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Biodeterioration of monumental rocks: decay mechanisms and control methods

January 1998

Authors:

Piero Tiano

Italian National Research Council

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Biodegradation of Cultural Heritage: Decay Mechanisms and Control

Methods

Piero Tiano

CNR - Centro di studio sulle "Cause Deperimento e Metodi Conservazione Opere d'Arte", Via G.

Capponi 9, 50121 Firenze, Italy.

1. INTRODUCTION

Our cultural heritage is made with almost all type of materials produced by the nature and

used by men to realize several types of artefacts from very simple mono-components to complex

structures integrating inorganic and organic matters. These cultural heritage objects, even if made

with the more “resistant” stones and metals, are influenced by the environmental parameters,

which can modify their structure and composition. Furthermore, being them inserted into the

“biosphere” can be decayed by biological mechanisms. The biotransformation process has a

worldwide diffusion and attains highest levels in warm-humid climates where the environmental

conditions are extremely favourable to the growth of most organisms.

The role of conservators is to try to avoid weathering processes and of restorers to repair the

damages undergone.

Cultural heritage assets are exposed to weather and submitted to influence of environmental

parameters. Physicals, chemicals and biological factors interact with constitutive materials inducing

changes both in its compositional and structural characteristics. A certain aspect of matter

transformation is due to the metabolic activity connected with the growth of living organisms. This

activity is needed to maintain in equilibrium the “matter transformation cycles” and contribute to

very important aspect of “life” such as to transform hard rocks in soft soils (pedogenesis) or to

reduce the complex biological structures into the originate simpler components. In fact, several

kinds of micro and macro organisms can find a suitable habitat for their growth either on

monumental buildings, archaeological remains and other works of arts made with wood, textiles,

paper etc. The living species dwelling on these materials are ranging from microscopical bacterial

cells to higher plants and animals. The growth process and vegetative development of organisms

have a direct consequence on the conservation of cultural assets. The main types of damages

derived from metabolic activity of organisms are related with physical, chemical and aesthetical

mechanisms, while the intensity of these damages are strictly correlated with: type and dimension

of the organism involved; kind of material and state of its conservation; environmental conditions,

micro-climatic exposure; level and types of air pollutants.

Various methods have been used to classify and quantify the micro and macro organisms

involved in the biodeterioration processes. The ecological and physiological parameters concerned

with the development of specific biodeteriogens have been investigated together with the effect of

their growth on material weathering. Conservative interventions using direct and indirect

methodologies have been applied in order to stop or slow-down the biodeterioration process. The

methodologies and products must be chosen taking in mind substrate conditions and species to be

treated, standing to not cause negative interference with materials and with low environmental

impact.

In the proceedings of the ARIADNE 9 WS

Institute of Theoretical and Applied Mechanics of the Academy of Sciences of the Czech Republic

Prague 4-10 February 2002

2. Inorganic substrata

The biodeterioration of inorganic materials used in the works of arts is essentially a process

that involves monumental rocks and archaeological remains, being other materials such as

glasses and metals practically unaffected by this mechanism. Only a case is reported in which the

ancient glass windows of a medieval church have been etched by algae, while some corrosion

pattern of buried metals can be attribute to an indirect action due to the surrounding microbial

activity.

2.1 Stones

The decay of rocks and freshly exposed monumental stones is a complex process in which

physical and chemical mechanisms are usually considered the main factors, it generally starts with

alteration processes due to the synergetic action of rain, wind, sunlight and freezing/thawing

cycles. Thus the initial smooth and clean stone surface becomes progressively rough and more

porous, with the formation of micro cracks and fractures within crystal grains. Moreover, the

pollutants, whose concentration level is continuously increasing in our atmosphere, form deposits

of particles, black encrustation and leave secondary reaction products on stone surfaces. The

consequence of these combined actions is a loss of cohesion with dwindling and scaling of stone

material and with general weakening of the superficial structural strength. |1-6|.

We consider in particular the biodeterioration mechanism and the role played by several

biological agents (Biodeteriogens) both in the weathering of rocks |7-10| as well as in the decay of

stone monuments |11-20|. In fact, different kinds of organisms can grow using the mineral

components of a stone and its superficial deposits. The main consequence of their metabolic

activity, such as the excretion of enzymes, inorganic and organic acids and of complex forming

substances, is the dissolution of minerals of the substratum. Moreover, the growth and the swelling

of some vegetative structures (e.g.: roots and lichenic thalli) induce physical stresses and

mechanical breaks.

2.1.1 Bioreceptivity Of Stone Materials

The concept of bioreceptivity was focused by Guillitte |21| as the aptitude of a material to be

colonised by one or several groups of living organisms. Otherwise, the mere occurrence of

organisms on stone surfaces does not automatically imply destructive action but in some cases

could be considered: as only aesthetic detrimental appearance |22|; perceived aesthetically

pleasing or credit these with a protective role against weather induced aggression |23|.

The bioreceptivity must be related with the totality of material properties that contribute to the

establishment, anchorage and development of flora and/or fauna. In stony material it relates mainly

with surface roughness, moisture content, chemical composition of the outer layers and with the

structure-texture of the rock.

The "Primary or Intrinsic Bioreceptivity" is connected with the initial potential of colonization of

a sound stone. Then, following the evolution over time of surface characteristics, under the action

of colonising organisms and environmental factors, it becomes the more important "Secondary

Bioreceptivity". The conservative treatments applied on stone induce a "Tertiary Bioreceptivity".

Moreover, we can have "Extrinsic Bioreceptivity" when the stone colonization is essentially due to

the presence of settled matter not related with the beneath stone |21|.

One can assess the bioreceptivity of a material to an organism by artificially inoculating the

material with the diaspores of the organism itself and placing them under optimal environmental

conditions (growth chamber). Thus a specific "Bioreceptivity Index" could be determined for a stony

material related to its susceptibility to biodeterioration. To achieve this result a multidisciplinary

team must operates to conduct integrated studies, as standard as possible, choosing the most

suitable parameters for measuring the bioreceptivity either for quantify the microbial mass or to

characterise the stony material |24|. However these studies are always indicatives, as many types

of colonisation are part of a synecological mechanism and not all the organisms present on

exposed stone surfaces can develop on artificial conditions, even if very close to those present in

nature. Hence the test made with a single organism or few isolated strains can become either

impossible or completely atypical. In fact, the results reported in few lab experiments can be

considered pertinent only for the stones used, the organisms tested and the incubation conditions

applied |24-26|.

2.1.2 Ecology of stone substrata

We must take into consideration that the chemical composition and physical characteristics of

stone surfaces change over the time of exposure to weather. The initial exclusively mineral

composition progressively is implemented with organic matter, contemporary the physical structure

changes from almost smooth and whenever polished to a rough finish with increase of the stone

porosity, and in the worse cases "sugar" decay with sandy transformation of the rock can takes

place. In general, the kind of biological agent that develops on a stone is affected by the rock type

(e.g.: alkaline limestone or acidic sandstone) and its surface conditions, together with the

environmental location and the climate to which the monument and the stone are exposed. The

various nutritional and finishing conditions of an exposed stone substratum influence the temporal

and spatial distribution of the biocoenosis, which can develop on it.

2.1.3 Ecological succession

The pioneering micro organisms that develop on a surface, which is strictly mineral, without

any presence of organic matters, are those lithophilous, photoautrophic, chemiolithotropic and the

lichens. A fertilized surface with very small amounts of organic matter and growth factors

(Oligotrophic or Copiotrophic,), indifferent of their origin either from airborne particles or derived

from previously developing organisms, can support the growth of oligotrophic, heterotrophes

bacteria, actinomycetes and microfungi. The development and the death of successive

generations of micro organisms, together with the accumulation of ammonia and phosphate

compounds coming from the atmosphere, enriches the stone with organic matter and essential

growth factors (Eutrophic) allowing the development of heterotrophic micro organisms. Then with

the progressive worsening of the physical surface conditions, a “soiling” effect is induced which

allows the implantation and germination of the most common vegetal reproductive bodies deposed

from the air on surfaces. These range from cryptogames spores up to higher plants seeds. Finally

a more complete ecological niche is established with the presence of some very adaptable

specialized microfauna. Some micro organisms can growths only on stone surfaces (Ephilitic)

others prefer more protect habitats like small crevices and fissures (Chasmolitic) or inside the

stone structure (Endolithic).

Favourable environment and the presence of energy sources with organic and inorganic

nutrients allow the biological settlement of an exposed stone surface. When we analyse this

colonization we find that a photosynthesis-based consortium of organisms generally composes it. It

shows seasonal changes, following the different levels of humidity and the life cycles of the single

species, taking into account that many survival strategies have been developed to survive

unfavourable living conditions |18|. The pioneering colonization steps, of a single or very few

strains, can be followed in time observing fresh exposed stones or cleaned surfaces after

restoration interventions.

The metabolic activities of these organism such as the production of extra cellular polymers;

the liberation of chelating compounds and of organic/inorganic acids; together with the presence of

coloured pigments and to the mechanical pressure exerted by growing structures or

shrinking/swelling phenomena induces different types of damages:

physical (abrasion, mechanical stress);

chemical (solubilisation, new-reaction products);

aesthetical (coloured patches or patinas and crusts).

The consequence of the biological activity on stone surfaces is the formation of biofilms,

coloured patinas, encrustations and the presence of vegetative and reproductive bodies.

Moreover, the stony structure can be interested to dwindling, erosion, pitting, ion transfer, and

leaching processes.

