Rate of formation of black crusts on marble. A case study

Journal of Cultural Heritage 1 (2000) 111 – 116 © 2000 E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved S1296-2074(00) 00161-8/FLA

Rate of formation of black crusts on marble. A case study Roberto Buginia*, Marisa Laurenzi Tabassob, Marco Realinia a

Centro CNR ‘‘Gino Bozza’’, p.za L. da Vinci 32, 20133 Milan, Italy b ICCROM, via di S. Michele 13, 00153 Rome, Italy Received 15 May 1999; accepted 24 October 1999

Abstract – The formation of black crusts on stone monuments is an important process in stone deterioration. The aim of this work is to study the rate of formation of crusts in an urban area for which pollution levels are well known. Samples of crust were collected from measured areas of two sculptural groups (made from white Carrara marble) inaugurated in 1937 on the front of Milan General Hospital and never restored. Analyses were carried out on ground samples by XRD, ionic chromatography and SEM. Gypsum is the main component followed by carbonaceous particles and iron oxides. The rate of formation of the crust, calculated considering the average crust thickness, the sample weight, the area of sampling and the length of exposure to the atmospheric pollution (54 years), is 2 – 5 mm per year. The amount of gypsum formed per unit surface (5–13 mg/cm2) has been calculated from the sulphate content and the sample weight per unit surface; the rate of gypsum formation in the black crust is about 0.2 mg/cm2 per year. © 2000 E´ditions scientifiques et me´dicales Elsevier SAS Keywords: stone / marble / decay / black crust / gypsum / pollution

1. Introduction The effects of air pollutants on stone monuments have been observed and studied for many years, especially in northern European countries, where the industrial development had a negative impact on air quality. The increasing presence of carbonaceous particles suspended in the air, coupled with a damp, cold atmosphere, brought about a more rapid blackening of building surfaces and more intense stone decay, with a concomitant need for intensified maintenance [1]. According to Brimblecombe [2] people since ancient times have been concerned by the smoke produced by burning organic combustibles and its negative effects, but only much later did the distinction between particulate and gaseous pollutants begin to be understood. * Correspondence and reprints: [email protected]

With the development of industrial activities and the corresponding increase in air pollution, scientific studies on its effects became more frequent. Already in the 1930s several papers on this specific topic were published [3, 4], and after the Second World War the correlation between sulphur compounds in the air and gypsum present on the surfaces of deteriorated stones was clearly described by Camerman [5, 6]. Since then, studies on pollutant-induced deterioration have been aimed not only at investigating in greater detail the mechanisms of stone–pollutant reactions but also at evaluating the deterioration rate of stones exposed to a polluted atmosphere. These studies attempt to identify ‘damage functions’ through which it is possible to describe the rate of deterioration of given stones when exposed to a given atmosphere.

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Several methods can be followed for this purpose. In the majority of cases, however, the evaluation is based on the loss of stone surface, as observed in those parts of monuments and buildings that are directly exposed to rain washing. Some mathematical functions – the so-called ‘damage functions’ – have been proposed to describe the rate of surface loss as a function of the concentration of gaseous pollutants (mostly sulphur dioxide) and of the pH of rain. In a recent publication [7], the interaction between CaCO3 in stones and rain was thoroughly discussed and the damage functions proposed by different authors were examined on the basis of statistical analysis. As a conclusion, a new function has been proposed, valid for marble and limestone in the range of 3 –5 rain pH:

if not impossible, to hypothesise any time frame for the formation of black crusts unless one knows whether, when and how the stone surface has been cleaned. A conservation treatment recently carried out on two contemporary marble groups offered the opportunity to attempt such an evaluation. These two groups were carved by two renowned Italian artists (Arturo Martini and Francesco Messina) to decorate the main entrance of a huge, new hospital built in Milan (Italy). The official inauguration was held in 1937 [8]. Since then, no cleaning was ever carried out until 1992, when a conservation treatment was decided upon. The two sculptural groups were not sheltered from rain, but, due to the presence of rather deep undercuts where rain could not reach, thick black crusts had been formed. The washed surfaces, in contrast, showed the typical corroded pattern produced by the solvent action of rainwater. In December 1991, after the statues had been exposed to the Milan environment for 54 years, some samples were taken from the two marbles to carry out the diagnostic study, prior to conservation treatment. This unusual situation was considered a unique opportunity to attempt an evaluation of the rate of formation of the black crust, and suitable samples were collected for this purpose.

