Cristina Sabbioni

Wetting, deterioration and visual features of stone surfaces in an urban area

Abstract The deterioration of carbonaceous rocks in urban areas can be roughly classified in three kinds of feature patterns visibly distinguishable on the rock surface. In this paper the three different kinds of deterioration are described and interpreted in terms of the principal mechanisms leading to their formation. The way liquid water controls both the rate and the morphology of the deterioration crust formed on the stone as observed in the field, indicates that local rainfall is a critical factor in determining the type of deterioration. It appears that the different weathering observed actually depends on the way the interaction of rain with the surface occurs, at one extreme a complete washout of the surface, to the other where the surface is only very slightly wetted. The wetting resulting from moisture condensation on the stone surface appears to play a very minor role compared with that due to rainfall.

The origin of calcium oxalates on historical buildings, monuments and natural outcrops

Abstract Calcium oxalates (whewellite and weddellite) occur widely in nature. They usually form a patina on the marbles and limestones of monuments, historical buildings and artifacts, as well as natural outcrops. Whewellite and weddellite are produced by a pervasive transformation of the calcium carbonate, occurring in the presence of lichens (or more rarely of other microorganisms such as blue algae and fungi) which provide the oxalic acid required to precipitate the calcium in the form of oxalates. In the past, when the quality of the atmosphere was more suitable for lichen support, the formation of oxalate patinae was a widespread weathering phenomenon in urban areas. Today, in highly polluted sites, weddellite and whewellite are found only as a testimony to such a past transformation. The limited thickness of the oxalate patina must be related to the shallow penetration of the fungi hypae into the rock. Because of this, the phenomenon of oxalate formation: (1) has been first observed on monuments, where superficial weathering processes are better studied; and (ii) has been found on the vertical surfaces of monuments where this phenomenon is not masked by other weathering processes, as usually occurs on natural outcrops. Finally, regarding the oxalates present on monuments, such as the so-called “scialbatura” of Roman imperial marble: their origin has been widely reported in the literature as being due to protective treatments. Our experimental results firmly exclude this.

Origin and growth mechanisms of the sulfated crusts on urban limestone

Building stone surfaces exposed to the urban environment are subjected to rapid and differentiated deterioration, due to various chemical and physical attacks. Apart from a chemical deterioration below the sufated crusts, which mainly grow outwards at the unaltered gypsum/carbonatic rock interface, another physical deterioration was observed. The latter is, at times, even more important than the rapid chemical attack typical of the zones which are subjected to heavy washing by rainwater. The formation and growth of three different kinds of deterioration layers are discussed with reference to the main kinds of deterioration characterized by white, grey and black external appearance.Analyses of airborne particulate matter and of deterioration layers of urban monuments led to the conclusion that not all the gypsum of crusts which can be observed on marble monuments is derived from the transformation of the underlying rock, but it is also partially transported by aerosols.

Atmospheric deterioration of ancient and modern hydraulic mortars

Different types of ancient and recent hydraulic mortars were collected from well-documented archaeological, historic and modern buildings in various geographical locations (urban, suburban, rural and maritime) of Italy, Spain and Belgium, representative of different environmental impacts, types and degrees of deterioration. A synthesis of the characteristics of the collected samples is presented, along with the identification of the formation products that occurred on the sample surfaces as a result of the reaction of the mortars with atmospheric pollutants. The analyses were performed by means of optical microscopy (OM), X-ray diffractometry (XRD), scanning electron microscopy (SEM-EDX) and ion chromatography (IC). The results obtained prove that sulphation processes takes place on hydraulic mortars, leading to gypsum formation on the external surface of the samples. Through the reaction of gypsum with the aluminate hydrate of the binder, ettringite formation was found to occur on a cement-based restoration mortar sampled in Antwerp.