2.1.4 Organisms

The principal groups of organisms investigated in relation with the biodeterioration of

monumental stones, with the specific methodologies of study and their relative importance in

decay processes are:

Bacteria

The researches have been mainly directed to make a statistical survey of the possible

functional groups present on various monuments and to study their ecological and physiological

significance |27-33|. In fact several species of bacteria have been detected and isolated from a

stone monument either from its surface or at a depth down to several millimetres, using

microbiological techniques. The micro organisms investigated can be divided in two main

nutritional groups:

a) Chemiolithoautotrophes

b) Chemioorganotrophes

a) The first very specialized group contains almost exclusively micro organisms belonging to

the sulphur and nitrogen cycle. Various species in these groups give out, as their end-metabolic

product, a strong inorganic acid (sulphuric and nitric respectively). Initially the attention was

focused upon the sulphur-oxidizing bacteria (Thiobacillus spp.) |34-38| on the basis of the high

level of slightly soluble calcium sulphate normally detected on decayed stones. Afterwards their

importance was reshuffled |39|, except for very particular substrata |40|. In fact new results seem

to confirm the purely chemical origin of the gypsum |41,42| and no experimental evidence has

succeeded in demonstrating the direct action, on a specific monument stone decayed example, of

this group of bacteria. Positive results have been obtained in lab experiments |43-45| in which the

substratum was concrete and the reduced sulphur compound utilized by the bacteria was

essentially H2S. Even if this is a typical sewage environment, very different from that one of an

exposed monument.

The nitrifying bacteria |46-48| on the other hand were investigated for their ability to oxidize

reduced nitrogen compounds especially ammonia substrata (by Nitrosomonas spp.) originating

from the atmosphere. While the very soluble nitrates are hardly detected on stone surfaces, it has

been demonstrated in lab experiments the high level of nitric acid produced by p. from nitrites and

its direct action on the binding material of a concrete block. Besides the nitrite has a catalytic effect

on the SO2 oxidation to sulphite |49|.

However the high level of NOx existing in the polluted atmosphere could have a prevalent role

in stone decay processes |50|

b) The occurrence of Chemioorganotrophes bacteria, has been investigated for their capacity

to produce several organic acids that can solubilize the minerals components of stones

|8,14,17,18,20,51-59|. Among the wide range of heterotrophes isolated, most could be assigned to

the genera Flavobacterius and Pseudomonas |60|. The genesis of some red staining of stone

could instead be attributed to species of genus Microbacter |61,62|.

All these microscopical organisms must be identified and quantified using laboratory methods,

as they are macroscopically undetectable on stone surfaces. The most used procedure is the

simple microbiological colture technique. But the data output must be correctly interpreted since

the synthetic media normally used are quite different from the real nutritional conditions; hence

there is always the doubt that the isolated strains are qualitatively and quantitatively very different

from those effectively developing on stone monuments.

To directly detect the activity of bacteria on stone samples collected from monuments several

instrumental methods have been applied such as: the Fluorescent Antibody Technique (FAT) |63|

and the Enzime-linked Immunosorbent Assay (ELISA) |64| using antisera, raised on rabbit, of

bacterial cell surface antigens present on previously isolated species from the same decayed

stone, together with: staining |65| or enzymatic methods measuring the DHA |66,67| alone or

coupled with the determination of protein content in filtrates solutions of powdered rock samples

|68|, or through the determination of the metabolic activity with FDA and INT |69| colorimetric

assays or with bioluminescent techniques quantifying the ATP content alone |70| or coupled with

Differential Flux Calorimeter determinations (DFC) |71|. Also the application of fatty acid methyl

ester analysis (FAME) |72| was used to identify the isolated strains. The direct visual observation

of stone surfaces, either with optical epifluorescence |73| or scanning electronic microscope (SEM)

|74-76| is surely an useful tool to ascertain the biological presence on stone samples, even if there

is always the doubt that the organisms observed are viable.

The results obtained with all these investigations are surely very precise but, as they arisen

from pure isolated strains incubated in laboratory conditions on highly energetic culture media,

these cannot give a complete idea of the complex processes involved in stone deterioration. In

fact, an exposed stone surface has generally a mixed interacting population, a copiotrophic

nutritive level and it is subjected, in European climates, to long periods of unsuitable weather

conditions. Therefore these results are very useful to understand the biodeterioration process in

laboratory, but hardly can be fully extrapolated to an "in situ" situation.

Lithophilous Algae and Cyanobacteria

These atmophytis photoautotropic organisms can develop well on exposed stone whenever a

suitable combination of dampness, warmth and light occurs. Growth is most prominent in the

spring and autumn. During dry and cold weather these organisms will die leaving a dirt deposit of

dead cells and reproductive bodies, which permit rapid new growth as soon as suitable weather

returns. These micro organisms are very sensitive to moisture content as they develop and

reproduce in a water film on the stone's surface. Bright green coloration develops on all areas that

are sufficiently moist and not subjected to direct sunlight. Their presence always indicates a high

moisture content of the substrate |9|. Depending on the kind of organism and on the cycle phase,

dark green, brown, grey and pink coloured patinas may also occur |77|. In urban atmospheres

pollution may prevent the development of algae and other organisms |78|.

The algae and cyanobacteria can be considered the pioneering inhabitants of a stone surface.

The latter can develop with just CO2, N2 and salt minerals traces. In fact they are, together with

some very specialized bacteria, the only organisms capable to directly do nitrogen fixation |79|.

Survival strategies of terrestrial algae include the formation of a polysaccharide sheath that allows

both fast absorption and slow releasing of the humidity. Moreover this sheath facilitates

cell/surface adhesion and the overcoming of long dry periods |80|.

These organisms can adapt themselves to very particular substrata, changing in colour and

morphology. The use of synthetic media could favour the development of one species at the

expense of another, leading to wrong conclusions on their respective ecological importance. In this

case, direct observation under optical or scanning electronic microscope can easily overcome

these drawbacks |81|.

These micro organisms usually participate in numerous lichenic associations |82| and the

most common genus for Cyanobacteriaceae are Nostoc, Calothrix and the boring species Hyella.

Epiphytic green algae, Pleurococcus spp and Chlorococcum spp, that utilize humidity of the air for

their hydric needs, can also develop on stone surfaces |83|.

The occurrence of micro algae and cyanobacteria on monuments |84-87| or plaster |88| has

been investigated both for their ability to etch mineral components |89| and to form anaesthetically

coloured patinas and in some case a reddish staining of marble |90|. But while their aggressive

action in relation to the substrate is still a matter of controversy |84| the presence of algae patinas

can assume great relevance, especially for monuments exposed in wooden environments |91|,

even if, the metabolic activity and the remains of the dead cells promote the development of

heterotrophic organisms and lower plants. |92|.

More recently new approaches for direct identification of phototropic organisms have been

experimented using biomolecular methods such as the Polymerase Chain Reaction (PCR) |93| and

the application of Fluorescence Lidar technique for the remote monitoring of the fluorescence

spectra of photosynthetic pigments |94|.

Lichens.

The lichens are of great ecological importance because of their pedogenetic action on the

lithic surfaces |94-106|. Through respiration they produce CO2, which it is transformed, within the

thallus into carbonic acid. With their metabolism release organic acids with chelating properties,

which complex mineral cations present in the rock substratum.

These metabolic substances cause compositional change in the stone surface layer

immediately beneath the lichen structure, with depletion of its main chemical elements (Al, Mg, Mn,

Zn, Si, Ca, K, Fe) and the accumulation of some of these, especially Ca, inside the thallus |107|.

Moreover they can cause physical damage |108| as a result of the hyphal deep penetration (up to

15 mm |89|) and periodical thallus contraction and swelling (up to 35 time its dry weight |110|) as a

consequence of climatic changes. With their slow constant growth (about 1 mm per year |111|),

they can cover large stone areas with undesired chromatic patches.

With respect to their morphology, crustose lichens are closely attached to the substratum and

are more significant in stone weathering than foliose and fruticose lichens. Among crustose lichens

the ephi and endolithic strains are very important for stone biodeterioration |112|.

A close relationship has been found between the growth and the distribution of the identified

lichenic species and some environmental characteristics of an examined area |113,114|. In fact,

these organisms have been proposed and used as biomonitors |115|. Different lichenic species

develop selectively upon specific lithotypes |116| and their colonization can be supported by the

contribution of the organic nitrogen present in bird excrements such as crows and pigeons |117|.

The genera of ephi and endolithic crustose lichens, frequently isolated from stone

monuments, are Protoblastenia, Verrucaria, Caloplaca, Aspicilia, Lecanora and Xanthoria

|118,119|.

Like algae these organisms are also sensitive to air pollution which exerts a selective action

on the species present |120|. When lichens die they leave behind a pitting corrosion with etched

negative image pattern due to the metabolic activity and to the incorporation of mineral fragments

into the thallus |109|. In some cases the lichen crust can act as a protective cover against other

decaying agents, like atmospheriles, substance dwindling and temperature and humidity

fluctuations |109,120|. Moreover the lichens can have a plugging effect on very porous and soft

stones |110|. For these reasons it is sometimes better to avoid eradicating lichenic thalli from stone

surfaces |120|.

One of the most studied acids produced by lichens has been oxalic acid. Saxicolous lichens of

crustose species can produce up to 50% of their total weight of this acid |121|, which being toxic

for the organisms is neutralized forming a very insoluble compound, the calcium oxalate |122-124|.