surface loss (mm/m rain) =18.8 + 0.016 H+ +0.18 (Vd × SO2/R) where Vd is the deposition rate of SO2 (cm/s), SO2 is the concentration of SO2 (mg/m3) and R is the amount of rain (m). For a condition in which R =1 m/year, SO2 = 40 mg/m3 and Vd = 0.3 cm/s (typical of the NE United States), Lipfert calculates that limestones lose an average of 22 mm/year. Even if very useful for a quantitative evaluation of the surface loss on rain-washed surfaces, the proposed damage functions do not consider other important types of deterioration, such as the formation of black crusts and the subsequent catastrophic events of their detachment and the loss of the powdering material underneath. These events are rather frequent and their significance in the deterioration rate is very high. It is, however, very difficult to assess the rate of formation of black surface crusts and the time required for a crust to detach from the substrate and fall away, as evaluation through direct measurements on monuments is highly uncertain. In fact, the history of maintenance or restoration treatments on monuments is rarely complete. Consequently, it is difficult,

2. Sampling The two sculptural groups, which flank the main entrance to the building, were studied. Black crust samples from four sheltered, flat areas were carefully collected by scraping the surface with a sharp knife; the intermediate layer was collected in all cases where the black crusts were still adhering well to the marble beneath. The area where the sample was taken was measured and the corresponding sample weight was registered without any conditioning or drying (table I).

Table I. Sampling of black crusts. Statue Pio II Bianca M. Sforza Deputati Ospedalieri San Carlo

Sample 1 2 3 4

Weight (mg)

Area (cm2)

448.9 1 067.0 294.4 404.0

50 60 21 34

Weight/area (mg/cm2)

(Weight/area) per year (mg/cm2) per year

8.98 17.80 14.02 11.88

0.17 0.33 0.26 0.22

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Table II. Ion content in black crust (meq in 100 mg) (–, absent). Sample

F−

Cl−

NO− 3

SO− 4

Ca++

Mg++

Na+

K+

NH+ 4

1 2 3 4

0.0017 0.0033 – 0.0111

0.0012 0.0041 0.0010 0.0101

0.0002 0.0002 – 0.0013

0.6417 0.8699 0.6678 0.6972

0.8542 1.0092 0.7258 0.7923

0.0008 0.0012 0.0007 0.0008

0.0003 0.0003 0.0002 0.0009

0.0009 0.0010 0.0011 0.0025

0.0002 – – –

The differences in size and shape among the samples are due to the availability of regular, flat surfaces where area measurements could easily be carried out. Several fragments of stone with black crust were collected from other areas as well, in order to carry out the necessary petrographic characterisation of marble and to study the morphology of the system crust/surface.

3. Analytical techniques The powdered samples, scraped off from the black crusts, were first analysed by XRD in order to detect the crystalline phases. The same samples were subsequently conditioned at 60 °C and solutions containing 100 mg of sample in 100 cc water were prepared, according to Raccomandazione NORMAL [9]. After filtering on a cellulose filter (0.45 mm pore size), the solutions were analysed by ionic chromatography to carry out the quantitative evaluation of the ions coming from water-soluble salts. From the fragments collected, thin and polished cross-sections were prepared for observation under a polarising microscope and a scanning electron microscope equipped with an EDS X-ray spectrometer.

