Conservation of Volcanic Tuffs Bearing Rock Paintings at La Pintada, Sonora Archaeological Site

Abstract

The Archaeological Site La Pintada is located near Hermosillo, Sonora, Mexico; this is one of the most important sites in the region due to its extension and wide archaeological evidence. This site was a gathering point for various indigenous groups since the pre-Hispanic age to the first contacts with the Spanish conquerors; they left their live experiences and worldview in the form of rock paintings along a wide natural canyon. Several of these paintings are in risk due to weathering processes of their rock support: the volcanic tuff.

In order to preserve these paintings the characterization of the volcanic tuff was made with petrographic analyses and X-Ray Diffraction, and among with the results obtained from environmental monitoring, different degrees of alteration were established. According to these results, the conservation treatment proposed was the consolidation of the tuff.

The consolidants used for this treatment were alkoxysilanes in two variations: Silicate and alumino-silicate which were developed in Mexico for volcanic tuff consolidation. The consolidants were synthetized in the laboratory, and afterwards were tested on powders and cubic probes of the rock in Hermosillo’s environmental conditions (24°C, 45%RH).

The consolidants were tested in two different types of application: aspersion and injection. The consolidated probes were evaluated for hardness, water absorption and color change. Afterwards, in situ tests were performed. The results of this evaluation showed that the Silicate consolidant was the best for this treatment, since it was easy to manipulate, improved the rock’s properties and did not cause a change appearance; making this Mexican material suitable for the conservation of the rock paintings.