This and its different hydrate forms (Whewellite and Wedellite) have been greatly investigated for

their relevance upon the formation of patina on stone monuments |109,110,113,116,125-129|. This

patina in some cases was extremely continuous and homogeneous like a paint (scialbatura)

|125,127|. A very interesting debate has been made, in two international specific congresses, upon

its origin (biological or chemical) and significance and whether it has a protective function or not

|128,129|.

Artificial laboratory growth is very difficult for these organisms and the use of optical

microscopy must be used for their direct study on stone specimens |130,131|.

Actinomycetes and Fungi

The actinomycetes preferentially develop where a growth of algae and chemosynthetic

nitrogen fixing micro organisms exists |132|. With their metabolic activity contribute to weathering

and to enrich in organic matter the stone surfaces. Species have been isolated belonging to the

genus Nocardia and Streptomyces. They prefer to grow in subterranean environments (crypts,

grottoes, tombs) with high and constant level of relative humidity |133,134|.

The second group of strictly heterotrophes micro organisms, the fungi, also need organic

matter to grow. The acidolytic and complexing activity shown by the lichens was due to the fungal

component of this symbiotic association. This, as well as the reduction and oxidation of mineral

cations are characteristics activities of fungi |135-138|. Some fungal species (Penicillium and

Cephalosporium) have a greater biochemical decay potential than lichens |99|. The genera which

have been demonstrated being more abundant on the monuments investigated are: Cladosporium,

Penicillium, Trichoderma, Fusarium and Phoma |139,141|. They chiefly utilize the polysaccharides

coming from: microbial cell walls, metabolic intermediates and storage materials that are available

on stone surface.

Metal (Mn and Fe) bio transfer, operated by some fungal species, is involved in crust

formation |142| as well as the formation of rock varnish |143|. The former can induce crustose

exfoliation processes, while the latter hardens the surface layer, acting as a protective coating

|144|.

Several fungal metabolic products are strongly coloured and the phenomenon of staining of

damaged areas on monuments can be the consequence of fungal development. Dark spots are

attributed to the presence of fungi of the family of Dematiaceae which contains, water and organic

solvents insoluble, melanin pigments inside the mycelium |141,145|. Even if the formation of brown

or black colour patinas could derived from the oxidation of cell component debris due to the

cyanobacterial and algal colonization |146|.

Lower Plants and Weeds

The primary vegetal colonization of mineral substrata usually originates from more primitive

lower plants which have a much greater capacity to extract mineral ions from "unavailable" sources

such as rock surfaces |147|. This is due to the presence in their rootlets of higher concentration of

H+ ions |148|.

Liverworts and Mosses develop less rapidly and rarely on stone that is free from humus

deposit. The principal source of such a deposit is the accumulation of atmospheric particles and

from dead micro organisms. The mechanical and chemical action shown by these organisms for

stone monuments is negligible, as they do not have true roots systems but only rhyzines which are

not directly in contact with stone surface, even if the capacity of mosses to accumulate Ca++ ions

could be related with a biodeterioration capacity |149|. These lower plants are indicators of wet

conditions as they need very damp environment to complete their reproductive phase |150|. Heavy

accumulation (cushion) of growth can obstruct the passage of rainwater from gutters causing

stagnation, and consequently increase damage through freezing |111|.

Higher plants with a real root system and aerial vegetative apparatus may be of greater

importance in the conservation of monuments, especially for the archaeological remains where

they can obstruct both sight and accessibility |151| or for subterranean structures |152|. The action

of these organisms may be physical, through the pressure exerted by roots growth (up to about 15

atm. |153|) and chemical through the production of acidity and exudates from their rootlets |154|.

The transfer of nutritive metal cations through a network of colloidal particles, by a contact-

exchange mechanism with the H+ ions of rootlets, starts with the attack of silicates and carbonates

structures |155|. Among the organic exudates, secreted by roots, are found the chelating acids 2-

ketogluconic, oxalic and citric |156|.

The pH measurements made directly on the rootlets of some species present on monuments,

(Parietaria diffusa, Capparis spinosa, Cymbalaria muralis and Sonchus tenerrimus) have shown

values from 5.2 to 6.3 |158|.

In temperate climates the most frequently recovery of ruderal plants are herbaceous or shrub

species, with calciophilous, xerophilous and nitrophilous habitus (Cynodon dactylon, Melica

minuta, Parietaria officinalis, Capparis spinosa, Ceterach officinarum, Hedera helix |151|) and very

rarely arboreal species (Allianthus altissima, Ficus carica, Robinia pseudoacacia |158| or Ulmus

minor |159|.

The development of micro and macro flora is heavier and more noxious in tropical climates

where high rainfall and temperature greatly increase the growth of vegetal, especially higher

plants, on monuments |160,161|.

In some cases the roots can reach several metres (10 - 20) deep, as in the case of the very

common rupicolous species (Capparis spinosa) which is able to penetrate into very compact

substrata |162|. The deep penetration of the root system of tree species such as: Pinus pinea and

Quercus ilex can be dangerous for hypogeous structures, provoking the detachment of plaster and

the collapse of walls |158|.

Other factors must be taken in consideration as visual obstructions and the risk of fires

|163,164|.

The biodeterioration of calcareous stones induced by plants, which accumulates high amounts

of calcium inside, was checked using Energy-Dispersive X ray analysis (EDX) |165|, while the

Scanning Electron Microscopy (SEM) was employed to study stone surfaces colonized by plants

|166|.

Animals

Arthropodes communities as: the red mite Balaustium murorum with others acari (Phauloppia

lucorum, Trichoribates trimaculatus) and the psocid Cerobasis lucorum were observed on stone

building supporting algal and lichenic growth. This microfauna feeds on detached lichen particles

and contribute to the diffusion and propagation of new colonies being the spore, soredia and algal

cells entrapped among their legs and body hairs |167|.

The avifauna, especially pigeons and crows, can directly or indirectly damage stone substrata.

The direct destructive action of birds is both physical, caused by trampling and grazing, and

chemical, caused by the dropping of acid excrement (guano) containing high amount of nitrate and

phosphate compounds. Indirect damage is made by organic substances accumulated on stone

surfaces, which can serve as nutritive substrata for heterotrophic micro flora (bacteria and fungi)

|154|. For grotto and caves the presence of bats colonies can induces discolouration of painted

surfaces.

The passage of sheep and goats in archaeological sites with their trampling and feeding

action can compromise floor and mosaic wholeness on structures with poor cohesion |154|.

The action of an insect known as the mason bee (Hymenoptera) which bores, for the purpose

to building his nest, into stones and removes clay ground material from them has also been

reported |168|.

Black spots due to the dense mosquitoes deposits (Diptera) excretes have been found on

white marble surface |169|

The decay action played by sea boring animals |170| such as bivalves (Molluscs) or marine

worms (Polychetes) is confined to the case of submerged monuments |153|.

3. Plaster, Mortar and Frescoes

The same organisms as on natural stones can develop and bio deteriorate on these substrata,

at higher on lower extension depending if the object made with these materials is placed in

outdoors or indoors exposure. A particular case is represented by the frescoes, which composition

should be fully inorganic, but either for their collocation or for past restoration interventions some

organic materials can be present in their structure, consequently them can be attacked also by the

organisms developing on organic substrata |171|.

4. Organic substrata

The easel and panel paintings, wooden sculptures, library materials, prints and textiles are

artistic objects whose composition is based on natural organic materials. All of them, due to their

biodegradable nature, are usually maintained in confined environments. The biological attack of

these materials occurs only under poor conservation conditions: high humidity level, soil contact,

poor ventilation, and rare maintenance operations.

4.1 Wood

The susceptibility to the microbial attack of this material is depending on its moisture content.

Microbial deterioration, operated mainly by microfungi, can starts when the water content is above

the 20%. These can develop on surface or within internal structures inducing, through the

production of exoenzymes, change in cell integrity. The structural biological polymers are

destroyed chiefly by the activity of fungal strains |172-175|:

White rot (Pholiota sp., Fomes,sp., and Pleurotus sp);

Brown rot (Merulis lacrymans, Poria spp. and Coniophora puteana);

Soft rot (Chaetomium, Xylaria, Alternaria, Humicola Stenphylium

While the role of bacteria (Pseudomonas and Achromobacter) and actinomycetes

(Micromonospora) is less important because they require an higher water content and have been

found mainly in outdoor and marine environments.

Insects, however, are the most serious source of damage for wooden objects kept in

museums or indoor environments; they use wood as a nutrient source, for shelter and egg deposit.

In feeding, some insects utilize only the compounds obtained from the cell contents (sugars and

starch) while others utilize even the cellulose. Among insects able to use cellulose, the way in

which adult insects and/or their larvae digest wood varies. Some of them are able to digest partially

decomposed cellulose, while the generality have a complex symbiotic relationship with some micro

organisms that secrete cellulase enzyme complex in their gut (yeasts in Anobidi and protozoan in

Termites). The main insects involved in wood decay are reported in Table 1 |174|.

Among Coleoptera or beetles, many species have larvae that destroy wood by living inside it

and feeding on it, The life cycle of the beetles comprises four phases: egg, larva, pupa and adult.

The beetles lay their eggs in cracks or sheltered places. After few days (10) the larvae emerge,

bore into the wood and live into it (for 1 to 5 years) until they mature. After a short pupal stage (six

weeks), they become adult, bore to the surface and fly away. The sexually mature beetle

reproduces and the life cycle begins again. For most of their life cycle, these insects are not visible

and infestations are usually detected only by the small flight holes or by the formation of little bore

dust deposits.