component, while calcite and quartz are present in low quantities. The results of the ionic chromatographic analyses carried out on the water solutions are given (table II) as milli-equivalent of the ions in 100 mg of dry sample. As observed in the polished cross-sections, the thickness of the black crust is irregular, ranging from 300 mm to a minimum of approximately 100 mm. It is mainly composed of gypsum crystals and carbonaceous black particles; iron oxides are present as well. The calcite crystals of the very external marble surface are frequently corroded along their cleavage planes, where newly formed micro-crystals are also visible. Moreover, some isolated calcite macro-crystals are enveloped by the black crust (figure 1 ). The same cross-sections, when analysed by EDS, showed that the crust is mainly composed of sulphur and calcium. SEM observations (coupled with EDS analyses) of the external side of the black crust fragments showed the presence of gypsum plaques, crossed by a network of microcracks (figure 2 ). A closer view of the crust shows the well-known ‘desert rose’ pattern of gypsum crystals together with spongy, rounded carbonaceous particles. It is worth noting that the latter also contain sulphur, silicon and aluminium, with minor amounts of iron and titanium.

4. Experimental results The samples taken from both groups are finegrained marble, characterised by equi-dimensional calcite crystals. The crystal size ranges from 0.1 to 0.2 mm. It is reasonable to hypothesise that white marble from Carrara was used by both artists. From the microscopic observations, however, minor differences were observed between the two groups. The marble used by Martini has tiny veins running parallel to the vertical axis of the sculptures; in Messina’s statues, no veins are seen but several areas with larger calcite crystals are clearly visible. The XRD of the powdered samples scraped from the black crusts show that gypsum is the main

Figure 1. Calcite crystals in the crust (polished cross-section).

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Figure 2. Gypsum plaques on black crust with microcracks (SEM).

Figure 3. Spongy particles on the internal surface of the black crust (SEM).

strong similarity of the marbles used and the identical exposure conditions. The elemental, mineralogical and structural properties of the black crusts are very similar to those described in many scientific reports concerning the effects of air pollution on marbles and compact limestones [10–15]. Gypsum is the main component, followed by carbonaceous particles and iron oxides. Therefore the samples examined in the present study can be considered, at least in a first approach, as representative of the conditions of marbles exposed to a polluted, urban atmosphere, in a temperate climate with rainy cold seasons. As reported (table I), the amount of black crust per unit surface ranges between 9 and 18 mg/cm2, the difference among the samples being due to the difference in thickness and morphology of the collected crusts. Actually, the mechanism of crust formation seems to be influenced by the slightly different conditions that may exist in the different parts of the statues (e.g. verticality of the surface, orientation, thickness of marble block, etc.). In fact, the crust in sample 2 has a dendritic morphology, whereas in the other three samples it is flatter. The thickness measured in the cross-sections varies from 100 to 300 mm. Considering the elapsed time between the inauguration of the statues and the collection of the samples (1937–1991), a rate of 2–5 mm/year can be calculated for the formation of the crust. Such an appraisal, however, considers a constant rate during the whole period, but this is probably not true also because of the variations in pollutants concentrations in Milan in the period concerned. The amount of gypsum formed per unit surface and per year can be calculated (table III) assuming

The spongy particles are present both on the external surface of the black crust and on the internal surface, at the boundary line with the marble crystals. These inner particles seem to have a more regular, spherical shape (figure 3 ). Compared to the morphology of the black surfaces, samples taken from rain-washed parts are free of gypsum deposits but are severely corroded along the intergranular surfaces and along the cleavage and twinning planes of the calcite crystals (figure 4 ).

5. Discussion of the experimental results The samples collected from the two sculptural groups can be considered as a unique set, due to the

Figure 4. Corroded calcite crystals on washed marble surface (SEM).

