Case 4
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- Introduction
- Causes of Defects
- Good Practices
- Standards
- Maintenance and Diagnostics
- Remedial
- Similar Cases
- References
Cause of Defects
The mechanism of soiling follows that as described:
Soiling on stone is hydrophobic in nature. Most of the water held in pores in the stone would be available to microorganisms. As soiling builds up on stone, it gradually reduces the access of moisture into the stone
If soiling builds up to a sufficient level it may completely cut off fluid movement into and out of a stone through its face. Although moisture may still be able to enter and leave the stone in the vapour phase, moisture movement can still occur internally. With moisture and sunlight, algae perpetuate its growth.
Another possible cause for such defect:
- Treatment to stones may alter some of the characteristics of stone and affect the growth of algae and other organisms:
- Removal of soiling may reduce the levels of some toxins (e.g. sulphate) and some nutrients (e.g. hydrocarbons) at the stone surface
- Surface roughening caused by abrasive cleaning may affect the susceptibility of a surface to biological growths.
- Residues from chemical cleaning may increase algal and lichen growth by providing nutrients which are normally in limited supply on stone surfaces
- The removal of the hydrophobic soiling layer by abrasive cleaning may increase the amount of moisture which can enter a stone. Any increase in moisture availability is likely to increase the amount of biological growth. Increased movement of fluids through a stone may mobilise a previously stable mineral assemblage in the outer layers of the stone altering its suitability for particular organisms.
- Removal of many of the pollutants (e.g. sulphates) present in the soiling may allow organisms to colonise the stone which were previously inhibited by the toxicity of particular components of the soiling layer. Alternatively, some organisms (e.g. bacteria) may have previously utilised some components of the soiling (e.g. hydrocarbons) as nutrients.
- Removal of a normally dark soiling layer will increase light penetration below the surface of a stone which, in a porous, light-coloured sandstone, may allow colonisation by photosynthetic organisms to greater depths than was previously possible.