Natural stones in buildings, like all rocks outcropping on the surface of the Earth, are subject, with the passing of time, to more or less intense transformations which may even only be evidenced by macroscopic analysis. Of all available materials, mankind has made extensive use of stone. It is often considered to be "eternal" but in reality is even subjected to far-reaching transformations of its characteristics.

 

These alteration/deterioration phenomena are the result of interaction between stone material and the atmosphere: water plays a decisive role, as well as atmospheric gases and the various particulate components.

 

For the lithotypes used in historical and contemporary buildings, it is easy to note, for example, variations in colour or full-scale stains, a lack of cohesion between the various components of the rock, detachment of portions of material, formation of saline substances, appearance of fractures and the growth of various vegetal organisms.

 

Each kind of rock often presents a particular morphology of deterioration but examples of stone materials characterised by more than one form of alteration/deterioration are by no means rare. The installation environment plays a fundamental role in the development of such effects.

 

Historic rocks in general present a worse conservation status than materials installed in more recent times. In certain conditions (climatic, installation site, type of installation, form and thickness of the stone element), however, even contemporary natural stones may present alteration and deterioration.

 

This article outlines the mechanisms and forms of deterioration, with reference to standardised terminology (Normal Recommendation 1/88, Lexicon of the macroscopic alteration of stone materials).

 

Meteorological deterioration and erosion are the geologic processes that mainly affect rocks on the surface of the Earth. The action of natural agents (weathering) is influenced by chemical-mineralogical-structural composition, climatic conditions and exposure time. Chemical and physical mechanisms may significantly modify rock and subsequent transport of these products may even give rise to a new stone.

 

Meteorological deterioration and erosion mechanisms have intensely modified the nature and appearance of the Earth’s surface and, with tectonic and volcanic factors, were and still are the most important geological processes in the lithogenetic cycle.

 

Evidence of such modifications are before the eyes of everyone: sand on a beach and the pebbles in a river bed are created by the crushing and morphogenetic re-elaboration of rocks subjected for long geological periods to these processes. Even "natural stone" used in buildings present such action, for example through far-reaching surface transformations up to significant loss of material in monuments and various architectural works.

 

Climate plays a fundamental role: conditions of high humidity and temperature can rapidly generate such modifications; a dry climate involves lesser transformations (or in any case of a different type). Frost, wind, fog and the action of sea waves give rise to modifications that are readily seen in rocky outcrops everywhere.

 

In any case, it is only a question of time (on both human and geological scales): a rock will always disintegrate.

 

For rocks used in buildings, the decisive role played by lithological nature is immediately apparent: the different response of rocks to natural agents can be seen, for example, in tomb headstones of the same age and in the same place made with different types of rock, or by considering the different conservation status of marble, granite and stone in general in the same construction and the same installation position. As regards, for example, the heating of rock in sunshine, one can consider the behaviour of calcite, a typical mineralogical component of a great many marbles: the linear thermal expansion of this crystal is anisotropic, positive parallel to the main crystal axis and negative at right angles to it. The crystal, inasmuch, because of temperature variations, expands in one direction and contracts in the other, with the outcome that, because of cyclic expansion and contraction, micro-fractures are generated between the granules, with an increase in the porosity of the rock until loss of cohesion and detachment of the granules takes place. The larger the grain structure of marble, the more intense the thermal deterioration. The minerals in natural stone are subjected to chemical transformations - just as all rocks on the surface of the Earth - are subjected to chemical processes. Their minerals react with water, oxygen and carbon dioxide to generate new materials. For example, potassium feldspar (an important component of many granites) in the presence of water is modified into the clay-like mineral kaolinite, releasing silica and potassium ions into the water solution. The process (hydrolysis) involves breakage of the original crystalline structure and dissolution of the potassium ion; it is very slow but the presence of carbon dioxide accelerates it alongside increase in temperature. A granite in a warm and humid climate is significantly degraded and about one-third of its content in potassium feldspar is totally turned into clay.

 

Carbonates, the main components of marbles, essentially include calcite, calcium carbonate and dolomite, calcium carbonate and magnesium, that dissolve and respectively release ions of calcium and calcium+magnesium. Carbon dioxide, in the presence of water, transforms calcium carbonate into bicarbonate, that is soluble and inasmuch expelled from the rock.

 

The dissolution of carbonates is very evident on the surface of marble artefacts exposed to the action of acid rain, that involves attack of carbonatic minerals, while silicatic minerals resist better and protrude in relief on the surface of the stone.

 

In a simplified scale of chemical stability, rock salt is the most easily degraded mineral, while quartz is extremely stable. A rock such as quartzite, in general, is rather long-lasting.

 

Chemical reactions such as hydration/oxidation of iron oxides bring about the formation of hydroxides ("limonite") with yellow-brown spotting of the rock.

 

Just like the rocks of the crust of the Earth, even natural stones - used in different fields of application - are involved in a state of loss of equilibrium from the time they are installed compared with the initial conditions of their formation and are inevitably converted, often with forms comparable to those seen in the original outcrops. Over and above such "natural" modifications, such materials are also exposed to the chemical action of the pollutants present in the atmosphere: since the end of the 1800s, this process has become increasingly evident and in many cases is the primary cause of the conversion of natural stones. A vital role is played by pollutants such as sulphur dioxide (SO2), nitrogen oxides (NOx), carbon dioxide (CO2), a normal component of the atmosphere which may nevertheless become a pollutant following increased concentration, ozone, particulates (metal oxides, carbon-based material, ash, gas deriving from the combustion of oils and various mineralogical powders). In particular, sulphur dioxide (developed to a great extent by the combustion of fossil fuels) reacts with water to form sulphuric acid; contact with calcite in marbles then forms CaSO4.2H2O. The use of methane in cities has currently helped significantly reduce the concentration of SO2 in urban areas, although high percentages of particulates remain - that blacken the surface of stone where there is no rainwater run-off, thus also promoting the absorption of other pollutants by the stone material itself. In particular, in urban areas, there is very evident deposition of particulates caused by road traffic, emitting pollutants at ground level, affecting the facades of buildings that become intensely blackened, especially if the roads are narrow and the buildings are high.

 

Natural stones also undergo bio-deterioration caused by animals (particularly birds, whose droppings mechanically damage the surface of stone in installation sites) and plants (musk, algae, lichens, weeds, larger plants).

 

Particular natural events, such as earthquakes, exceptional rainfall and strong winds may also involve important transformations of stone building materials.

 

The history of the rock from quarrying to processing thus plays a vital role as regards its durability. The type of installation of the stone element is another significant factor, together with the design characteristics and, especially, the thickness of the rock utilised.

 

The cause of deterioration in natural stone may also involve the direct action of people: acts of vandalism spoiling decorative stone materials, damage by terrorist action, warfare, lack of maintenance or unsuitable intervention.

 

For all these causes, natural stone may take on an appearance and properties that are very different from their original features: related terminology inasmuch defines alteration when the modification process of a natural stone does snot imply worsening of its characteristics from a conservational point of view, while deterioration always implies worsening (Normal - 1/88). In reality, it is difficult to define a clear distinction between the two: to what extent, for example, is the process involving chromatic alteration of the material also reflected in its durability? If petrographical techniques are used to investigate stone materials that have undergone transformations after installation, it is extremely difficult to establish a limit between the processes that do or do not worsen their conservation status, although from a macroscopic point of view the distinction between the two types of process is more evident.