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74 ABSTRACT Biodeterioration is one of the most serious problems faced by conserva- tors of the Athens Acropolis monuments. In this contribution the authors present the strategy designed for its control on a large scale outdoor monument, as well as the complications encountered in situ because of numerous environmental and logistical parameters. Reference is made to the bibliographic and research phases of the project, the testing of selected biocides for their compatibility with the stone and conservation materials and their efficacy against micro-organisms in laboratory tests and in situ both over time and when used in combination with current conservation treatments. Co-operation with environmental microbiologists and botanists, the need for an interdisciplinary, holistic approach to causes and control of biodeterioration and the ceaseless role of conservation are also discussed. ÖzeT Biyolojik bozulma, Atina Akropol anıtları konservatörlerinin karşılaştığı en ciddi sorunlardan biridir. Bu çalışmada yazarlar açıkta yer alan büyük bir anıtta bu sorunun kontrolü için tasarladıkları stratejiyi, ayrıca sayısız çevresel ve lojistik parametreler nedeniyle ortaya çıkan in situ komplikasyonları sunmaktadır. Projenin kaynakça ve araştırma aşamalarına, bazı seçme biyosidlerin taş ve konservasyon malzemesiyle uyumunun sınanmasına, laboratuvar testlerinde hem zaman içerisinde hem de güncel konservasyon işlemleriyle bir arada kullanıldığında mikro organizmalara karşı ne kadar etkili olduklarına atıf yapılmaktadır. Çevreci mikrobiyologlar ve botanikçilerle işbirliği ile, biyolojik bozulmanın nedenleri ve kontrolüne disiplinler arası ve bütünselci yaklaşım ihtiyacı tartışılacak ve konservasyonun hiç bitmeyen rolü ele alınacaktır. InTRoduCTIon Micro-organisms, plants and animals cause serious decay to the building stones of the Acropolis monuments of Athens, Fig. 1. Biodeterioration results in material loss and undesired mechani- cal, chemical and aesthetic alterations as well as affecting most conservation interventions. In the past, Acropolis Restoration Service (YSMA) conservators concerned with biodeteriora- tion have collaborated with the eminent environmental micro- biologists S.B. Curri, R. Palmer, W.e. Krumbein, C. urzi, A. Pantazidou and their associates, who identified a series of micro-organisms and described the mechanisms that control bio- deterioration. Since 2003 a research project between YSMA and the Botany department of the Faculty of Biology at the national and Kapodistrian university of Athens ( nKuA) has aimed to control biodeterioration caused by micro-organisms and plants. This contribution outlines the problem of biodeterioration as encountered on the Acropolis monuments, describes the control strategy designed by the YSMA conservators and the environmental biologists from nKuA and discusses the practi- cal complications that emerge in situ. emphasis will be given to the conservators’ work on control of biodeterioration by micro- organisms using biocides. Finally, the indispensable need for an interdisciplinary approach and continuous evaluation of the treatments applied is highlighted. THe BIodeTeRIoRATIon ConTRoL STRATeGY The objectives of the research project aimed at controlling bio- deterioration from plants and microbes were: 1. to describe the most characteristic biodeterioration patterns and investigate their controlling parameters; 2. to identify the main plants and microbes with biodete- rioration potential; and 3. to select an efficient method for controlling the bio- deterioration of the Acropolis monuments’ stone surfaces and sealing mortars. oBJeCTIVe 1: BIodeTeRIoRATIon PATTeRnS And ConTRoLLInG PARAMeTeRS Plants, animals and micro-organisms are agents of biological decay for the Athens Acropolis building stones, which include Pentelic marble (mainly used for the superstructure) and lime- stones (used for foundations). decay due to animals is principally from pigeons and their acidic metabolic products. Plants develop between and near building stones and degrade them by exerting mechanical pressures, excreting corrosive metabolic products or providing nutrients and niches for micro-organisms [1]. Micro-organisms develop epilithically (on stone surfaces) as coloured mats, and chasmoendolithically (within the stone in cracks, fissures, gaps and exfoliations). Krumbein et al. have demonstrated that a vast variety of bacteria, fungi, algae, cyanobacteria and lichens degrade the Acropolis’ marbles and limestones through secretion of corrosive organic compounds and exertion of micropressures [2]. Micro-organisms also favour the development of plants and animals. epilithic micro-organisms form thick biological mats consist- ing mainly of colourful lichens and bryophytes that cause ‘hon- eycomb’ and ‘pitting’ decay patterns. Comparison with historic images shows that the epilithic mats have now been lost from most of the monuments’ surfaces, probably due to the increase of organic pollutants in the atmosphere [3]. Their presence is cor- related with orientation and humidity: most lichen and bryophyte mats are observed on the northern surfaces of monuments or on ancient inorganic mortars. Mats become more intense where water retention is higher due to the inclination of a surface or because of limited exposure to the sun, Fig. 2. Conservators are more concerned with chasmoendolitho- trophs, which cause greater material loss to the marble. Black and green microbial masses develop underneath surfaces, darkening them and causing exfoliation and eventual material loss by expansion, Fig. 3. Although chasmoendolithotrophic decay is common in all the Acropolis monuments, its particular prevalence in the Pentelic marble seems to be connected to the presence of aluminosilicate veins that encourage the develop- ment of green and black microbial growth. The veins expand and deteriorate faster than the calcite matrix, perhaps because aluminosilicate compounds and micro-organisms both have high surface to volume ratios, ion exchange properties and water and organic material retention potential [4]. BIodeTeRIoRATIon ConTRoL FoR THe ATHenS ACRoPoLIS MonuMenTS: STRATeGY And ConSTRAInTS Sophia Papida, dionysis Garbis, evi Papakonstantinou and Amalia d. Karagouni Fig. 1 The Athens Acropolis monuments. Photo: S. Mavromatis.

