Historical Quarries, Decay and Petrophysical Properties of Carbonate Stones Used in the Historical Center of Madrid (Spain)

The carbonate stones that make up the four fountains of the 18 century located in the Paseo del Prado of Madrid (Spain) are studied. The documentary search in historical archives, together with the petrographic, cartographic and paleontological studies permitted to determine that the fountains have been built with dolostone of the Castrojimeno Formation, with gastropods of the Trochactaeon Lamarcki specie of the Santonian (Upper Cretaceous). The historical quarries from which the ashlars have been extracted is located in Redueña Village. The petrophysical properties of this dolostone (effective porosity, bulk density, mercury intrusion porosity, ultrasound wave propagation velocity, micro-roughness and color) have been calculated and compared with Colmenar de Oreja limestone. Each of the four fountains has a circular pylon at the base, a central column that holds a smaller pylon and is topped by a sculpture that serves as a spout. A bomb destroyed three ashlars of the basal pylon, column, small pylon and the sculpture of the SE fountain, during the Spanish Civil War, in 1936. These damaged elements were replaced by other carved limestones from Colmenar de Oreja in 1944. The four sculptures had been replaced in 1996 with resin replicas and the originals are preserved in the San Isidro. Los orígenes de Madrid museum. The study of the petrophysical properties of the sculptures located in the museum allowed us to determine the decay of different stone types. The analysis of micro-roughness was employed to define that the dissolution


Introduction
Carbonaceous stones have been used in the carving of sculptures throughout history [1], however the passage of time makes it sometimes unable to know the type of stone used in monuments. The location of historic quarries is one of the disciplines of geology applied to heritage conservation necessary for the conservation of heritage assets [2]. In order to locate the original quarries of the stones used in the monuments an exhaustive documentary search must be done, accompanied by a cartography and fieldwork.
The four fountains, also known as "Las This pylon is made of 12 ashlars and in the center containing a column carved by Narciso Albedo, in which sculpted vegetal motifs appear and animals: eight leaves superimposed in the lower part and four bear heads, sculpted by José Rodríguez. Each column is topped by a small circular pylon with 20 leaves on which is placed a sculpture of a triton child holding upright a dolphin whose mouth flows the water (spout). These four sculptures were finally elaborated by Alfonso Bergaz and Roberto Michel. All were carved out of carbonate stone ( Figure 1) and they were installed in 1782.
The carbonatic building stones traditionally used in Madrid have been mainly Redueña dolomite and Colmenar de Oreja limestone. Redueña stone includes limestone and dolomicrites, normally of cream colour tones. Its historical quarries were very dispersed, notably the villages of Redueña, Guadalix de la Sierra, El Molar, Venturada and Torrelaguna, as well as other towns in Guadalajara [2].
The Castrojimeno Formation out crops in Redueña and has several members of dolomites with different concentrations of cretaceous fossils. Members without fossils are those that have been studied so far. This stone has been widely used in Madrid´s monuments due to its whitish color.
The fountains have suffered aggressions throughout its long history. However, the petrographic studies, aimed at the characterization and conservation of the stones, have not been carried out so far. Therefore, the aim of this study is to figure out the characteristics of the stones used in the sculptural complex known as the four fountains of the Plaza Murillo in Madrid (Spain) and determine their origin and compare the type of decay of each stone. Moreover, this study focused on future restoration works. In addition, the petrophysical properties are compared between two levels of Redueña dolostone and Colmenar de Oreja limestone.
Geographic, orographic and especially geology show great importance when it comes to the selection of construction materials. The stones used for sculptures must have characteristics of proper durability, styling, polishing and cost determined for each need. Mass media, transportation and tools of the 18 th century were precarious and the work with stones were hard, thus some works had prolonged for years to complete. The architects usually chose the stone according to the distance of it quarries, the price, the use, the finish required for the stone. The geology of the area near Madrid contains different types of igneous, metamorphic and sedimentary rocks.
The petrographic characterization is a classic stone characterization technique [3,4], which together with cartographic study and paleontological study allows us to localize the historical quarries of stones used in the construction of historical buildings or monuments. Ultrasonic auscultation is used to obtain the stones characterization by identifying the ultrasonic wave propagation velocity, which has a strong dependence on porosity, fractures and alteration. Ultrasound is useful to determine the anisotropy of rocks, which in turn is also closely related to their durability and decay [5,6]. In general, ultrasonic velocity is directly proportional to the durability of the stone and inversely proportional to its anisotropy [7,8].
Effective porosity and apparent density also provide information on the durability of a stone. In general, higher porosity corresponds to lower durability and higher density to higher durability [9].
The distribution of pore size has great importance to know the durability of the rocks [10,11].

