The preservation of sandstone reliefs at the archaeological site of Tajin, Mexico, using colloidal silica

Abstract

The Archaeological Site of Tajín was the biggest and most important prehispanic city of the north coast of the Mexican Gulf; it reached its peak from the early ninth to the early thirteenth century C.E. Currently, it is surrounded by jungle and stands in the vicinity of areas of petroleum extraction. At this World Heritage Site several constructions have detailed religious and symbolic scenes carved on carbonate- cemented sandstone, which suffers from diverse deterioration, mainly weathering, flaking, and often extensive loss. The first plasters applied in the conservation of these reliefs were made of synthetic polymers or lime without good results. These materials differed in color, texture and porosity from the original material; they were difficult to apply and often stained the stone during application and after weathering. After time, they also suffered from cracking and detachment. Fills prepared with colloidal silica showed good initial results for the treatment of the sandstone reliefs subject to tropical environmental conditions and air pollution. Colloidal silica-based plasters have been tested for one year on site and have shown better endurance, stability, and appearance than the previously applied plasters. The plasters were also tested as protective surface coatings for the sandstone against environmental deterioration and acidic pollution at the site. Petrographic studies and nitrogen adsorption-desorption techniques were used to characterize the plasters in order to evaluate their long term durability and compatibility with the original sandstone, as well as future alteration.