Weathering impact on the colour of building stones of the Gateway of India monument

Environ Geol (2005) DOI 10.1007/s00254-005-0020-4 ORIGINAL ARTICLE L. B. Tiwari C. J. Jahagirdar V. D. Deshpande R. Srinivasan G. Parthasarathy Weathering impact on the colour of building stones of the Gateway of India monument Received: 21 December 2004 Accepted: 07 June 2005 Ó Springer-Verlag 2005 L. B. Tiwari (&) Æ C. J. Jahagirdar V. D. Deshpande Applied Physics Division, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, Maharashtra, India E-mail: lalchandra@hotmail.com R. Srinivasan Æ G. Parthasarathy National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, Andhra Pradesh, India L. B. Tiwari B18, B304/402, Gulshan, Gokuldham, Goregaon (East), Mumbai 400063, Maharashtra, India Abstract To understand the effect of monsoon and marine environment on the colour of building stones of the Gateway of India monument, 133 samples from different areas were assessed. Colour changes were evaluated using Microflash 200d colour spectrophotometer over a period of 1 year. The alteration in the stones effecting the colour change was studied by petrographic thin section, which shows that the colour change is associated with weathering of the feldspars. Keywords Monument Æ Colour Measurement Æ Stone degradation Æ Petrograph Æ Colour change Introduction Colour change of the stone used for monuments due to degradation has been a perennial problem, as the beauty of a monument is dependent on the surface texture and colour. Any degradation in texture and/or colour can reflect the damage to the stone. Most natural stone monuments are affected by weathering (Fitzner et al. 1997). Colour measurement of the stone may quantify a change that has occurred. Visualizing the change of colour in the stones can mark the effect and gives a picture of deterioration. Interaction between decaying agents and any monument stone is a very complex phenomenon (Krumbein 1993), therefore determination and interpretation of the colour change are very important to assess weathering problems (John 1988). Many researchers have carried out studies on weathering and other factors contributing to the degradation of monument stones (Ascaso et al. 1982; Bernardi et al. 1985; Dorn 1994; Garcia-Valles et al. 2000, 2002; Keller 1957; Lal 2000; Marsh 1926; Marshall 1923; Plenderleith 1957; Prakash and Rawat 1965; Schaffer 1932; Schiffer and George 1977; Singh 1987; UNESCO 1972). The nature of the stones and the minerals present must be documented so that conservation of the monument can be carried out. Distribution of minerals within stones contributes to their appearance (Gore 1938; Winkler 1966). Combination of different colours within stones is due to saturated colours of the individual minerals. The degree of brightness also depends on overall colour of the stones. For sustainable monument preservation, a suitable technique should be adopted so that the degree of alteration can be easily documented. The colour measurement technique has been found to be useful in the assessment of colour comparison and quantification

of the level of encrustation removal during and after cleaning (Jankowska and Sliwinski 2004; Mignani et al. 2004; Skrzeczanowski et al. 2004). Colour changes in the Gateway of India in Mumbai were documented in an Indian newspaper (Upadhyay 2001). A study was performed on the monument using colour measurement techniques to investigate the degree of colour changes due to different seasonal weather conditions. Historical background Gateway of India is the most inspiring monument of historical significance to Mumbai city. It was erected in 1911 to commemorate the visit of the first British monarch King George V, accompanied by his consort, Queen Mary. The design of the monument was prepared by George Wittet during 1912 1913 and was approved in August 1914. Although the foundation was laid on 31 March 1913, the monument was not erected until 1924 (Dwivedi and Mehrotra 1995, 2001). Research methodology With permission from the Government of Maharashtra, Department of Archaeology and Mumbai Municipal Corporation, measurements were made during the period June 2001 to May 2002. The skeletal diagram of the layouts of the structures of Gateway of India was drawn to better document the monument (Figs. 1 and 2). The stones in the monument were washed with water and dried gently to free the dust particles; then, they were marked according to their specific row and column location. The vertical horizontal measurement technique was used for identification of the position of the stones selected for measurements. Samples from 133 different portions were collected to study the effect of variation in weather conditions on the stone. Care was taken so that all portions represented the marine environment while others not facing the effect of the marine environment could be checked with respect to colour change. Colour-measurement technique To quantify the colour change in the stones, the colour parameters were calculated using CIELAB 1976 formula (Wyszecki and Stiles 1982). Reflectance values of the stone were measured with a Microflash 200d spectrophotometer supplied by Datacolor International, USA, to measure the colour in the field. The wavelength pitch (band width) was 10 nm in the range of 400 700 nm during the measurements. All the measurements were made with the D65 illuminant and 10-degree observer condition. The readings were taken three times and an average value was derived to minimize the error. Error bar was also taken into consideration, as it can arise during the positioning of instruments on the spots. Error bar for L*, a*, b* andde is considered 0.5, 0.4, 0.4 and 0.7 unit, respectively (Jahagirdar et al. 2002); where L* is the lightness/darkness coordinate; a * the red/green coordinate, with + a* indicating red and ) a* indicating green; b* the yellow/blue coordinate, with + b* indicating yellow and ) b* indicating blue; DE is the total colour difference calculated from CIELAB 1976 colour difference formula. Results and discussion Table 1 shows the colour parameters of a few selected spots measured on 8 June 2001. The stones used for building the monument were of different colours. This was also noticed on the monument (Table 1). The variation may be due to original variations in the quarried stone. When stones are cut into small blocks, the portion exposed to the outside atmosphere is darker than the inner portion. It may also be observed from Table 1 that the stones positioned at lower levels (first and second row) from the earth s surface are darker than those positioned at upper levels (third row onwards). To study the weathering effect, investigations were conducted for a period of 1 year from June 2001. Figures 3 and 4 demonstrate the variation in colour parameters of stones of the outer portion of the monument (Figs. 5 and 6, respectively) facing the marine environment. Figure 3 elucidates the effect of the stones lying at the lower level, i.e., below 0.5 m, and Fig. 4 represents the stones lying above 2 m high from the earth s surface. As shown in Fig. 3, DE value increases considerably in July, which is attributed to the change in the environmental condition due to heavy rainfall in June. As there is rainfall in July also, a small rise in DE value could be seen in August. However, clear monsoon without rainfall at the Gateway region in August signifies a drastic decrease in DE value during September. Further, heavy rain in September results in a sharp rise in DE value in October. Winter season begins mid- October resulting in a decrease in temperature and relative humidity, and a continuous decrease in DE value is found during October to February. The atmospheric conditions change from mid-february due to the summer season, resulting in a slight rise in temperature and humidity; and an increase in DE value could be seen up to May (Fig. 3). This change in DE value is ascribed to the change in colour parameters (L*, a* and b* values) with variable weather conditions.