Transcript of 11 papida-biodeterioration control for the athens acropolis

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ABSTRACTBiodeterioration is one of the most serious problems faced by conserva-tors of the Athens Acropolis monuments. In this contribution the authors present the strategy designed for its control on a large scale outdoor monument, as well as the complications encountered in situ because of numerous environmental and logistical parameters. Reference is made to the bibliographic and research phases of the project, the testing of selected biocides for their compatibility with the stone and conservation materials and their efficacy against micro-organisms in laboratory tests and in situ both over time and when used in combination with current conservation treatments. Co-operation with environmental microbiologists and botanists, the need for an interdisciplinary, holistic approach to causes and control of biodeterioration and the ceaseless role of conservation are also discussed.

ÖzeTBiyolojik bozulma, Atina Akropol anıtları konservatörlerinin karşılaştığı en ciddi sorunlardan biridir. Bu çalışmada yazarlar açıkta yer alan büyük bir anıtta bu sorunun kontrolü için tasarladıkları stratejiyi, ayrıca sayısız çevresel ve lojistik parametreler nedeniyle ortaya çıkan in situ komplikasyonları sunmaktadır. Projenin kaynakça ve araştırma aşamalarına, bazı seçme biyosidlerin taş ve konservasyon malzemesiyle uyumunun sınanmasına, laboratuvar testlerinde hem zaman içerisinde hem de güncel konservasyon işlemleriyle bir arada kullanıldığında mikro organizmalara karşı ne kadar etkili olduklarına atıf yapılmaktadır. Çevreci mikrobiyologlar ve botanikçilerle işbirliği ile, biyolojik bozulmanın nedenleri ve kontrolüne disiplinler arası ve bütünselci yaklaşım ihtiyacı tartışılacak ve konservasyonun hiç bitmeyen rolü ele alınacaktır.

InTRoduCTIonMicro-organisms, plants and animals cause serious decay to the building stones of the Acropolis monuments of Athens, Fig. 1. Biodeterioration results in material loss and undesired mechani-cal, chemical and aesthetic alterations as well as affecting most conservation interventions. In the past, Acropolis Restoration Service (YSMA) conservators concerned with biodeteriora-tion have collaborated with the eminent environmental micro-biologists S.B. Curri, R. Palmer, W.e. Krumbein, C. urzi, A. Pantazidou and their associates, who identified a series of micro-organisms and described the mechanisms that control bio-deterioration. Since 2003 a research project between YSMA and the Botany department of the Faculty of Biology at the national and Kapodistrian university of Athens (nKuA) has aimed to control biodeterioration caused by micro-organisms and plants.

This contribution outlines the problem of biodeterioration as encountered on the Acropolis monuments, describes the control strategy designed by the YSMA conservators and the environmental biologists from nKuA and discusses the practi-cal complications that emerge in situ. emphasis will be given to

the conservators’ work on control of biodeterioration by micro-organisms using biocides. Finally, the indispensable need for an interdisciplinary approach and continuous evaluation of the treatments applied is highlighted.

