Open Access

Precipitation of manganese oxides on the surface of construction materials in the Khmer temples, Cambodia

Heritage Science20164:16

DOI: 10.1186/s40494-016-0086-1

Received: 17 December 2015

Accepted: 10 May 2016

Published: 27 June 2016

Abstract

Background

In addition to the blackening caused by blue-green algae growth, other black areas on the surface of construction materials (e.g., sandstone, laterite and bricks) are frequently observed in the Khmer temples in Cambodia. A non-destructive on-site investigation was carried out using a portable X-ray fluorescence analyzer (pXRF). In addition, samples were taken from the buildings and were analyzed using an X-ray diffractometer (XRD) and a scanning electron microscope with an energy dispersive X-ray spectrometer, and were observed using a field emission scanning electron microscope.

Results and conclusions

A non-destructive investigation using the pXRF revealed that this blackening was caused by manganese oxide precipitates. The precipitates contained small amounts of Ni, V, Zn, Y, K, Cl, S, Pb, and Cr. The XRD analysis indicated that the manganese oxides were mainly present as an amorphous phase, but some formed birnessite and todorokite. The manganese precipitates were mostly in a hexagonal plate form (100–300 nm), but some were in a rod-shape, which may have been caused by the activity of manganese oxidizing microbes. Preliminary experiments on removal of manganese oxide precipitates were conducted. The manganese oxide precipitates could be easily removed using a reducing agent such as an oxalic acid solution.

Keywords

Manganese oxide Manganese oxidizing microbe Birnessite Todorokite Khmer temple Cambodia

Background

Sandstone, laterite and bricks were the major construction materials used by the Khmer people in their temples, including the Angkor monuments in Cambodia during the ninth to fifteenth centuries [1, 2]. With the passage of time, the surfaces of these materials have blackened. Most of this discoloring is caused by blue-green algae (cyanobacteria), which gradually damage the bas-relief carvings on these temples [3]. In addition, black areas with more luster than the blue-green algae are frequently observed on the surface of the construction materials in the Khmer temples. Similar blackening has been also observed on the surface of Mesopotamian clay tablets, and analysis indicated that this was caused by manganese oxide precipitates [4]. This is similar to desert varnish, which contains manganese and iron oxides and is found on stone surfaces in the desert [57]. To date, no study has been conducted to investigate if the blackening on the Khmer temples is caused by manganese oxide precipitates. Although this black material does not seem to damage the construction materials, it has changed the color of the temples so they no longer appear as they did in their original condition. To prevent further blackening, this study aimed to determine if the blackening on the surface of construction materials in the Khmer temples is caused by precipitation of manganese oxides, and also to elucidate the precipitation mechanism.

A non-destructive on-site investigation was carried out using a portable X-ray fluorescence analyzer (pXRF). In addition, samples were taken from the buildings and analyzed using an X-ray diffractometer (XRD) and a scanning electron microscope with an energy-dispersive X-ray spectrometer (SEM-EDX), and examined using a field emission scanning electron microscope (FE-SEM).

Methods

Sample site

The investigation was carried out at the Angkor, Koh Ker and Sambor Prei Kuk monuments in Cambodia, which were selected as sites that were representative of the Khmer monuments (Fig. 1). The Angkor monuments are located 250 km northwest of Phnom Penh, the capital of Cambodia. They were constructed during the ninth to fifteenth centuries. The Koh Ker monuments are located 85 km northeast of the Angkor monuments. This site was the capital of the Khmer empire from 921 to 944 AD. The Sambor Prei Kuk monuments are located 140 km southeast of the Angkor monuments, and construction at this site occurred in the pre-Angkorian period. The Sambor Prei Kuk monuments were the capital in the seventh century.
Fig. 1

Map showing the locations of the Angkor, Koh Ker, and Sambor Prei Kuk monuments in Cambodia

Blackening, suspected to be from manganese oxide precipitates, is observed on the surface of all types of construction materials used in the Khmer temples, including sandstone (feldspathic sandstone and siliceous sandstone), laterite, and bricks. This discoloration is extensive on the surface of laterite at the Koh Ker monuments, especially on the Neang Khmau (Fig. 2a), Chrap (Fig. 2b), and Banteay Pir Chan (Fig. 2c, d) temples. Blackening is also evident on the surfaces of bricks and sandstone used in the southern brick towers situated on the eastern side of the Pre Rup temple at the Angkor monuments (Fig. 2e, f). Also at the Angkor monuments, there is blackening on the east side of the north face of the inner gallery of the Bayon temple (Fig. 2g). At the Sambor Prei Kuk monuments, there are black areas on the walls of the towers C1 (Fig. 2h) and S1 (Fig. 2i). Black areas are observed on both the inner and outer surfaces of the buildings (Fig. 2d, f). In addition to these sites, blackening is frequently observed at other Khmer architectural sites.
Fig. 2

Photographs showing blackening suspected to be from manganese oxide precipitates on the surfaces of construction materials in the Khmer temples at the following sites: a Neang Khmau, b Chrap, c Banteay Pir Chan, d Banteay Pir Chan (inner wall), e Pre Rup, f Pre Rup (inner wall), g Bayon (inner gallery), h the tower C1 at the Sambor Prei Kuk monuments, and i the tower S1 at the Sambor Prei Kuk monuments

Analytical methods

The black materials, which were thought to be manganese oxide precipitates, were analyzed non-destructively on-site using a pXRF analyzer (Delta Premium, Innov-X Systems, Waltham, MA, USA) with an X-ray tube with a Rh target (4 W). The analysis was conducted in a soil mode for 60 s at each point. Before analysis, the pXRF analyzer was calibrated for Mn, Ni, V, Zn, K, Pb, Cr, Sr, Cu, Ti, Rb, Ca, Fe and Zr using 10 rock standards obtained from the Geological Survey of Japan [8]. However the analysis for the black materials may be semi-quantitative because the black materials are as thin as less than 0.2 mm and X-ray penetrates into the under layer such as sandstone, laterite and bricks. Analyses were conducted on the blackened areas of the construction materials and also on non-blackened areas of the construction materials as controls. Five points were analyzed at each building, and the average values were calculated.

