ABSTRACT
The Hueco Tanks State Park & Historic Site, near El Paso, Texas, USA, is culturally and spiritually significant for several Native American Tribes. It has the largest concentration of prehistoric mask pictographs (rock paintings) in North America. This National Historic Landmark, also a popular recreational destination, was vandalized numerous times over the past century. The pictographs and rock formations were disfigured by extensive graffiti paint. Laser ablation was used to remove the graffiti without affecting the original pictographs and rock formations. This process has restored the legibility and beauty of the pictographs while respecting their sacred legacy.
To carefully assess the laser cleaning treatment, it was necessary to analyze the pictograph and graffiti. The site is considered sacred, so physical sampling of the pictographs was not permitted. A mobile lab was set up at the cave sites to perform in situ, non-invasive Raman (Bravo and Sentinel, Bruker Scientific, LLC), infrared (Alpha II, Bruker Scientific, LLC), XRF (Tracer 5i, Bruker Nano), and microscopic analysis of the pictographs. Goethite, hematite, quartz, gypsum, feldspar, and calcium oxalate were detected. The graffiti paints were sampled and analyzed using FTIR, Py-GCMS, and SEM-EDS. Cellulose nitrate and alkyd-based paints with chrome yellow, Prussian blue, iron oxide, burnt umber, zinc oxide, calcium carbonate, and barium sulfate pigments/fillers were detected.
Based on this information, Conservation of Sculpture and Objects Studio Inc. prepared mockups on local stone substrates to simulate the pictographs and graffiti paint environment and optimize the laser parameters. After the optimal parameters were determined, they were applied in the field using 1064nm pulsed lasers to remove the graffiti safely and effectively without affecting the original works. This paper describes the research, preparation, and graffiti removal from the Hueco Tanks pictographs and rock formations using selected laser systems.