THe BIodeTeRIoRATIon ConTRoL STRATeGYThe objectives of the research project aimed at controlling bio-deterioration from plants and microbes were:

1. to describe the most characteristic biodeterioration patterns and investigate their controlling parameters;

2. to identify the main plants and microbes with biodete-rioration potential; and

3. to select an efficient method for controlling the bio-deterioration of the Acropolis monuments’ stone surfaces and sealing mortars.

oBJeCTIVe 1: BIodeTeRIoRATIon PATTeRnS And ConTRoLLInG PARAMeTeRSPlants, animals and micro-organisms are agents of biological decay for the Athens Acropolis building stones, which include Pentelic marble (mainly used for the superstructure) and lime-stones (used for foundations). decay due to animals is principally from pigeons and their acidic metabolic products. Plants develop between and near building stones and degrade them by exerting mechanical pressures, excreting corrosive metabolic products or providing nutrients and niches for micro-organisms [1].

Micro-organisms develop epilithically (on stone surfaces) as coloured mats, and chasmoendolithically (within the stone in cracks, fissures, gaps and exfoliations). Krumbein et al. have demonstrated that a vast variety of bacteria, fungi, algae, cyanobacteria and lichens degrade the Acropolis’ marbles and limestones through secretion of corrosive organic compounds and exertion of micropressures [2]. Micro-organisms also favour the development of plants and animals.

epilithic micro-organisms form thick biological mats consist-ing mainly of colourful lichens and bryophytes that cause ‘hon-eycomb’ and ‘pitting’ decay patterns. Comparison with historic images shows that the epilithic mats have now been lost from most of the monuments’ surfaces, probably due to the increase of organic pollutants in the atmosphere [3]. Their presence is cor-related with orientation and humidity: most lichen and bryophyte mats are observed on the northern surfaces of monuments or on ancient inorganic mortars. Mats become more intense where water retention is higher due to the inclination of a surface or because of limited exposure to the sun, Fig. 2.

Conservators are more concerned with chasmoendolitho-trophs, which cause greater material loss to the marble. Black and green microbial masses develop underneath surfaces, darkening them and causing exfoliation and eventual material loss by expansion, Fig. 3. Although chasmoendolithotrophic decay is common in all the Acropolis monuments, its particular prevalence in the Pentelic marble seems to be connected to the presence of aluminosilicate veins that encourage the develop-ment of green and black microbial growth. The veins expand and deteriorate faster than the calcite matrix, perhaps because aluminosilicate compounds and micro-organisms both have high surface to volume ratios, ion exchange properties and water and organic material retention potential [4].

BIodeTeRIoRATIon ConTRoL FoR THe ATHenS ACRoPoLIS MonuMenTS: STRATeGY And ConSTRAInTS

Sophia Papida, dionysis Garbis, evi Papakonstantinou and Amalia d. Karagouni

Fig. 1 The Athens Acropolis monuments. Photo: S. Mavromatis.

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The condition of stones is another important parameter. Cracks, fissures and gaps provide ideal shelters for chasmoendo-lithotrophs and there is a synergistic effect where they are found in damaged substrates, including:

• sheltered areas or lower parts of the monuments;

• gaps between surfaces of architectural members; and

• cracks, fissures and exfoliations from fire, earthquakes or corroded and expanded iron reinforcements from a previous restoration intervention.

dark chasmoendolithotrophs also colonize conservation mortars based on sulphate-free white Portland cement, calcium hydrox-ide, silicate sand and pozzolans that are used for sealing joined fragments and various surface discontinuities (gaps, cracks, fissures and exfoliations). These become ideal microbial niches: their placement and porosity favours increased water content and their composition provides micro-organisms with readily available inorganic nutrients that render mortar surfaces dark and friable. The mortar–marble interfaces are attacked, cohe-sion loss results and the integrity of the substrate is endangered, Fig. 4.

epiliths and chasmoendolithotrophs degrade stones and mortars aesthetically and encourage water retention, leading to deterioration by physico-chemical and biological mechanisms.

oBJeCTIVe 2: IdenTIFICATIon oF PLAnTS And MICRo-oRGAnISMS CAuSInG BIodeTeRIoRATIonTo identify the principal plants that are present on and near the Acropolis monuments and understand their biodeterioration

potential, YSMA worked with botanist dr Vallianatou (nKuA) to produce a long list of damaging perennial (phanerophytes, chamaephytes, hemicryptophytes and geophytes) and annual (therophytes) plants [1].

