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CHARACTERISATION OF OOLITIC TEXTURED STONE ACCRETIONS AND EROSION PHENOMENON OF AN EXCAVATED STRUCTURE, RANI Kl VAV, PATAN

SHARMA, R.K. and GUPTA, H.O.

Archaeological Survey of India, Dehradun (India)

KEY WORDS : Vav (Stepwell), Patan, Excavation, Oolithic textured, Accretions, Erosion, Colour change.

SUMMARY The painstaking work of careful excavation revealed the largest and most sumptuous Vav, an 11th century structure built by queen Udayamati, The exposed sand stone sculptures were observed to have deposited soil accretions and oolithic type texture embedded with mud. In some cases, localised erosion phenomenon wi.th change in colour of the stone has also been observed, The detailed analytical studies of different types of stones used in the Vav (stepwel.1) show that the phenomenon of localised erosion of stone sculptures.with colour change is generally due to the presence of veins or streaks whereas the ool­

ithic textured surface is not original part of the sandstone but the hard accretionary deposit formed by the calcium leached from the stone and clay particles. SEM micrographs of a few sandstone samples and the accretionary deposits corroborate the observations regarding the characterisation of oolith-alc textured stone accretions.

1 INTRODUCTION

The Vavs (Stepwells) distinguished by a series of steps and platforms as important architectural­features are different from ordinary wells. Rani ki Vav at Patan constructed by Udayamati, queen of Bhimadeva is reported to have taken some fifteen to twenty years to build and structurally completed by 1090 A.D .. The Rani stepwell derived its name from its builder patron, the queen addressed as Rani. The British traveller Arthur Malet visited this site in the early 19th century; in 1837, Colonel James Tod visited the site and an account of Rani ki Vav is found in his book 'Travels in Western India' stating that the material from this stepwell was taken to build another one in the modem part of Patan. When the archaeologists James Burgess and Henry Cousens saw the stepwell in 1883, it was completely filled up with earth and only the back wall of the well was visible (Plate 1 ), which they called a huge pit 285 feet in length, with the portion of the masonary of the well shaft at one end and fragments of a pillar at the other

end.

Plate 1 General view of the silted up Vav

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1.1 Cause of silting up

In the earlier years of the nineteenth century, .i!Js reported that the upper levels of the stepwell were still visible, though it is not clear whether they were intact or in ruins. for both Tod and Alexander Kinloch

Forbes (Notes of a visit to Goozerat) have reported that materials from the Rani ki Vav were carried off

and used in the construction of the Barot Vav. It then raises the question under what conditions, the Rani ki Vav was silted up ? According to Mankoti, on the basis of evidence provided by the exposed structure

and the recovery of some unserviceable metal objects during the excavation purported to have been

consigned by the local Jaina laity, most likely a great flood of the Sarswati river which fl .ows less than

one kilometre away, silted up the well and this submergence must have taken place before the establ­

ishment of Muslim rule in Patan about 1300 A.O. , but probably late enough for Merutunga himself to have seen the stepwell before he completed his .Prabandhacintamanil" in 1304 A.O. The enormous

quantity of sand, on record since at least the nineteenth century, indicates that one or more floods filled

up the pit of the stepwell, bringing down the storeys of the pavillions in the process.

1.2 Excavation and Structural Restoration

After independence, the Archaeological Survey of India declared the Rani ki Vav stepwell as a protected

monument of national importance. In 1960's, initially the excavation was carried out on a small scale.

The systematic painstaking work of excavation was jointly taken up in late seventi es by the Archaeol­ogical Survey of India and M.S.Uni,versity, Vadodara. As the excavation progressed, a number of

difficulties were encountered due to the presence of excessive water. Four water pumps had to be put

into operation for dewatering to enable the excavation work to be completed up to the seventh step, At this stage, it is interesting to observe that in the architectural landscape of Patan, the Rani ki Vav as

reclaimed after excavation stands out consipicuously as the sole survival of Hindu Monuments dating to any period prior to the arrival of Muslims (Plate 2 & 3). The structural repairs to this Vav were

undertaken by the Vadodara Circle of Archaeological Survey of India by constructing the fallen retaining wall , resetting of the out of plumb parts of the side walls and other conservation measures. Another important feature noted in this ashlar masonary structure is the use of wooden dowels as well.

