# Assessment of sandstone deterioration at Ta Keo temple (Angkor): first results and future prospects

Marie-Françoise André, Samuel Etienne, Denis Mercier, Franck Vautier, Olivier Voldoire

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Marie-Françoise André, Samuel Etienne, Denis Mercier, Franck Vautier, Olivier Voldoire. Assessment of sandstone deterioration at Ta Keo temple (Angkor): first results and future prospects. Environmental Geology, 2008, 56, pp.677-688. 10.1007/s00254-008-1408-8. hal-00347801

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# Assessment of sandstone deterioration at Ta Keo temple (Angkor): first results and future prospects

Marie-Françoise André · Samuel Etienne ·

Denis Mercier · Franck Vautier · Olivier Voldoire

**Abstract** A first application of geomorphological methods to the assessment of sandstone deterioration at Angkor is presented. Damage diagnosis was carried out on the first eastern tier of the central pyramid of the 1,000 year-old Ta Keo temple. Methods combine field observations and measurements at 230 sampling points, high-resolution lasergrammetry and stereophotogrammetry on a 2-m² test zone, and SEM observations. The first results indicate that decay operates through a synergistic combination of weathering phenomena dominated by scaling and solution, and exhibits a high spatial variability. Percentages of deteriorated surfaces vary from 17.6 to 93.8%, and average stone recession values from 0.00 to 2.71 cm (minimum) and 0.34 to 5.49 cm (maximum). On the test-zone, stereophotogrammetry and 3D-mapping of the present and reconstructed initial states using lasergrammetry indicate that erosion scars up to 6 cm deep have formed since 1963. On the whole, the amount of deteriorated surfaces more than tripled between 1963 and 2008. The degree of implication of salts in stone decay remains unclear for most efflorescences are composed of calcite (CaCO3), with secondary

importance of barite (BaSO4) and gypsum (CaSO4·2H2O). Future prospects aim to evaluate the impact on stone decay of the clearing out of the temple from the forest in the 1920s.

**Keywords** Cambodia · Sandstone decay · Weathering rates · Photogrammetry · Lasergrammetry

## Introduction

In the last 25 years, geomorphologists involved in the study of rock weathering have shown a growing interest in stone deterioration (Pope et al. 2002). This shift from natural to cultural contexts was motivated by the interest of monuments as ‘natural laboratories’ to assess erosion rates. This geomorphological approach was applied to various monuments, including 5,000 year-old megaliths (e.g. Sellier 1997), 3,000 year-old rock carvings (e.g. Swantesson, 1992a, b), 2,000 year-old Roman temples (e.g. Paradise 1998), 800 year-old mediaeval churches (e.g. Robinson and Williams 1996; André et al. 2008; Vautier et al. 2008), and 150 year-old tombstones (e.g. Meierding 1981; Cooke et al. 1995; Inkpen 1999; Augeyre 2008).

The contribution of geomorphologists and biogeomorphologists to the multidisciplinary approach of stone deterioration and conservation is threefold: (a) it provides the conservators with data on long-term stone degradation in the real world, that is on the monument itself, (b) it proposes innovative methods and techniques to monitor stone decay and evaluate the positive versus negative influence of restoration measures, and (c) it demonstrates that weathering has not always had destructive effects but happens to protect the stone surface through case hardening and lithobiontic coatings (see implications on stone cleaning strategies).

<footnote>M. F. André (✉) · S. Etienne · F. Vautier · O. Voldoire
GEOLAB, Laboratory of Physical and Environmental
Geography, UMR 6042 CNRS Blaise Pascal University,
MSH 4 rue Ledru, 63057 Clermont Ferrand Cedex 1, France
e mail: m francoise.andre@univ bpclermont.fr</footnote>

<footnote>D. Mercier
GEOLITTOMER, UMR 6554 CNRS/Nantes University,
Chemin de la Censive du Tertre, BP 81227,
44312 Nantes Cedex 3, France</footnote>
A variety of methods have been used by geomorphologists to quantify the amount of stone deterioration having occurred since the building of monuments (see review in Pope et al. 2002). They include direct measurements of surface recession based on datum provided by carved or polished surfaces and tooling marks, ranking of inscription legibility, 2D and 3D mapping of decayed surfaces.