Seasonal sampling and identification of the main microbial populations responsible for biodeterioration was undertaken by the environmental microbiologist Professor Karagouni and her colleagues (nKuA) using a sterile scalpel and sterile agar plates or (where sampling was not permitted) slides in contact with the sampling surface. Sampling took place from marble bearing characteristic biodeterioration features including exfoliations, chasmoendolithic dark microbial presence, epilithic coloured microbial mats, black crusts and brown-orange and grey layers of unknown origin.

In addition, limestones, sealing mortars and plants were sam-pled and analysed using traditional and molecular techniques. A vast variety of bacteria, streptomycetes, fungi and yeasts were identified. Their large populations (103–1011 per millilitre of suspension) explain the high degree of stone decay [5].

oBJeCTIVe 3: BIodeTeRIoRATIon ConTRoLVallianatou stressed the stone decay potential of plants and the importance of their systematic elimination by manual methods, since chemical herbicides can be dangerous for both the stone and the environment [1].

However, the use of biocides was agreed by both conservators and microbiologists to be necessary for control of the aggressive biodeterioration problem. Methods that are more friendly to the user, monument or environment — such as irradiation, fumiga-tion or laser cleaning, as used on the Parthenon frieze — were rejected due to the logistical constraints of using them on large-scale monuments and the limited number of trials conducted on microbial mats.

The project related to the selection of suitable biocides was divided into four phases:

A. Preliminary selection of biocides;

B. Laboratory and in situ tests of biocides on sound specimens;

C. In situ evaluation of biocides on biodeteriorated surfaces;

d. development of an application methodology.

Recovery of active microbial populations from the monuments’ building stones and evaluation of the efficacy of selected bio-cides have taken place since 2003 using traditional and molecular techniques [5]. That work is still in progress and so is mentioned here only briefly. This paper will focus on the conservation, work and concerns.

Fig. 2 epilithic microbial presence on the north side of the erechtheion marble surface. Photo: S. Papida.

Fig. 3 Chasmoendolithic microbial presence under marble surface exfoliations. Photo: S. Papida.

Fig. 4 Biodeteriorated sealing mortars. Photo: S. Papida.

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Phase A: preliminary selection of biocidesTen biocides effective against a vast spectrum of micro- organisms and based on active ingredients of various chemical groups were selected for testing, based on the criteria below and on extensive bibliographic and market research, Table 1. All had been safely used on various stone types before and according to the manufacturers each biocide was only moderately to slightly hazardous, would not cause aesthetic, chemical or mechani-cal alteration to the stone and was compatible with inorganic materials.

The compatibility of the biocides with stone and conservation materials and their efficacy against micro-organisms would be tested in vitro and in situ.

Phase B: laboratory and in situ assessment of biocides on sound specimensThe criteria set for the biocides to be used on the Acropolis monuments were:

• no undesirable mechanical, chemical or aesthetic altera-tions to the stone substrates or the inorganic conservation materials used;

• efficacy against the microbial consortium for two to four years;

• safety for the environment and user (World Health organisation (WHo) toxicity class of 2–3); and

• ease of preparation and in situ application.

For assessment of the first criterion, the conservation team carried out preliminary tests of the biocide solutions (made up as recommended by the manufacturers) for their pH and their effect on the colour and weight of sound dionysus marble (which is similar to Pentelic marble), Piraeus (Actitis) limestone and sealing mortar specimens. Colour change of treated specimens was measured with a Sheen Micromatch Plus colorimeter using the CIe L*a*b* colour space after a 3-month in situ exposure. dry weight change (%) was measured after immersing the specimens in the biocide solutions overnight. In both cases, acceptable changes were considered to be those similar to those produced by deionized water [6]. Biocidal efficacy against micro-organisms was examined by the environmental micro-biologists using molecular techniques [5]. Ld50 oral (rat) data indicated the toxicity class of the product and its safety for the user and the environment when used in situ. ease of application was evaluated by taking into account a product’s state (solid or liquid), the solvent type needed (water or other) and any pre-caution measures required.

