sections.
Petrographic Microscopy (PM)
Two thin sections of M1, M2 and M3 were observed under a polarisation microscope Leica DM 4500 P, equipped with a digital FireWire Camera Leica DFC 290 HD that worked with the Leica application suite software LAS 4.
Linear microcrack density (LMD) was calculated as the number of microcracks per millimetre. It was found for the first 5 mm from the surface to the inside of another thin section (M4) by counting the number of microcracks intersecting a line of 5 mm.
Mercury Intrusion Porosimetry (MIP)
The micropore size distribution was measured by mercury intrusion porosimetry (MIP; Pascal 140 and Pascal 440 from Thermo Scientific) using irregular samples (ca. 1.0 g).
The analysis was run on M1, M2 and M3 samples oven-dried to a constant weight at 70 °C.
249Colour
After oven-drying at 70 °C to a constant weight, 15 colour measurements were taken in each sample, and the mean for each sample were calculated. Afterwards each sample was submerged in water for an hour and 20 colour measurements were taken again in the wet samples. The colour was assessed with an X-Rite colorimeter (model 964), with 45°/0° geometry and specular component included, D65 illuminant and 8 mm aperture. Colour was expressed using the three chromatic coordinates of CIE-L*a*b* system. The colour difference (ΔE*) between the dry and wet state was calculated for each sample.
Results and Discussion
Petrographically, Montescaros marble is dolomitic with crystals visible to the naked eye with coarse equigranular blasts and granoblastic texture. The blasts’ boundaries are very sinuous, and microcrystals fill cracks and blast boundaries (Fig. 6).
Figure 6: Montesclaros marble. a: Altered surface. b: Parallel Nicol microscopic image. c: Crossed Nicol microscopic image.
The porosity is very low. Fig. 7 shows the pore size distribution for the three samples M1 = 0.09 %, M2 = 0.29 %, M3 = 1.46 %.
Figure 7: Montesclaros marble. Pore size distribution determined by MIP for three tested Montesclaros marble samples (M1, M2 and M3).
LMD decresses with the depth. LMD = 17 in the first millimeter and LMD = 9 in the fourth millimeter. The rhombohedral crystals that M4 presents on the surface are due to the alteration, which accentuates the rhombohedral exfoliation, decreasing the equidistance between the microcracks (Fig. 8, red ellipse).
The alteration produces a reduction in the luminosity of the marble. Marble becomes more yellow and red (table 1). The colour difference (ΔE) between dry and wet state has been 18.0, 14.1 and 12.3 for M1, M2 and M3 respectively.
Table 1: Colour parameters of M1, M2 and M3 L*: lightness; a*: red-green value; b*: blue-yellow value.
| Dry color | |||
| L* | a* | b* | |
| M1 | 82.3 ± 1.8 | 0.7 ± 0.2 | 4.8 ± 0.6 |
| M2 | 76.6 ± 1.6 | 1.9 ± 0.8 | 8.2 ± 1.5 |
| M3 | 72.9 ± 1.5 | 1.9 ± 0.6 | 8.5 ± 1.4 |
| Wet color | |||
| L* | a* | b* | |
| M1 | 64.4 ± 15.3 | 0.8 ± 0.2 | 6.7 ± 0.6 |
| M2 | 63.6 ± 2.8 | 2.9 ± 0.9 | 13.5 ± 1.6 |
| M3 | 61.2 ± 2.5 | 2.2 ± 0.9 | 12.3 ± 1.9 |
Conclusions
Knowledge of stones, historic quarries, modifications suffered in the monuments and causes of 250stone decay are necessary for conservation interventions, especially for reintegration and replacing the original stone with compatible materials.
Figure 8: PM micrograph mosaics of Montesclaros marble (M4 sample) above: parallel Nicols. A 5 mm line indicates the LMD. below: crossedNicols. A red ellipse indicates the tightly rhombohedral exfoliation in the most superficial part of the sample.
Fuente de Cibeles was originally at ground level, protected by 20 granite bollards.
All water-spouts and bollards were removed in 1862, and a perimeter cast-iron fence was installed.
The monument was moved, rotated and raised in 1895 and the rocky platform was expanded to add two putti.
A hand, the keys, the sceptre and the nose of the goddess Cybele were damaged and restored in 1931. The left lion lost its snout and suffered damage to the left front leg and tail in 1936.
The perimeter fence was removed after the Spanish Civil War and the perimeter of the fountain was landscaped. Two granite basins with cascading water from the upper basin to the new external basins were added and the flint rock covering the base of the rocky promontory was removed in 1968. The last restoration was in 2016.
The dissolution of the smaller calcite crystals that border the dolomite crystals causes increased porosity, colour change and superficial disintegration of the blasts.
Tightly rhombohedral exfoliation is in the most superficial part of the alterated sample.
Acknowledgements
Stimulus of Scientific Employment Individual Support 2017. CEECIND/03568/2017. Fundação para a Ciência e a Tecnologia of Portugal (FCT).
References
Bednarik M., Moshammer B., Heinrich M., et al. 2014. Engineering geological
