|University Federico II
of Naples, Italy
Column capital voluteMarble surfaces exposed to rain develop a rough "sugary" texture because the calcite grains are loosened as the edges dissolve in the rain water.
Jefferson Memorial, Washington, D.C.
Marble, like all calcareous rocks, is particularly sensitive to degradation by acid chemicals and to weathering. Acid rains are one of the main degradation agents for marble artifacts.
Acid rain contains carbonic, nitric and sulfuric acid, that are produced by oxidation and dissolution in water of gaseous oxides (CO2, NO2 and SO2), present in the air as chemical pollutants.
Head of an ancient statue in calcareous material with clear signs of environmental corrosion
Calcium carbonate, the main component of marble, is not soluble in water. Nevertheless the acids contained in the acid rain are capable to transform CaCO3, by chemical reactions, into soluble salts which are washed away, giving rise to the formation of holes on the surface of the artifacts, due to loss of material.
A heavily degraded statue in compact calcareous materialThe photograph was taken in former DDR, where the large and uncontrolled use of coal combustible contributed to create an acidic, chemically highly aggressive environment.
Dresden, Zwinger terrace
Natural rain is weakly acidic, because of the presence of gaseous acid compounds in the environment. In normal conditions, marble degradation goes on slowly, it may take centuries. Human activities bring about emission of stronger acids in the atmosphere, that react with marble artifacts giving rise to faster degradation phenomena. Of course the rate of degradation depends on the concentration of such acid pollutants in the air.
The main chemical reactions leading to environmental degradation of marble artifacts are hereafter described.
Carbonation - Carbon dioxide (CO2), one of the air components, is weakly acidic when dissolved in water. Rain containing CO2 has a pH about 6 and it may transform calcium carbonate into water soluble calcium bicarbonate. The chemical reaction is:
CO2 + H2O + CaCO3 → Ca(HCO3)2
George Washington, New YorkThis statue of George Washington was first put outside in New York City in 1944. During the next 58 years, acid rain caused significant damage to the statue.
A statue located in the garden of Castelfranco Veneto (Italy)Loss of face characters and presence of black crusts are evident.
Sulfation - Calcium carbonate, the major component of marble, easily reacts with sulfuric acid (transported by acid rains), producing calcium sulfate according to the reaction:
CaCO3 + H2SO4 → CaSO4 + CO2 + H2OCaSO4 is thousandfold more soluble in water than CaCO3, so it is easily washed out by the rain. Removal of material from the artifact surfaces erases its characters.
High relief in white Carrara marbleLeft: the black crust on the surface.
Right: the same artifact after a treatment of restoration and cleaning with removal of the black crust.
Black-crust formation - A mixture of crystals of calcium carbonate, nitrate and calcium sulfate makes up so-called black crusts. Such salts, which are formed on marble surface due to acid rains, are first washed away and later, because of water evaporation, are precipitated again, often incorporating black carbon particles.