When talking about weathering processes the way in which the surface is is very important ........

Read the passage given below and answer the questions that follow :

The problem of the conservation of historical monuments (statues, columns, ancient buildings) requires an accurate study of the many environmental agents which bring about the
1o deterioration process and affect the actual rate of deterioration. Among the most active causes of weathering, we can cite : (i) the weakening of the outer layer of the artwork, which is caused by complex microphysical effects and is in particular due to wetting of the outer layer of the artwork (a comparison between the effects of rainwater and
condensation is made); (ii) the chemico-physical action of pollutants captured in both the dry and wet phase.

The destructive processes depend on many factors : the past history of the monument, the capture and nature of the pollutants, and the frequency of some microclimatic conditions, particularly those supplying the water necessary for chemical reactions. To this end studies of diurnal and seasonal variations on the monument of both the heat wave and the fluxes of moisture and energy are very important.

Some of these processes have been shown with microclimatic studies and microphysical investigation associated with mineralogic and chemical analyses of samples of weathered material. Mathematical models in some cases (especially for solar radiation and temperature) permit physical simulation, reconstruction of past data and extrapolation into the future observation of many monuments from different epochs; situated at various sites, gives examples of the fundamental processes that are often associated with, or masked by, other effects.

The main weathering processes are due to the combined action of rainwater and atmospheric pollutants (particularly the carbonaceous particles due to combustion) deposited on the surface of the monument. The way in which the surface is wetted is very important in fact, a short drizzle can activate the dry deposit without washing it away, and in this case the pH of rain droplets is of secondary importance; on the other hand, showers supply abundant water which favours dissolution of the stone and removal of the solute, resulting in
a thinning of the original rock. Another important factor is the dynamic regime (i.e., laminar or turbulent) of the water flowing over the surface of the monument.

In zones where the surface of marble or calcareous monuments is only wetted, but protected from run- off, black crusts (characterized by crystals of gypsum and calcite with carbonaceous particles embedded in the crusts) are formed. Zones subjected to heavy run- off are also subjected to a thinning of the rock and small authentic calcite crystals form that are white in appearance.

In the case of relatively unpolluted towns, where marble and limestone sulphation is not the main cause of the deterioration process, biological deterioration mechanisms are often very important, due to the activity of epilithic and endolithic microflora and microfauna. 

The case of particularly precious mortars, e.g., murals or frescoes, is discussed in order to clarify the thermodynamic method proposed for the analysis of experimental campaigns. Finally actions are described that act on the causes and not only on the effects.




 

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