To date, this geomorphological approach of stone deterioration was not applied to the Angkor site, except for a pioneer attempt by Delvert (1963) to provide tentative rates of surface recession having occurred since the building of the temples (ninth to thirteenth centuries). It is for this reason that a research programme was launched in 2004, based on previous dating of monuments by the EFEO researchers (e.g. Jacques 1990), characterisation of sandstone material by geologists (Saurin 1952; Lille 1967; Uchida et al. 1999, 2003), and analyses of biological colonisation and microclimate conditions at the stone surface (e.g. Hyvert 1968; Fusey 1991; Waragai 2003).

This programme is being developed in close cooperation with the GACP team involved in the conservation of the Angkor Wat carvings (Leisen et al. 1996; Leisen and von Plehwe-Leisen 2005; Reucher et al. 2007; Warscheid 2006).

Following a preliminary survey of 17 monuments (André 2006), the 1,000 yr-old Ta Keo temple was selected for a detailed study. This aims to characterise weathering forms and quantify the sandstone decay at two timescales (since the building of the temple and since its clearing out from the forest in the early 1920 s). Quantification is based on direct measurements of historical decay from reference surfaces ('zero-datum levels'), high resolution laser scanning, and stereophotogrammetry.

The present article reports on the first results obtained on the sculpted surfaces of the first eastern tier of the central pyramid concerning: (1) the overall amount of stone deterioration and the associated weathering forms, (2) the controlling factors of damage intensity, and (3) the temporal frame and the responsible processes of stone deterioration.

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Location map (source: Jacques and Freeman, 2000)</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Fig. 2** Ta Keo temple mountain (a) and its ornamented sandstone wall affected by severe decay (b)</figcaption></figure>

## Study site and stone material

Ta Keo (Fig. 1, 2a) is one of the great temple-mountains at Angkor, which measures 122 × 106 m at its base and rises to a height of 42 m above the ground level. While the two enclosure walls of Ta Keo are made of laterite, the walls of the three-tiered central pyramid are entirely covered with sandstone stones, up to 0.80 m thick. This sandstone facing is sculpted on the eastern side of the temple and exhibits much more severe stone decay (Fig. 2b) than the gopuras and the upper towers, consisting of greywacke.

According to epigraphic sources and stylistic studies, the construction of Ta Keo has been achieved by the early 1,000s (de Coral-Rémusat et al. 1934). The temple was abandoned at the end of the thirteenth century, and remained buried below the tropical forest for six centuries. It was rediscovered in 1873 by Doudart de Lagrée, and until the early 1900s, trees and bush vegetation were colonising up to the upper platform (Dieulefils 1909). The clearing out of the temple was carried out by Henri Marchal and Charles Batteur in 1920-1923, and its architectural surveys were conducted in 1967 by Jacques Dumarçay (Dumarçay 1971).

The Ta Keo material used in the sculpted facings belongs to the main sandstone family used as building material at most of the Angkor temples, including Angkor Wat.

Its composition and properties have been described by Saurin (1952), Delvert (1963), and Lille (1967). This grey to yellowish fine-grained micaceous arkose contains numerous feldspars (plagioclase, orthoclase, microcline) as quartz, and exhibits often a preferred orientation of biotite flakes. Its cement mainly consists of calcite and clay minerals, and its porosity reaches up to 18.7%, that is, ten times higher than one of the green greywacke of the upper towers (Uchida et al. 1999). A peculiarity of the Ta Keo arkosic sandstone is the abundance of iron oxides, responsible for the yellowish to rusty colouration of this originally light grey sandstone. Opaque minerals are mainly interpreted by Saurin (1952) as the product of biotite alteration, with a chain of transformations leading from biotite to magnetite, haematite, and finally limonite. Recent analysis by Uchida et al. (1999) on the same type of sandstone indicate alterations from magnetite to goethite, and from biotite to chlorite and goethite. Alteration from feldspar to kaolinite has also been reported. These transformations are mainly related to alteration prior to quarrying (Saurin 1952), but they are enhanced by capillary action as demonstrated by magnetic susceptibility studies (Uchida et al. 1999).

