PRESERVATION OF PREHISTORIC PETROGLYPHS: INFLUENCE OF SILICATE SOLUTIONS ON ROCK PROPERTIES, SAPAKSI AND POOL ROCK SITES, LOS PADRES NATIONAL FOREST, CALIFORNIA

Rock art sites across the U.S. are threatened with deterioration and destruction as the result of the natural erosive forces of wind, water, and freeze/thaw cycles and anthropomorphic forces. Internationally-known petroglyphs were incised in sandstone caves and outcrops throughout the Los Padres National Forest, California by Chumash, Sailden, and Esselen Indians over thousands of years. The sandstones these artists used as a canvas are medium-coarse grained, Oligocene-Miocene aged, subarkosic-quartzose with minor non-quartz cement, that are interpreted to be fluvial deposits. For quartzose rocks, treatment by infusion of ethyl silicate solution into the rock pores offers the potential to enhance rock strength and extend the preservation of these cultural artifacts. Following saturation of the pore space with ethyl silicate solution, evaporation and contact with water results in precipitation on pore body and pore throat surfaces of microcrystalline quartz and/or siliceous cement and may form a chemical bond with the quartz grains. Fundamental questions about the effects of this process on the rock must be addressed to insure that remediation of rock art sites will be safe and last millenia. To evaluate the influence of ethyl silicate treatments on basic rock petrophysical properties and the nature of the cementation, sandstone samples from the Sapaksi and Pool Rock sites were tested. Sandstone blocks from each site were cored and divided into four groups representing no treatment, one, two and three treatments. Each treatment resulted in an average porosity decrease of 2.2 porosity percent with a total average decrease of 6.6 porosity percent for three treatments. Air permeability (k), critical for stone "breathing," negatively correlates with porosity but decreases less with decreasing porosity for treated samples than untreated (Treated: logk (md)=0.093*Porosity (%) + 0.6; Untreated: logk (md)=0.13*Porosity (%) - 0.29). Compressive strength increases with decreasing porosity and both untreated and treated samples exhibit a similar trend: for Sapaksi, Unit Load (psi)=-360* Porosity (%) +8311. These, and other tests, can be interpreted to indicate that ethyl silicate treatment closely mimics natural cementation and could prove to be an effective treatment for rock art preservation.