Construction materials covered with black materials were also taken from the buildings (sample nos. 3401–3414 in Table 1). The samples were taken from different places, which were analyzed using the pXRF analyzer. The black materials on the surface of the samples were removed using a drill. The samples of the black materials were analyzed using an XRD (RINT-RAPID, Rigaku, Tokyo, Japan) with an X-ray tube with a Cu target. The current and voltage were 30 mA and 40 kV, respectively. Each scan was conducted from 2θ = 2°–60° with a scan speed of 1°/min.
Table 1

List of the samples of black materials taken from the Khmer temples

Sample no.

Base material

Temple

Sampling point

3401

Sandstone

Bayon

East side of the north face of the inner gallery

3402

Laterite

Prasat Neang Khmau

Outer wall of the central tower

3403

Laterite

Prasat Chrap

Inner wall of the southern tower

3404

Laterite

Prasat Banteay Pir Chan

Inner wall of the central tower

3405

Laterite

East Mebon

Northwestern library

3406

Laterite

Pre Rup

Outer wall of the inner enclosure

3407

Sandstone

Pre Rup

Outer wall of the southernmost tower in the east towers

3408

Laterite

Ta Keo

Second platform

3409

Laterite

Phnom Krom

Outer wall of the enclosure

3410

Laterite

Wat Athvea

Inner wall of the enclosure

3411

Bricks

Sambor Prei Kuk

Outer wall of the tower S1

3412

Sandstone

Sambor Prei Kuk

Outer wall of the tower S1

3413

Bricks

Sambor Prei Kuk

Outer wall of the tower C1

3414

Sandstone

Sambor Prei Kuk

Outer wall of the tower C1

The black materials were also analyzed using a SEM-EDX (JSM6360, JEOL, Tokyo, Japan equipped with Inca Energy, Oxford Instruments, Abingdon, UK). The SEM-EDX analysis was conducted on a polished cross section of each sample. Samples for the SEM-EDX were first embedded in an epoxy resin (EpoFix, Struers, Ballerup, Denmark), and then polished with waterproof SiC paper (#180, #800, #1200, and #2400 μm, Struers) and diamond paste (3 μm, Struers). Finally, the samples were coated with carbon. The accelerating voltage was 15 kV, and the beam current was adjusted so that the X-ray count was 2000 count/s on the Co surface. The measurement time was 60 s.

In addition, the surface of each black material sample was examined using a FE-SEM (4500S, Hitachi, Tokyo, Japan). The accelerating voltage was 15 kV, and the sample surface was coated with platinum-palladium (Pt80Pd20).

Results and discussion

pXRF analysis

Because the pXRF analysis was conducted in the air and in a soil mode, light elements such as Na, Mg, Al and Si which are major components of rocks, could not be analyzed. The pXRF results for the average chemical compositions at each building are summarized in Table 2. The ratio of each element in the black area to that in the non-black area is shown in Fig. 3. This shows that Mn was highly concentrated in the black areas (22,700–247,000 mg/kg) compared to the non-black areas. In addition, Ni, V, Zn, Y, K, Cl, S, Pb, and Cr were also concentrated in the black areas compared with the non-black areas. The average V content range was 1000–10,800 mg/kg, except for the tower C1 (sandstone) at the Sambor Prei Kuk monuments. The Ni content was high at the Koh Ker monuments (1400–2800 mg/kg). The Zn and Cr contents were high at the tower S1 at the Sambor Prei Kuk monuments, with concentrations of 2400 mg/kg (Zn) and 6000 mg/kg (Cr) recorded on bricks and 850 mg/kg (Zn) and 3600 mg/kg (Cr) on sandstone. The Fe content in the black areas was similar to that in the non-black areas. The Mn content is generally less than 0.3 % in sandstone, laterite, and bricks [1, 2]. However, laterite at the Koh Ker monuments contains more Mn (0.3–2.4 %) [9]. This may be one of the reasons why precipitation of manganese oxides is remarkable on the surface of laterite at the Koh Ker monuments.
Table 2

pXRF results for the black materials on the surface of the construction materials used in the Khmer temples

Temple

Mn ± s.d. (mg/kg)

Ni ± s.d. (mg/kg)

V ± s.d. (mg/kg)

Zn ± s.d. (mg/kg)

Y ± s.d. (mg/kg)

K ± s.d. (mg/kg)

Cl ± s.d. (mg/kg)

