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