Belonging to the Coleoptera order, the Anobiidae are the insects most frequently found in

deterioration of furniture, sculpture and other wooden objects. High RH and moderate temperature

favour their development. Unlike the Anobiidae, the powderpost beetles (Lyctidae) even thrive in

dry conditions and attack mainly sapwood. Longhorn beetles (Cerambycidae) can cause serious

damage, mainly to structural timber such as roofs or floors.

Table 1 - Insects frequently found on wooden materials

Orders Families Common names Type of damage

Anobiidae furniture beetle Winding and circular tunnels; circular

emergence holes

Lyctidae powderpost beetle Tunnel with oval section

Botrychidae wood borer Circular holes and tunnels

Coleoptera

Cerambycidae longhorn beetle Large, oval tunnels and holes

Kalotermitidae termites or white

ants

Deep and crater-shaped holes;

sometimes entire interior of object is

destroyed but outer surface is left intact

Isoptera

Rhinotermitidae “ “

Hymenoptera Siricidae wood wasp Circular tunnels and holes of wide

dimension

Termites are most common in the tropics and subtropics because they require high

temperature and humidity. They are social insects, forming communities that include reproductive

individuals (queen and males), nymphs and wingless specialized sterile forms (workers and

soldiers). Workers are confined to the ground or wood where they devote their energy to feeding

and foraging for the community. They often hollow out the wood completely, leaving a thin outer

shell of wood undamaged. Termite attack is often not detected until the whole structure collapses.

This happens because many termite species are photophygous (heliophobic).

There is also a range of omnivorous insects that can cause incidental damage while

scavenging for other food; cockroaches (Dictyoptera) belong to this group. These insects only

cause surface erosion of variable depth.

A different situation occurs with Dermestidae, a family of Coleoptera, which are destructive for

dry materials of animal origin and occasionally for wood and materials of vegetable origin; the

larvae frequently bore "refuges" into timber adjacent to a food source.

In contrast, the damage caused by insects living inside the wood includes emergence holes of

different shapes and sizes as well as superficial or deep tunnels. The presence of holes does not

necessarily indicate an active infestation, but the bore dust produced from holes may give an

indication. The main insect damage is due to the breakdown of the wood fibres, which affects

structural strength.

Waterlogged wood is the term used to describe wood kept under soil or water, for example

archaeological wood such as shipwrecks or pile-dwellings. Under these particular conditions (high

water content and lowered oxygen pressure), wood can easily be attacked by microaerophilic and

anaerobic heterotrophic microorganisms and, in sea sites, by some marine organisms.

The micro flora able to attack waterlogged wood varies in relation to the maintenance site

(sea-water, fresh-water, water-saturated soil, etc.) and specific environmental conditions (oxygen

concentration, temperature, concentration of mineral salts, etc.). In sea water, bacterial

degradation of wooden materials can occur in water or in sediment up to a maximum of 60 cm

deep; fungi occur under aerobic conditions in the sediment-water interface |176|.

The micro organisms that can degrade waterlogged wood are either some wood destroying

species such as soft rot fungi, sea-water fungi or bacterial strains.

Among bacteria, the most common are cellulolytic aerobic (Cytophaga, Cellvibrio,

Cellfalcicula) or anaerobic species (Plectridium, Clostridium) and common microorganisms such as

strains of Bacillus, Pseudomonas, Arthrobacter, Flavobacterium, and Spirillum. Bacteria are also

categorized according to the type of alteration they cause. Thus, there are erosion bacteria,

cavitations bacteria and tunnelling bacteria |177|.

In sea-water and fresh-water environments, some micro-algae strains can also cause damage

to immersed woods. They form slimy green patinas and cause superficial modification of wood

characteristics. The alterations caused by these micro organisms are not too serious, but they

contribute to chemico-physical decay.

In sea water, marine borer animals are the most destructive organisms for waterlogged wood.

They are widely distributed in salt water, prevalent in warm climates, and include different species

of Mollusca and Crustacea.

The main genera of Mollusc are Teredo, Bankia and Martesia, which produce tunnels that

penetrate deeply within the wood. The tunnels may have a chalky-white shelly (calcareous) lining.

The first two genera comprise a number of species known as "shipworms". After a free-swimming

larval stage, the young organisms become able to attack timber and make very small entrance

holes in the surface of wood, but once within it they increase rapidly in size. They use wood for

shelter and feed on the minute organic particles and plankton found in the sea water. Their whitish,

worm-like bodies are equipped with a pair of shelly valves at the anterior end that excavate wood.

The molluscs of the genus Martesia are small clams with their bodies entirely encased within the

bivalve shell. The damage caused is relatively small compared to the other two genera.

Within Crustacea the main wood-destroying genera are the isopods Limnoria, Sphaeroma

(common name "wood lice") and Cherula (common name "sand fleas"). The wood-boring

crustaceans differ from the molluscan borers either in their methods of attacking wood or in their

general structure. They do not become imprisoned in the wood but, instead, are able to move

about. The young and the adult are equipped with a pair of strong-toothed mandibles designed for

chewing wood, which provides nourishment; they burrow in the timber, making galleries of various

depths with narrower bores than those of Mollusca. They attack the timber in such great numbers

that the infested wood becomes honeycombed and can be broken by the mechanical action of

water at the point of attack. The damaged area is usually concentrated at the water line. The

damage caused by these marine borers is more evident to inspection and proceeds more slowly

than that of shipworm, so it is less spectacular and serious |178,179|.

4.2 Paper

Paper is primarily composed of cellulose and other substances related to the origin of the raw

materials used in its manufacture: lignin, hemicelluloses, pectins, waxes, tannins, proteins and

mineral constituents. In fact, paper can be manufactured from textiles or from wood through

various and complex operations. The content of these components varies according to the

papermaking process, the type of paper and the period of production. In the middle Ages, for

example, the quality of paper was particularly good; it was manufactured from carefully selected

cotton rags and contained a great amount of cellulose and only a few impurities. Subsequently, the

quality of paper changed for the worse modern papers produced by industrial manufacturing

processes since the end of the seventeenth century. These are derived from the stem of wood or

wood pulp and contain a great amount of polymers and non-fibrous material other than cellulose,

as well as impurities. Modern papers are thus more vulnerable to micro organism attack than older

ones |180|.

Paper is a good source of nourishment for heterotrophic micro organisms and organisms. The

indispensable condition for a microbial attack is a high water content, and the hygroscopicity of

paper makes it more sensible to the biodegradation.

The main biodeteriogens micro organisms for paper are bacteria, micro-fungi and

actinomycetes, either cellulolytic strains damaging the chemical structure of paper or non-

cellulolitic ones with non-specific action. But, the most common are fungi because they show a

great tolerance to environmental conditions and can live with lower water content than bacteria and

actinomycetes.

Among cellulolytic strains of micro-fungi, many species of Deuteromycetes (e.g., Alternaria

spp., Aspergillus spp., Fusarium spp., Humicola grisea, Myrothecìlim verrucaria, Penicillium spp.,

Stachybotrys atra, Stemphylium spp., Trichoderma spp., Ulocladium spp., etc.) and Ascomycetes

(c.g., Chaetomium spp.) are frequently isolated from books, documents and prints |181,182|.

Certain cellulolytic species of Aspergillus and Penicillium are particularly harmful to paper because

they are able to grow on substrates having a 7-8% moisture content, which can be reached in

some types of paper at RH levels as low as 62-65%; such conditions are not rare in many places

where paper is kept |182|.

The binding is the first part of a books to uptake moisture from the air, thus fungal growth

spreads more quickly on bindings than on sheets of paper |208|. Some species of Chaetomium,

Trichoderma viride and Stackybotrys atra cause a deep decay of cellulosic material, which then

lose its mechanical properties.

In general, fungi cause alterations on paper producing various kinds of stains, either round or

irregular in shape and coloured in red, violet, yellow, brown, black, etc. These stains are due either

to the presence of pigment mycelium or to the release of coloured metabolites |182|. The colour of

pigments can change depending on the conditions of growth and the properties of paper (e.g., pH,

presence of starch or gelatine sizing, presence of metal, etc.). A particular kind of alteration due to

fungi is the discoloration of inks due to tannase, an enzyme that catalyses the hydrolysis of

gallotannate, produced by some strains of Aspergillus and Penicillium.

Bacteria attack paper less frequently than fungi, but several bacteria have been isolated from

papers maintained in environments with RH higher than 85% |180,184,185|. The cellulolytic

species are obviously more harmful than non-cellulolytic ones; the genera Cytophaga, Cellvibrio

and Cellfalcicula belong to the first group, the latter one is rarely found.

During metabolism, all micro organisms produce different organic acids (oxalic, fumaric,

succinic, citric, etc.), which reduce the pH of paper, conditioning the dynamics of bacterial and

fungal growth in secondary attacks. Frequently, bacterial and fungal attack makes the paper feltish

and fragile.

One particular and very common chromatic alteration of paper is foxing; it appears as rust-

coloured marks of different shapes, frequently as spots. The causes of foxing have not yet been

clarified, many authors consider fungi as possible cause, but others believe that the presence of

heavy metal, most commonly iron, might be among the factors favouring this kind of alteration

|186-188|.