Bayon

 Blackened area

51,500 ± 28,800

156 ± 52

1030 ± 551

454 ± 188

57 ± 24

16,600 ± 4500

260 ± 155

 Sandstone

297 ± 63

<6

378 ± 392

33 ± 9

16 ± 2

17,500 ± 2230

366 ± 181

 Ratio

173.4

2.7

13.7

3.5

0.9

0.7

East Mebon

 Blackened area

93,900 ± 21,400

560 ± 289

4570 ± 1090

111 ± 63

72 ± 134

7650 ± 2920

1840 ± 503

 Laterite

3185 ± 817

<18

3090 ± 1250

42 ± 49

43 ± 124

1910 ± 1700

613 ± 120

 Ratio

29.5

1.5

2.6

1.7

4.0

3.0

Pre Rup

 Blackened area

81,100 ± 53,500

522 ± 359

3820 ± 1520

92 ± 48

128 ± 141

8400 ± 7130

523 ± 103

 Laterite

2171 ± 2270

<26

1908 ± 738

42 ± 15

26 ± 11

6750 ± 7490

995 ± 476

 Ratio

37.4

2.0

2.2

5.0

1.2

0.5

 Blackened area

47,400 ± 12,600

85 ± 43

2530 ± 1750

41 ± 8

173 ± 142

10,200 ± 3890

3560 ± 1030

 Sandstone

474 ± 119

<8

276 ± 358

33 ± 8

29 ± 19

16,600 ± 2400

1030 ± 532

 Ratio

99.9

10.7

9.2

1.3

6.1

0.6

3.5

 Blackened area

137,000 ± 61,400

123 ± 42

2660 ± 1360

593 ± 278

475 ± 153

18,700 ± 7790

10,500 ± 3530

 Bricks

339 ± 102

17 ± 7

202 ± 126

15 ± 3

17 ± 8

1890 ± 174

1270 ± 883

 Ratio

404.0

7.3

13.2

40.1

28.3

9.9

8.3

Ta Keo

 Blackened area

93,900 ± 33,400

560 ± 238

4570 ± 2440

111 ± 36

72 ± 57

7650 ± 5130

1840 ± 1930

 Laterite

3185 ± 1701

<28

3090 ± 1450

42 ± 6

43 ± 28

1910 ± 1400

<613

 Ratio

29.5

1.5

2.6

1.7

4.0

3.0

Phnom Krom

 Blackened area

188,000 ± 53,600

925 ± 350

2130 ± 2300

300 ± 76

30 ± 11

18,400 ± 5090

<469

 Laterite

3130 ± 1670

<31

1216 ± 255

163 ± 54

14 ± 5

2700 ± 1900

424 ± 115

 Ratio

41.5

3.9

1.7

2.4

4.2

2.8

Wat Athvea

 Blackened area

74,900 ± 16,700

247 ± 109

2300 ± 915

187 ± 121

445 ± 370

5100 ± 2460

1210 ± 531

 Laterite

682 ± 399

<13

573 ± 174

73 ± 31

103 ± 54

2250 ± 632

507 ± 291

 Ratio

110.0

4.0

2.6

4.3

2.3

2.4

Pr. Neang Khmau

 Blackened area

146,000 ± 73,000

1480 ± 804

4800 ± 1220

27 ± 98

116 ± 80

16,900 ± 20,400

<629

 Laterite

12,700 ± 1340

<34

1842 ± 661

46 ± 9

17 ± 10

2420 ± 820

833 ± 148

 Ratio

20.0

2.6

2.0

7.2

12.6

1.4

Pr. Chrap

 Blackened area

214,000 ± 129,000

2750 ± 1700

3180 ± 1390

453 ± 348

498 ± 332

9890 ± 5900

2530 ± 1370

 Laterite

3670 ± 1040

<40

1800 ± 308

83 ± 68

49 ± 44

4020 ± 2510

1130 ± 273

 Ratio

58.3

1.8

5.4

10.2

2.5

2.2

Pr. Banteay Pir Chan

 Blackened area

248,000 ± 68,600

2320 ± 707

3740 ± 1430

99 ± 66

197 ± 81

24,500 ± 17,200

<629

 Laterite

24,000 ± 20,300

103 ± 76

2770 ± 515

89 ± 21

48 ± 28

3330 ± 1720

1000 ± 142

 Ratio

10.3

22.5

1.3

1.1

4.1

7.3

Sambor Prei Kuk Tower S1

 Blackened area

22,700 ± 8170

178 ± 156

10,800 ± 6150

2420 ± 2560

648 ± 494

5660 ± 3660

1190 ± 312

 Bricks

460 ± 180

10 ± 4

142 ± 84

35 ± 7

31 ± 8

5100 ± 1370

279 ± 43

 Ratio

49.4

18.8

76.4

68.7

21.0

1.1

4.3

 Blackened area

79,500 ± 44,400

182 ± 143

7740 ± 2620

846 ± 728

338 ± 195

16,000 ± 3940

1580 ± 1130

 Sandstone

572 ± 365

<14

132 ± 100

38 ± 9

22 ± 12

19,100 ± 1990

343 ± 196

 Ratio

139.0

13.0

58.5

22.5

15.5

0.8

4.6

Sambor Prei Kuk Tower C1

 Blackened area

188,000 ± 178,000

47 ± 28

1650 ± 1300

103 ± 54

85 ± 105

28,400 ± 24,900

2940 ± 2840

 Bricks

1012 ± 756

13 ± 12

150 ± 82

53 ± 31

31 ± 8

6750 ± 4750

823 ± 681

 Ratio

186.0

3.7

11.0

1.9

2.7

4.2

3.6

 Blackened area

39,200 ± 18,000

32 ± 15

181 ± 62

30 ± 4

17 ± 1

16,900 ± 2130

6910 ± 4300

 Sandstone

463 ± 45

<7

129 ± 32

42 ± 9

22 ± 2

22,500 ± 1880

1030 ± 184

 Ratio

84.5

4.5

1.4

0.7

0.8

0.8

6.7

Temple

S ± s.d. (mg/kg)

Pb ± s.d. (mg/kg)

Cr ± s.d. (mg/kg)

Co ± s.d. (mg/kg)

Sr ± s.d. (mg/kg)

P ± s.d. (mg/kg)

As ± s.d. (mg/kg)

Bayon

 Blackened area

1540 ± 618

40 ± 19

398 ± 318

5 ± 4

215 ± 18

2730 ± 1170

7 ± 3

 Sandstone

5450 ± 7380

16 ± 4

64 ± 22

12 ± 1

220 ± 6

<2270

4 ± 2

 Ratio

0.3

2.5

6.2

0.4

1.0

1.2

1.8

East Mebon

 Blackened area

5510 ± 2270

67 ± 27

1270 ± 308

155 ± 23

121 ± 297

5020 ± 6050

62 ± 174

 Laterite

2890 ± 4180

38 ± 133

1500 ± 570

254 ± 73

70 ± 461

16,500 ± 3690

55 ± 241

 Ratio

1.9

1.8

0.8

0.6

1.7

0.3

1.1

Pre Rup

 Blackened area

3760 ± 1180

80 ± 85

987 ± 695

323 ± 100

140 ± 117

5220 ± 911

55 ± 37

 Laterite

2530 ± 1020

25 ± 7

654 ± 533

289 ± 92

69 ± 44

8870 ± 2860

56 ± 76

 Ratio

1.5

3.1

1.5

1.1

2.0

0.6

1.0

 Blackened area

3050 ± 670

35 ± 13

1400 ± 1030

<2

241 ± 80

5220 ± 2000

7 ± 1

 Sandstone

863 ± 502

8 ± 1

172 ± 219

13 ± 3

194 ± 21

3630 ± 2400

<2

 Ratio

3.5

4.4

8.1

 