The outcome of this work led to the selection of d/2 Architectural Biocide, Preventol R80 (quaternary ammonium compounds) and Algophase (organochlorinated compound) that matched the stated criteria relatively well, Table 2.

Phase C: in situ evaluation of biocides on ancient biodeterio-rated marble surfacesIn situ environmental conditions include numerous param-eters that cannot be accurately simulated in the laboratory.

Table 1 Active ingredients, Ld50 and toxicity class of the biocides

Product Active ingredients LD50 oral (rat)(mg/kg of body weight)

WHO toxicity classa Manufacturer

Algophase 2,3,5,6-tetrachloro-(methylsulphonyl)pyridine 1508b 3 Phase

Architectural d2 octyldecyldimethylammonium chloride dioctyldimethylammonium chloride didecyldimethylammonium chloride Alkyl (C14 50%, C12 40%, C16 10%) dimethylbenzylammonium chloride

>5000 3 Cathedral Stone

Kimistone Benzalkonium bromide >50%Biguanide derivative <5%

1800 3 Kimia

Microtech 25X 3-iodo-propynyl-n-butylcarbamate 1470b 3 Wykamol

Polybor na2B8o13·4H2o 2550 3 Borax

Preventol R80 Benzalkonium chloride 80% 240b 2 Bayer

Rocima 101 didecyldimethylammonium chloride 645 3 Rohm and Haas

Rocima 110

didecy-dimethylammonium chloride Tributyltin naphthenate nonylphenoxypoly(ethylene-oxy)ethanol

810 3 Rohm and Haas

Wykabor 10 10% na2B8o13·4H2o2% Benzalkonium chloride

2550b

240b 2 Wykamol

Vancide 51 Sodium dimethyldithiocarbamate 27.6% Sodium 2-mercaptobenzothiazole 2.4% 3120 3 Vanderbilt

notes aClass 2: moderately hazardous; Class 3: slightly hazardous.bThe Ld50 oral (rat) of the active ingredient(s) is indexed when the Ld50 oral (rat) for the overall product is not available from the manufacturer.

Table 2 Laboratory assessments for the selected biocides

Product Concentration (w/w %)

pH Ease of application Dry weight change % (24-hour immersion)

Colour difference (ΔE76)a In vitro microbial inhibition(majority of 46 samples) [5]

deionized water 100% 6.7 excellent 0.015 11.40 –

Algophase 0.5% ethanol solution 5.5 Good 0.005 2.66 Very good

Architectural d2 100% 9.5 excellent 0 3.19 Good

Preventol R80 3% aqueous solution 7.0 Good 0.01 3.57 Very good

noteaColour difference between sample before application and after a three-month in situ exposure, given in Δe units in the Commission Internationale de l’eclairage (CIe) 1976 system.

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The microbiologists were concerned that biocides usually have a different elimination effect on a mixed in situ micro-population compared with an in vitro isolated micropopulation. Conservators were concerned about the possibility of undesired side effects on biodeteriorated ancient stones after the use of biocides.

The in situ efficacy of the three biocides when applied on biodeteriorated marble surfaces bearing a dark chasmoendolithic presence amongst dense exfoliations was measured relative to time and conservation effects:

• effect over time All biocides were applied to chasmoendolithically bio-

deteriorated marble surfaces during the dry season and caused a staining effect during the first 24–72 hours that gradually disappeared; a reduction in the microbial pres-ence was observed on the surfaces after the rains started. After 26 months all surfaces are satisfactorily clean and no recolonization has been noted macroscopically, Fig. 5. In vitro monitoring by the microbiologists is still in progress but, as expected, recolonization time is directly related to the toxicity of the products [7].

• conservation effect The surface consolidation and cleaning processes that

take place during systematic conservation were thought to prolong recolonization time so the three biocides were applied to both treated and untreated chasmoendolithi-cally biodeteriorated marble surfaces. Surface consoli-dation, the first conservation stage for stones suffering from intergranular disaggregation and exfoliation, took place via 40 successive limewater suspension sprays. The surface was cleaned using hydrogen peroxide solutions, deionized water and tools, Fig. 6.

As with the previous test applications, the conservators noted no aesthetic alteration to the surfaces and there was a satisfactory removal of the microbial presence after the rain. The ongoing in vitro research, however, suggests that conservation treatment does not prolong the recolonization time [7].