A particular type of damage “Consolidation” can affects books that have been flooded or

heavily moistened. The alteration is brought about by growth of bacteria and fungi. This is related

to the production, during cellulose degradation, of oligosaccharides with mucous properties and,

on substrates particularly rich in sugar, to the formation of secondary metabolic products of a slimy

nature. It is clear that the composition of paper (fibre content and type of glue) affects the intensity

of the degradation |189|.

Insects are frequently involved in the deterioration of paper. Many insects are able to attack

cellulose with processes like those used on wood. Others also cause damage by utilizing fillers,

glues, boards, textile fibres, leather or other constituent elements of paper materials. Based on

frequency, there are customary and occasional insects. The former are using this material for

nourishment, while the latter (e.g., wood or textile borers) sometimes damage the paper only when

feed different materials present on it, such as leather, starch and wood. The morphology of insect

damage ranges from tiny superficial abrasion and surface erosion to holes and sometimes tunnels.

In Table 2 the main orders and families of insects responsible for deterioration of paper works

of art are listed |182,190|

Table 2 - Insects frequently found on paper materials

Orders Families Common names Type of damage

Thysanura Lepismatidae silverfish Small surface erosion

with irregular outline

Isoptera Kalotermitidae

Rhinotermitidae

Termitidae

termites Deep crater-shaped

holes and erosion;

destruction of the

interior of object while

the outside remains

intact

Anobfidae furniture beetles Winding, circular

tunnels

Lyctidae powder post

beetles

Tunnel with oval

section

Coleoptera

Dermestidae skin beetles Short blind tunnels

with circular section

and irregular

perforation

Corrodentia Liposcelidae book lice Tiny surface abrasion

Blattoidea Blattidae

Blattelidae

cockroaches Surface erosion

Anobiidae, Lyctidae and Dermestidae complete their life cycle inside books. Dermestid

beetles mainly eat leather bindings, but they burrow into books, making tunnels where they pupate.

Blattidae, Blattelidae, Lepismatidae and Termitidae live in environments where books are kept, and

paper, cardboard and glue of vegetable or animal origin represent their source of nourishment.

Liposcelidae are the smallest insects in book biodeterioration (1-2 mm), and are very common.

They feed on paper, glue, etc., and live especially on bindings. These insects also feed microfungi

developed on deteriorated surfaces, so they appear frequently under high-humidity conditions.

4.3 Textiles

Textiles can be of either vegetable or animal origin. Those of vegetable origin are cotton, flax,

hemp, jute and sisal. They are mainly composed of cellulose derived from plant fibres; for example,

flax and jute are phloem fibres, sisal and manila are leaf fibres and cotton is made from seed

fibres. Like other materials of vegetable origin, the susceptibility or resistance of textiles to

biodeterioration depends on the content of cellulose, lignin and other organic constituents. The

presence of non-cellulosic components such as lignin and waxes decreases susceptibility; in

contrast, pentosans and pectins increase susceptibility. Natural cotton contains about 5% of non-

cellulosic products, flax 15%, so cotton is less susceptible to microorganisms than flax.

Fibres with a high amount of lignin are more resistant to microbiological attack than purified

fibres, while textiles contain sizing (starch and dextrin) are more susceptible. The following

structural features of fibres and textiles also affect biodegradation: degree of polymerisation, length

of cellulose chain, fibre crystallinity and orientation. Furthermore, mechanical, and photochemical

degradation increase susceptibility to biodeterioration. Loosely woven fabrics are less resistant

than tightly woven fabrics because they hold more dirt and biological pollutants between fibres,

creating favourable conditions for biological attacks|191|.

The most frequent biodeteriogens of cellulosic textiles are micro-fungi (Ascomycetes and

Deuteromycetes) and bacteria of cellulolytic and non-cellulolytic species, many of which have been

mentioned for the wood and paper biodeterioration.

The most active agents of textile deterioration are fungi. Among Deuteromycetes, we find

several species of Alternaria, Aspergillus, Fusarium, Memnoniella, Myrothecium, Neurospora,

Penicillium, Scopulariopsis, Stachybotrys and Stemphylium, etc. Quite frequent and particularly

harmful because of its high cellulolytic activity is the genus Chaetomium of Ascomycetes

|181,191,192|. Zygomycetes such as Mucor and Rhizopus sometimes occur on textiles.

Bacteria and actinomycetes need high water content in fabrics, and their occurrence in

museums should be rare. But in damp environments or when textiles are buried in the soil (e.g.

archaeological textiles), the most serious decomposition of cellulose is caused by bacteria

belonging to the species of Sporocytophagamyxococcoides and then by Cellvibrio, Cellfalcicula,

Microspora and anaerobic Clostridium |206|. Degradation of fabrics exposed outdoors has also

been associated with species of Myxomycetes and actinomycetes |191|.

Under conditions of damp, heat, lack of ventilation and contact with decaying matter, the

microbiological attack can occur in a few days. The biodeterioration of textiles can take several

forms, such as discoloration, staining and loss of strength.

The metal fibres, used for embroidery in the manufacture of high cost textiles objects such as

religious furniture, noble dressing and ancient costumes, can inhibit microbial growth, particularly

when copper or other heavy metals are present.

Cellulosic textiles are also susceptible to attack by insects such as silverfish (Lepisma

saccharina) and cockroaches (Blattelidae). The probability of attack is increased when this material

contain glues made of starch or dextrin. The main insect families involved in textile biodeterioration

are Blattidae, Lepismatidae and Mastotermitidae, Hodotermitidae, Rhinotermitidae (termites), but

many others may occur occasionally |193|. In general, their attack ranges from surface erosion to

destruction of parts of the object.

4.4 Parchment and leather

Parchment as a writing support was produced from sheep or goatskin. It is composed of

collagen, some amount of keratin and elastin, and a minimal amount of albumin and globulin.

Collagen is one of the proteins most resistant to micro organisms attack it can only be hydrolysed

by specific enzymes, collagenases, produced by some anaerobic bacteria of the genus Clostridium

|180,181|. The mechanical disintegration of skin during the manufacture of parchment,

depolymerise the collagen. Non-specific proteolytic enzymes produced by many bacteria and

some microfungi in aerobic conditions can decompose this denatured collagen. Other factors such

as temperature (increases), humidity (violent change), pH and exposure to UV rays can also

influence the stability of collagen in parchment; these factors cause changes in its structure, and

decrease the resistance to the biodeterioration. In addition to collagen, some other components

(other proteins, lipids, carbohydrates, mineral constituents and impurities) either derived from the

original skin or are added during manufacture, are involved in the decay of parchment.

The parchment's susceptibility to biodeterioration depends: by the raw material, its method of

production and its conditions of preservation. Under aerobic conditions, only bacteria can attack

partially decomposed collagen, among which are strains of Bacillus (e.g., B. mesenthericus),

Pseudomonas, Bacteroides, and Sarcina. Ancient parchment materials can also be attacked by

certain species of fungi of the genera Cladosporium, Fusarium, Ophiostoma, Scopulariopsis,

Aspergillus, Penicillium, Trichodernia, etc., |194|. As a result of biodeterioration, parchment loses

its original properties and becomes hard and brittle, often with deformation of the structure. The

microbial attack also causes variegated spots, white films and fading of the texts.

Leather has a chemical composition similar to parchment and its susceptibility to

biodeterioration and the species involved are almost the same. To produce leather, skins are cured

by subjecting them to tanning, which causes chemical and physical changes. The attack of micro

organisms varies depending on whether the skins were tanned or not. Bacteria under conditions of

high humidity attack untanned skins. Tanned leathers are not readily subject to bacterial attack, but

are degraded by fungi. Usually, after tanning, leather has a pH of about 3 to 5, which is more

suitable for fungal growth. Vegetable-tanned leathers are more susceptible to fungal growth than

chrome-tanned types because they contain some amount of glycosides. Chrome-tanned leather

seems to afford some fungistatic activity and protection against microbial activity but sometimes

this type of leather shows the growth of fungi of the genera Penicillium and Paecilomyces, which

are tolerant to the chromium compounds. Fungi that attack tanned leather often belong to lipolytic

species and utilize the fats present in leather as a source of carbon. In this case the proteins are

not directly affected, but can be damaged by organic acids released as a metabolic end products

and the artefact becomes stained and stiff |195,196|. The principal effects of microbial deterioration

on proteic materials are the presence of different stained spots, loss in tensile strength, and, if the

proteins are attacked, hydrolysis of the leather. Proteinaceous materials like parchment and

leather are also susceptible to attack by insects: Dermestidae and Tineidae are the main families

that can selectively attack either collagenous or keratinous materials |195,196|. Occasionally,

cellulose and proteinaceous feeders cause damage to parchment and leather materials. The insect

damage appears as superficial erosion, deep erosion, holes or loss of material.

4.5 Wool and silk

The wool is produced from the hair of different mammals (e.g., sheep, alpaca, vicuna, etc.),

consists mainly of keratin, a highly insoluble protein containing sulphur linkages in the molecule.

The commercial silk is a proteinaceous filament secreted by the domesticated "silkworm"

Bombyx mori for making its cocoon. A caterpillar of the Antheridae family produces wild silk. The

silk fibre is composed of two filaments made of a protein (60-70%), fibroin, linked together by

another protein (25-30%), sericin. Fibroin is a highly crystalline scleroprotein, which is very

resistant to chemical agents and highly insoluble. Sericin is an albuminoid substance. In some

manufacture of silk fibres, it is removed by treatment with warm water and soaps (degumming

process); this treatment increases silk's resistance to microbial attack.