1.2

1.4

 Blackened area

7110 ± 5030

40 ± 26

1350 ± 743

19 ± 15

66 ± 23

31,600 ± 21,500

<3

 Bricks

1810 ± 746

9 ± 5

79 ± 44

<1

30 ± 8

3940 ± 1850

<3

 Ratio

3.9

4.2

17.2

14.5

2.2

8.0

Ta Keo

 Blackened area

5510 ± 1900

67 ± 66

1270 ± 619

155 ± 23

121 ± 40

5020 ± 1300

62 ± 27

 Laterite

2890 ± 1290

38 ± 18

1510 ± 942

254 ± 53

70 ± 26

16,500 ± 5930

55 ± 24

 Ratio

1.9

1.8

0.8

0.6

1.7

0.3

1.1

Phnom Krom

 Blackened area

3400 ± 4900

126 ± 41

660 ± 873

214 ± 45

51 ± 18

31,300 ± 16,300

93 ± 30

 Laterite

2760 ± 1950

76 ± 50

362 ± 257

298 ± 83

44 ± 12

57,400 ± 35,700

76 ± 23

 Ratio

2.6

1.2

4.6

0.7

1.2

0.5

1.2

Wat Athvea

 Blackened area

6773 ± 9340

157 ± 82

944 ± 534

<11

229 ± 29

8660 ± 7590

148 ± 67

 Laterite

7440 ± 12,900

48 ± 23

210 ± 63

180 ± 103

190 ± 22

3850 ± 1200

163 ± 53

 Ratio

0.9

3.3

4.5

0.1

1.2

2.2

0.9

Pr. Neang Khmau

 Blackened area

<1720

<10

2340 ± 569

<15

134 ± 35

14,700 ± 5170

12 ± 4

 Laterite

4070 ± 1160

<14

1470 ± 332

150 ± 63

85 ± 32

19,100 ± 10,900

18 ± 7

 Ratio

1.3

1.0

1.6

0.8

0.7

Pr. Chrap

 Blackened area

6230 ± 1720

<21

701 ± 423

207 ± 127

515 ± 153

25,000 ± 9050

13 ± 5

 Laterite

7620 ± 3290

<14

2000 ± 1080

192 ± 78

307 ± 269

18,200 ± 6960

32 ± 11

 Ratio

0.8

0.3

1.1

1.7

1.4

0.4

Pr. Banteay Pir Chan

 Blackened area

1620 ± 97

<10

917 ± 302

<16

424 ± 116

23,400 ± 5720

14 ± 6

 Laterite

4100 ± 1190

<10

2280 ± 791

121 ± 68

582 ± 312

25,900 ± 7720

28 ± 14

 Ratio

0.4

0.4

0.7

0.9

0.5

Sambor Prei Kuk Tower S1

 Blackened area

21,200 ± 25,187

31 ± 20

6000 ± 3510

29 ± 12

73 ± 24

16,200 ± 11,300

9 ± 5

 Bricks

1737 ± 1423

18 ± 6

69 ± 35

3 ± 1

38 ± 6

<3330

5 ± 0

 Ratio

12.2

1.7

87.3

9.4

1.9

4.8

1.8

 Blackened area

10,890 ± 7470

48 ± 51

3600 ± 1930

35 ± 23

314 ± 120

13,000 ± 10,600

15 ± 14

 Sandstone

6590 ± 11,300

16 ± 8

55 ± 16

9 ± 2

248 ± 15

<1140

<2

 Ratio

1.7

2.9

65.3

4.1

1.3

7.7

Sambor Prei Kuk Tower C1

 Blackened area

3790 ± 2360

14 ± 7

541 ± 476

<2

369 ± 598

<2860

5 ± 2

 Bricks

947 ± 544

16 ± 6

56 ± 18

5 ± 2

60 ± 18

<1250

10 ± 2

 Ratio

4.0

0.8

9.7

6.2

0.6

 Blackened area

4940 ± 1970

14 ± 2

56 ± 8

<2

244 ± 8

<4390

<2

 Sandstone

681 ± 271

12 ± 1

77 ± 11

14 ± 2

240 ± 5

<1550

<4

 Ratio

7.3

1.2

0.7

0.1

1.0

2.8

Temple

Cu ± s.d. (mg/kg)

Ti ± s.d. (mg/kg)

Rb ± s.d. (mg/kg)

Ca ± s.d. (mg/kg)

Mo ± s.d. (mg/kg)

Fe ± s.d. (mg/kg)

Zr ± s.d. (mg/kg)

