The advanced condition of deterioration of the column’s bases of the courtyard of the Bishop’s Palace of Tarazona (Spain) built in the middle of 16th century required its restoration with
Alabaster is a stone that has been used since centuries in buildings, mainly for decorative purposes, altarpieces and window enclosures (
The isotopic analysis (sulfur, oxygen, strontium) in medieval and Renaissance artworks allowed to determine that the Nottingham quarries supplied Sweden, Iceland and Spain, and the Catalonian quarries (Spain) provided the material for the South of France (
The main goal of this study is to evidence the alabaster decay agents when exposed indoors but mainly outdoors. This study is focused in the alabaster found in the court columns of Bishop’s Palace of Tarazona (Aragon region, Spain), due to the permanent exposure of the columns bases to rainwater that caused loss of mass and alteration of the original shape. Although the restoration of these columns was already carried out, no reports about the weathering agents were done before it. Thus, this study will allow to assure if the interventions were adapted to the deterioration agent and they really will contribute to the long-term alabaster preservation.
The court columns were carved by the French stonemason Guillaume Brimbeuf during the pontificate of the bishop Juan González de Munébrega between 1556 and 1558, under the direction of the Piacenza artist Pietro Morone (
a. Removal of mortar on base. b. Removal of reinforcement mortar that covered lower part of carved ring and shaft. c. Alabaster quarry. d. New bases. e. Restored alabastrine gypsum columns.
In the restoration works carried out between 2016 and 2017 (
Three parts of the column NE of the court were sampled in order to characterize the different alabaster features. The samples were obtained from the basis, the lower shaft and the upper shaft. XRD analysis was carried out with a Bruker D8 Advance equipment and the results analyzed with the DIFFRACplus software. Density and open porosity were obtained following the standard EN 1936 and water absorption by immersion following the standard EN 13755.Two samples, one from the column bases and other from the upper shaft were selected for a petrographic study with the polarized optical microscope Olympus AX-70. The water used for the hydric test was not distillated water as mentioned on the standard due to the low pH and the high solubility of alabaster stone. Water was enriched in Ca with a measured pH = 7 to avoid dissolution.
From fresh quarried stones, water absorption and density were determined following the mentioned standards. In addition, capillary water uptake, p-wave velocities and elasticity modulus, and uniaxial compression strength were obtained following the standards EN 1925, EN 14579, EN 14146 and EN 1926 respectively. The water used for the hydric tests was not distillated water as mentioned on the standard due to the low pH and the high solubility of alabaster stone. Water was enriched in Ca with a measured pH = 7.2 to avoid dissolution. In addition, the drying stage was carried out at 40°C until weight stabilization. The dynamic parameters were measured with a Pundit Plus avec transductors with a frequency of 54MHz. Although alabaster did not show preferential orientation or bedding, p-waves were measured in three directions.
Three different ageing tests were undertaken in order to simulate the weathering agents that may attack the alabaster bases and columns.
This test was conceived to assess the effect of temperature changes on the alabaster stone. These variations can be of different nature, and due to that, the samples and the tests were adapted to obtain the maximum information. The first aim was to assess the deformation suffered by slabs (bowing) exposed to UV at high temperature conditions. For these purposes the selected samples were 6 slabs of dimensions7cm × 3.5cm × 0.5 cm (named G) and 6 slabs of 7cm × 2.5cm × 0.5cm (named P). These two different widths were used to evaluate the influence of the slab dimension in the deformation.
The second aim was to determinate the color changes. Color was measured on the bigger slabs (G) with a Minolta colorimeter CR-400 using the D65 illuminant, beam of diffuse light of 8-mm diameter, 0° viewing angle geometry, specular component included and spectral response closely matching the CIE standard observer curves. Measurements are expressed following the CIE L* a* b* systems. ∆E* is introduced as the total color change, to compare the variations before and after the tests as follows: ∆E* = [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2. A threshold of ∆E=3 is generally considered as the limit above which homogeneous color change is visible to the naked eye (
The third aim was to study the microfissures evolution due to thermal fatigue. For this test, eight cubes of 3.5cm edge were used.