Plate 2 Exposed and

structurall.y conserved Vav Plate 3. Well at the western side of the Vav

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2.MATERIALS

The stone used in the construction of Vav is generally sedimentary sandstone having creamy colour varying from fine grained to coarse grained texture. The medium grained and coarse grained stones have generally been used for the construction of pillars, niches and slabs whereas the fine grained stones have been used for the sculpture work (Plate 4). Only a very small number of sculptures are observed to be eroded significantly (Piate 5). The wall of the well is most profusely ornamented with sculptures of deities in recessed and projected niches. These sculptures have mostly been carved from reddish colour sandstone and do not have any accretionary deposit, like those at different steps of the Vav. These sculptures are comparatively in much better state of preservation.

Plate 4 Row of stone sculptures

2.1 Investigations The stone sculptures in general have been observed to have hard accretionary clayey deposits giving measles type appearance (Plate 6). In some cases, a part of the sculpture has eroded with colour

change giving reddish appearance (Plate 7) . The stone sculptures were cleaned for the removal of superficial muddy accretions using non-ionic mild detergent and dilute ammonia solution. During the course of cleaning, it was noted that the accretionary deposit having oolithic textured appearance has formed a hard crust on the stone sculptures tenaciously

holding the fine clay particles.

Plate 5 Sculpture showing significant erosion

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Plate 6 Accretionary deposits on the stone sculptures giving oolitic textured appearance

From a few locations where the accretions were discontinuously bonded, a small chip of the accretionary deposit was taken for the analytical studies (Plate 8). The physical appearance of the stone sculptures from where this hard crust was taken out, clearly showed that the measles type appearance of the accretionary deposit is not an original part of the stone but specific characterisation of the accretionary deposit. The chemical analysis shows that the accretionary deposit has unusually high content of calcium carbonate (Table 1),

Table 1. Chemical composition of Oolithic textured accretions

Compositon (%) Sample Location Si02 CaC03 MgC03 R203 number

1 llnd 40.18 48.94 3.13 8.32 2 steps 36.29 59.30 1.34 2.17

3 Ill rd 19.00 76.80 1.73 2.14 4 steps 25.19 71 .40 1.78 1.40

5 IVth 14.60 79.57 4.12 1.51 6 steps 15.53 76.48 3.18 4.62

The physical characteristics of a few stone samples from the Vav indicate that their apparent density varies from 2,37 to 2.50 whereas the apparent porosity varies from 7.0 to 12.90 indicating that the grains

of stone are somewhat loosely packed with poor binding matrix. The chemical analysis of these stone samples shows the predominant presence of silica upto 90,85 percent (Table 2). The calcium oxide

content in case of sample No 4 having R2 03 content as 8,38 percent is very low being less than 1 %,

Though this sample is fine grained with dark red streaks, the banded zone is more prone to weathering indicating the inherent weakness of the stone having presence of reddish streak, The sculptures carved from such stones are seen to have suffered localised erosion with colour change.

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Table 2. Chemical composition of sandstones

Sample Appearence

Number

1 Fine grained

Creamy Colour 2 Medium grained

Light yellow

3 Creamy colour

4 Fine grained

Dark red streaks

Plate 7 Localized eroded surface with colour change

App. Density

2.50

2.42

2.37 2.47

Plate 8 Accretionary deposit sample

showing oolitic textured surface

App. Porosity

7.90

7.06

12.90 4.76

Composition(%)

Si02 R20 3 cao MgO

27.68 1.52 2.80 2.00

90.85 2.85 2.24 1.02

92.30 3.00 2.24 0.80 85.22 8.38 0.84 0.80

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2.2 Electron Microscopy

Examination of the oolitic textured accretionary layer with an electron microscope shows cavernous structure of the upper surface (Plate 9a) at 1 OOx and on further magnification (450x) shows that there are no true oolites but a fine layer of clay deposit over the surface (Plate 9b). The lower surface of the

accretionary deposit observed to be hard and relatively smooth, shows the calcium carbonate layer under SEM (Plate 10). From the same side at one location, the presence of organism was also scanned

(Pate 11). The probable cause for the intrusion of the microorganism at the lower surface of the accreti­onary deposit seems to be the remains of some microvegetational growth which might have taken place on the stone surface before the Vav got silted up towards the end of the 13th century.

