Potential Preservation of Native American Petroglyphs at Steamboat Butte, Montana, Using Ethyl Silicate Solution Treatments

Article excerpt

ABSTRACT

Samples of the Tongue River Sandstone, collected from the top of Steamboat Butte in central Montana, were treated with an ethyl silicate solution. The samples showed a large increase in compressive strength and freeze-thaw resistance, relative to untreated samples, and indicates the treatments) significantly consolidate(s) the stone, thus providing a method to increase the lifetime of the petroglyphs carved onto the stone.

Keywords: Ethyl silicate, sandstone consolidation, petroglyph preservation, Steamboat Butte, Montana. site preservation

Native American rock art sites provide links to the past and should be preserved for future study. Although many sites have been vandalized, the destructive force acting on all rock art is natural weathering. Such sites are attacked by freeze-thaw, wet-dry and heat-cool cycling; wind and water erosion; biological growths; salts; atmospheric pollution; and mineral weathering. Naturally, climatic characteristics, the degree of protection from the elements, and the properties of the stone play important roles in determining which of the agents is detrimental at a given site. The placement of a suitable bonding agent in liquid form into the stone will improve the durability of the stone and retard the effects of such weathering, thus prolonging the life of the petroglyphs.

The Steamboat Butte Site is located in central Montana in Musselshell County. Rising about 300 feet above area drainages, Steamboat Butte was an ideal lookout tower. The petroglyphs are carved into the cap rock or the top layer of sandstone of the butte. The petroglyphs have a vertical orientation, meaning they are carved onto the cliff face. The location is exposed to most of the weathering elements mentioned above. Figure 1 shows the approach to the cap rock at Steamboat Butte. Figure 2 shows a portion of the petroglyphs. Despite the isolated location, the petroglyphs share space with modern graffiti.

In order to be effective at consolidating the stone and thereby increasing its resistance to weathering, any potential chemical treatment should meet the following criteria:

1) The treatment must penetrate the stone so that the entire zone of weathering is treated.

2) The treatment should improve the compressive strength of the stone, an indication of successful bonding.

3) No discoloration of the stone should occur. This means that no coloring ions should be present and that the solution should have a neutral pH to avoid possible reaction with the mineral components of the stone.

4) The treatment should not completely seal the pore system of the stone; the stone should be able to breathe and rid itself of excess moisture.

At present, one chemical system that meets the above criteria is known commercially as Conservare OH; the chemical solution is primarily composed of ethyl silicate (silicic ethyl ester) dissolved in a methyl ethyl ketone/acetone mixture. The ketone mixture acts as both a solvent and a volatile inert carrier and the low molecular weights of the compounds and low viscosity of the solution promote penetration into the stone. The solution has a neutral pH. Developed by Wacker Chemie in Germany, this solution in the United States is supplied by ProSoCo, Inc. or Process Solvents Company, Inc., of Lawrence, Kansas. This solution has been successfully used for consolidation purposes on many historic buildings. Treatments have produced increased strength of the stone and no appreciable discoloration has occurred. Based on improvements in the compressive strengths of different types of stone treated with Conservare OH, the stone consolidation is particularly effective when used on sandstones. An example of the effect on the properties of sandstone has been reported by Zinsmeister et al. (1988). Perhaps most important is the fact that with proper application, the pore system of the stone is not sealed, and the stone can rid itself of any excess moisture. …