Two consecutive tests were carried out, the UV exposure and the thermal fatigue. In the first test, all the samples were introduced at climatic chamber Suntest XXL+ for sunshine ageing test in which three cycles were undertaken by day. Each cycle consisted in four hours of heating at 60°C plus UV radiation that involved a total temperature around 90°C-105°C followed of four hours of cooling at 20°C and in the darkness. The duration of this test was 20 cycles and the energy irradiated during this time was 1800kWh/m2, equivalent to five years of real exposition in north of Spain. Capillary uptake, color, and deformation in the respective samples were measured after 10 cycles and at the end of this test.
After that, all the samples were exposed to the second test. They were introduced in a Votsch climatic chamber and submitted to 70 cycles of thermal fatigue, with one cycle consisting in four hours at 90°C followed by four hours at 20°C and the same parameters were measured at the end of this test. The heating and cooling rate were less than 2°C/min, to avoid fracturation due to a sudden temperature change. Capillary water uptake, color and deformation were measured again after the tests. XRD analyses were carried out to determine if new mineral phases such as bassanite or anhydrite appeared. In addition, mercury injection porosimetry was measured with a Micromeritics Autopore IV 9500 to observe if porous network distribution varied with heating.
Two acid rain tests were carried out following the methodology developed in Eyssautier et al. (
The first test carried out was named passive immersion (PI). It reproduced the effect of stagnated water at the basis of the column by the simple immersion of the cubes in the water solution. The second test was named active immersion (AI). It simulated the drivingrain and summer storms by the immersion of the samples in the water mixture and by placing the recipient in a shaking table with a movement frequency of 180 rpm. The concentrations of Ca2+ were measured from the solutions every day by Inductively Coupled Plasma - Atomic Emission Spectrometry (ICP-AES, Iris Advantage, Thermo Fisher Scientific). The analysis of Ca2+ was conducted at wavelength of 184.006 through three measurements of each solution. A 50 ml aliquot of all the immersion solutions was sampled and stored at 4°C until chemical analyses.
A third test was developed to simulate real damage in the Tarazona palace columns. The test consisted in placing a prism of 7cm height and 4.5cm base in a film of distilled water. The film rises only 1mm simulating capillary test. A cooling plate Tetech CP061 set at 60°C was placed in contact to one 7cm × 4.5cm side. These two setups simulated the heating of the column during the day and stagnated water after rain. Cycles were performed with 6 hours of drying at 40°C in an oven followed by 18 hours of test. Due to the low capillarity of fresh stones, 6 hours at the selected temperature were enough to dry the surface humidity and cool down the sample. After four cycles of heating and cooling, the samples were dried in an oven at 40°C for 72 hours. Then the weight and dimensions as well as visual observation were measured. After the whole test, capillary water uptake was determined.
