Black layers on historical architecture

Background, aim and scope

The external surface of any building in urban polluted environment is unavoidably destined to be covered with layers that assume a grey to black colour and are generally called ‘black crusts’. These, according to standard protocols and glossary, are deteriorated surface layers of stone material; they can have variable thickness, are hard and fragile and can detach spontaneously from the substrate, which, in general, is quite decayed. Plain visual examination may lead to consider ‘black crusts’ all similar, whilst only a careful diagnostic investigation can distinguish ‘black crusts’ and the consequences of their formation on stone substrates. In this paper, various black layers on marble are studied and compared and the morphological and compositional characteristics discussed according to the related mechanisms of formation. Differences between old (hundred years) and recent crusts (30 years) are investigated and pointed out.

Materials and methods

Samples of black crusts collected from the Milan Cathedral façade (Candoglia Marble) have been studied and compared with the careful and synergic employ of traditional techniques: optical (transmission and reflected VIS light) and electron microscopy, X-ray spectrometry and micro-Fourier transform infrared spectroscopy.

Results

Visual examination of loose fragments does not allow to point out outstanding differences amongst the various samples; black layers have similar main mineral components, gypsum and airborne particles, with different spatial distribution. The microscopic studies allowed to point out the porosity differences, the gypsum crystallisation habit, different amount of embedded particles, level and progress of marble decay.

Discussion

The observations lead to define three main types of black crusts: black crust deriving from marble sulphation, compact deposit and encrustation due to exogenic materials deposition. Black crusts show evidence of sulphation in progress, without a clear continuity solution between crust and marble; the lack of separation is particularly evident in ‘recent’ crust, where the sulphation process is more active. Black compact deposits show a higher porosity than black crusts because gypsum is not coming from the chemical corrosion of the substrate but from outside; actually, in the former case, the substrate is sound. Encrustations show a highly regular crystal organisation of gypsum (close packed tabular crystals) that cannot be traced back to casual atmospheric deposit or to corrosion of the substrate but rather to the crystallisation of a solution coming from an external source. Also in this case, the marble is sound; evidence of the effect of some protection treatment is pointed out.

Conclusions

In spite of the apparent similarity of the examined samples, analytical results have evidenced three main types of black crusts: black crust with decayed substrate, compact deposit and black encrustation showing a sound substrate underneath. Experimental evidence of calcite grains sulphation in progress, taking place according to a model recently proposed, has been observed. Sulphation process is prevented where particular conservation treatments had been applied in the past.

Recommendations and perspectives

New experimental studies can be focussed to understand the specific conditions (measurements of micro-climatic and thermodynamic parameters) and mechanisms for black crusts formation in situ. The problem of the kinetic of the sulphation process of marble, the assessment of black layers formation in the case of different carbonate stone materials and the study of acid attack in presence of surface protecting layers deserve further investigation.     

Origin,characteristics and morphology of weathering crusts on Istria stone in Venice

To evaluate the effects of the environment on weathering of monuments of Istria stone in Venice, systematic mineralogical, petrographical and elemental analyses of depth profiles were performed on samples of surficial crusts (dendritic black, compact black, grey and white). Decay products of deposition and interactions between gases and stone, including wind blown dust, marine salts, anthropic aerosol, gypsum and nitrates, are incorporated into the mineral matrix down to a depth of 10 mm. In areas sheltered from rain, where black and grey crusts are observed, the high general humidity, carbonaceous particles and deposition of aerosols lead to the development of gypsum and other salts. In compact and grey crusts decay products decrease; in grey crusts carbonaceous particles are reduced, but in compact black crust products of previous treatments are found. In white crusts, formed in areas experiencing effective wash-out, chemical and mechanical attack cause surface recession.     