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Table III. Rate of formation of black crust. Sample

Black crust (mg/cm2)

Black crust (mg/cm2) per year

Gypsum (%)

Gypsum (mg/cm2)

Gypsum (mg/cm2) per year

1 2 3 4

8.98 17.80 14.02 11.88

0.17 0.33 0.26 0.22

55.2 74.8 78.9 75.5

4.95 13.31 11.06 8.97

0.09 0.25 0.21 0.17

that all the SO− detected is related to gypsum, 4 because no others minerals containing SO− 4 have been found by XRD analyses of the crust. From these data, the rate of gypsum formation in the black crust is in the range of 0.1 – 0.2 mg/cm2 per year. Considering that gypsum is present not only in the collected surface portions but also penetrated inside the calcite crystal of marble, the above values could be slightly underestimated. ++ The correlation among SO− concen4 and Ca trations is quite good (table II). Even if there is a sensible excess of Ca++ equivalents in samples 1 and 2 and none of the analysed anions can balance this Ca++ excess. The samples were taken just at the black crust/marble interface; this fact can partly justify the amount of CaCO3 in the samples, but the solubility of CaCO3 is too low to justify the imbalance completely, and the discrepancy remains unsolved. Similar imbalance, however, was detected in other marble monuments of different ages, exposed in urban polluted environments and different climatic conditions [16]. This could possibly be attributed to the presence of a metastable calcite phase, therefore with higher solubility, formed through a dissolution/reprecipitation process. In order to verify the above experimental results with the concentration of sulphur-containing pollutants in Milan, some of the available data from a monitoring station not far from the Niguarda hospital have been used. Even if data are not available for the entire period of exposure, it can be presumed that air pollution increased fairly continuously until the 1970s; from that period on, the SO2 concentration started to decrease (mainly due to the growing use of methane in domestic heating) but it still remains higher than 200 mg/m3 in winter. Standard particulate matter concentration on the other hand remained rather uneven for the same period and ranged from 100– 200 mg/m3 (figure 5 ). Several hypotheses have been proposed for the formation of the black crust [17, 18]; in all of them,

however, besides the suspended particulate matter (s.p.m.), SO2 and water are also considered to play an important role. Once formed, the crust cannot be considered a stable layer, merely covering the original stone surface. Black crusts are always rather porous. From the experimental results (weight per unit surface and thickness of the crust) the bulk density of the crust can be roughly estimated: it ranges from 0.4 to 1.8 g/cm3. These values are much lower than 2.7 g/cm3 which is the bulk density of white Carrara marble [19]. The continuous network of microfractures in the crust is the way through which aggressive gaseous and liquid products can penetrate and reach the calcite crystals underneath. The penetration is confirmed by the presence of gypsum inside the intercrystalline spaces and by the increase in those spaces. Also the rather layered structure of the crust could be interpreted as the result of a continuous dissolution/crystallisation process, which brings about the slow accumulation of s.p.m. on the surface of the first calcite row or even its penetration into the microfractures (figure 6 ).

6. Conclusion The study of modern marble (or limestone) sculptures, exposed for a known period in a known

Figure 5. Yearly mean concentration values of SO2 ( ) and s.p.m. ( ) in Milan.

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Figure 6. Gypsum penetration along microcracks (thin cross-section).

environment, could be a useful way to make a quantitative evaluation of the rate of black crust formation. A very careful collection of samples, aimed at that specific purpose, is of the utmost importance. The rate values reported in the present study indicate a rapid process for the formation of the black crust (2–5 mm/year; 0.2 – 0.3 mg/cm2 per year) and the rate of gypsum accumulation (0.1 – 0.2 mg/cm2 per year) is rather high. In this area the trend of s.p.m. concentration is very irregular (yearly average values range from 120 to 170 mg/m3) and it is correct to expect similar values and similar trends for the non-detected years as well; on the contrary the average SO2 concentrations show an evident decrease in the last year, but it is correct to assume that during the non-detected period the values were higher. From these data we cannot calculate how many more years are required for the black crust to fall away and expose an already deteriorated marble surface. This ‘catastrophic’ part of the deterioration process is perhaps the most difficult to evaluate in terms of velocity and would require the study of slightly older, untouched monuments where the detachment phenomenon can be observed in its initial stages. The experimental results of the present study are therefore not exhaustive for a complete evaluation of the damage produced by sulphur-containing pollutants on calcareous stones sheltered from rain. However, they can represent a first contribution towards a deeper understanding of marble deterioration in a polluted, urban environment.

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