The petrographic characterization of the column samples (base and upper shaft) revealed that visually both samples showed a granular texture, with gypsum crystals of variable size (0.1 mm to few mm) as white and translucid crystals, both encompassed by a clayey brownish matrix. By microscopic assessment, three crystal distributions were observed in the base sample. The first type were gypsum aggregates of 1–2.5 mm with a crystal sizes of 20 to 100 μm with the typical alabastrine texture; the second one were allotriomorphic crystals with sizes comprised between 50 μm and 0.5 mm; the last type were subidiomorphic crystals of higher size, from 0.5 to 1.5 mm with anhydrite inclusions. This base sample showed 34% of alabastrine texture, 56% of granular subidiomorphic texture, 8% of marly matrix (clay and carbonate) and 2% or relict anhydrite. In the upper shaft, 75% of the sample revealed a granular subidiomorphic texture with crystals of very variable size, from 100 to 300 μm generally, but up to 2500 μm in rare cases. The alabastrine original texture was only represented in a 13%, with gypsum crystals under 50 μm. In this sample, the marly matrix proportion with clays and carbonate was of 11% while relict anhydrite was only 1% (
Alabastrine texture in the centre of upper shaft (
Both samples from the base and the upper shaft came from the same rock block as deduced from its similar texture. The alabastrine texture can be considered as the original of the stone although the proportion of granular idiomorphic texture is more difficult to predict, it could belong to the original texture or the recrystallized one. In any case, the stone was a low quality alabaster due to the 10% of clays and carbonate matrix and commonly found and used in larger pieces than pure alabaster (
XRD analysis confirmed that the three samples obtained from the column (two from theshafts and one from the northeast base) were gypsum in its majority, with almost pure composition in the upper shaft (93,4%) and a more clay-rich and partially transformed to bassanite in the base (gypsum 74%, bassanite 3%, phyllosilicates 14%, quartz 4% and calcite 5%). The porosity obtained by water absorption (EN 1936) revealed that the upper and bottom shaft showed values of around 14% and 6.5% respectively. The base sample had the lower porosity (around 2.5%) explained by the phyllosilicates and clays filling the fissures.
The main petrophysical properties of the material are presented in
Aragonese alabaster properties. Bulk density (D), Porosity accessible to water (P), Water absorption by immersion (WAbs), Capillary suction (C), Dynamic elastic modulus (Edyn); P-waves velocity (Vp); Flexural strength (σf), Compressive strength (σc).
Source | Alabaster type | D (g/cm3) | P (%) | WAbs 24 h (%) | C g/cm2.min0,5 | Edyn (GPa) | Vp (m/s) | σf (MPa) | σc (MPa) |
---|---|---|---|---|---|---|---|---|---|
ENTECSA ( |
Alabaster from Jiloca, Aragon | 2.31 | - | 0.11 | - | - | - | 7.10 | 24.3 |
Alfonso et al. ( |
Champagne and Tobacco types | - | - | - | - | - | - | 5.3–5.7 | 35.6 |
Bardillo type | - | 3.9 | 24.6 | ||||||
This study | Base (original) | 2.20 | 2.58 | 1.17 | ( |
( |
( |
( |
( |
Bottom shaft (original) | 1.77 | 6.45 | 3.64 | ||||||
Upper shaft (original) | 1.91 | 13.75 | 7.19 | ||||||
Aragonese Bardillo type (restored bases) | 227 | 0.67–0.72 | <1 | 146±5 | 22.20 ±0.5 | 3128 (x) |
- | 26.6± 1 |
Very small samples to test.
Under microscope, the studied samples showed a granular texture with gypsum crystals of variable size from 70–350 µm approximately and a proportion of around 70–75%%, and some aggregates with alabastrine texture and a proportion around 25%. Also relict anhydrite with size around 80 µm and a proportion of 1% was observed. DRX analyses of the sound sample revealed a composition of pure gypsum.
The physical properties calculated by hydric methods did not give valuable results. The extremely low porosity and fragility after water immersion of the alabaster induced a low error during manipulation, nevertheless enough to consider the results not reliable. Thus, mercury intrusion was used with coherent results as an open porosity of 0.67–0.72% and an apparent density of 2.3 g/cm3. The pore distribution revealed a high open porosity over 10 μm and few pore access radii of lesser size with some samples showing voids up to 0.005 μm.
After only ten cycles of UV exposure, the color variation due to the dehydration of gypsum to bassanite was flagrant. The samples showed a white powdery layer that spread out between 10 and 20 cycles, up to cover approximately 80% to 90% of the sample surface. After measuring the color in dry conditions, the samples were dampened with a sponge to recreate the initial measuring conditions. The wet slabs showed a lighter color than initial slabs but due to the new white lines around the preexistent fissures, more evident after 20 cycles. After the second test, carried out afterwards on the same samples, the slabs become completely white with almost no differences between the dry and the wet conditions. The small differences were located on the fissures, with light brown color which turned in darker colors after wetting. The aspect of the samples is shown in
Surface appearance after thermal fatigue test.