Materials and Methods
A visual inspection has been performed on the four fountains stones, focusing on fossils and forms of decay. Two small chips of the basal pylon (SE fountain) have been obtained. One from an original ashlar and another of a replacement ashlars. Two thin sections of these samples were made and characterized under a polarized light microscope Olympus BX 51 equipped with a digital DP coupled camera (6 V/2.5 Å) Olympus DP-Soft software Olympus (version 3.2).
A mosaic was constructed with thirty microphotographs and an approximate surface area of 1 cm 2 .
Once the sample was characterized, a following study has been conducted in historical archives and cartography of cabinet and field, following the methodology proposed by to locate the quarries that have provided the building stones.
A stone block was obtained in the historic quarry from which seven cubic specimens of 5 cm of side were cut for petrophysical analysis and a cylinder for analysis of porosimetry by mercury intrusion. Specimens were cut at low speed (120 rpm) and low strain.
A thin section of a sample obtained in Redueña quarry and a mosaic of photomicrographs was made, the procedure used for its study was the same as for the samples obtained in the basal pylon of the SE fountain.
A visual inspection has been made at the sculptures located in the San Isidro. Los orígenes de Madrid museum.
For the XRD analysis A Philps analytical PW 1752 diffractometer operated at 40 KV and 30 mA was used with copper anode tube, graphite monochromator and PC-ADP Diffraction software.
The dust samples were analyzed with Cuα radiation. The measurements were performed in a range between 2 and 68 with an interval of 0.02 and 2 /min in continuous mode.
A Niton Series XL3t portable X-ray fluorescence kit has been used to identify the elements present in Redueña dolostone and Colmenar de Oreja limestone. This portable elemental analysis technique is non-destructive and fast. Ten measurements were made on the both fresh surfaces stones and the average for each stone has been calculated.
Effective porosity test (Pe) was performer using the natural stone method described in European standard UNE- EN, 1936EN, , 2007 in the seven cubic samples of Redueña dolostone. After the samples had reached a constant weight, they were placed in a vacuum chamber at 2 kPa for 2 h and they were slowly submerged in water (room temperature) and then stored at atmospheric pressure for 24 h, reaching water saturation. The Pe values were calculated from Equation 1: Ws is the weight of 24-h water-saturated sample, Wd is the sample dry weight, and Wh is the submerged in water sample weight.
The bulk density (ρb) mean of the same samples was also found as per European standard UNE-EN, 1936, 2007 as the ratio between specimen mass and its bulk volume, from equation 2: Ultrasonic pulse velocity (V p ) was measured for each of the seven cubic specimens of Redueña The anisotropy indices dM and dm were obtained [12] for Redueña dolostone following equations 3 and 4: (V pmax , V pmin y V pmean ) refer to the V p in the three orthogonal directions of space. In this way, V pmax is the maximum value, V pmin is the minimum value and V pmean is the average value of the ultrasonic pulse velocity.

Mercury intrusion porosity (MIP) is an indirect and relatively fast and simple method for
determining the distribution of pore size diameter [10].
MIP was conducted on a single prismatic specimen (12 ± 2 mm in diameter and 20 ± 2 mm high) cut from a Redueña quarry specimens. The analysis was run on a sample oven-dried at 70 C to a constant weight. A Micromeritics Autopore IV 9520 porosimeter with maximum pressure of Five measurements of color have been made on each side of the seven dry samples of Redueña dolostone and one of the Colmenar de Oreja limestone. The mean of these measures was calculated for each stone. The same number of measurements were made on the wetted surface with water from the same specimens.
The spectrophotometer used was a Minolta CM-700D, with a CM-S100W DATA Software SpectraMagic COLOR NX. The measure of change in visual perception of two given colors,

Results
Due to difficulties in financing of Carrara marble for the fountains, Ventura Rodríguez    The XRD allows identifying the constituent mineralogy. Redueña dolostone with gastropods has a higher dolomitic content than calcite ( Figure 5). The elements in which the Redueña dolostone differs with Colmenar de Oreja limestone are mainly Ti, Fe and Mg as shown in Table 1.
In Table 2, the petrophysical properties of the Redueña dolostone analyzed are similar or even better than those of Colmenar de Oreja limestone. Redueña dolostone presents higher porosity, V p and lower porosity due to mercury intrusion and less anisotropy.
The pore diameter distribution of Colmenar de Oreja limestone and Redueña dolostone without gastropods is concentrated at a pore diameter range of approximately 0.5 to 1 μm [2]. The pore diameter ranges of the Redueña dolostone with gastropods are between 0.01 and 1 μm and 100 to 400 μm. As can be seen in figure 6.

Sr (%) Cu (%) Fe (%) Ti (%) Al (%) Si (%) Cl (%) S (%) Mg (%)
Colmenar  and b* indicates yellow/blue coordinates (+b indicates yellow, -b indicates blue) in Table 3. L* has in general high values, between 66.9 and 83.5, being more light Colmenar de Oreja limestone. Both stones experience a reduction of the light when getting wet. a* and b* lower for Colmenar de Oreja limestone, indicating that this rock is whiter than the Redueña dolostone, whose colors are closer to the red and yellow. Redueña dolostone experiences greater change of total color when wet.
The micro-roughness varies considerably according to the type of stone from the sculptures located in the museum. The three original sculptures, carved from Redueña dolostone arount 1782, have greater micro-roughness than the fountain of Colmenar Viejo limestone installed in 1944, seen in Table 4.
R a is the arithmetic mean of the absolute values of the deviations from the mean; R q is the square root of the deviation and R z is the sum of the vertical distances between the five highest peaks and five lowest valleys found in the sample.