## Methods

The wall of the first tier of the central pyramid was selected for a methodological test, for it exhibits large erosion scars of which evolution over time can be reconstructed based on iconographic documentation covering the 1909-2008 period. An extensive study was carried out along the 15 m-long wall, whereas lasergrammetry and photogrammetry were tested on a 2 m² area at the southern side of the wall (Fig. 3).

## Sampling design and data analysis

In order to propose a qualitative and quantitative diagnosis of stone deterioration, geomorphological observations and

<figure><img src="image_4.png" /><figcaption>**Fig. 3** Study site on the first eastern tier: investigation zone (black) and test zone for lasergrammetry and stereophotogrammetry (white). The wall is 15 m long and 6 m high. The test zone surface is 2 m²</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 4** Moulding of the investigated temple wall. Numbers 1 7 correspond to dimension stones. *Letters* indicate subdivisions of dimension stones according to sculpted levels. (source: Dumarçay 1971)</figcaption></figure>

<table><caption>**Table 1** Quantitative and qualitative data collected at 230 sampling points on the first eastern tier of Ta Keo temple</caption><thead><tr><th colspan="2">Quantitative data</th></tr></thead><tbody><tr><td>Minimum stone recession (cm)</td><td></td></tr><tr><td>Maximum stone recession (cm)</td><td></td></tr><tr><td>Stone thickness (cm)</td><td></td></tr><tr><td>Vertical distance to ground level (cm)</td><td></td></tr></tbody><thead><tr><th colspan="2">Qualitative data</th></tr><tr><th>Variables</th><th>Modalities</th></tr></thead><tbody><tr><td>Stone shape</td><td>Rectilinear<br>Convex<br>Concave</td></tr><tr><td>Stone type</td><td>Hewed stone (no moulding)<br>Sculpted stone (profiled moulding)<br>Ornamented stone (chiselled moulding)<br>Stone standing vertically on edge</td></tr><tr><td>Colouration (Munsell code)</td><td>Grey (10 YR 5/1)<br>Greenish grey (6/5 GY)<br>Yellowish (10 YR 6/3)<br>Rusty (2.5 YR 6/8)<br>Blackish braun (10 YR 4/1)<br>Black (10 YR 2/1)</td></tr><tr><td>Weathering forms</td><td>Multicoloured<br>Single flaking<br>Multiple flaking<br>Scaling<br>Blistering<br>Radial cracking<br>Horizontal cracking<br>Granular disintegration<br>Solution<br>Induration</td></tr><tr><td>Salt occurrence</td><td>Efflorescences<br>Crust</td></tr><tr><td>Vegetal colonisation</td><td>Black biofilm<br>Crustose lichens<br>Foliose lichens</td></tr><tr><td>Human impact</td><td>Graffiti</td></tr></tbody></table>

measurements were made at ten fixed points located every 1.50 m, along a transect following the sculpted wall. For accessibility and security reasons, the upper part could not be reached. The surveyed lower half of the wall, which is 6 m high, is composed of 7 dimension stones subdivided in 23 sculpted levels numbered from 1A to 7 (Fig. 4). At the 230 sampling points, quantitative and qualitative data were collected, that are listed in Table 1. Of special importance are: (1) the inventory of weathering forms (including <5 mm
flaking and >5 mm scaling), and the occurrence of salts and/or biofilms, and (2) the measurements of stone recession, made with a metal ruler (accuracy: 0.5 mm), based on zero-datum levels provided by the 1,000 yr-old tooling marks and sculpted surfaces. Besides this sampling, the length of the deteriorated zones was measured along the whole transect to calculate the decayed percentage for each of the 23 sculpted levels. A preliminary data analysis was carried out to provide an overview of the spatial variability of the erosion values and test some potential controls such as the stone thickness and the height above ground level. Student's *t*-test was used to test the significance of the differences observed between recession values, with and without salt, and biofilm using PAST statistics software package.