The color evolution from the fresh stone slightly dampened to the dry samples with the white powdery layer was very evident during the first cycles and only small variations were measured from then. Initial values of lightness (L*) were medium/dark due to the sample’s translucency and changed to values around 90 with the bassanite transformation. The decrease in b* from yellow colors to white ones was also remarkable (
Color evolution after thermal tests in dry and wet measurement conditions.
The color evolution between wet samples showed the same trend but with bigger differences from 20cycles to the end of the following 70 cycles. The tone of the final samples (a* and b* parameters) was more vivid in the wet samples due to brownishing of the fissures filling in contact with water.
DRX analyses confirmed the bassanite presence, with around 65% for the slabs and 40% for the cubes, which indicated that the transformation was not complete. Microscopical observations of the sound stone before testing and the slabs and the cubes after thermal tests, revealed notable differences between them (
Microscopical observation with non-polarized (up) and polarized (bottom) light. Sound alabaster with alabastrine texture (left); cubes after insolation test and the textural transformation to bassanite (center); slabs after insolation test showing the progression of the textural transformation (right).
The slabs did not show any measurable bowing deformation. The stone texture adapted to temperature changes without any difference through the sample. Capillary water uptake was measured after 20 and the following 70 cycles of thermal variations. The initial capillary coefficient was null and so was after 20 cycles. The fissure network had not suffered any variation that would allow the water entering through the stone. At the end of all the tests, the average capillary coefficient increased to 146 +-5 g/m2 s1/2. The porosity calculated by the hydrostatic measure method (EN 1936) revealed a porosity at the end of the test of 24–28%, very different from the <1% of the fresh samples. Mercury porosimetry revealed an increase of porosity up to 8% with a unimodal distribution and a peak around 0.15 μm (
Pore radii access distribution obtained by Hg porosimetry of the fresh samples (left) and compared after thermal fatigue (right, vertical axis x20).
The visual differences denoted a great change between samples and between tests (passive and active immersion). For all the samples, the surface increased its roughness. For both tests, the samples with a white homogeneous texture dissolved uniformly while the samples showing fissures experimented a differential dissolution. The clays that filled the fissures dissolved or fell mechanically slower that the gypsum and that created, a relief on the surface.
After the passive immersion test, the cubes showed a shape of slightly truncated tetrahedra, keeping their dimension unaltered at the bottom and decreasing linearly through the top. That is due to the greater dissolution of the cube vertices completely exposed to the solution. During the active immersion the samples exhibited a homogeneous dissolution (
Comparison of surface condition after passive (up) and active (down) immersion tests.
The weight loss in all the samples followed a linear trend, with an 8% of weight loss in passive immersion samples and more than 13% in active immersion, values comparable to the volume loss (
Weight loss in passive and active immersion tests.
The dissolved cations of the solutions were analyzed every day. Ca2+ values corresponded to gypsum dissolution and Mg2+ to marly matrix components. The results showed that gypsum dissolves in a homogeneous way with similar values every day (
Evolution of the dissolved cations on the solution.
After only 15 days some variations were observed in the prism, exposed to acid water on the basis and high temperature on one vertical face. Visual color change was observed in both exposed areas. The first centimeter from the bottom lost the powdery aspect due to the sawing process and obtained a wax gloss and darker colors. The face exposed to the heat showed a slight variation to white in some areas although not changing the stone general aspect. A dissolution and later precipitation in the water-air boundary was observed.
The weight loss measured every day revealed a sharp decrease during the first three days due possibly to the lack of pH equilibrium. After these days the loss was less accentuated but continuous with time (
Weight loss during the combined test.
