CHIARI, GIACOMO. Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy. - Lichens on a sandstone: Do they cause damage?
Seven samples of sandstone, covered with lichens and old petrogliphs
were studied by optical microscopy, X-ray diffraction, ESEM, water
absorption, mercury porosimetry, and a new porosimetry measurement
based on color mode analysis of images, obtained from thin sections
with impregnating resin colored with a blue dye. This technique
measures the porosity gradient as well. The image is divided, on the
computer, into parallel slices from the outside to the core, and
porosity changes of each section are a function of depth. In our case,
porosity was smaller toward the outside, since in the outer layer
lichens occluded the pores. Counting them, the porosity was the same.
The pure quartz sandstone has homogeneous grain size, which differ
with rock type. ESEM imaging of lichens interaction with the sandstone
(David Carson, GCI), showed a superficial layer (live lichens with
smaller airborne particles) clearly distinguishable from the bulk
(larger grains and pores). Consolidation tests were carried out using
ethyl silicate [Wacker OH (with) and Monsanto Silbond (without
catalyst)], on samples with the largest and smallest grains. Porosity
decreased less for Silbond, which, without catalyst and an active
surface, did not polymerize. For an ethyl silicate treatment it is
advisable to use catalyzed products. Porosity depends on grain size:
the larger the grains, the larger porosity, water absorption,
fragility and de-cohesion of the rock. Lichens cannot etch quartz. The
grains dislodgment seems not important from thin sections
observations. Lichens fill the gaps between grains, which are large
enough to accommodate them without them exercising a relevant
pressure. This result may help to decide whether or not to eliminate
the lichens from the surface. Of course surface appearance is
important, but, at microscopic level, a serious damage due to the
lichens can be ruled out.
Key words: environmental SEM, image processing, mercury porosimetry, porosity gradient, stone biodeterioration