Feasibility of Removing Surface Deposits on Stone Using Biological and Chemical Remediation Methods

Abstract

The study was conducted on alterations found on stone artwork and integrates microbial control and a biotechnological method for the removal of undesirable chemical substances. The Demetra and Cronos sculptures are two of 12 stone statues decorating the courtyard of the Buonconsiglio Castle in Trento (Italy). An initial inspection of the statues revealed putative black crusts and highlighted the microbial contamination causing discoloration. In 2006, the Cultural Heritage Superintendence of Trento commissioned us to study and remove these chemical and biological stains. Stereomicroscopy characterised the stone of the sculptures as oolitic limestone, and infrared analyses confirmed the presence of black crusts. To remove the black crusts, we applied a remediation treatment of sulphate-reducing bacteria, which removes the chemical alteration but preserves the original stone and the patina noble. Using traditional and biomolecular methods, we studied the putative microbial contamination and confirmed the presence of biodeteriogens and chose biocide Biotin N for the removal of the agents causing the discolouration. Denaturing gradient gel electrophoresis fluorescent in situ hybridisation established that Cyanobacteria and green algae genera were responsible for the green staining whereas the black microbial contamination was due to dematiaceous fungi. After the biocide Biotin N treatment, we applied molecular methods and demonstrated that the Cyanobacteria, and most of the green algae and dematiaceous fungi, had been efficiently removed. The reported case study reveals that conservators can benefit from an integrated biotechnological approach aimed at the biocleaning of chemical alterations and the abatement of biodeteriogens.

Cryptogam covers on sepulchral monuments and re-colonization of a marble surface after cleaning

“…Though directly after cleaning with biocides, no remaining organisms could be detected (Polo et al 2010), a long-term experiment documented re-colonization after cleaning treatment over 6 years (Lamenti et al 2000). However, just enrichment culturing was used in this approach, which will greatly alter the microbial community before analysis.…”

“…Since the monument is situated under a tree, bird droppings (Hallmann et al 2011a) may contribute to favorable conditions for Stichococcus. Generally, these photosynthetic microorganisms cause staining by photosynthetic pigments and may be involved in stone decay by secretion of organic acids and indirectly by supporting growth of other microorganisms like fungi and bacteria (Ariño et al 1997;Polo et al 2010).…”

“…Open-air sculptures and historic buildings exposed to weather and pollution are deteriorated by physical and chemical factors, as well as by microbial contamination (Saiz-Jimenez 1995;Zanardini et al 2000;Polo et al 2010).…”

“…Different cleaning methods and conservation procedures were developed, based on chemical compounds (Young and Urquhart 1998;Moropoulou et al 2002;Polo et al 2010), excimer laser cleaning (Maravelaki-Kalaitzaki et al 1999), protective synthetic resins (Cappitelli et al 2007) or application of biomimetic apatite (Yang et al 2012). Even biological treatments with sulfate reducing bacteria for black crust removal were established (Cappitelli et al 2006;Konkol et al 2008).…”

“…Lamenti et al (2000) observed the re-colonization of marble statues by phototrophic biofilms for 6 years after restoration by culturing method. Polo et al (2010) analyzed microbial communities by denaturing gradient gel electrophoresis (DGGE) before and immediately after biological and chemical remediation of oolithic limestone sculptures. Lan et al (2010) stated obvious differences between the eukaryotic community of an old and a newly developed biofilm on sandstone of a Temple building of Angkor Thom in Cambodia, while the bacterial communities of both biofilms were almost similar.…”