Ageing tests revealed that alabaster was very susceptible to rain and insolation exposure. For the same intensity of the ageing test (360 hours of thermal fatigue and 384 hours for immersion), rainwater that lead to stone dissolution produced more decay than the exposure to high temperatures. This fact was probed by the combined test.
Regarding rain events, two different processes occur when in contact with the outer column of Bishop’s Palace of Tarazona. In the shaft, rainwater enters in contact with the stone and then the water run-off the column by gravity. Due to the low porosity of these stones, water does not enter but reacts with the surface while dropping. The ageing test that corresponded with this phenomenon was the active immersion. This test revealed that dissolution was more important that stationary immersion because salt dissolution involved two processes, i) the chemical reaction of the surface and ii) the fluid flow. When the rain water interacts with the stone, it has a pH around 5.6 due to the CO2 contained in the water. This acidity starts a reaction leading to dissolution during the run-off water along the stone surface. The solution pH increases slightly during the way down though it remains in acid values due to the short time needed to arrive to the column basis. The velocity and the intensity of the run-off water influence the reactions kinetics, with higher dissolution for higher velocities (
Then, the rain water reached the basis of the column and stagnate there. Its pH is less acidic before the contact with the stone due to its saturation in gypsum (
Even with all the parameters for being less weathered than the shaft (stone more resistant to water, run-off water with higher pH, stationary flow) the basis of the columns looked more weathered than the shaft. Due to the climatic conditions of Tarazona’s area, with several rainy days even during the dry months, the cycles of wetting-drying seemed to produce more damage that dissolution by water stagnation. During the cycles, a flow is produced during the wetting (arrival of run-off water), and also a molecules movement during evaporation. This flow activates the dissolution reaction but also produces a mechanical effect on clays that are detached from the stone. This creates an increase on specific surface in which the new run-off water may act, enhancing the basis deterioration. In addition, dissolution is a linear process for natural stones, however, close to equilibrium the dissolution is inhibited (
Regarding the deformation due to the insolation exposure observed in other studies (
The most important deterioration linked to temperature changes was the progressive dehydration, i. e. bassanite transformation, translated visually as color variation from light brown to white. To simulate the transformation rate, the stequiometric balance was calculated relating the molar masses and the sample weight as follows (
Mg and Ma are the molar mass of the minerals, 172.17 g/mol for gypsum and 145.14 g/mol for bassanite; m0 is the mass of the fresh samples and mt the mass after the complete test of thermal fatigue that is 90 cycles corresponding to 360 hours. The results gave a value of αb of 0.88 with a deviation of 0.04 indicating that the transformation to bassanite was not completed and in agreement with XDR results. This transformation involved an increase in porosity by hydric methods from 0 to 28% but not in volume (in agreement to
Alabaster exposed to environment is strongly affected by dissolution from rainwater, by run-off water but also by stagnated water in plinths or copings. The loss of material is evident, but porosity does not change, keeping the structural properties of the inner core intact. The temperature variation produces a long-term decay that involves the dehydration to bassanite, which can start at 40°C and entails an increase in porosity. This change produces a higher dissolution and also structural weakness if the transformation reaches the inner core. The early stages of dehydration reveal an evident change in color that will allow to detect the transformation and to adopt prevention measures before an irreversible decay. The main solution to avoid the dissolution and the permanent solar exposure consists of the physical protection of alabaster building elements as well as drainage of water mainly on horizontal surfaces in contact with them. Other alternatives are treatments to avoid surface dissolution, use in areas of less sunshine and frequent maintenance measures.
The authors acknowledge the Universidad Politécnica de Madrid for the EEBB mobility fellowship and the Fundación Tarazona Monumental and architect José M. Valero. Also, to Alexandra Guillaneuf for the chemical analysis, Julien Hubert for the Hg porosimetry analysis, Xavier Drothiere for the photographic support and Enrique Areces and Nicanor Prendes for XRD collaboration.