Mapping the impact of climate change on surface recession of carbonate buildings in Europe

Climate change is currently attracting interest at both research and policy levels. However, it is usually explored in terms of its effect on agriculture, water, industry, energy, transport and health and as yet has been insufficiently addressed as a factor threatening cultural heritage. Among the climate parameters critical to heritage conservation and expected to change in the future, precipitation plays an important role in surface recession of stone. The Lipfert function has been taken under consideration to quantify the annual surface recession of carbonate stone, due to the effects of clean rain, acid rain and dry deposition of pollutants. The present paper provides Europe-wide maps showing quantitative predictions of surface recession on carbonate stones for the 21st century, combining a modified Lipfert function with output from the Hadley global climate model. Chemical dissolution of carbonate stones, via the karst effect, will increase with future CO(2) concentrations, and will come to dominate over sulfur deposition and acid rain effects on monuments and buildings in both urban and rural areas. During the present century the rainfall contribution to surface recession is likely to have a small effect, while the increase in atmospheric CO(2) concentration is shown to be the main factor in increasing weathering via the karst effect.

Airborne carbon particles and marble deterioration

Abstract Oil-fired carbonaceous particles are consistently found inside gypsum-calcite layers covering some marbles located in northern Italy urban areas. A series of analyses performed on the particles and on thin sections of sulfated layers show that these particles are probably the most important, if not the determinant, agents of the observed deterioration. Some observations seem also to indicate the way they act. The present results, combined with the origin of these particles, as indicated by their nature, support the hypothesis that the reported marble deteriorations affecting our artistic heritage actually started after the last war.

Contribution of atmospheric deposition to the formation of damage layers

Abstract A study was performed to evaluate the contribution of atmospheric deposition to the formation of damage layers on stone monuments. Elemental concentrations and evaluation of the enrichment factors with regard to carbonate stone and soil dust are reported for the site of Ancona. A discrimination between natural and anthropogenic components has been obtained.

Urban stone sulphation and oil-fired carbonaceous particles

Abstract There is evidence that oil-fired carbonaceous particles play an important role in the gypsum formation on urban limestone. The course of the stone decay registered in Venice strongly supports this view. A model simulating the impact of atmospheric factors on this mechanism justifies the unusually high rate of stone damage observed in Venice.

Exposure tests of building materials in urban atmosphere

Samples of stones and mortars (air setting and hydraulic) characteristic of ancient masonry and modern conservation works were exposed in Milan and Ancona for a period of 6, 12 and 24 months. After exposure, the samples were analysed. As previously observed on carbonate rocks, the data obtained highlight that sulphation also occurs on mortars (which are more reactive than stones), producing an intermediate product, i.e. calcium sulphite. Sulphate and sulphite, alongside nitrate and nitrite, typical of urban atmospheric deposition, were found in greater amounts in the samples exposed in Milan, while chloride, a typical sea-site tracer, was found higher in Ancona.

Organic anions in damage layers on monuments and buildings

Abstract This article is focused on small (C1–C2) organic anions present in the damage layers on historic monuments and buildings. Formate, acetate and oxalate are consistently found in black crusts, where atmospheric deposition accumulates along with the products of the chemical transformation of stone and mortars. While sulphation processes affecting building materials have been extensively studied, the importance of carbon compounds in black crusts is only recently being realised. Recent data show carbon to be the second most important airborne element after sulphur in damage layers on building exteriors. Total carbon is composed of carbonate-, organic-, and elemental carbon. The organic fraction includes formate, acetate and oxalate; these are always detected in black crusts. Their origin, role and measurement in the atmosphere and in the museum environment have been the subject of many studies, but little has been reported concerning their presence in building exterior damage layers. This paper presents data on these anions in damage layers on stones and mortars sampled on monuments and buildings at different urban, suburban and rural European sites. Oxalate encountered in black crusts likely originates from the metabolism of micro-organisms and protective treatments on surfaces. Primary and secondary atmospheric pollutants are likely the main sources of formate and acetate anions.