Discussion
The four fountains preserve original building stones of Redueña dolostone. The use in construction of Montejo member of Castrojimeno Formation with Trochactaeon Lamarcki gastropods is the first time being described in a scientific literature. Although Redueña dolostone has been studied in previous scientific articles [19][20][21] as a building stone. For example, in [2] and [19] the petrophysical characteristics of Redueña dolostone without Trochactaeon Lamarcki gastropods are described. The level with gastropods shows a lower porosity and highest ultrasound velocity [2] that makes it more resistant to the decay.
Geological history, petrophysical and petrographic characteristics, use, environmental conditions such as humidity, temperature, presence of salts and contamination, in conjunction with other factors, determine the durability of building stones [22,23]. Madrid has a climate with frequent frosts [26].
An important point related to the decay (figure 4C) is the action of mechanical strength due to low temperatures. A study on the response to freeze-thaw of Redueña dolomite is necessary. The basal pylons present large fractures, with loss of material. Also biological colonization [20], black crusts [24], use of Portland mortar, metal staples and improper coatings on fractures (Figure 4). In addition, these stones are susceptible to decay due to exposure to aggressive agents [25], mainly due to them being located in one of the most touristic areas of the city of Madrid with heavy vehicular traffic.
As indicated in petrography, fossils and fractures are filled with calcite ( Figure 7) and the matrix of Redueña stone is dolomitic. Fossil molds are visible on the top horizontal surface of the basal pylon ashlars ( Figure 7B). The calcite has greater dissolution in these horizontal pylon surface ashlars because drops of water fall on them and remain for a few minutes until their evaporation.
However, in the basal pylon outer surfaces (vertical planes), no differential dissolution is observed ( Figure 7D) because the exposure to water is minimal. There are different techniques for measuring the weathering of the stones [27]. Dissolution by water is the most important decay in the sculptures with water spouts preserved in the San Isidro.
Los orígenes de Madrid museum. Colmenar de Oreja limestone has a higher degree of dissolution and it has less micro-roughness than Redueña dolostone, since Colmenar de Oreja limestone is mostly composed by calcium carbonate [18]   The gastropods level of Redueña dolostone has similar petrophysical properties than Colmenar de Oreja limestone (Table 2) and better than Redueña dolostone without gastropods [2].
The sedimentary structures can cause anisotropy in carbonate stones [30], which in turn influences in the durability. It is observed that the anisotropy is lower in the Redueña dolostone analyzed in this study than in Colmenar de Oreja limestone. a* and b* parameters are higher in Redueña dolostone with Trochactaeon Lamarcki gastropods than in the other two stones ( The main type of the fountain pylons decay is due to the use of incorrect mortars, staples and conservation treatments. These treatments have different durability and color than the original stone ( Figure. 4C, D and E). The chromatic parameters of Redueña dolostone are provided in this study.
The color of restoration treatments should be similar to the original stone color.

Conclusions
The petrography has provided us with additional information to the historical data and allowed to determine the historical quarry of the four fountains of the Plaza Murillo in Madrid (Spain).
The original carbonate stone used in the four fountains come from a Redueña quarry. It is located at the coordinates 40.80208, -3.59141. In this place, the dolostone of the Montejo member of Castrojimeno Formation presents Trochactaeon Lamarcki gastropods and greater cementation than in other levels of the same formation. Which generates a different porosity and increases its durability or resistance to decay.
The four stone sculptures with water spout of the studied fountains are preserved in the San Isidro. Los Orígenes de Madrid museum. The study of their petrophysical properties allowed to determine that three of them are carved out of Redueña dolostone and one out of Colmenar de Oreja limestone.
The analysis of micro-roughness was employed to define that the dissolution effect on the sculptures is different between Redueña dolostone and Colmenar de Oreja limestone. Dolostone is more resistant to dissolution effect than limestome. The micro-roughness is greater in the Redueña dolostone, and however, macro-roughness is greater in Colmenar de Oreja limestone.
Redueña dolostone with gastropods has good quality, with low porosity and with good durability, carving, polishing and degree of dissolution that are adjusted to the needs of a carbonate stone to be used in places with the presence of water.
The SE fountain presents replacement stones (Colmenar de Oreja limestone) in three of the ashlars that constitute its basal pylon, in part of the central column, in the small pylon and in the sculpture with the water spout (Now in the San Isidro. Los Orígenes de Madrid museum). Now days the four water spout sculptures are epoxy resin replicas in the four fountains of the Plaza Murillo.
The petrographic characteristics of building stones give good petrophysical properties and also provide very useful scientific data for other disciplines such as history, archeology, restoration, fine arts and architecture.
It is necessary to carry out petrophysical studies with non-destructive techniques to detect fractures in the four fountains and their materials added should be removed on future conservation and restoration works. The stones must be from the original quarries to ensure their durability and compatibility of materials.