Air pollution records on selenite in the urban environment

Black crusts (BC) on selenite, i.e. gypsum stone used in buildings and monuments in the city of Bologna (northern Italy) have been studied.The crusts have been compared with genetically identical ones encountered on gypsum outcrops in the area surrounding the same city, in a relatively clean rural environment.Because of gypsum's high solubility, also in the past, BC began to form and embed particles of atmospheric aerosol as soon as the stone was used in construction, even though the SO₂ levels in the air were probably much lower than today. Unlike other types of stone (sandstone, limestone, marbles), selenite is currently undergoing a deceleration of dissolution processes, following the acidification of the environment caused by industrialisation. For this reason the BC on selenite are older (up to 580±45 yr), thicker (up to 4 cm) and far more stable than those encountered on other types of building stones used in the same area.The BC are stratified: at the base, in contact with the stone, the crust contains numerous prismatic black particles originating from wood combustion; immediately above the base layer these particles progressively increase; finally, in the outer part of the crust one encounters the characteristic black, spongy spherical fly-ash originating from the combustion of mineral oils.From the chemical point of view, a comparison between urban and rural crusts shows that elements such as C, Si, K, Fe, V, Cr, Ni, Zn, Ga, As and Pb are due to present-day and/or past pollutant sources within the city itself.Moreover, the chemical data from the stratigraphy of the BC show that, while C and K were more abundant in the past than now, elements such as Fe, V, Cr, (Mn, Ni), Cu, (Zn), (Ga), As, Pb characterise recent and current urban atmospheres.     

Application of spectrometric analysis to the identification of pollution sources causing cultural heritage damage

Black crusts are recognized to have been, up to now, one of the major deterioration forms affecting the built heritage in urban areas. Their formation is demonstrated to occur mainly on carbonate building materials, whose interaction with an SO₂-loaded atmosphere leads to the transformation of calcium carbonate (calcite) into calcium sulfate dihydrate (gypsum) which, together with embedded carbonaceous particles, consequently forms the black crusts on the stone surface. An analytical study was carried out on black crust samples collected from limestone monumental buildings and churches belonging to the European built Heritage, i.e., the Corner Palace in Venice (Italy), the Cathedral of St. Rombouts in Mechelen (Belgium), and the Church of St. Eustache in Paris (France). For a complete characterization of the black crusts, an approach integrating different and complementary techniques was used, including laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), Fourier transform infrared spectroscopy, optical and scanning electron microscopy. In particular, the application of LA-ICP-MS permitted to obtain a complete geochemical characterization in terms of trace elements of the black crusts from the inner parts to the external layers contributing to the identification of the major combustion sources responsible for the deterioration over time of the monuments under study. In addition, the obtained results revealed a relation between the height of sampling and the concentration of heavy metals and proved that the crust composition can be a marker to evaluate the variation of the fuels used over time.     

Embryonic sulphated black crusts on carbonate rocks in atmospheric simulation chamber and in the field: role of carbonaceous fly-ash

Experimental conditions characteristic of the urban pollution in many European cities over the last decades were reproduced in a simulation chamber in which samples of limestone were exposed for a period of 12 months, both naked or sprinkled with carbonaceous fly-ash. In agreement with the findings reported in the literature the fly-ash emitted by heavy fuel combustion are highly reactive, undergoing chemical and morphological transformations which give rise to crystals growth, especially gypsum. The development of gypsum crystals was observed overall in close proximity of fly-ash anchoring them to the limestone surface. Samples of the same limestone exposed in the field in a polluted environment for the same period of time led to similar results. The preliminary mechanisms leading to the genesis of sulphated black crusts in polluted environments were thus highlighted. Because of their roughness the embryonic black crusts increase the development of the crust by trapping new particles. This trapping is also facilitated by the wetness of the stone surface leading to the development of hydrated mineral (gypsum) in the water meniscus between fly-ash and stone surface.     