Phase D: developing an application methodologyAs well as selecting suitable biocides, the conservators, advised by microbiologists, also aimed to develop an application meth-odology compatible with the in situ materials, methods and working conditions. A principal question was whether biocide application should be before or after consolidation, cleaning,

joining of fragments, filling of discontinuities and sealing. Little related information was found in the literature. The microbiol-ogists suggested that biocide application should follow conserva-tion procedures and that biocides should not be removed after application. In contrast, the manufacturers’ guidelines regard-ing both issues were variable. An additional concern was that biocides may gradually be converted to nutrients and enhance recolonization [8].

The conservators therefore conducted further testing on bio-deteriorated stone and sealing mortar surfaces with the aim of macroscopically monitoring the microbial removal potential and the recolonization time for each product as well as assessing its applicability. Some trials were supported by in vitro microbio-logical examinations.

The descriptions below focus on the rationale rather than the results of the processes since biocide efficacy and recolonization time will always depend on the substrate, environmental param-eters, deterioration mechanisms and conservation materials and methods involved.

The YSMA systematic conservation protocol for treatment of the marble includes the following steps:

1. surface consolidation with repeated limewater suspen-sion sprays;

2. mapping and safe removal of loose exfoliations and fragments;

3. cleaning of the interior surfaces, exfoliations and dis-continuities using deionized water, hydrogen peroxide solutions and tools;

4. joining of fragments using Portland cement;

5. filling of surface discontinuities by injecting inorganic grouts; and

6. sealing of joined fragments and filled surface disconti-nuities with inorganic mortars, based on white Portland cement, calcium hydroxide, pozzolans, silicate sand and inorganic pigments.

• the effect of consolidation and cleaning Consolidation via limewater suspension sprays is the

first step towards the conservation of stone surfaces suf-fering from intergranular disaggregation or exfoliation. In an attempt to detect the best stage for biocide applica-tion, biocides were applied on treated (consolidated and cleaned) and untreated biodeteriorated marble surfaces (with exfoliations and a black and green chasmoendo-lithic presence) with the aim of measuring the effects of the treatments on the biocide’s efficacy. Macroscopic

Fig. 5 Marble surface treated with Algophase, d/2 Architectural Biocide and Preventol R80, 21 months after treatment. After 26 months all surfaces were still satisfactorily clean and no recolonization was macroscopically noted. Photo: S. Papida.

Fig. 6 Marble exfoliations being cleaned using hydrogen peroxide. Photo: S. Papida.

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observations showed a remarkable removal of the microbial presence when the biocide was applied after consolidation and cleaning. Moreover, biocide applica-tion on unconsolidated surfaces inevitably causes greater material loss. However, raw microbiological data up to now suggest that the action of biocides, applied after the treatments, was limited since recolonization time was shorter [7]

• the effect of cleaning alone Knowing whether biocides facilitate cleaning or if

cleaning prolongs biocide efficacy would indicate to con-servators whether biocides should be applied before or after cleaning. Biocides were applied on marble bearing epilithic coloured mats or chasmoendolithic microbial presence and on darkened sealing mortars, both cleaned and uncleaned. Macroscopic observations showed that:

– cleaning before biocide application facilitated removal of organic remains, especially the epilithic mats, and had aesthetically better results, Fig. 7. Surfaces are being monitored macroscopically for recolonization but clean-ing before biocide application is expected to retard it.

– biocide applications on untreated marble surfaces resulted in an acceptable removal of the chasmoendo-lithic microbial presence that was more apparent at a macroscopic scale after rainfall, Fig. 8. In all cases it was understood that removal of the biocide along with the dead cells some days later — either mechanically or by

the rain — was indispensable for improving the aesthetic result. Moreover, failure to remove these organic remains provides micro-organisms with a nutrient substrate.