The main agents of deterioration are micro organisms (bacteria, actinomycetes and fungi) and

insects, but insects are considered the most serious pests for cultural objects kept in museums.

Although protein fibres, such as silk and wool, are not as susceptible to micro organism

deterioration as cellulosics, they can be attacked if they contain a high degree of impurities (such

as sericin for silk or sizing and suint for wool) and if they are stored under warm and humid

conditions |191|.

In general, wool fibres are more subject to attack by bacteria than cotton fibres and less

subject to fungal attack. An indispensable condition to start a bacterial attack is a high water

content; this condition is favoured by the high hygroscopicity of wool. Bacteria may be

keratinophiles, able to produce enzymes that break down keratin, such as some dermatophytic

fungi (Trichophyton and Microsporum), which can cause disease in human beings.

The micro organisms most frequently mentioned in degradation of wool and other protein

fibres are bacteria of the genus Bacillus (B. mesenthericus and B. subtilis), Proteus (P. vulgaris)

and some species of Actinomycetes (Streptomyces albus and Streptomyces fradiae) |181,191|.

Pseudomonas aeruginosa frequently causes green, red or other coloured stains according to the

pH of the substrate.

Micro-fungi are not very frequently involved in biodeterioration of protein fibres but species of

Aspergillus, Fusarium and Trichoderma are sometimes reported |181,191|.

Silk is fairly resistant to micro organisms if impurities and sericin are removed in

manufacturing processes (degumming). The strains degrading silk include the species that attack

other protein fibres, but specific deteriogen micro organisms for silk are not identified. The

microbiological damage appears as discolorations, stains of different shapes, colour and diffusion.

Tensile strength may be reduced: for example, silk becomes brittle.

The most serious damage of animal-derived textiles, in museums in moderate climates, is

related to insects, as microbial attacks on protein fibres rarely occurs. The insects most frequently

found are some species within the families Dermestidae, Oecophoridae (brown house moth) and

Tineidae (clothes moth). The better-known species of moths are Tinea pellionella, T bisselliella and

Hofmannophila pseudospretella, which degrade extensively wool and rarely unscoured silk.

Larvae, which use wool as food and to build a protective sheath, do the damage. Carpet beetles,

too, cause damage to textiles. Among most frequent species of Dermestidae are Anthrenus

verbasci, A. museorum, Attagenus pellio, which cause damage only through their larvae because

the adults usually spend their life elsewhere.

Infestation by these insects is rather widespread and not confined to textiles. Susceptibility is

increased and the range of deteriogen insects can become larger if these materials contain sizing,

composed of starch or dextrin, and other organic components. The type of damage involves the

production of holes of irregular shape, superficial abrasion or deep erosion.

5. COMPOSITE MATERIALS

Works of art are often made of a combination of different organic and inorganic materials,

rather than just one. It is impossible to list all the types of cultural heritage assets and to give

information about their susceptibility to biodeterioration. In general, the risk of biological attack is

linked to the most susceptible component. The biodeterioration phenomena on composite

materials are similar in their character and effect to those of the single component.

5.1 Paintings

Paintings are composed of a support (canvas, wood, paper or parchment), a preparation layer

and a paint layer, the chemical composition of which varies according to the mode of painting, the

kind of paints used (oil paints, distemper or watercolour), the historical period and the author.

In canvas paintings, the preparation is usually made with lime or gypsum with addition of

animal or vegetal glue. On this smooth surface several layers of colour are present, which consist

of pigments mixed with binders of oil or distemper (egg or glue). The surfaces are usually covered

with a thin, translucent protective varnish. In paintings on wooden supports, a similar multilayer

structure is observed.

Paintings on paper can be watercolours, gouaches or pastels. The paint is laid directly on

paper and the state of preservation of the paper determines the durability of the whole painting. In

watercolours, the transparent paint layer also contains a small amount of binder, usually gum

arabic. Pastels are made with crayons consisting of pigment without binder and are, therefore,

extremely difficult to store and preserve.

In painted works of art, the biodeterioration processes can involve either a portion of the

painting or all of its components. Thus, paintings may show traces of a biological attack on the

reverse side, the support, or on the painted side, and a part or all components may be damaged.

The organic components in paintings represent a good source of nutrition for a wide range of

heterotrophic micro organisms and organisms. But, biological attack occurs only when there are

favourable environmental conditions, and such conditions are often found in museum rooms, old

churches or in deposits without any control of the humidity and temperature.

The micro flora attacking paintings include virtually all species of micro-fungi because the

variety of organic components of these works of art can represent a carbon source for practically

all species. In addition, they show a great tolerance for environmental conditions and can use

condensation moisture. In contrast, the moisture content of these objects is rarely so high as to

favour development of bacteria, which are unable to use condensation water |184|.

Generally, the first part subjected to the microbial deterioration is the support. In fact, in

paintings on canvas, the microbial attack usually starts from the reverse side, because the glue

sizing increases the natural susceptibility of textiles. Then the biodeteriogens penetrate inside

canvas reaching the back side of the paint layer, causing cracks and detachment, while the

cellulose hydrolysis creates differences of adhesion between the paint layer and the canvas itself

|197|. In paintings on wooden board, the decay of the support can be different from the paint layer.

The micro organisms involved in biodeterioration processes are those mentioned for

biodeterioration of materials of vegetable and animal origin. Sometimes the presence of

substances, such as sizing glue or lining paste used for different treatments of the support, can

increase susceptibility |184,198|.

Biological attack on the paint layer is less frequent than on the support and depends on the

nature of pigments. The most susceptible to biological attack are casein and egg distemper,

emulsion distemper and linseed oil in this order. In contrast, the presence of heavy metals in some

pigments, such as lead, zinc or chromium, can increase the resistance of the paint layer.

Watercolours contain only a small amount of organic binder and are, therefore, as susceptible to

microbial deterioration as pastels |184|.

The growth of a microfungal mycelium by a microscopical germinating spora is quite rapid,

with radial development, and it become macroscopically visible in few days. Among the species of

micro-fungi most frequently involved in deterioration of the paint layer, species of Penicillium,

Aspergillus, Trichoderina and Phomapigmentovora disintegrate distemper and oil binders,

Aureobasidium decompose oil binders, Geotrichum develop on casein binders, Mucor and

Rhizopus attack glue |183,184,199|.

The development of micro-fungi on the surface of paintings induces aesthetical, mechanical

and biochemical decay. In fact, the growing mycelium spread over the paints, masking design and

colour, while the growth of hyphae and fruiting bodies inside the support can cause, friability and

loss of the paint layer. Exoenzyme activities can cause more serious damage by decomposition of

some polymers both of the paint layer and of the support, whereas the presence of coloured

fruiting bodies and the production of coloured metabolites provokes the formation of permanent

stained patches while organic acids produces irreparably modifications in the structure.

In biodeterioration of painted materials, insects are also often reported but they only

occasionally damage the paint layer, preferentially attacking the wood of the frame or support and

the vegetable glue.

5.2 Restoration Materials

The material used during the restoration procedures (animal and vegetal glues, re-lining

paste, varnishes, temperas and other materials used for cleaning and soaking) can contribute to

the biological risk being these fresh and with high water content.

6. METHODS OF INTERVENTION

In order to control biodeterioration processes the most suitable methods and products must be

used. With regard to the principles and the nature of the means employed, control methods can be

classified as mechanicals, physicals, biologicals, or biochemicals, even if those based on active

principles in solution (chemicals) are the more frequently applied (either as wide-spectrum active

principles, or more specific fungicides, algaecides, herbicides, insecticides and repellents for

birds).

6.1 Direct Methods

6.1.1 Mechanical Methods

All the techniques described as "mechanical" have in common a method of displacing the

biodeteriogens. Traditional mechanical methods involve the physical removal of biodeteriogens

either by hand or with tools such as scalpels, spatulas, scrapers, air abrasive or vacuum cleaners.

Although frequently used in the past, these methods do not produce lasting results and the

eradication of vegetation could do not completely arrest its vegetative activity. Moreover,

mechanical methods can damage the substrate, even if they have the advantage to not add on it

any substance that might cause further deterioration. In some cases the mechanical methods

alone can be sufficient to eradicate some of these biodeteriogens such as, mosses, foliose lichens

and annual herbaceous plants. Nevertheless a preliminary biocide treatment is surely

advantageous to facilitate the removal of the biological growth and to hinder the diffusion of the

colonisation of lichens, reproducing by means of soredia, through their dispersal due to mechanical

cleaning methods |200|. The mechanical removal of lichens on stone can be facilitated by applying

beforehand some alkaline solutions (e.g., 5% of ammonia) in order to swell and soften the thalli

6.1.2 Physical Methods

The methods used experimentally are: ultraviolet rays (UV), gamma rays, low frequency

electrical current systems, heat, deep-freeze temperatures and ultrasonic.

Ultraviolet rays have been used especially against bacteria, algae and fungi in the treatment

of renders and plasters |201|. The part of the UV spectrum with germicidal activity is between 300-

200 µm, with a maximum of activity between 275-230 µm. Micro organisms vary in sensitivity

depending on their growth phase (greater during the logarithmic phase) and the nature of the

substrate on which they are found. UV radiation is more effective at low RH values (less than 50-

60%). It has a poor penetration power and can modify some materials (e.g., cellulose, proteins)

and the colours (natural or dyes) of surfaces.

Gamma rays are a form of electromagnetic radiation. They are extensively used for sterilizing

micro flora and killing insects, especially on organic materials such as paper, parchment and wood

|202,203|. A dose of 500 Gy is required to kill larvae and to prevent the emergence of adult insects.