## Lasergrammetry

The KONICA MINOLTA VI 910 scanning laser was selected to obtain the very high resolution required for stone decay monitoring. It is a short range scanning laser (0.5 2.5 m) with a high accuracy (X axis: ±0.22 mm; Y axis: ±0.16 mm; Z axis: ±0.1 mm). The measurement is very fast (2 s for a 1 m² scan), but the scanning laser can only be used with a very low light intensity (<500 lux). Therefore, the surveyed zone must be covered by a shadowing canvas cover (Fig. 5). The high density point cloud (300,000 points for 1 m² scan) is visualized in the field with the Polygon Editing Tool software (PET), and post-processed with the Geomagic Studio software.

## Stereophotogrammetry

This method is used to reconstruct the scenario and calendar of damages since the clearing out of the temple from the forest, based on a collection of photographs covering the 1909 2008 period. Overlapping adjacent photos are

taken with a CANON EOS 400 D camera, which has a fixed focal distance and a calibrated lens. Control points are measured in XYZ with a PENTAX W-800 reflectorless total station. Photos and control points are post-processed in TOPCON PI3000 software to obtain 3D data (accuracy: 1 cm). Er Mapper software is used to rectify distortion of old pictures, which are imported in the MapInfo G.I.S. to calculate the eroded surface areas at each date.

## SEM observations

Observation of stone fragments has been performed under a scanning electronic microscope (JEOL JSM 5800LV). Samples were air-dried and coated with gold-palladium before observations under SEM. Element depletion was identified by energy-dispersive X-Ray microanalysis (EDS) operated by a PTS IMIX microprobe with a germanium detector at 15 kV.

## Results

### Weathering forms associated with stone decay

All weathering forms are represented in the decayed sections of the sculpted facing, except for iron induration (Fig. 6). Scaling, solution and flaking are the most frequent phenomena, whereas blistering, horizontal cracking and radial cracking are present only at places. Granular disintegration of sandstone occupies an intermediate position. Data analysis indicates that scaling and solution frequently coexist, suggesting a possible synergistic combination of processes and/or a temporal succession of processes. Salt efflorescences are widespread over the study area in association with weathering phenomena. Salts are especially abundant at sampling points where the sandstone facing is affected by blistering and radial cracking (Table 2). Over 90% of salt efflorescences and biofilms are not present at the same sampling points, whereas they coexist at less than 10% of sampled points.

<figure><img src="image_6.png" /><figcaption>**Fig. 5** Short range 3D laser scanner used at the study site</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 6** Representation of weathering forms at the study site</figcaption></figure>
<table><caption>**Table 2** Weathering forms and salt efflorescences: a widespread association</caption><thead><tr><th>Weathering forms</th><th>Salt efflorescence occurrence (%)</th></tr></thead><tbody><tr><td>Blistering</td><td>80</td></tr><tr><td>Radial cracking</td><td>61</td></tr><tr><td>Granular disintegration</td><td>56</td></tr><tr><td>Flaking</td><td>45</td></tr><tr><td>Horizontal cracking</td><td>44</td></tr><tr><td>Scaling</td><td>42</td></tr><tr><td>Solution</td><td>17</td></tr></tbody></table>

## Spatial variability of stone decay and controlling factors

Linear measurements of stone deterioration along the temple wall indicate a high variability, with decayed percentages ranging from 17.6 to 93.8% according to the sculpted levels (Table 3). Similarly, data analysis indicates that stone recession values measured at fixed points highly vary between levels and within a same level (Fig. 7). Ornamented levels 3F, 6A and 6D are the most severely deteriorated, with extreme recession values at 5.6 7.5 cm

(minimum) and 8.5 17.1 cm (maximum). On the whole, average recession values along the temple wall range from 0.00 to 2.71 cm (minimum) and from 0.34 to 5.49 cm (maximum). The height above ground level does not seem to significantly influence the intensity of stone decay (Fig. 8a, b), whereas the stone thickness appears as a major control, with the thinnest sculpted levels (<15 cm) being the most severely damaged (Fig. 8c,d). The role of salt efflorescences and biological colonisation was investigated based on the working hypothesis of a deteriorative action of salts versus a protective action of biofilms. Results indicate that stone recession values are up to 5 times higher at points where no biofilm occurs than at points where it does (Table 4) and student's t-test indicate that this difference is statistically significant (P = 0.001). The role played by salts remains unclear.