Fig. 1 Gateway of India Mumbai, India Fig. 2 Layout of Gateway of India Mumbai, India As shown in Fig. 4, DE values are comparatively less than those in Fig. 3, mainly due to the positioning of stones. The rainwater accumulates at the lower levels by creeping into the upper levels, causing the fast degradation of the stones at the lower levels compared to those at the upper levels. It further breeds vegetation or microorganisms with consequent effect on the rapid colour change in the stones. Colour variations during July to October are mainly due to the effect of monsoon, which causes the stones to absorb water and makes them appear darker (Figs. 3 and 4). Thus, the lightness value (L* value) decreases comparatively to other seasons and further attributes to the increase/decrease in a* and b* values with increase in DE values. The temperature and humidity are comparatively low in winter; which allow the stones to expel the absorbed water by evaporation, resulting in increased L* values. Also a* and b* values increase/decrease depending on the stone s colour and variation in colour parameters, which causes a decrease in the DE value of the stones. The actual process of colour change begins in the summer season, when the temperature is comparatively high to promote oxidation process, which may result in an increase in L* value if the stone is lighter and also decrease in L* value if it becomes darker due to alteration. The variations in colour parameters depend on the state of the altered condition of the stone. Figure 7 exhibits variation in colour parameters of stones from the inner layer, which do not face the marine environment and sunlight. A sharp increase in DE value is observed due to the change in humidity, temperature and weather conditions, which cause colour change in the stones. Also, when b* values of March are compared to those of June, increase in yellowness is seen, which further leads to darkness of the stone. To study the degree of alteration and behaviour of colour change in fresh (unaltered) stone, the broken portion of the inner stone of the monument was considered to be fresh (although partially altered), which does not either face the seawater or the sunlight (Fig. 8). The colour variation is more than that in previous results, which may be due to the presence of higher active minerals in fresh stone, further reacting with the humidity and resulting in colour changes at a faster rate. Also, the presence of kaolinized plagioclase is susceptible to reaction with change in humidity. Thus, the rate of colour change in fresh stone is more than in any altered stone. This further confirms the finding that the chances of algal growth on monument stones are very high and in abundance on any fresh stones or followed by cleaning, which further indicated rapid colour change in the monument compared to those which are not fresh or prior to cleaning (Young 1997). Study was further carried out on the thin section of stones using a petrological microscope to assess their