Bayon

 Blackened area

13 ± 4

2780 ± 597

56 ± 5

11,600 ± 2720

<2

17,100 ± 1480

114 ± 27

 Sandstone

13 ± 5

2630 ± 928

62 ± 3

20,300 ± 8410

<2

15,800 ± 2150

127 ± 27

 Ratio

1.0

1.1

0.9

0.6

1.1

0.9

East Mebon

 Blackened area

87 ± 22

13,400 ± 1220

21 ± 5

14,100 ± 1950

7 ± 2

355,000 ± 274,000

167 ± 48

 Laterite

117 ± 15

11,600 ± 3180

13 ± 6

16,000 ± 1320

6 ± 2

430,000 ± 191,000

195 ± 33

 Ratio

0.7

1.2

1.7

0.9

1.2

0.8

0.9

Pre Rup

 Blackened area

105 ± 163

10,400 ± 2580

21 ± 10

11,320 ± 4120

8 ± 3

292,000 ± 172,000

145 ± 25

 Laterite

130 ± 78

9180 ± 2970

15 ± 5

9710 ± 2250

6 ± 3

333,000 ± 113,000

126 ± 41

 Ratio

0.8

1.1

1.3

1.2

1.3

0.9

1.2

 Blackened area

40 ± 16

4670 ± 928

58 ± 7

10,700 ± 958

3 ± 0

19,300 ± 2250

181 ± 10

 Sandstone

15 ± 3

2830 ± 492

68 ± 9

9900 ± 346

3 ± 1

18,200 ± 2300

227 ± 62

 Ratio

2.7

1.6

0.8

1.1

1.0

1.1

0.8

 Blackened area

45 ± 9

3390 ± 467

15 ± 6

20,100 ± 4510

3 ± 1

8400 ± 1010

193 ± 23

 Bricks

16 ± 4

2860 ± 332

14 ± 3

8130 ± 476

3 ± 1

9960 ± 1870

201 ± 23

 Ratio

2.8

1.2

1.1

2.5

0.8

0.8

1.0

Ta Keo

 Blackened area

87 ± 26

13,400 ± 3560

21 ± 7

14,100 ± 1200

7 ± 3

355,000 ± 132,000

167 ± 32

 Laterite

117 ± 37

11,600 ± 2470

13 ± 2

16,000 ± 6960

6 ± 3

430,000 ± 144,000

195 ± 29

 Ratio

0.7

1.2

1.7

0.9

1.2

0.8

0.9

Phnom Krom

 Blackened area

45 ± 16

8220 ± 1920

13 ± 7

9070 ± 1360

5 ± 2

309,000 ± 76,200

110 ± 10

 Laterite

83 ± 4

5400 ± 1100

22 ± 12

9750 ± 1590

5 ± 1

209,000 ± 90,500

113 ± 29

 Ratio

0.5

1.5

0.6

0.9

1.1

1.5

1.0

Wat Athvea

 Blackened area

75 ± 22

6830 ± 2260

20 ± 13

11,200 ± 3490

7 ± 1

124,000 ± 100,000

127 ± 29

 Laterite

76 ± 15

3760 ± 721

20 ± 5

14,500 ± 7200

7 ± 2

93,000 ± 51,900

120 ± 30

 Ratio

1.0

1.8

1.0

0.8

1.0

1.3

1.1

Pr. Neang Khmau

 Blackened area

<16

19,700 ± 2490

9 ± 1

7785 ± 1420

8 ± 1

403,000 ± 48,000

173 ± 17

 Laterite

52 ± 35

20,800 ± 3680

<7

31,900 ± 9600

7 ± 1

529,000 ± 36,300

208 ± 22

 Ratio

0.9

1.3

0.2

1.1

0.8

0.8

Pr. Chrap

 Blackened area

161 ± 85

13,900 ± 4650

12 ± 2

8750 ± 2430

10 ± 5

193,000 ± 51,000

179 ± 54

 Laterite

59 ± 45

26,300 ± 8040

13,200 ± 4320

14 ± 8

656,000 ± 127,000

199 ± 39

 Ratio

2.7

0.5

0.7

0.7

0.3

0.9

Pr. Banteay Pir Chan

 Blackened area

<15

23,500 ± 4640

12 ± 4

7990 ± 2130

10 ± 4

402,000 ± 59,700

216 ± 30

 Laterite

46 ± 20

33,800 ± 4100

6 ± 2

13,300 ± 3780

22 ± 12

681,000 ± 161,000

194 ± 41

 Ratio

0.7

2.0

0.6

0.5

0.6

1.1

Sambor Prei Kuk Tower S1

 Blackened area

54 ± 29

6370 ± 2470

57 ± 18

13,400 ± 9070

3 ± 1

9220 ± 444

273 ± 23

 Bricks

16 ± 2

2760 ± 263

60 ± 9

8480 ± 1930

4 ± 2

10,400 ± 346

279 ± 32

 Ratio

3.3

2.3

1.0

1.6

0.8

0.9

1.0

 Blackened area

37 ± 19

6800 ± 3650

74 ± 13

13,700 ± 4680

4 ± 1

13,500 ± 1790

136 ± 93

 Sandstone

13 ± 2

1950 ± 286

67 ± 2

21,100 ± 16,100

21 ± 26

18,700 ± 2890

276 ± 357

 Ratio

2.9

3.5

1.1

0.6

0.2

0.7

0.5

Sambor Prei Kuk Tower C1

 Blackened area

32 ± 21

3690 ± 1510

58 ± 18

23,300 ± 16,100

9 ± 4

8500 ± 1650

225 ± 86

 Bricks

22 ± 5

2750 ± 282

70 ± 13

8000 ± 1310

5 ± 1

10,500 ± 810

277 ± 65

 Ratio

1.4

1.3

0.8

2.9

2.0

0.8

0.8

 Blackened area

8 ± 3

198 ± 178

67 ± 3

19,700 ± 4430

3 ± 1

16,600 ± 629

142 ± 21

 Sandstone

22 ± 13

3150 ± 208

93 ± 10

11,100 ± 453

3 ± 1

25,200 ± 1220

253 ± 84

 Ratio

0.4

0.6

0.7

1.8

0.8

0.7

0.6

Fig. 3

pXRF results expressed as ratios for the concentrations of the elements in the black area to those in the non-black area on the surface of construction materials in the Khmer temples

The enrichment of Mn, Ni, V, Zn, Y, Pb, and Cr in ferromanganese crusts against seawater was reported by Hein et al. [10]. In addition, the enrichment of K, Ni, Zn and Pb in rock varnish was reported by Dorn et al. [11]. The enrichment of Cl and S in the manganese oxides on the surface of the construction materials in the Khmer monuments may be attributable to bat guano [12].