Moulds are less sensitive to ionising radiation than insects, and different strains show different

levels of sensitivity; generally most fungi are killed by a total dose of 10 kilo Grays (kGy) |204|.

Despite its power, gamma irradiation does not induce any secondary radioactivity, does not

damage oil and tempera polychromy, leaves no hazardous residues and penetrates completely

into the objects. Moreover, a large quantity of materials can be treated at one time. Paper is much

more sensitive than wood to the effects of gamma radiation. A possible negative effect of gamma

rays on musical instruments has been verified in the modification of the “sound” of a violin after

treatment. Repeated treatments are not recommended because the effects are cumulative.

Gamma rays have recently been employed to cause the polymerisation of resins used in the

impregnation of decayed wooden objects; in this case, wood-boring insects are exterminated at the

same time.

High-frequency current can be used to kill wood-destroying insects (Anobiidae), but only in the

absence of metals |202|.

Low-frequency electrical current systems have recently been used to keep birds from roosting

on monuments |205|. This method is completely harmless for animals and humans.

Heat (dry or wet) is used in the disinfestations and disinfections of organic materials. The

application of moist heat for disinfections of books is still one of the most widely used techniques

|206|. A temperature of 95°C and 40% RH for 4 hours is recommended.

Other methods used to eliminate infestations of books are exposure to blast-freeze

temperatures or to substitute the atmosphere in close container with N

2 or with O

2 scavengers

(concentration below 0,05 %) |207|. Reduction in pressure, using vacuum dryers, causes cell’s

explosion.

Finally, we might mention the suggested use of light traps for controlling insects attracted by

light, in museums, and the exclusion of light, using opaque plastic sheets, to prevent the

photosynthesis by biodeteriogens present on statues in wooden environments.

6.1.3 Biological Methods

This application is based on the presence of parasitic or antagonistic organisms of the

biodeteriogens. Bacteria, insects and phage might be used, but up to now this kind of intervention

has been applied chiefly in the agricultural sector. Antagonistic or predators species have been

introduced for pull away the birds from cities.

6.1.4 Biochemical Methods

In this group, we consider biodeterioration control systems that use chemical compounds of

biological origin even if many of these are now synthetically produced.

Antibiotics are substances produced by micro organisms during their growth in order to avoid

the competition of other species, which are inhibited or killed |208|. These compounds are active at

very low doses, but can lose effectiveness after storage.

Enzymes are proteins that catalyse biochemical reactions. They have been used on rare

occasions as biocides and are sometimes mentioned for cleaning of adhesives |209|.

The proteolytic enzyme trypsin has been used for the removal of lichenic crusts, but reports

have indicated disadvantages in the practical difficulty of maintaining a good enzymatic activity, as

a function of temperature and pH |210|.

Pherormones are volatile substances produced by one individual, which have a specific action

on the opposite sex of the same species. Some experiments have been made with one of this for

controlling insects in museums with sexual attractants traps. These substances could be very

useful as preventive, taking into account the difficult for liquid biocides to deeply penetrate inside

the object attacked by boring insects.

6.1.5 Chemical Methods

Many organic and inorganic compounds have been used, as biocide agents, to eliminate the

biodeteriogens from cultural objects (see Table 3).

Pesticides are chemicals used for killing undesirable biological growth. They have a biocide

action with a specific toxicity for the species to be eliminated (it is also possible to use the term

biocides). These are classified in different ways depending on their chemical nature (e.g., organic

or inorganic) or on the target pest species. It is also possible to classify pesticides according to

their chemical groups or their mode of action. In addition to the pesticide, other ingredients, such

as carriers or additives to improve the effectiveness of the product or to facilitate its application, are

present in the chemical formulation. These are called coformulants and could have negative

effects on the objects to be treated.

Disinfectants are chemicals that destroy vegetative forms but are not always effective against

survival resistant or quiescent phase structures (bacterial spores, fungal conidia and insect eggs).

A side problem with the use of pesticides is the persistence of the product in the soil or water.

This problem is especially prevalent with herbicides that are applied or dispersed in external

environments, with high risk for soil and water contamination.

6.1.6 Inorganic Substrata

The biodeterioration of monumental stones, rock and archaeological remains is a worldwide

problem. In fact, the growth and development of many organisms, belonging to different systematic

groups, acting alone, together or in ecological succession, surely enhance the stone decay and

makes their conservation more difficult. For these reasons adequate interventions must be taken to

stop or at least slow down the biodeterioration process.

Once assessed the active presence of organisms on stone monumental surfaces and before

to adopt any remedial measures, is necessary to properly evaluate the entity of damages induced

by this presence. In fact, when this is very slights and a regular maintenance is impossible, any

biocide treatment is useless. The same when the biological patina (lichens) could act as protective

layer against major injuries due to physical-chemical factors. Otherwise is necessary to intervene

in order to eliminate or control the biodeteriogens.

Many authors have used several products having biocide characteristics, in order to eliminate

the biodeteriogens developing on stone monuments or rock sites. These products are

commercially available both as active principle or formulates and cover a wide range of chemical

classes, from very simple inorganic compounds such as Na and Ca hypochlorite to very complex

organic ones such as the Quaternary Ammonium Compounds (Preventol R50, Neo-Desogen).

Furthermore, they can possess a strictly specific mode of action such as the urea derivatives

(Diuron, Karmex) that block the photosynthetic process, or a broad toxic spectrum like the

Organotin compounds (TBTO). A list of the most frequently applied products and the different

methodologies used for their application are reported in some articles |17,19,78,211-216| and in a

recently published book, in which are reported detailed technical forms for 31 of these biocides

|217|. Any biocide intended for use on historic monuments and rock sites must be not only effective

against biological growths but at the same time cause no damage to the stone material either by

direct action or by leaving deposits on it which may result in successive damage |17,111|.

Furthermore, must be underlined the necessity of testing these products under similar “real case”

conditions as the interactions among type of substrates, biocides and organisms can give a more

realistic indication of how a biocide will perform in situ |213|.

The treatments of weeds and higher plants can be made either using pre or post-emergence

products. In the first case the treatment (Simazine, Monuron) must be done in winter in order to

block the seed’s germination; in the second case the treatment is made in summer by spraying the

product (Picloram, Secbumenton) on leaves |215|.

The problem represented by the birds roosting on monuments can be easily solved by placing

on these either low current wires or simple plastic needles (unaestethical). The more general

problem of the invasion of the urban areas by this fauna has been fronted either with the use of

feeding materials mixed with anti pregnant products together with their periodic capture and

release in other sites.

The following steps must be assessed before the application of any treatment in this field of

intervention:

a) Effectiveness of action of biocides

The product should be chosen on the basis both of the organisms to be eliminated and of its

highest efficacy at lowest dosage |218|. Normally the data reported in bibliography are very

accurate on the type, concentration and mode of application of the biocide used, while rarely is

mentioned the effective amount of biocide applied for square meter of surface. Thus, it is very

difficult to extrapolate the results obtained with the same biocide in different applications. Besides,

a general lack of a common test organism |219| to be used as internal standard for lab tests

contribute to the impossibility to correlate the real efficiency of different biocides used in various

laboratories. Normally the results of positive biocide treatments are clearly visible on the "in situ"

biological growth with the exception of lichens which can remains macroscopically unchanged for a

long time even if the treatment was effective |217|. The lasting of a treatment in outdoor conditions

is ranging from 1 to 3 years depending both on type of the chemical used and environmental

conditions to which the treated stone is exposed. Moreover, if the biocide must be wash off, after

its action, or leave as preventive on the surface is a matter of discussion |217,220,221|, included

the extreme suggestion made for the conservation of the Acropolis of Athens, where the preventive

biocide treatments of the critical spots, perhaps even the whole buildings, seem to be as

unavoidable as daily tooth brushing for the biological status of human teeth |222|.

b) Interaction biocide/stone/organisms

Possible negative effects on stone surface treated with biocides have been detected either

with SEM observations or determined by colorimetric measurements |223-225|. It was ascertained

the influence (positive or negative) that a lithotype exerts on the efficacy of a biocide |26,226|. In

addition, the same biocide can be more or less active on different strains present in the consortium

to be eliminated |227|. Hence the selected biocide should always be tested on test specimen made

with the same material of the monument, possibly colonized with the same biocoenosis developing

on it. More recently some applications have been made using protective or consolidating polymers

alone or mixed with biocides in order to enhance their efficiency |228-230|, while simple cleaning

procedures with water-jet have been tentatively used for eradicate biological growth |231|. The

ultimate approach is to increase the susceptibility of organisms to biocides prior to treatment, using

product as EDTA |232| or ionising radiation |233| which could lead to the use of even lower

concentrations of biocides with low environmental impact |232|. Some attempt have been made

using enzymes such as protease instead of biocides for the elimination of lichens |19,217,234|.

6.1.7 Organic Substrata

For organic materials even more care should be followed when biocide treatments are

required and these must be applied only as last resource. Most of these objects are maintained in

closed environment and the development of biodeteriogens, especially microfungi, is always due to

unexpected increasing of the water content of the air or of the material itself. Thus the first steps is

to individuate the source of the humidity and to block it, then the material could be dried in a

ventilated environment, and successively the biological infestation can be gently brushed without

any chemical treatments. The application of biocides is foreseen in very special cases, when a

material is heavily attacked and for different reason the drying is very difficult or too slow.