• sealing mortars Inorganic sealing mortars are also affected by micro-

organisms. Removal and replacement of affected mortars is possible, but may be constrained by lack of time and personnel and by the risk of material loss when joined fragments, exfoliations or intergranular disaggregation are involved. Minimum intervention is preferred and the prolongation of the mortars’ service life is sought. effective biocidal elimination of organic remains on mortars was macroscopically observed after rainfall. Partial removal of both recently applied and older biode-teriorated mortars showed that their decay and cohesion loss was rather superficial, so biocide application could probably prolong their lives. Biocides can be used before and after new mortar applications to better safeguard the interventions.

dISCuSSIon Biodeterioration of stone monuments is a complicated problem that needs an interdisciplinary approach by experienced con-servators and specialized biologists. Its control strategy should involve careful in vitro and in situ examination of the stone substrates, (micro)flora and fauna and biocides.

on the Acropolis monuments, manual plant elimination was suggested to avoid the danger of herbicides, but for control of extensive microbial decay the decision was taken to use biocides. Test applications were undertaken to develop the best meth-odology for various substrates, depending on their properties, condition, micro-environment, compatibility with conservation materials, application methods and in situ constraints. It is pos-sible to draw a few conclusions from these tests regarding the use and application of biocides:

• applying biocides after consolidation seemed to prevent their in vitro efficiency [7]. It is worth further investi-gation into why limewater consolidation prevents the interaction of micro-organisms with the biocides.

• cleaning before biocidal application seemed to facilitate removal of organic remains resulting in aesthetically better surfaces. The prolongation of recolonization time is anticipated.

• biocidal treatment of both older and newly applied mor-tars prolongs their service life as well as the integrity of joined fragments and loose exfoliations.

• repeated applications are suggested depending on the recolonization time, the condition of the substrate, the toxicity of the product and the substrate’s accessibility.

Biocides are definitely not a panacea for biodeterioration for a number of reasons. Micro-organisms soon become unaffected by the biocides’ active ingredients and more toxic products are required, leading to safety hazards for the user, environment and monument during their in situ preparation, use and proper disposal [8]. Accessibility is another significant factor for large-scale monuments such as the Acropolis. Biologists suggest that biocides are applied to the largest possible area in order to prevent contamination. However, working conditions on the monuments mean that biocides can be applied only where res-toration and systematic conservation are currently taking place and scaffolding permits access. on fully accessible building stones, where conservation methods and materials can be moni-tored over a long period, less toxic products (e.g. Architectural

Fig. 7 Cleaning before biocide application resulted in effective removal of epilithic mats (second and third columns of squares). Photo: S. Papida.

Fig. 8 Rain resulted in effective removal of chasmoendolithotrophs on biodeteriorated marble surfaces that were treated with a biocide (right fragment). Photo: S. Papida.

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d/2) can be tested and re-applied or abandoned. The opposite is true for architectural members conserved and replaced high on a monument: access is impossible after the end of restoration projects and the removal of scaffolding, so biocides with a longer recolonization time (e.g. Preventol R80 or Algophase) may be necessary. In all cases, safety precautions must be taken by the user and the minimum amount necessary should be carefully prepared and applied during the dry season in order to prevent release of toxic materials into the environment.

In practice, biocide re-application is also affected by the general conservation principle of minimum intervention since every conservation treatment has the potential to cause damage. Conservators may have to supplement the microbiologists’ data for recolonization with their own macroscopic observations and re-apply a biocide only when the process seems less harmful than the biodeterioration type encountered.

Last but not least, plant and animal elimination is an indispen-sable part of an integrated biodeterioration strategy. The seasonal manual elimination of plants and the exclusion of pigeons need to become a priority for the management of a site. Some plant elimination efforts have recently been initiated at the Acropolis monuments with the co-operation of specialized mountaineers but progress is still needed until it becomes a regular part of the annual schedule.

ConCLuSIonSThe co-operation of conservators and biologists towards a biodeterioration control strategy produced useful information for the preservation of the Acropolis monuments. Given their supreme cultural importance, this project has been the focus of much attention.

Results include the identification of plants and micro-organisms that adversely affect the monuments. Constraints that inevitably affect choices and treatment potential have also been identified and include the varying condition of the stone, co-existing and synergistic deterioration mechanisms, the scale of the monuments and their exposure to the natural environment. evidence from both laboratory and in situ tests, combined with environmental and working conditions on the one hand and with conservation theory and practice on the other, are some of the parameters that raise complications and combine to affect the choice of suitable application methods.

In this work, no ‘wonder’ materials or methods have been proposed. Instead it is argued that a strategy based on practical testing, ceaseless monitoring and preventive care should take place in order to develop a suitable methodology for each type of substrate, biodeterioration pattern or working environment. Systematic biodeterioration control processes are still being developed within our routine care programme but we are keen to share our experiences and results to date with other conser-vators in order to promote the best possible preservation of our monuments.

