Lichens on Wyoming Sandstone
Seven samples of sandstone covered with lichens from a Wyoming canyon that support petroglyphs were studied using a number of different techniques including COLORMOD, a porosimetry measurement based on color mode analysis of images, obtained from thin sections with impregnating resin containing a blue dye. This technique also allows one to measure the porosity gradient. Porosity was less toward the outside, since in the outer layer lichens occluded the pores. When the lichen body was counted as well, the porosity was proved to be the same as in the core of the rock. The sandstone composed largely of quartz, is homogeneous in grain size within a single rock, but differs greatly from one rock to another. ESEM imaging of lichen interaction with the sandstone showed a superficial layer (live lichens and small mineral 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 greatest and least porosity. Porosity decreased less for Silbond, since, without a catalyst and an active surface, it hardly polymerized. Therefore, if an ethyl silicate treatment is planned, it is advisable to use catalyzed products. The grain size distribution of the sandstone controls the physical properties: the larger the grains, the greater the porosity, water absorption, fragility and de-cohesion of the rock. Based on observations of thin sections, grain dislodgment due to lichens is difficult to demonstrate because lichen hyphae fill the gaps between grains, which under dry conditions seem to be large enough to accommodate the hyphae without exercising undue pressure on the structure of the rock. This result may help to determine the advisability of removing the lichens from the rock surface.
KeywordsSandstone lichens porosity image processing consolidation conservation
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