Laser cleaning of historical limestone buildings in Bordeaux appraisal using cathodoluminescence and electron paramagnetic resonance

BACKGROUND: Most historical buildings in Bordeaux city are made of limestone. This yellowish-white rock is rather porous and highly sensitive to pollution. As a consequence of local weathering conditions, these buildings present a dark appearance due to the development of a superficial dark grey to black crust. METHODS: For the last decade, a campaign has been underway to clean these buildings. Eleven techniques of surface treatment have been used, including laser beam technology. As a contribution to the study of laser beam effects on stone buildings, two analytical methods have been used on clean versus unclean surfaces: Cathodoluminescence (CL) and Electron Paramagnetic Resonance (EPR), in addition to SEM-EDX and XRD. RESULTS: The black crust is composed of different types of particles: carbon porous micro-particles of industrial origin, atmospheric dust due to the erosion of soils and rocks, alumino-silicate particles from urban pollution; all these particles being cemented by gypsum. DISCUSSION: As far as heritage conservation is concerned, the laser surface treatment not only preserves the original patina of the stone, but also leaves surface smoothness unaltered. CONCLUSIONS AND PERSPECTIVES: CL and EPR data confirm that lasers--with highly controlled parameters--only get rid of the black crust and, thus, reveal the underneath layer, the so-called patina. This patina shows no luminescence, whereas the limestone on which it has grown shows a bright orange emission of CL. This indicates CL to be a fast and easy way to provide a high quality control for the restoration of polluted ancient stones.     

Sandstones of unexpectedly high diffusibility

Measurements have been made of diffusion coefficients (D(i)=-mass flux/concentration gradient) using a double reservoir, steady-state method with two tracers, CaBr(2) and amino-G-acid, on intact samples of Triassic red-bed sandstone from northwest England. Diffusibility (D'=D(i)/diffusion coefficient in water) averages 0.124, ranging between 0.075 and 0.215 (porosity 0.1 to 0.24), very similar for the two tracers. Implied tortuosities (actual path length/straight line length) average 1.21 (range 1.06 to 1.47), with constrictivities close to 1. In comparison with limited red-bed sandstone data from elsewhere, these D' values are up to 4 times greater, and tortuosity correspondingly lower. Re-interpretation of formation factor data from previous studies on shallow sandstone samples also from northwest England confirms that diffusibility is significantly higher in these sandstones than others from similar palaeoenvironment/stratigraphic units. The lower tortuosities appear to result from the relatively high permeability, open fabric of the rock, properties likely to be present in shallow sandstone systems used for water supply. It is concluded that diffusion rates may, in some shallow freshwater-containing continental sandstone systems, be significantly greater than is implied by estimates of sandstone diffusibility current in the literature.     

Preliminaries on pollution risk factors related to mining and ore processing in the Cu-rich pollymetallic belt of Eastern Carpathians, Romania

The present study focuses on the mineralogical and geochemical patterns of mining and ore-processing wastes from some occurrences in the Eastern Carpathians; its aim is to identify the main factors and processes that could lead to the pollution of the environment. In this respect, the following types of solid waste were investigated: efflorescent salts developed on the surface of rock blocks from a quarry, ore-processing waste from two tailings ponds, and salt crusts developed at the surface of a tailings pond. The potential risks emphasized by these preliminary investigations are the following: (1) the risk of wind-driven removal and transport of the waste from the surface of tailings ponds, given that fine grains prevail (up to 80 %); (2) the risk of tailings removal through mechanical transport by water, during heavy rainfall; (3) the appearance of hydrated sulfates on the rock fragments from the mining waste, sulfates which are highly susceptible to the generation of acid mine drainage (pH?<?4); (4) the high amount of toxic elements (Pb, Cd, Cu, Zn, As, etc.) that acid mine drainage leachates contain; and (5) the development of a salt crust on the flat, horizontal surfaces of the waste deposit, due to this very shape. Statistical data regarding the amount of both major and minor elements in the tailings have revealed two statistical populations for nearly all the toxic metals. This suggests that, beyond the effect that the tailings have upon the environment through their mere presence in a given area, there are alleged additional factors and processes which intensify the pollution: the location of the waste deposit relative to the topography of the area; the shape of the waste deposit; the development of low areas on the surface of the deposit, areas which favor the appearance of salt crusts; and the mineralogy of efflorescent aggregates.     