Table 1 Colour parameters and their specification of spots of the selected stones The alphabets used in the specification column indicates the following terms: D down, R row, L left, H horizontal, V vertical, C column * The terms inside the bracket [] in specification columns are the position chosen for the measurement and unit for which is centimeter (cm) Specification L * a * b * L1O-R1/C1 [V18-LH3] 53.98 3.48 10.97 L1O-R6/C2 [V5-LH18] 59.93 4.91 14.89 L1O-R9/C2 [V8-LH8] 56.95 5.48 16.01 L1O-R8/C2 [V9-LH15] 59.39 5.63 16.36 A1O-R1/C2 [V5-LH6.5] 49.45 4.60 13.76 A1O-R7/C2 [V28-LH11] 54.35 4.13 11.04 B1O-R1/C2 [V17-LH13] 37.31 3.28 8.46 B2O-R1/C2 [V13-LH85] 49.03 5.05 14.24 B1O-R6/C1 [V32-LH15] 57.59 5.23 13.66 C1O/RDn1/C3 [V30-LH80] 48.15 6.51 15.97 C2O-RL1/C2 [V15-LH11] 54.70 4.00 12.92 D1O-R1/C2 [V20-LH63] 53.86 4.52 12.75 D1O-R6/C1 [V28-LH23] 58.56 3.75 11.70 E1O-RL1/C2 [V16-LH49] 46.94 3.98 11.80 E1O-R6/C2 [V30-LH33] 53.68 5.27 11.98 F1O-R4/C3 [V13-LH26] 50.01 3.80 12.26 F2O-R2/C2 [V14-LH22] 47.19 3.69 13.63 G2O-RL1/C2 [V11-LH36] 46.90 3.53 10.86 G2O-R6/C1 [V30-LH24] 51.79 4.21 11.12 H1O-R1/C2 [V28-LH12] 49.38 3.95 12.95 H1O-R6/C1 [V34-LH28] 53.99 3.73 12.00 I1O-RL1/C1 [V20-LH45] 47.73 4.33 12.56 I2O-R6/C1 [V30-LH8] 59.68 4.42 12.45 J1O-R1/C2 [V14-LH52] 48.81 3.34 11.83 J1O-R6/C1 [V34-LH12] 54.85 4.76 12.51 K1O-RL1/C2 [V17-LH95] 49.63 4.54 13.50 Fig. 3 Variation in colour parameters of stones of the outer portion of the monument corresponding to Fig. 5 degree of alteration. The objective was to confirm the degree of alteration of the outer (facing sunlight and seawater) as well as the inner (not facing sunlight) Fig. 4 Variation in colour parameters of stones of the outer portion of the monument corresponding to Fig. 6 portions and also to investigate the type of stones used for the monument. Figures 9 and 10 are the photo-micrographs of the thin section of stones of the outer and inner portions of the monument. Plagioclase phenocryst has been altered

Fig. 5 Stone of block L (outer) first row/second column Fig. 7 Variation in colour parameters of stone of the inner portion of the monument Fig. 6 Stone of block I (outer) sixth row/first column partially in more altered groundmass, which appears to be brownish grey when compared to the fresh part, which is white. Pyroxene has been altered partially, with iron oxide released from the minerals (Fig. 9). From Fig. 10, it can be observed that kaolinized plagioclase (grey) and pyroxene (dark brown) have been altered strongly. Also, clinopyroxene has been altered strongly Fig. 8 Stone of block L (inner) with the iron oxide released during alteration. The degree of alteration in the inner portion is found to be more than the outer portion.

Figure 11 represents the micrographs of the thin section of the fresh stone. It is found that Basalt shows phenocryst of clinopyroxene in a matrix of plagioclase and clinopyroxene. The clinopyroxene phenocryst shows a low degree of alteration along cleavages and fractures. The matrix of brownish black mineral is altered pyroxene, and pale brownish white mineral is altered plagioclase. The matrix minerals are more altered than the clinopyroxene phenocryst. Plagioclase phenocryst showing a low degree of alteration relative to clinopyroxene in the matrix can also be seen. From a petrographic study, it was found that the rock samples are of basalt (Figs. 9, 10 and 11). Conclusions The results of the present study demonstrate that the proposed method of the non-destructive colour measurement technique is found to be useful for the assessment of degradation of stones, and it can also be used as an analytical technique for monitoring the colour change in the monument stones with changing weather conditions. Colour measurements give an idea about weathering and alteration of the stones. The changing colour of monument stones is mostly due to the reaction of minerals present in the stones under different climatic conditions. Stones at lower levels facing different sources of degradation such as sunlight, marine environment, rainwater, etc., degrade at a faster rate than those at upper levels. The stone colour changes according to change in weather conditions, for example, it appears darker during monsoon than other seasons. The colour change in the inner portion of the monument, which does not face sunlight, marine environment and rain- Fig. 10 Thin section of stones of the inner portion of the monument. [Magnification: 63, 1 cm=250 lm; Nicol Position: Plane polarizer] Fig. 11 Thin section of fresh stones of the inner portion of the monument. [Magnification: 56, 1 cm=200 lm; Nicol Position: Plane polarizer] Fig. 9 Thin section of stones of the outer portion of the monument. [Magnification: 56, 1 cm=200 lm; Nicol Position: Plane polarizer] Fig. 12 Trend of total colour variation (DE) in the outer portion of the monument due to weathering

water, is continuously rising according to the variations in temperature and humidity in different seasons. Fresh stones (unaltered) result in faster degradation than altered stones due to the presence of more active minerals. The overall change in colour parameters (June to May) in the outer portion stones is not very high (DE0.5 3) unlike the inner portion stones (DE3 6). The colour deviation of the outer portion resembles the trend shown in Fig. 12. The degree of alteration in the inner portion is more than in the outer portion and basaltic rocks have been used in building the monument. Acknowledgements The authors express their sincere thanks to the Government of Maharashtra, Department of Archaeology and Mumbai Municipal Corporation, especially to P.T. Gaud, Deputy Secretary, Social Welfare Affairs and Sports Department and B.S. 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