation ANGKOR VAT

'E 4 .-------------------,

i~ I ll~H~Willmml~l~u~J >. J: 1v 2v

1p 2p 5v 14v 15v 17v

Sp 14p 15p 17p Sample No.

Ip: vertical/parallel to layerin ~ K61n/H L s,gfj

Fig. 12: Hygric dilatation of sandstone samples from Angkor Vat

The water vapour diffusion is very important during drying processes, when capillary water transport mechanisms are no longer in function. Conservation operations may not lead to a high increase in water vapour diffusion resistance coefficient (µ value).

The water vapour diffusion coefficient was tested by the wet cup method. The samples show low water vapour diffusion resistance between 20 to 25, thus having a good permeability for water vapour. Therefore it is not likely that a stone conservation treatment using Si organic consolidants or water repellents deteriorates considerably the drying behaviour.

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4.2.5 Mechanical properties

Determination of mechanical properties on sample material by ultrasonic pulse method and drilling resistance measurements confirmed the results of the field studies. Here again the weathered surface zones are characterized by a distinct loss of strength. In these areas stone consolidation treatments may lead to an egalization of the strength profile.

5. CONCLUSION

The Apsara reliefs are prominently damaged by contour scaling with total loss of the sculptured sur­face. Consolidation of the surface zone leads to a densified and hardened crust, that eventually deta­ches from the underground. Besides, structural degradation of the stone material occures preferen­tially at the more fragile parts of the figures. The contamination with soluble salts is extremely high in different types of crusts. The influence of thermal load of the material is still under investigation. The individual reliefs vary extremely in their degradation state and their weathering forms as well as in their alteration caused by former conservation measures.

The building material is a clay-rich sandstone with a high total porosity. Its water transport and mechanic properties let a successful conservation seem probable. Its hygric behaviour, however, demonstrates an extreme susceptibility to hygric dilatation, due to a considerable content of swelling

minerals. The results presented decide the further advance to the conservation operations. Necessary conservation measures will be consolidation, backfilling of scales, pointing and gluing and, with reservation as to further investigations reduction of hygric dilatation and desalination. Adapted materials for these conservation treatments have to be selected, tested and applied on testing areas. To compete the extremely variable conditions of the reliefs a flexible conservation system has to be developed with a stock of conservation steps, that may be combined as building block system in order

to garanty a specially adapted treatment for each relief.

6. ACKNOWLEDGEMENTS

The investigatons presented have been funded by the Ministery of Foreign Affairs of Germany, Cul­

tural Department and the Fachhochschule Koln.

7. REFERENCES

[1] STIERLIN, M. - Angkor; Architektur der Welt. Taschen Verlag, KOln, 1970.

[2] GITEAU, M. - Khmer - Kunst und Kultur von Angkor. Office du Livre, Fribourg, 1969.

[3] HYVERT, G. - Les alterations biogeochimiques des arkoses et gres des monuments Khmers.

These Universite de Paris; Fae. des Sciences; Paris, 1969.

[4] FUSEY, P. - Alteration Biologiques des Gres Cambodians et Recherche de Moyens de Pro­

tection.- Pub. Hors. Serie de L'Ecole Fran~aise d 'Extr~me-Orient, Paris, 1991 .

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[5] NARASIMHAIA, B. - Angkor Vat - India's contribution in Conservation 1986-1993, New Dehli

(ASI), 1994.

[6] DELVERT, J . - Recherches sur I' "erosion" des gres des monuments d'Angkor.- Bulletin de l'Ecole

Fran<;aise d' Extreme-Orient, Vol. 51, No. 1-2, Paris, 1963, pp. 453-534.

[7] WENDLER. E. & SNETHLAGE, R. - Der Wassereindringprufer nach Karsten - Anwendung und

Interpretation. Bautenschutz + Bausanierung, 12/1989, Koln. 1989, pp. [110-115).

[8] WENDLER, E .. SA TILER, L. - Bohrwiderstandsmessung als zerstorungsarmes Prufverfahren zur Bestimmung des Festigkeitsprofils in Gesteinen und Keramik. 4. Int. Koll. Werkstoffwissenschaften

und Bausanierung, 12, 1996, Technische Akademie Esslingen, in prep.

[9]KOHLER, W. - Structure changes of weathered Carrara Marble Sculptures as a function of their

destructioned position. ICOM 9th Triennal Meeting Dresden, Rome, 1990, pp. 330-335.

[1 OJ FITZNER, B. - Internal Report on Pore Space Parameters, 1996, 9 pp.

[11]WENDLER, E. , KLEMM, D.D. & SNETHLAGE, R: - Contour scaling on building facades -

. dependence on stone type and environmental conditions. MRS Symp. Proc. Vol. 185, San Francisco,

Materials Issues in Art and Archeology II Pittsburgh, 1991 , pp. 265 - 272.

[12) SNETHLAGE, R.,WENDLER, E. & KLEMM, D.D. - Tenside im Gesteinsschutz- bisherige

Resultate mit einem neuen Konzept zur Erhaltung von Denkmalern aus Naturstein. Denkmalpflege

und Naturwissenschaft: Natursteinkonservierung I, Ernst & Sohn, Berlin, 1995, pp. 127-146.

[13) PLEHWE-LEISf=N, E.,WENDLER, E., DAVID CASTELLO BRANCO, H. & DOS SANTOS, A.F. -

Climatic influences on longterm efficiency of conservation agents for stone - a German - Brazilian

oUtdoor exposure program - Proc. 8th Int. Congr. on Deterioration and conservation of stone, 1996, this volume.