SEM-EDX analysis

The black materials obtained from the different sites were less than 0.2 mm thick, but in some cases as thick as 0.5 mm (Fig. 4). The results from chemical analyses of these samples by the SEM-EDX are summarized in Table 3. In addition, SEM and X-ray images in the cross section of sample no. 3405 from the inner wall of East Mebon were shown in Fig. 4. The dominant element in the black areas was Mn, with a range of 4–48 %. Compared with the non-black areas, Mn was 10–390 times more concentrated in the black areas. The Al, Si, and Fe content ranges in the black areas were 3–19, 3–34, and 2–52 %, respectively. These elements could be attributed to the construction materials. P is distributed just on the surface of construction materials and is covered by black materials (Fig. 4). P is considered to have been derived from bat excrement. Trace elements detected by the pXRF such as Ni, V, Zn, Y, Pb, and Cr were not detected by the SEM-EDX because these elements were contained in the black materials under the detection limit of the SEM-EDX.
Fig. 4

Cross section of sample no. 3405 (laterite) from the inner wall of East Mebon covered with black precipitates, and SEM and X-ray images. The analyzed area was surrounded by a white frame

Table 3

SEM-EDX results for the black materials on the surface of the construction materials used in the Khmer temples

Temple

Sample no.

 

Mn ± s.d. (%)

Fe ± s.d. (%)

Na ± s.d. (%)

Mg ± s.d. (%)

Al ± s.d. (%)

Si ± s.d. (%)

K ± s.d. (%)

Ca ± s.d. (%)

Ti ± s.d. (%)

P ± s.d. (%)

S ± s.d. (%)

Cl ± s.d. (%)

Bayon

3401

Blackened area

28.55 ± 15.73

10.59 ± 7.74

0.52 ± 0.47

5.71 ± 4.21

6.41 ± 4.02

22.64 ± 14.48

1.42 ± 3.23

1.67 ± 0.92

0.83 ± 0.48

Sandstone

0.37 ± 0.18

5.22 ± 1.26

3.55 ± 1.19

2.60 ± 0.43

8.67 ± 2.13

39.08 ± 4.96

2.41 ± 1.24

1.67 ± 0.37

0.37 ± 0.14

0.31 ± 0.38

Ratio

78.01

2.03

0.15

2.19

0.74

0.58

0.59

1.00

2.65

East Mebon

3405

Blackened area

21.67 ± 5.91

2.76 ± 0.91

0.24 ± 0.09

10.59 ± 3.41

29.26 ± 2.78

0.88 ± 0.12

0.57 ± 0.22

1.03 ± 0.52

0.30 ± 0.84

0.16 ± 0.41

0.52 ± 0.30

Laterite

0.18 ± 0.13

28.87 ± 2.89

0.09 ± 0.05

16.24 ± 0.46

20.40 ± 0.90

0.05 ± 0.09

0.09 ± 0.12

0.97 ± 0.23

0.59 ± 0.25

0.32 ± 0.19

0.12 ± 0.15

Ratio

123.13

0.10

2.59

 

0.65

1.43

19.04

6.06

1.06

0.51

0.51

4.38

Pre Rup

3406

Blackened area

6.81 ± 2.66

2.29 ± 2.63

0.11 ± 0.04

0.21 ± 0.05

4.20 ± 2.29

33.84 ± 4.41

0.34 ± 0.06

0.28 ± 0.03

0.19 ± 0.25

0.05 ± 0.12

0.27 ± 0.28

0.05 ± 0.12

Laterite

0.22 ± 0.11

29.80 ± 2.07

0.14 ± 0.03

10.63 ± 0.75

15.00 ± 1.89

0.42 ± 0.05

0.02 ± 0.05

0.02 ± 0.04

0.06 ± 0.07

Ratio

31.54

0.08

0.81

0.40

2.26

0.44

2.17

15.00

0.96

3407

Blackened area

12.36 ± 4.45

4.67 ± 3.47

2.80 ± 1.43

3.14 ± 2.30

7.48 ± 1.89

20.84 ± 6.57

1.82 ± 1.96

0.88 ± 0.44

0.88 ± 0.23

1.50 ± 0.72

Sandstone

0.03 ± 0.04

1.80 ± 0.45

3.44 ± 1.06

1.21 ± 0.38

7.10 ± 1.31

32.90 ± 2.26

3.77 ± 2.13

0.40 ± 0.34

0.21 ± 0.00

0.88 ± 0.13

Ratio

386.38

2.59

0.81

2.60

1.05

0.63

0.48

2.19

1.71

Ta Keo

3408

Blackened area

18.29 ± 10.23

10.73 ± 5.68

0.18 ± 0.12

12.79 ± 3.00

14.17 ± 1.65

1.23 ± 0.51

0.67 ± 0.38

1.00 ± 0.62

0.75 ± 0.60

Laterite

0.56 ± 0.14

43.03 ± 0.96

0.14 ± 0.08

9.86 ± 0.66

8.51 ± 0.43

0.27 ± 0.23

Ratio

32.78

0.25

1.28

1.30

1.66

2.46

Phnom Krom

3409

Blackened area

10.07 ± 3.63

4.28 ± 5.79

0.16 ± 0.09

19.28 ± 2.58

10.28 ± 3.57

2.39 ± 1.07

0.85 ± 0.39

0.53 ± 0.16

Laterite

0.18 ± 0.17

21.33 ± 5.03

0.08 ± 0.05

6.77 ± 0.82

24.56 ± 4.80

0.40 ± 0.09

0.15 ± 0.13

Ratio

56.57

0.20

1.93

2.85

0.42

5.57

Wat Athvea

3410

Blackened area

7.34 ± 1.26

3.30 ± 1.25

0.19 ± 0.15

9.42 ± 1.78

31.70 ± 3.05

0.63 ± 0.25

0.38 ± 0.20

0.48 ± 0.29

Laterite

0.33 ± 0.27

3.05 ± 0.29

0.10 ± 0.04

12.42 ± 0.28

32.74 ± 0.27

0.68 ± 0.10

0.39 ± 0.21

0.37 ± 0.19

0.30 ± 0.00

Ratio

22.24

1.08

1.88

 

0.76

0.97

0.91

0.99

1.31

   