## 3D-mapping of stone decay

The 2-m² test zone selected for lasergrammetry and stereophotogrammetry can be seen in Fig. 9. It comprises eight sculpted levels, numbered 4E 7 (see above, Fig. 4). Of special interest is the main torus of the sculpted

<table><caption>**Table 3** Results of measurements of stone decay and recession on the first eastern tier of Ta Keo temple</caption><thead><tr><th rowspan="2">Level no</th><th rowspan="2">Stone thickness (cm)</th><th rowspan="2">Total length (m)</th><th colspan="2">Length affected by stone decay</th><th colspan="2">Average stone recession (cm)</th></tr><tr><th>m</th><th>%</th><th>Minimum</th><th>Maximum</th></tr></thead><tbody><tr><td>1a</td><td>4.00</td><td>15.00</td><td>3.00</td><td>20.00</td><td>0.10</td><td>0.40</td></tr><tr><td>1b</td><td>33.5</td><td>15.00</td><td>8.10</td><td>54.00</td><td>0.00</td><td>1.34</td></tr><tr><td>1c</td><td>3.50</td><td>15.00</td><td>2.75</td><td>18.33</td><td>0.00</td><td>0.28</td></tr><tr><td>2a</td><td>12.0</td><td>15.00</td><td>4.80</td><td>32.00</td><td>0.00</td><td>0.55</td></tr><tr><td>2b</td><td>30.0</td><td>15.00</td><td>11.75</td><td>77.33</td><td>0.13</td><td>1.96</td></tr><tr><td>3a</td><td>4.50</td><td>15.00</td><td>11.13</td><td>74.20</td><td>0.00</td><td>0.76</td></tr><tr><td>3b</td><td>10.0</td><td>15.00</td><td>13.22</td><td>88.13</td><td>0.00</td><td>2.46</td></tr><tr><td>3c</td><td>8.00</td><td>15.00</td><td>12.91</td><td>86.06</td><td>0.00</td><td>1.91</td></tr><tr><td>3d</td><td>10.0</td><td>15.00</td><td>10.44</td><td>69.60</td><td>0.02</td><td>1.26</td></tr><tr><td>3e</td><td>4.00</td><td>15.00</td><td>12.41</td><td>82.73</td><td>0.14</td><td>2.62</td></tr><tr><td>3f</td><td>11.0</td><td>15.00</td><td>14.07</td><td>93.80</td><td>2.33</td><td>3.57</td></tr><tr><td>4a</td><td>19.0</td><td>15.00</td><td>12.07</td><td>80.46</td><td>0.45</td><td>1.43</td></tr><tr><td>4b</td><td>6.50</td><td>15.00</td><td>11.08</td><td>73.86</td><td>0.00</td><td>1.10</td></tr><tr><td>4c</td><td>2.50</td><td>15.00</td><td>13.33</td><td>88.86</td><td>0.09</td><td>0.73</td></tr><tr><td>4d</td><td>6.00</td><td>15.00</td><td>13.36</td><td>89.06</td><td>0.05</td><td>1.13</td></tr><tr><td>4e</td><td>10.0</td><td>15.00</td><td>12.53</td><td>83.53</td><td>0.01</td><td>0.35</td></tr><tr><td>5a</td><td>12.0</td><td>15.00</td><td>11.07</td><td>73.80</td><td>0.83</td><td>2.30</td></tr><tr><td>5b</td><td>27.5</td><td>15.00</td><td>7.06</td><td>47.06</td><td>0.04</td><td>3.30</td></tr><tr><td>6a</td><td>13.0</td><td>14.60</td><td>6.66</td><td>45.61</td><td>2.71</td><td>5.49</td></tr><tr><td>6b</td><td>9.00</td><td>14.60</td><td>2.57</td><td>17.60</td><td>0.00</td><td>0.34</td></tr><tr><td>6c</td><td>6.00</td><td>14.60</td><td>12.07</td><td>82.67</td><td>2.06</td><td>2.66</td></tr><tr><td>6d</td><td>9.00</td><td>14.60</td><td>10.12</td><td>69.31</td><td>1.53</td><td>4.35</td></tr><tr><td>7</td><td>42.0</td><td>14.60</td><td>11.06</td><td>75.75</td><td>0.24</td><td>2.82</td></tr></tbody></table>

