University Federico II
of Naples, Italy
European Chemistry
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Degradation factors

This stone sculpture is highly degraded because of environmental factors

Artifacts in stone, due to the action of various environmental factors, suffer deterioration processes, which are quite often irreversible. The mechanisms of such processes strongly depend upon the chemical/physical nature of the constituent materials, as well as their structure and morphology.

The factors of deterioration of stone artifacts are conveniently classified into Physical, Chemical and Biological factors.

  1. Physical factors
    • Wind may produce:
      • erosion processes due to transport of hard rock granules (for example quartz);
      • disintegration, due to fast evaporation of water from the pores and crystallization of salts.
    • Hot/cold cycles cause phenomena of expansion/contraction, which give rise to micro-fractures through which water transports dissolved salts and reactive chemicals inside the rock.
    • Freeze-thaw cycles damage stones mainly because water increases in volume when it freezes. When water is contained in rock pores, this determines pushes on their walls, and in time it may produce cracks and fractures.
  2. Chemical factors - Acid Rains
    The presence in the environment of pollutants such as carbon, nitrogen and sulfur oxides (CO2, NO, NO2, SO2) is the consequence of liquid, gaseous and solid substances emitted in the air by industries, motor vehicles, and domestic heaters.

    The mechanism of acid rain formation is transformation of the above mentioned (gaseous) oxides into the corresponding acids and their dissolving in water. The chemical reactions involved are hereafter depicted.

    for Carbon:

    H2O + CO2 → H+ + HCO3-

    for Sulfur:

    2 SO2 + O2 → 2 SO3
    this reaction occurs in the presence of microscopic particles of metal oxides, acting as catalysts, and of light
    SO3 + H2O → H+ + HSO4-
    in presence of water sulfur trioxide SO3 transforms into sulfuric acid

    for Nitrogen:

    the nitrogen oxides, following various reactions, give rise to the formation of nitric acid H+ + NO3-

    Under some particular conditions, following different mechanisms, hydrochloric acid (HCl) also forms. It must be pointed out that hydrochloric, nitric, and sulfuric acids all are very strong acids.

    Water is one of the most effective degradation agents. As a matter of fact water not only acts as a solvent (acid rains), but it also directly takes part in many reactions leading to degradation of stone materials.

    Acid rains are particularly noxious to calcareous stones, such as marble statues, and to siliceous rocks whose cement phase has a calcareous composition, such as sandstones. Siliceous rocks are generally resistant to acid rains; their degradation is mainly due to the action of physical and biological factors.

    Particles of carbonic, metallic and siliceous nature contained in the envirobmental particulate, with a damaging action to stone artifacts
    Atmospheric particulate is constituted of very small particles of carbonic, metallic and siliceous nature, produced by combustion of fossil fuels. It contributes to degradation of stone artifacts, mainly by promoting the formation of black crusts.
     
  3. Biological factors
    They include bacteria, fungi, algae, lichens, mosses, molds, arboreal and herbaceous plants. Some lichen species are effective in damaging calcareous stone artifacts; others only form black or green colored patinas on the surfaces, with little bulk effect. Plants, by penetrating their roots deep into the stone material, may determine heavy mechanical damages that can lead to formation of fractures.

Examples of degradation in stone artifacts are seen in the next page.