Pr. Neang Khmau

3402

Blackened area

4.91 ± 3.01

52.07 ± 8.03

0.27 ± 0.29

5.51 ± 2.09

3.22 ± 1.56

0.70 ± 0.42

0.87 ± 0.13

0.51 ± 0.23

0.89 ± 0.77

Laterite

0.25 ± 0.18

54.92 ± 4.59

0.11 ± 0.05

4.16 ± 0.20

6.23 ± 1.70

0.65 ± 0.07

0.40 ± 0.04

0.03 ± 0.07

Ratio

19.79

0.95

2.39

 

1.32

0.52

  

1.09

2.18

15.00

 

Pr. Chrap

3403

Blackened area

31.72 ± 13.36

6.36 ± 15.86

0.35 ± 0.29

0.41 ± 0.18

6.05 ± 1.78

9.21 ± 4.00

1.44 ± 0.61

0.45 ± 0.19

0.78 ± 0.40

0.46 ± 0.31

0.33 ± 0.18

0.40 ± 0.21

Laterite

0.31 ± 0.10

51.78 ± 0.86

0.20 ± 0.15

6.09 ± 0.23

2.43 ± 0.24

0.05 ± 0.06

0.10 ± 0.13

0.89 ± 0.05

1.17 ± 0.10

0.05 ± 0.06

0.34 ± 0.10

Ratio

102.99

0.12

1.77

0.99

3.78

27.66

4.32

0.87

0.39

6.58

1.19

Pr. Banteay Pir Chan

3404

Blackened area

10.85 ± 6.22

 

0.62 ± 0.69

11.05 ± 2.77

12.03 ± 4.81

0.92 ± 0.42

0.32 ± 0.04

1.30 ± 0.42

0.99 ± 0.31

0.53 ± 0.34

Laterite

1.03 ± 0.79

24.30 ± 12.48

0.67 ± 0.56

9.09 ± 1.70

8.64 ± 1.25

1.10 ± 0.44

0.96 ± 0.20

Ratio

10.55

0.99

0.93

1.22

1.39

1.19

1.04

Sambor Prei Kuk S1 tower

3411

Blackened area

3.87 ± 2.93

1.50 ± 0.73

0.39 ± 0.18

0.81 ± 0.23

6.10 ± 1.51

33.27 ± 9.41

1.14 ± 0.24

0.71 ± 0.82

2.73 ± 2.04

1.59 ± 1.42

0.46 ± 0.27

Bricks

0.09 ± 0.06

0.37 ± 0.46

0.32 ± 0.12

2.46 ± 0.39

44.84 ± 1.11

0.67 ± 0.24

0.39 ± 0.21

0.24 ± 0.20

Ratio

45.05

4.01

1.21

2.48

0.74

1.71

4.07

1.90 

3412

Blackened area

8.71 ± 3.19

0.96 ± 3.24

2.95 ± 1.70

0.66 ± 1.28

3.98 ± 3.82

13.63 ± 11.33

1.18 ± 1.62

4.37 ± 2.71

5.62 ± 3.63

4.28 ± 1.05

Sandstone

0.22 ± 0.10

2.46 ± 0.63

3.39 ± 0.93

0.93 ± 0.22

6.88 ± 1.55

36.51 ± 2.13

2.97 ± 1.08

0.47 ± 0.28

0.33 ± 0.00

Ratio

38.88

0.39

0.87

0.71

0.58

0.37

0.40

9.26

Sambor Prei Kuk C1 tower

3413

Blackened area

48.41 ± 1.69

0.30 ± 0.19

4.18 ± 0.57

0.54 ± 0.50

3.84 ± 1.30

1.90 ± 0.16

2.84 ± 0.23

0.39 ± 0.14

0.22 ± 0.20

Bricks

0.09 ± 0.07

1.19 ± 0.22

0.30 ± 0.09

0.13 ± 0.11

3.66 ± 0.81

45.76 ± 2.14

1.26 ± 0.17

0.07 ± 0.14

0.43 ± 0.06

0.49 ± 0.00

0.36 ± 0.21

Ratio

562.93

0.98

31.18

0.15

0.08

1.50

40.54

0.61

3414

Blackened area

16.12 ± 6.89

2.05 ± 3.80

0.74 ± 1.49

6.23 ± 1.78

3.06 ± 2.14

20.60 ± 8.45

1.68 ± 1.54

3.17 ± 4.48

1.49 ± 4.52

0.38 ± 0.20

0.51 ± 0.20

Sandstone

1.06 ± 0.44

1.55 ± 1.90

3.72 ± 3.52

1.21 ± 0.47

7.34 ± 3.96

36.99 ± 6.09

1.68 ± 1.11

1.59 ± 2.02

0.20 ± 0.25

Ratio

15.21

1.32

0.20

5.16

0.42

0.56

1.00

2.00

7.46

Each analysis was conducted on a polished cross section of the sample

The concentration ratio of each element in the black area to that in the non-black area is shown in Fig. 5. This figure shows that Mn was considerably concentrated in the black area. In addition, slightly higher levels of Mg, Ca, and K were found in the black areas than those in the non-black areas (Fig. 4).
Fig. 5

SEM-EDX results expressed as concentration ratios for elements in the black areas to those in the non-black areas on the surfaces of construction materials used in the Khmer temples

XRD analysis

The results of the XRD analysis are shown in Table 4.
Table 4

XRD results for the black materials on the surface of the construction materials used in the Khmer temples

Temple

Sample no.