Stone thickness refers to the height of each sculpted level
<figure><img src="image_8.png" /><figcaption>**Fig. 7** Mean and standard deviation stone recession values of ten measurements per sculpted level. **a** Minimum values. **b** Maximum values</figcaption></figure>

facing, which exhibits one of the major erosion scars of the study site.

The DEM of the present deterioration state was obtained from ten overlapping laser scans and a 1-mm spaced point grid (Fig. 10a). The DEM of the initial state could be reconstructed based on scans from the nearby well preserved section of the wall displaying the same repetitive ornamentation patterns (Fig. 10b). Superposition of present and initial states made possible to produce a 3D erosion map quantifying the overall amount of decay (Fig. 11). While most of the test zone experienced rather minor recession (<1 cm), two sculpted levels appear severely damaged. The erosion scar of the upper torus corresponds to the detachment of a 3-cm thick sandstone scale, whereas stone recession reaches up to 6 cm on the thin ornamented level near the base of the test-zone. The sharp external limits of these two scars are controlled by the stone module. Last, a 2-cm thick losangic erosion scar can be seen, of which limits are controlled by the ornamentation pattern.

## Evolution of damages over the 1963 2008 period

The contemporary evolution of erosional damages in the test zone was roughly evaluated based on a preliminary diachronic analysis of two photographs. The

first photograph was taken in 2008, and this photograph and the associated control points were post-processed in TOPCON PI3000 software to obtain 3D data. The second photograph had been taken in 1963 by Guy and Jacqueline Nafilyan (EFEO). Once orthorectified with Er Mapper, this photograph was imported in MapInfo for comparative purposes. The resulting 2D-diachronic mapping of stone decay (Fig. 12) suggests that the amount of deteriorated surfaces more than tripled between 1963 and 2008 (from 8 to 26% of the test-zone). Interestingly, they also indicate that the two major scars of this 1,000 yr-old sculpted facing have formed very recently, that is during the second half of the twentieth century.

## SEM observations on biofilms and salts

Biological structures were analysed under SEM. The interface between sandstone and the atmosphere is occupied by a black lichenic biofilm, well developed in the lower part of the sculpted facing. At microscale, a synergistic modulation of the stone structure by biological activity and chemical denudation can be observed (Fig. 13a). Fungal spores and mycelium are coated with silica globules, indicating a greater chemical activity. Several dissolution figures are encountered, from small
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 8** Controls on stone decay. **a, b** Minimum and maximum stone recession versus height above ground level. **c, d** Minimum and maximum stone recession versus stone thickness</figcaption></figure>

<table><caption>**Table 4** Stone recession, vegetal colonisation and salt occurrence</caption><thead><tr><th rowspan="2">Average stone recession (cm)</th><th colspan="2">Black biofilm</th><th colspan="2">Salt efflorescences</th></tr><tr><th>Yes</th><th>No</th><th>Yes</th><th>No</th></tr></thead><tbody><tr><td>Minimum</td><td>0.1</td><td>0.5</td><td>0.4</td><td>0.6</td></tr><tr><td>SD</td><td>0.6</td><td>1.4</td><td>1.1</td><td>1.6</td></tr><tr><td>Maximum</td><td>1.2</td><td>2.2</td><td>2.3</td><td>2.1</td></tr><tr><td>SD</td><td>1.9</td><td>2.7</td><td>2.4</td><td>2.9</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>**Fig. 9** Test zone used for lasergrammetry and stereophotogrammetry (2 m²)</figcaption></figure>

bourgeons (less than 1 µm) to larger silica sheets (5 10 µm).