For the elimination of bacteria, microfungi and actinomycetes have been used either specific

products such as antibiotics or broad-spectrum biocides, in the first group, streptomycin pimafucin,

kanamycin, echonazole and nystatin |208|, together with pimaricin, for the treatment of textiles with

good results |235|. In the second group have been used the same products used for inorganic

substrata (TBTO and Benzalkonium chloride) together with some chlorine derivatives (Sodium

Pentaclorophenate, O-Phenyl-Phenol, P-cloro-m-cresol) very active against fungi but with possible

drawbacks due to the liberation of chlorine in the substrate.

For the elimination of insects developing inside wooden structures the best solution is the use

of poisonous gases, highly penetrating and effective also against eggs, but this is practically very

difficult to do. This treatment needs special precaution and permission as for the gamma irradiation

system. Furthermore, only small dimensions and mobile objects can be treated. Normally these

biodeteriogens are eliminated with application of insecticides, either of synthesis (Dichlorovos and

Dieldrin) or natural derivatives (Resmetrine and Permetrine), in organic solvents. The efficiency of

the treatment is linked to a complete absorption of the product by the wooden structure because its

action (neural block) is effective only after the ingestion of treated wood by the larvae.

In deposit and unsafe confined environment there exist the possibility that small roditors can

deteriorate the objects made with organic materials. In this case specific products mixed with food

(rice or flour) can be leave inside.

6.1.8 Application Procedures

Spraying and brushing of diluted biocide solutions are the most common systems of

treatment. The spraying method is preferred for paintings with a very deteriorated surface layer

and, generally, when the component material is in poor condition. In the case of organic materials,

such as paintings on paper, prints, parchment or old books, spraying or brushing biocides is not

always possible because some liquid solutions can dissolve pigments and inks.

Application of poultices is carried out especially in the case of hard encrustations in order to

increase the contact time and use the dissolving action of water itself. These compresses are

made of carboxymethyl cellulose, paper pulp or inert materials that are soaked in the biocide

solution. They are covered with sheets of polyethylene or something similar in order to reduce

dehydration of the compress itself. The length of application ranges from one day to several days.

Sometimes the chosen biocide can be added to a gelatinous solvent paste used for cleaning stone

and mural paintings. In some cases, the removal of biological or chemical incrustations on stone is

obtained through non-selective absorbent clays (e.g., sepiolite or attapulgite).

Injection of pesticides, using the larvae’s tunnels, can be used to increase their diffusion for

infested wooden structures. When trees or bushes have high root system and their eradication can

be dangerous the injection of concentrated antigerminative substances inside stumps, is needed to

completely block new emergencies. In the case of subterranean termites, injection is performed

under pressure into the soil.

Fumigation methods are widely employed for organic materials. The treatment consists of

distributing the fumigant (gas) in the air and through materials. This method has rapid

effectiveness and reach deep penetration inside the object. Due to the high toxicity of fumigants,

they must be applied in airtight chambers or in perfectly sealed spaces (sometimes created with

polyethylene sheets), where the pressure can be modified to improve the penetration of the gas.

Fumigants require highly trained applicators and have no residual effects after aeration. As an

alternative to conventional fumigation techniques, researchers have recently investigated the

possibility of employing inert gases, such as nitrogen, together with low RH for controlling museum

pests |236,237|. The advantage of this system is that it is safe, inexpensive and not invasive, but

more tests are necessary to assess its effect against the various species of micro organisms and

insects. Treatments with substances active as vapour have similar advantages and, moreover, do

not require complicated equipment because the products generally used in vapour form are not

very toxic. The object can be treated in a polyethylene bag or airtight cabinet to permit the

concentration of vapours.

When either spraying and brushing treatments or poultices are used, it is sometimes

advisable to wash off the biocide residues in order to avoid possible secondary reactions (due to

the degradation of products) or toxicological difficulties (high-persistence products on premises

open to the public or for operators).

The time necessary for a substratum to be reinfested by biodeteriogens is depending on the

preventive measures taken for its conservation. This can be very long (several years) if the

restored objects are maintain in conditioned and safe confined spaces Otherwise, it is surely very

short (1-2 years) when these objects can not be moved inside and are left exposed in outdoor

conditions, even if a biocide treatment is left as preventive.

6.2 INDIRECT METHODS

These should control the environment surrounding the objects. The control of weather

variables is practically inapplicable for monumental stones, while for rock sites the construction of

protective structures, preventing the water run-off of surfaces, can effectively reduce the biological

growth |238|. An effective and simple good practice for limit the biodeterioration of exposed stones

is their maintenance. In fact, the periodical simple cleaning of exposed surfaces eliminate the

“soiling” effect due to the deposit of environmental particles which can favour the development of

reproductive bodies. Furthermore, after stone cleaning interventions some chemical substances

(protective coatings or consolidants) can be applied to the object in order to increase its water

repellence and cohesion. These products may prevent or retard the recolonization of the substrate

decreasing its porosity, roughness and water content, but in critical environmental conditions, they

could favour the development of specialised micro flora.

In order to reduce the biological risk, for the organic objects maintained in confined

environments, taking into account that in our climates the temperature and humidity are for long

periods of time favourable to the development of the widespread biological reproductive bodies

(conidia, spores and eggs), these should be conditioned at values of temperature ranging between

16° e 18°C and at relative humidity level below the 60%. This humidity value can be a limiting

factor for the development of fungi while the temperature for the insects’ eggs.

Preventive measures against insects attack are the conservation of the objects inside

cardboards, boxes or glass cases treated with insecticides, further the circulation of flying insects

can be reduced with air filter and thin wires screens.

7. BIOREMEDIATION

Chemical and physical techniques have been widely used for the cleaning of cultural objects

but in order to decrease any possible risk for the materials treated, innovative systems related with

biological methods have been introduced in this sector starting from the biological pack |239|.

Enzymes, such as lipase, have been successfully used to remove aged acrylic resin coatings

in paintings |240|, while for the removal of sulphates, nitrates and organic matter from artistic stone

works, carefully selected microbial cultures have been used |241,242|. A similar methodology has

been applied for the elimination of insoluble calcium oxalate patinas from monuments. In fact,

some bacteria and actinomycetes living in soils use the oxalate salts as sole source of carbon and

energy |243|.

The biomineralization process has been investigated in order to develop a new conservative

method for monuments restoration. The precipitation of new calcite crystals inside stone samples

was primarily tested utilising the mineralisation process mediated by the organic matrix extracted

from mineralised hard parts of marine organisms. Preadsorption of soluble acidic matrix

glycoproteins extracted from the shell of the mollusk Mytilus californianus was found to increase

the amount and depth of penetration of calcite crystals formed inside the stone structure with a

significant decrease of its porosity |244|. Furthermore, such a treatment seems to improve the

stone superficial mechanical strength |245|.

Also the precipitation of calcium carbonate crystals produced in nature by bacteria has been

applied using specific bacterial strains |246| (Bacillus cereus), even if the use of heterotrophic

viable organisms does not seem appropriate for this purpose. In fact, chemical reactions with stone

minerals due to metabolic by-products and the growth of fungi, due to the application of organic

nutrients for bacterial development, can have negative effects on monumental stone materials. The

results obtained using Bacillus subtilis and Micrococcus sp., seem chiefly influenced by the

presence of a big microbial mass, due to bacterial growth, rather than to the presence of new

crystals due to calcite precipitation |247|. For these reasons the study of the genetic mechanism

that controls bacterial calcite precipitation has been started. The Bacillus subtilis strain has been

mutagenized and not-precipitating mutants isolated. This mutation has been successfully

transferred, by transformation, to another B. subtilis strain, showing that it affected one gene only.

The isolation of this mutant suggests the presence of a genetic control for crystal formation.

Characterization of this mutation, and of possible other mutations, should allow the identification of

the protein(s) involved in the process |248|.

Acknowledgement

The author wish to thank G. Caneva, M.P. Nugari and O. Salvadori for their valuable book

“Biology in Conservation” published by ICCROM in 1991, and from which most of information

related with organic materials and biocides have been taken.

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Zar Dheri Buddhist complex is one of the few sites systematically explored and scientifically excavated by professional archaeologists. The site is located to the right side of Shinkiari-Tambah road, a link road starting from the Karakuram Highway leading to Dadar and beyond. The site was for the first time documented by H. Hargreaves in 1922 (Hargreaves, 1923). It was later on excavated by the Tokyo National Museum Archaeological Mission (hereafter TNMAM) to Pakistan from 1995-99 in collaboration with the Department of Archaeology and Museums (henceforth DOAM), Government of Pakistan. These field investigations at the site revealed some remarkable discoveries including 146 stone sculptures/architectural elements, coins and ceramics. But the most important discovery revealed at the site was the exceptional cruciform stupa with flight of steps provided on each cardinal side (Yoshihide, 2011). This stupa has recently been declared as the earliest of all the cruciform stupas yet discovered from the entire South Asia (Hameed, 2018). But despite of its rich archaeological and historical potential, the site is constantly losing its authenticity due to natural disaster and human vandalism. The present study therefore aims to highlight the management issues and device mechanism to minimize the threats to the site.

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It is shown that Virginia Creeper ivy exudes calcium oxalate from its branches, stems and suckers, and organic compounds, predominantly chelate-type structures, from its leaves. These substances deposit on and impregnate the stone surface supporting the plant growth. The organic oxalate is not deleterious to the stone. The organic compounds may be cosmetically disfiguring, but can be destroyed by a bleaching agent. -from Authors

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