Towards a better understanding of tetrachloroethene entry pressure in the matrix of Permo-Triassic sandstones

The pressure required for a chlorinated solvent to enter a geological medium can be calculated given knowledge of the characteristic pore size of the medium and the interfacial tension (IFT) and contact angle of the solvent-water-rock system. Using a centrifuge-based method, capillary pressure-saturation curves have been determined for 30 water-saturated samples of Permo-Triassic sandstones for the solvent tetrachloroethene (PCE). These curves have been successfully fitted using the van Genuchten function to determine PCE entry pressure for each of the sandstone samples. A plot of PCE entry pressures against average pore diameter shows a linear relationship in log-log space; however, observed values for PCE entry pressure are significantly lower than would be expected theoretically for a sandstone-PCE-water system. This may be explained either by a decrease in the IFT or an increase in the contact angle. The IFT may decrease during contact with sandstones due to hysteresis effects during imbibition and drainage of fluids, but this is unlikely to be sufficient to account for the low entry pressures observed. Therefore, it is inferred that the low observed PCE entry pressures are due to higher than expected PCE contact angles, and that the average pore-throat surface of the sandstones is more solvent wetting than would be expected. A weak acid extraction indicates the presence of calcite and dolomite in the sandstone cores, and a correlation is observed between carbonate content per unit porosity and a reduction in PCE entry pressure. It is suggested that these mineral phases are responsible for observed wettability changes and a conceptual model is proposed. One consequence of the lower observed entry pressures is that solvents are likely to penetrate deeper into the matrix of water-saturated sandstones than previously expected.     

Origins of Manganese in Air Participates in California

The proportions of manganese to other metals in samples of airborne fine particles taken at some sites in California have increased greatly since the beginning of 1985. Here, data are presented which indicate that the addition of manganese to leaded gasoline is largely responsible for this increase. Concentrations of manganese, silicon, titanium, iron, lead and other elements in airborne particles were measured using energy-dispersive X-ray fluorescence analysis. Coefficients of correlation among levels of manganese, iron and lead measured at twenty sites in California were calculated. Levels of manganese and iron are generally highly correlated because of the presence of large amounts of these elements in the earth’s crust. Levels of airborne manganese and lead at sites In Southern California are often highly correlated, suggesting a vehicular source of manganese. Observed manganese concentrations are apportioned into two major sources: the earth’s crust and motor vehicles. The apportionment indicates that vehicular emissions of manganese may account for a significant part of the total at urban sites in Southern California. At most other sites, the contribution from vehicles is found to be far smaller than that from the earth’s crust.     

地衣对建筑物的生物破坏作用

大量的证据表明地衣可以诱发和加速矿质基质的物理、化学风化过程,着生于建筑物表面的地衣能对建筑材料导致明显的生物破坏.其中,地衣菌丝在矿质基质中的穿插生长、原植体随环境变化表现出的膨胀收缩、冻融交替等机制对建筑物造成机械破坏.而地衣分泌的草酸等简单有机酸以及大量的高分子有机化合物诱发和促进矿质基质的生物-化学风化过程.受其影响,矿质建材中原生矿物遭受溶解、蚀变,并产生多种次生矿物.最终破坏建筑物的坚固程度、缩短其使用寿命、减低其美学价值.     

Nitration of polycyclic aromatic hydrocarbons adsorbed to different carriers in a fluidized bed reactor

The reactivity of selected polycyclic aromatic hydrocarbons (PAH) adsorbed to various surfaces has been studied. PAH were adsorbed to silica, alumina or charcoal particles and exposed to ambient concentrations of NO₂ in air in a fluidized bed reactor. The reaction products were analyzed by capillary gas chromatography and gas chromatograpy/mass spectrometry.The results show that silica is a more reactive surface than alumina. The charcoal used showed to be very adsorbtive and too little material was recovered for analyses. The reactivity of the PAH compounds studied decrease in the order perylene > benzo(a)pyrene > pyrene > chrysene > fluoranthene and phenanthrene. The relevance of these results to the assessment of the possible formation of nitro-PAH in the atmosphere is discussed.     