The white efflorescences found on the sculpted wall in association with weathering forms (see above) were submitted to X-Ray and microprobe analyses. Most of the samples were identified as calcite (CaCO₃) coatings referred to as “calcin” (Fig. 13b). Other salts were rarely found, such as barite (BaSO₄) or gypsum (CaSO₄·2H₂O), which are of secondary importance but appear intermixed with calcite (Fig. 14).

Discussion

Based on comparisons with weathering phenomena in natural contexts (e.g. Ollier 1984), it is tempting to ascribe the different weathering forms observed at Ta Keo temple

to distinct responsible weathering processes: radial and polygonal cracking to thermal weathering, blistering to salt weathering, de-lamination (i.e. horizontal cracking) to wetting and drying weathering. However, data analysis points to stone thickness and ornamentation patterns as
<figure><img src="image_14.png" /><figcaption>**Fig. 10** DEM of the test zone: present state (a) and reconstructed initial state (b)</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 11** 3D mapping of the decayed parts of the test zone. Erosion values are expressed in mm</figcaption></figure>

major controls on weathering forms. For example, the thin non-ornamented layers are mainly affected by de-lamination following the orientation of micaceous layers, whereas the thick sculpted torus exhibits pronounced scaling. Moreover, different weathering forms coexist spatially and/or operate successively. At many sculpted levels, solution

seems to have played a prominent role in stone decay initiation, providing an avenue for later scaling.

Due to the probable existence of synergistic combinations of weathering processes, discriminating the role of each process proves to be very difficult. Salts, which are known to be key actors of stone deterioration (e.g. Winkler 1994; Goudie and Viles 1997; Doehne 2002), have been designated by previous authors as a main cause of stone deterioration at Angkor (e.g. Delvert 1963; Fusey 1991). Based on bulk chemical composition and S (sulphur) and Sr (strontium) isotopic ratios, Uchida et al. (1999) and Hosono et al. (2006) detected high levels of SO3 (>15%) in deteriorated sandstones, and they identified a variety of salts: sulphates (gypsum, bassanite, barite), phosphates (brushite, newberyite, whitlockite, taranakite) and carbonates (calcite). The S (sulphur) and P (phosphorus) components of salts were found to be primarily derived from bat guano, whereas the Ca component was suggested to be derived from calcite cement of building sandstone (Hosono et al. 2006).

The present study confirms the predominance of calcite at Ta Keo temple and the local occurrence of gypsum and barite. However, two questions remain open. First, the origin of sulphurs is still debatable, for Saurin (1952)

<figure><img src="image_16.png" /><figcaption>**Fig. 12** Diachronic 2D mapping of the decayed zones showing the expansion of damages between 1963 (**left**), courtesy of Guy and Jacqueline Nafilyan) and 2008 (**right**)</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. 13** SEM micrographs of the black biofilm colonising the lower part of Ta Keo wall (a) and of the carbonate efflorescences occurring in the middle and upper parts (b). Ca Calcite coating; Q Quartz grain</figcaption></figure>

identified at Ta Keo a sulphur-rich blue subtype of the grey to yellowish sandstone. Second, the ability of calcite crystallization to provoke stone deterioration, as suggested by Uchida et al. (1999) and Hosono et al. (2006), should be further demonstrated. Due to the high porosity and the low compressive strength of the grey to yellowish Ta Keo

sandstone, dissolution, wetting and drying weathering (and possibly thermal weathering) might be the key drivers of stone deterioration, with or without salts. This is suggested by the fact that differences in stone deterioration values with and without salts are not statistically significant and that the two major weathering forms, scaling and solution, are the less commonly associated with salt efflorescences. In contrast, the fact that stone recession values are up to 5 times higher at points where no biofilm occurs than at points where it does is statistically significant. This finding is in accordance with the protective role played by mature lichenic biofilms on building sandstone as demonstrated at Angkor Wat by Warscheid (2006).

