Strategies for increasing the scale of biocleaning treatment through sulfate crust bioremoval

During the last two decades the role of microorganisms in the conservation of monuments and stone works has been largely acknowledged. Recently, new efforts have been carried on to enhance bacterial performance (e.g.: by selecting natural improved strains or by engineering bacteria), to scale-up the productivity process of bacterial biomass and to improve the delivery system, in order to develop bio-treatments implementable at large scale.   Despite these promising progresses, the use of bacteria in restoration practice is still limited to small artworks or surface areas. In particular regarding the removal of gypsum encrustation by sulfate reducing bacteria, a major drawback is that the technique is time-consuming in the presence of thick and compact crusts.   Here, it is proposed a strategy to speed up the bio-cleaning of gypsum crusts by coupling with a chemical pre-treatment. A one-century-old artistic marble statue, affected by sulfate-based crusts and grey deposits, has been treated with a sulfate reducing bacteria (SRB)-based product and a non-ionic detergent, applied singularly and in combination. Both the biological and combined biological-chemical treatments resulted in an affective removal of black crusts but the synergic treatment, even retaining all the advantages of the biocleaning approach, was able to completely remove the alteration taking advantage of a 70% reduction of the number of biological applications. Moreover, the co-treatment removed also a black crust barium sulfate layer, a remnant of a previous restoration treatment.   The combined biological-chemical treatment offers an economic improvement for large scale restoration processes still retaining the advantages of the biological treatment, i.e. low environmental impact and preservation of the high value conservation patina developed on the sound stone.