Degradation of ancient Maya carved tuff stone at Copan and its bacterial bioconservation

August 04, 2021

Kerstin Elert (1), Encarnación Ruiz-Agudo (1), Fadwa Jroundi (2), Maria Teresa Gonzalez-Muñoz (2), Barbara W. Fash (3), William L. Fash (4), Nieves Valentin (5), Alberto de Tagle (6), Carlos Rodriguez-Navarro (1)
npj Materials Degradation, 5, Issue 44, August 2021. DOI: 10.1038/s41529-021-00191-4


Much stone sculptural and architectural heritage is crumbling, especially in intense tropical environments. This is exemplified by significant losses on carvings made of tuff stone at the Classic Maya site of Copan. Here we demonstrate that Copan stone primarily decays due to stress generated by humidity-related clay swelling resulting in spalling and material loss, a damaging process that appears to be facilitated by the microbial bioweathering of the tuff stone minerals (particularly feldspars). Such a weathering process is not prevented by traditional polymer- and alkoxysilane-based consolidants applied in the past. As an alternative to such unsuccessful conservation treatments, we prove the effectiveness of a bioconservation treatment based on the application of a sterile nutritional solution that selectively activates the stone´s indigenous bacteria able to produce CaCO3 biocement. The treatment generates a bond with the original matrix to significantly strengthen areas of loss, while unexpectedly, bacterial exopolymeric substances (EPS) impart hydrophobicity and reduce clay swelling. This environmentally-friendly bioconservation treatment is able to effectively and safely preserve fragile stones in tropical conditions, opening the possibility for its widespread application in the Maya area, and elsewhere.

How Our Software Was Used

Dragonfly was used to perform image analysis to quantify total, communicated, and occluded porosity in Copan stone samples.

Author Affiliation

(1) Department of Mineralogy and Petrology, University of Granada, Granada, Spain.
(2) Department of Microbiology, University of Granada, Granada, Spain.
(3) Peabody Museum, Harvard University, Cambridge, MA, USA.
(4) Department of Anthropology, Harvard University, Cambridge, MA, USA.
(5) Conservation consultant, Madrid, Spain.
(6)Conservation consultant, Cos, Cantabria, Spain.