From a methodological point of view, the present study has demonstrated the complementarity of field geomorphological investigations and of more high-tech methods such as high resolution laser scanning. The accuracy of this tool is remarkable, though it must be kept in mind that the reconstructed “initial” state of sculpted surfaces is underestimated (by 1 2 mm?) due to the effects of dissolution having operated since AD 1000, even on the best preserved 1000 year-old reliefs. Another limit of the study is due to the small size of the data set (230 sampling points) and test zone (2 m²). Last but not least, the causal relationships between stone deterioration, salt efflorescences and vegetal colonisation should be considered with caution, for the measured stone recession is the product of an historical evolution, whereas the spatial distribution of salts and biofilms reflects the recent if not strictly present conditions.

## Conclusion

Preliminary damage diagnosis

Based on data analysis, laser scanning, stereophotogrammetry, 3D modelling and SEM observations, the main results of the present study at Ta Keo temple indicate:

<figure><img src="image_18.png" /><figcaption>**Fig. 14** XRD analysis of efflorescences. Barite (BaSO₄) and calcite (CaCO₃) are intermixed at the sandstone surface</figcaption></figure>
(1) The occurrence of a variety of weathering forms dominated by a synergistic combination of scaling and solution: The importance of solution is due to the calcitic nature of the sandstone cement, and the dimensions of scales are controlled by the stone module and ornamentation patterns.

(2) The high spatial variability of sandstone decay: percentages of deteriorated surfaces vary from 17.6 to 93.8% according to sculpted levels, and data analysis from 230 sampling points indicate average stone recession values from 0.00 to 2.71 cm (minimum) and 0.34 to 5.49 cm (maximum). Stone thickness (i.e. height of sculpted levels) is of crucial importance as a control on stone decay, with the highest stone recession values (over 17 cm) being obtained on the thinnest levels.

(3) On the 2-m² test zone, stereophotogrammetry and 3D-mapping of the present and reconstructed initial states using lasergrammetry indicate that erosion scars up to 6 cm deep have formed since 1963. On the whole, the amount of deteriorated surfaces of the test zone more than tripled between 1963 and 2008.

(4) The degree of implication of salts in stone decay remains unclear, for most efflorescences are composed of calcite (CaCO₃), with secondary importance of barite (BaSO₄) and gypsum (CaSO₄·2H₂O).

(5) The black biofilm covering the lowest part of the sculpted wall might play a protective role, as suggested by the statistically significant differences observed in stone recession values with and without biofilm.

## Future prospects

The ongoing Ta Keo project aims to evaluate the impact on stone decay of the clearing out of the temple from the forest in the 1920s. Qualitative and quantitative damage diagnosis will be extended to the whole three-tiered central pyramid, with special attention to the eastern and southern sculpted faces, using the same combination of methods. Once quantified the amount of deterioration, the objective will be to discriminate between pre- and post-clearing decay based on a collection of old photographs dating back as far as 1909. Reconstructing the scenario of contemporary stone deterioration should help to assess the influence of the climatic stress induced at sandstone surface by forest clearing. If a post-clearing acceleration of stone decay is demonstrated, this should be taken into account for the future management of the Angkor archaeological park, which is submitted to increasing touristic pressure (Winter 2007).

Another objective is to contribute to propose standardised methods of quantification and monitoring of stone

decay to be applied in a variety of monumental contexts for comparative purposes. The ongoing development of interdisciplinary research programs on stone weathering, conservation and restoration should help to meet this objective.

**Acknowledgments** The Ta Keo research project is funded by the CNRS (Paris) and the Blaise Pascal University (Clermont Ferrand). The authors express their gratitude to the APSARA National Authority, the International Coordinating Committee for the Safe guarding and Development of the Historic site of Angkor (permanent secretariat: UNESCO) and the *Ecole Française d'Extrême Orient*, which encouraged and facilitated this undertaking. They are also grateful to Dov Corenblit (GEOLAB) for his contribution to statistical analyses, to Thomas Warscheid (GACP) for comments on biological structures, and to Alain Barreau and Nicolas Stephan for technical assistance at the *Service de Microscopie SMEBMN* (Nantes University).

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