Biogenic cements from rice hull ash doped with aluminum and iron

This work describes the use of rice hull as starting material for the synthesis of cements doped with iron and aluminum. Rice hull contains about 10-20% of silica along with organic material. In many countries rice hull represents an environmental problem since this material is merely burned at rice fields, rendering suspended silica particles in the air. Dicalcium silicate (beta-Ca(2)SiO(4)) is the second most important component of Portland cement and presents many environmental advantages over commercial cement. It can be prepared at lower temperatures saving energy and raw-materials. In this work we describe the synthesis beta-Ca(2)SiO(4) using silica derived from rice hull ash. Silica was obtained from heating rice hull at 600 degrees C. Starting materials (silica, calcium oxide, barium chloride, iron or aluminum oxide) were weighed in stoichiometric proportions and aqueous dispersions having water:solid ratio of approximately 20:1 were prepared and treated in an ultrasonic bath for 60min. After this, an intermediate silicate and the excess of calcium hydroxide were obtained. Finally solids were dried, grounded and heated up to 800 degrees C. It was observed that beta-Ca(2)SiO(4) was obtained when dopant concentration was limited to 1%.     

The assessment of particulate matter emitted from stone-crushing industry by correlating rock textures with particles generated after comminution and dispersed in air environment

The generation and emission of particulate matter from abrasion industry are subjects of the pollution monitoring by multidisciplinary study involving earth sciences and engineering disciplines. This work investigates the correlation between textural properties of in situ rock with class size distribution and morphology of particles generated after rock comminution and particles emitted in the air. A special comminution-dust sampling architecture was realised. The combined use of scanning electron microscopy and particle size analyser was considered in performing digital image analysis on both crushed products and airborne particles collected onto membrane filters. The results show that the size and morphology of crushed particles are linked to the petrographic rock properties. In particular, particles with fibrous morphology are prominent in rocks showing foliated textures where elongated minerals occurred, with implication for asbestos-bearing rocks. For what concerns the airborne particles, the results show that their aerodynamic diameters are independent of the crusher operating conditions. External parameters probably intervene in the distribution of the airborne particles emission, including the dynamic air fluxes, or environmental conditions. By applying mathematical models, the morphology and size range of airborne particles following the comminution processes can be predicted, and results has implication for pollutants contamination due to particulate matters emitted by crush stone industry.     

Preliminary indications from atomic force microscopy of the presence of rapidly-formed nanoscale films on aquifer material surfaces

The objective of this study was to determine if there is a nanoscale surface film on aquifer-like materials exposed to deep groundwaters, as has previously been found on surfaces exposed to surface and soil waters. Such surface films will modify surface properties that are so important in determining the mobility of many groundwater pollutants. Muscovite mica was used because a) it is a good analogue for the main sorbing phases of many clastic aquifers and b) its cleavage planes are atomically flat allowing high resolution imaging. Freshly-cleaved muscovite plates were exposed to groundwater from a sandstone aquifer for 30 min, and surface properties (morphology, coverage, roughness and tip–substrate force interactions) were measured using atomic force microscopy (AFM). A patchy surface film of several nanometres in depth, incorporating larger separate particles, was found on the mica surface. This film was associated with significantly increased roughness values and AFM probe–sample interaction forces compared with pure water and inorganic (synthetic groundwater) solution controls. Although the results reported are preliminary in nature, if confirmed, such films are likely to affect sorption reactions, surface-facilitated redox interactions, non-aqueous phase liquid wetting angles, and colloid–pathogen–rock attachment, and will thus be of importance in understanding natural attenuation and migration of dissolved, non-aqueous and particulate phases in groundwaters.     

Waste green sands as reactive media for the removal of zinc from water

Waste green sands are industrial byproducts of the gray iron foundry industry. These green sands are composed of fine silica sand, clay binder, organic carbon, and residual iron particles. Because of their potential sorptive and reactive properties, tests were performed to determine the feasibility of using green sands as a low cost reactive medium in permeable reactive barriers (PRBs). Serial batch kinetic tests and conventional batch sorption tests were conducted to determine the removal characteristics for zinc in aqueous solutions. Removal characteristics for zinc in the presence of green sands are comparable to those of Peerless iron, a common reactive medium used in PRBs. High removal capacities for zinc of green sands are attributed to clay, organic carbon, and residual iron particles, which are known sorptive media for heavy metals. Furthermore, high pH values in the presence of clay and residual iron particles enhanced sorption and precipitation of zinc.     

