Biodegradation of Cultural Heritage: Decay Mechanisms and Control
CNR - Centro di studio sulle "Cause Deperimento e Metodi Conservazione Opere d'Arte", Via G.
Capponi 9, 50121 Firenze, Italy.
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
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
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
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
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
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
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
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:
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
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
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|.
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
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
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
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
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
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
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
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
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
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.
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
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
Lyctidae powderpost beetle Tunnel with oval section
Botrychidae wood borer Circular holes and tunnels
Cerambycidae longhorn beetle Large, oval tunnels and holes
Kalotermitidae termites or white
Deep and crater-shaped holes;
sometimes entire interior of object is
destroyed but outer surface is left intact
Rhinotermitidae “ “
Hymenoptera Siricidae wood wasp Circular tunnels and holes of wide
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
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|.
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
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
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
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
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
termites Deep crater-shaped
holes and erosion;
destruction of the
interior of object while
the outside remains
Anobfidae furniture beetles Winding, circular
Lyctidae powder post
Tunnel with oval
Dermestidae skin beetles Short blind tunnels
with circular section
Corrodentia Liposcelidae book lice Tiny surface abrasion
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.
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
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.
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
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
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
(concentration below 0,05 %) |207|. Reduction in pressure, using vacuum dryers, causes cell’s
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
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.
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
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|.
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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