Plate 9a Cavernous structure of the upper surface of accretionary layer

Plate 1 O Formation of calcium carbonate layer at lower surface

Plate 9b Fine clay deposit devoid of oolites

Plate 11 Presence of the micro organism in lime layer

Plate 12a Presence of isolated localised cavity in fine grained

stone

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Plate12b Discontinuity in the laminar structure

SEM micrograph of fine grained creamy coloured stone sample shows compact packing with isolated localized cavities (Plate 12a), The presence of such cavities and discontinuation of planar structure (Plate 12b) shows that such regions are weak and can undergo preferentlal decay or losses, The medium grained sample having porosity close to fine grained stone has mainly difference in the size of the grains (Plate 13). The coarse grained sample has significantly large size quartz particles (Plate 14) and not

suitable for the fine carving work.

Plate 13 Formation of medium grained textured

stone (110x)

Plate 14 Size of coarse grained particles (40x)

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3. RESULTS AND DISCUSSION

The analytical investigations and electron micrographs of the tenacious accretionary deposit forming mi­crospherical concretionary bodies on the surface of the stone are due to the calcium carbonate alongwith

clay layer formed on the stone surface. The characterization of accretionary deposit having unusually high content of calcium carbonate is probably due to the leaching of calcium in the presence of water which has preferentially deposited on the surface of the fine grained stone sculptures.The results of Table 1 clearly reveal that the proportion of calcium carbonate in the accretionary deposits is much more in case of sampies taken from the third and fourth steps when campared to the samples taken from the second step, This phenomenon can be attributed to the presence of excessive water below the second step as was reported during the course of excavation. This hard crust, not uniform over the sculptures appearing to be forming a part and parcel of the stone itself is aesthetically not only distractive due to the embedded clay particles but undesirable as well. The presence of clay particles under favourable conditions of humidity especially during the rainy season which in fact would easily be the case, the growth of microorganisms over the years may lead to the erosion of the stone, Besides, the consol­idation measures can not be adopted till. the removal of this accretionary deposit. The analytical studies and SEM micrographs of a few stone samples have clearly established that the oolithic textured surface is not original part of the sand stone, but the accretionary deposit formed from the leached calcium with embedded clay particles giving blistering appearance. The phenomenon of localized erosion of stone sculptures with colour change is due to the presence of iron rich veins or streaks. The presence of such bands and discontinuous laminar structures indicates the inherent weakness of the stone. A few sculptures which had significantly eroded have been carved from stones with extremely poor binding matrix. The presence of voids in case of coarse grained structures and microcavities in fine grained stones indicate the requirement for consolidation of stone surface to slow down the weathering phenomenon.

On the basis of these investigations the appropriate measures for the chemical cleaning and preservation of the exposed exquisite sculptured Vav have been formulated, The exposed Vav has deposit of soil mass on the top level around the peripher at a slightly higher level than the surrounding ground surface. Though this area does not experience heavy rainfall and the subsoil water over the years has appreciably gone down which is reported to be more than 500 ft, below the ground level, the flow of soil over the stone surface especially the sculptures during rains would have significant abrasive action. Accordingly, for the conservation of this site, the proposed measures for improvement of environment include the paving of one metre stone slabs around the top storey of the Vav as an apron and pitching of stones on the sloped soil surface to prevent the erosion of soil. The rain water collected along the peripher at the top level would be drained out by an appropriate provision

4. CONCLUSIONS

The SEM micrographs of the stone samples as well as accretionary deposit having spheroidal or ovoid appearance deposited by calcium carbonate alongwith clay on the stone surface revealed that the oolithic textured surface is not original part of the sand stone, The phenomenon of localised erosion of the sculptures with colour change is generally due to the presence of iron rich bands or veins. These studies also indicate that the discontinuous hard crust cannot be regarded as a protective layer and it is desirable to remove it after the proper site management so as to affect surface consolidation and preservation of the loosely packed sediments.

ACKNOWLEDGEMENTS

The assistance provided by Shri S.K.Tewari in taking the electron micrographs is thankfully acknowledged. Authors are greatful to Mrs. Achala Moulik, Additional Director General & Chief Executive Officer for the support in carrying out these studies.

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