Base material

Quartz

Plagioclase

Goethite

Kaolinite

Birnessite

Todorokite

Brushite

Bayon

3401

Sandstone

  

 

Prasat Neang Khmau

3402

Laterite

 

  

 

Prasat Chrap

3403

Laterite

      

Prasat Banteay Pir Chan

3404

Laterite

  

    

East Mebon

3405

Laterite

  

   

Pre Rup

3406

Laterite

 

   

Pre Rup

3407

Sandstone

  

 

Ta Keo

3408

Laterite

 

   

Phnom Krom

3409

Laterite

 

   

Wat Athvea

3410

Laterite

 

   

Sambor Prei Kuk: Tower S1

3411

Bricks

      

Sambor Prei Kuk: Tower S1

3412

Sandstone

  

Sambor Prei Kuk: Tower C1

3413

Bricks

      

Sambor Prei Kuk: Tower C1

3414

Sandstone

  

Quartz, plagioclase, goethite and kaolinite were detected in many of the samples, and could be attributed to the construction materials (sandstone, laterite and bricks). Brushite (CaHPO4·2H2O) was found in the samples from the Sambor Prei Kuk monuments, and could be attributed to bat guano [12, 13]. Typical peaks for manganese minerals were not detected, which suggest the manganese precipitates exist as amorphous materials or nanoscale materials with low levels of crystallization. X-ray peaks corresponding to birnessite ((Na, Ca, K)x(Mn4+, Mn3+)2O4·1.5H2O) and 10 Å manganese oxide minerals were detected in several samples (Fig. 6). The 10 Å manganese oxide minerals could be buserite (Na4Mn14O27·21H2O) and/or todorokite ((Na, Ca, K)x(Mn4+, Mn3+)6O12·3–4.5H2O). It has been shown that buserite will convert to birnessite with dehydration on heating at 105 °C [7, 14]. Therefore, the black precipitates that showed X-ray peaks corresponding to 10 Å manganese oxide minerals were heated at 105 °C in an oven for 24 h to identify the mineral. After heating, no change in the X-ray peaks was observed. Consequently, the mineral was identified as todorokite. These findings are consistent with studies that have shown birnessite, todorokite, buserite, and vernadite are the major constituents of manganese nodules and crusts on the sea floor [1517]. Birnessite is also the main manganese mineral found in desert varnish [5].
Fig. 6

XRD patterns for the black materials taken from the surfaces of construction materials used in the Khmer temples. a Sample no. 3401 from Bayon (inner gallery), and b sample no. 3407 from Pre Rup. Q quartz, P plagioclase, Tod todorokite, and Bir birnessite

FE-SEM analysis

Stacks of hexagonal plates ranging in size from 100 to 200 nm in diameter were common in the samples of the manganese oxide precipitates (Fig. 7). In addition, some of the precipitates showed rod-shaped structures with lengths between 100 and 300 nm. The images for several of the samples showed areas with the rod-shaped structures incorporated into and forming the hexagonal plates. The rod-shaped structures suggest that the precipitation of manganese oxides may have been initiated by the activity of manganese oxidizing microbes. Manganese oxidizing bacteria have also been found in manganese nodules and crusts on the sea floor and in desert varnish [6, 18, 19]. In addition, Hariya and Kanari [20] reported that bacteria caused precipitation of manganese oxides in a spring.
Fig. 7

FE-SEM images of the manganese oxide precipitates on the surfaces of construction materials used in the Khmer temples. a Sample no. 3401 from Bayon, b sample no. 3402 from Neang Khmau, c sample no. 3404 from Banteay Pir Chan, d sample no. 3405 from East Mebon, e sample no. 3412 from tower S1 at the Sambor Prei Kuk monuments, and f sample no. 3413 from the tower C1 at the Sambor Prei Kuk monuments

Preliminary experiments on removal of manganese oxide precipitates

The solubility of tetravalent manganese ion is low, but that of divalent manganese ion is high. Therefore the reduction of manganese may be the best method to remove the manganese oxide precipitates on the surface of construction materials.

In this context, we carried out the preliminary experiment on removal of manganese oxide precipitates using an oxalic acid solution with a concentration of 0.1 mol/l as a reducing agent. The laterite sample from Prasat Chrap (sample no. 3615), covered with manganese oxide precipitates was immersed in the oxalic acid solution for 0, 1, 3 and 5 h. The photographs of the sample before and after the experiments are shown in Fig. 8. Almost all manganese oxides disappeared within 3 h after the immersion into the oxalic acid solution. No visible change of laterite was observed. The same result was obtained for sandstone covered with manganese oxide precipitates. Therefore the reduction using a reducing agent such as an oxalic acid is a useful method to removal of manganese oxide precipitates.
Fig. 8

Photographs of the laterite sample from the inner wall of Prasat Chrap (sample no. 3615) covered with manganese oxide precipitates after the experiment of immersion into the oxalic acid solution of a concentration of 0.1 mol/l for 0, 1, 3 and 5 h, respectively

Conclusions

Blackening caused by something other than the growth of blue-green algae is frequently observed on the surface of construction materials used in the Khmer temples in Cambodia. The results of this study showed that the black areas are caused by precipitation of manganese oxides. In addition to Mn, the precipitates contain small amounts of Ni, V, Zn, Y, K, Cl, S, Pb, and Cr. The manganese oxides mainly exist as an amorphous phase, but some are present as birnessite and todorokite. The precipitates are mostly present as hexagonal plates ranging in size from 100 to 300 nm, but some are rod-shaped. This suggests that precipitation of manganese oxides may have been initiated by the activity of manganese oxidizing microbes.

The manganese oxide precipitates could be easily removed using a reducing agent such as an oxalic acid solution.

Declarations

Authors’ contributions

EU designed this study. All the authors carried out the field investigation. RW and SO conducted the laboratory work. All the authors interpreted data. EU drafted the manuscript. All authors read and approved the final manuscript.

Acknowledgements

This research was conducted with permission from Authority for Protection and Management of Angkor and the Region of Siem Reap (APSARA National Authority), and was supported in part by a Grant-in-Aid for Scientific Research of the Japan Society for the Promotion of Science (Grant No. 23401001: E. Uchida, No. 25257303: T. Matsui of the University of Tsukuba). We would like to express our gratitude to all the members of JASA for their kind help during the research.

Competing interests

The authors declare that they have no competing interests.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
Department of Resources and Environmental Engineering, Waseda University
(2)
Department of Earth and Planetary Science, The University of Tokyo

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Copyright

© The Author(s). 2016