ACKnoWLedGeMenTSThe authors are grateful to the Committee for the Restoration of the Acropolis Monuments for their support. C. Vasiliadis (conservator, First ePCA) and dr e. Katsifas (environmental microbiologist, nKuA) are acknowledged for their co-operation; S. Cather and the preprint editors for their useful suggestions; and finally all biocide companies for kindly providing product samples. The project was co-financed by national and eu funds.

ReFeRenCeS 1 Vallianatou, e., Protection of the Athens Acropolis Monuments

Structures From Plants, report to Acropolis Restoration Service (YSMA) (2007) (unpublished) [in Greek].

2 Krumbein, W.e., Pantazidou, A. and urzì, C., Report of the First Phase of the Acropolis Project. Part II: Detailed Description of Samples, Microbial Communities and Photographic Documen-tation, report to Acropolis Restoration Service (YSMA) (1991) (unpublished).

3 diakumaku, e., Gorbushina, A.A., Krumbein, W.e., Panina, L. and Soukharjevski, S., ‘Black fungi in marble and limestones — an aesthetical, chemical and physical problem for the conservation of monuments’, Science of the Total Environment 167 (1995) 295–304.

4 Stotzky, G. and Burns, R.G., ‘The soil environment: clay-humus-microbe interactions’, in Experimental Microbial Ecology, ed. R.G. Burns and J.H. Slater, Blackwell Scientific, oxford (1982) 105–133.

5 Karagouni-Kyrtsou, A., Study of the Microbial Flora of the Acropo-lis Monuments, report to Acropolis Restoration Service (YSMA) (2005) (unpublished) [in Greek].

6 Papida, S., Garbis, d. and Papakonstantinou, e., Report Regarding the Conclusions Derived from the Study of the Microbial Flora of the Acropolis Monuments and the Work for the Biocides Selection and Control, report to Acropolis Restoration Service (YSMA) (2006) (unpublished) [in Greek].

7 Katsifas, S., Botany department, Faculty of Biology, university of Athens, personal communication, 14 September 2009.

8 Warscheid, T. and Braams, J., ‘Biodeterioration of stone: a review’, International Biodeterioration and Biodegradation 46 (2000) 343–368.

AuTHoRSSophia Papida trained as a conservator of antiquities and works of art at the Technological Institute of Athens. She has an MA in museum studies (university of Leicester, uK) and has conducted postgraduate research on stone biodeterioration (university of Portsmouth, uK). She has worked at various archaeological sites in Greece and since 2000 has been employed as a stone conservator at the Propylaea by the Acropolis Restoration Service (YSMA). She is interested in stone biodeterioration and the interpretation of conservation for the wider public. Address: The Acropolis Restoration Service (YSMA), 10 Polygnotou, 10555, Athens. Greece. email: [email protected]

dionysis Garbis trained as a conservator of antiquities and works of art at the Technological Institute of Athens. He has worked at the Vergina excavations and taught conservation in state institutions of professional training. Since 2000 he has been employed as a stone conservator at the erechtheion by the Acropolis Restoration Service (YSMA). He is inter-ested in stone biodeterioration, laser cleaning and management of ar-chaeological sites. Address: as for Papida. email: [email protected]

evi Papakonstantinou holds a diploma in chemical engineering from the national Technical university of Athens and has been head of the surface conservation project at the Acropolis monuments since 1987. Her research interests include stone deterioration mechanisms as well as stone conservation materials (consolidants, inorganic mortars, etc.) and methods (consolidation, cleaning, protection). Address: as for Papida. email: [email protected]

Amalia d. Karagouni, professor of microbiology at the Biology depart-ment, national and Kapodistrian university of Athens, holds a biology degree and a Phd from the university of Warwick (uK) and works in the fields of genetics, applied microbiology, microbial ecology and bio-technology. Her research activities include isolation of micro-organisms from particular ecosystems in Greece, their identification through phenotypic and genotypic characteristics, isolation and analysis of total dnA, identification of genes from environmental samples and analysis of environmental food samples for the identification of genetically modi-fied dnA. She participates in numerous funded research programs, has organized several international conferences and is a member of many international scientific associations. Address: department of Botany, Microbiology Group, national Kapodistrian university of Athens, Faculty of Biology, 157 81, Athens, Greece. email: [email protected]