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

Abstract

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.

Diversity and Composition of Culturable Microorganisms and Their Biodeterioration Potentials in the Sandstone of Beishiku Temple, China

“…The precipitation of calcium oxalate and calcium carbonate is influenced by fungal growth, as well as environmental conditions. Therefore, it is necessary to explain the precipitation process for calcium oxalate and calcium carbonate to provide a solid basis for subsequent microbial control and enable its use as a biotechnological tool for heritage protection [ 73 , 74 ].…”

The effects of cyanobacterial biofilms on water transport and retention of natural building stones

“…Moreover, biofilms affected the contact angle of a surface by changing the surface chemistry, roughness and re-entrant topography (Epstein et al, 2011;Tanaka et al, 2019). Increased water contact angles were also examined due to carbonatogenic bacteria (Elert et al, 2021) and Bacillus subtilis (Epstein et al, 2011). Polson et al (2002Polson et al ( , 2010 detected a change in wettability on quartz by a bacterial-fungal consortium that made the surface hydrophobic, while an algal consortium left the surface hydrophilic.…”

Degradation and conservation of clay-containing stone: A review

“…In parallel, mineralogical analysis of a range of building stone types showing surface flaking and scaling (i.e., detachment of sub-mm to mm-thick surface layers totally independent of the stone structure), exfoliation (i.e., detachment of multiple cm-thick surface layers along stone bedding/structural planes), spalling (i.e., detachment of up to cm-thick layers sub-parallel to the stone surface), delamination (i.e., detachment of multiple thin stone layers sub-parallel to the stone surface), or contour scaling (i.e., detachment of mm-to cmthick surface layers following the contour of stone surface features) (names of weathering forms after Vergés-Belmin [43]) (Fig. 1), revealed that, typically, they included clay minerals, normally in small quantities (commonly < 10 wt%) [8,9,[44][45][46][47][48][49][50][51].…”

“…Importantly, such stones displayed a significant hygric and/or hydric expansion (i.e., expansion in contact with vapor or liquid water, respectively), which led to the conclusion that clay swelling/shrinkage following wet-dry cycles had to be involved in their degradation [8,9,52,53]. Stone expansion is easily quantified in the laboratory using a linear variable differential transformer (LVDT), which measures changes in the dimension of a stone sample (i.e., free swelling strain ε = Δl/l, where Δl is the length increase of a sample with initial length l) with an accuracy of (at least) ± 1 μm upon changes in relative humidity or upon immersion in water [10,51]. Such measurements showed that the free swelling strain of clay-containing stones could reach values of up to ~ 0.01 (i.e., 1% = 10 mm/m) [50], and, in extreme cases, values of up to ~ 0.05 [9,50].…”