Real Time Measurement of the Size Distribution Of Particulate Matter by a Light Scattering Method

The body of information presented in this paper is directed to those individuals concerned with the measurement of the size distribution of particulate matter in air. The light scattering instrument described herein is characterized by the fact that it can accurately size particles almost independently of their index of refraction. The basic concept involves the simultaneous measurement of the intensity of light scattered by a single particle at two small scattering angles. The ratio of the two intensities is directly related io ine size of ihe pariicle, and for scattering angles of 5° and 10° the effective range of the instrument is 0.2 to 4 μm. The air flows through the optical system at such a rate that approximately 25 μs are required to determine the size of each particle, and concentrations as high as 10⁴ particles/cc can be measured without dilution and without serious coincidence effects. By employing a multichannel analyzer as the data storage and readout device it is possible to detect changes in particulate size distribution within a few seconds. Calibration of the instrument has been performed using polystyrene latex spheres and materials having a wide range of index of refraction and shape including carbon black, iron oxide and spores.     

Long-term measurements of iron-containing aerosols by Mössbauer spectroscopy in Poland

Mössbauer spectroscopy was applied to analyze the iron compounds present in atmospheric aerosol. As a significant part of air pollution, especially in winter months, iron appeared in the form of iron sulfides (FeS₂, FeS and Fe_1−xS), which were products of coal combustion. Also, iron oxyhydroxides and iron oxides, mostly α-Fe₂O₃ (bulk) and in the form of ultra fine particles in superparamagnetic state were observed. The concentration of iron in atmospheric air was calculated from the experimental spectra. Seasonal variations of iron concentration in atmospheric air measured over twenty years in the mountain region of Poland are discussed.     

The concentration, apparent molecular weight and chemical reactivity of silica from groundwater in southern Nevada

Sorption of radionuclides, metals and organic compounds to colloidal particles has been suggested to increase the mobility of these pollutants in groundwater. Because silicates and alumino-silicates can be important components of groundwater colloids, we have conducted a study to characterize the nature of silica in various springs and wells in Southern Nevada and to determine the extent that silica may be associated with colloidal particles that can participate in pollutant transport. The total silica content was measured using inductively coupled plasma emission spectroscopy (ICP). In addition, reactive silica was measured using the silica molybdate colorimetric technique. The apparent molecular weight of the silica was investigated using split-flow-lateral-transport-thin-cell-fractionation (SPLITT) which can readily distinguish between colloidal and low molecular weight associations. This study indicates that silica does not tend to form stable inorganic colloids in Southern Nevada groundwaters but exists as low molecular weight species. However, water from one of the test facilities on the Nevada Test Site (NTS) did contain stable siliceous colloids that could have important implications for the modeling the transport of radionuclides at this site.     

Effect of amorphous silica and silica sand on removal of chromium(VI) by zero-valent iron

The effect of two surfaces (amorphous silica and silica sand) on the reduction of chromium(VI) by zero-valent iron (Fe(0)) was investigated using batch reactors. The amendment of both surfaces significantly increased the rate and extent of Cr(VI) removal. The rate enhancement by amended surfaces is presumed to result from scavenging of Fe(0)-Cr(VI) reaction products by the provided surfaces, which minimized surface deactivation of Fe(0). The rate enhancing effect was greater for silica compared to sand, and the difference is attributed to silica's higher surface area, greater affinity for reaction products and pH buffering effect. For a given mass of Fe(0), the reactivity and longevity of Fe(0) to treat Cr(VI) increased with increasing dose of silica. Elemental analyses of the reacted iron and silica revealed that chromium removed from the solution was associated with both surfaces, with its mass distribution being approximately 1:1 per mass of iron and silica. The overall result suggests reductive precipitation was a predominant Cr(VI) removal pathway, which involves initial reduction of Cr(VI) to Cr(III), followed by formation of Cr(III)/Fe(III) hydroxides precipitates.