PMG Mold Remediation

Conservation Specialties

Conservation Specialties Analysis and Imaging Preventive Care Education and Professional Development Workplace Practices About Conservation Lexicon

• Photographic Material

Group

• Photographic Materials Group

Section

• Deterioration Issues
• Emergency Preparedness and Recovery
• Tools, Equipment, Materials, and Supplies
• Treatment Techniques and Procedures

Page Information
Date initiated	September 2009
Contributors	Chloé Lucas (Page Compiler), Cecilia Salgado, Susana Hoyos, Vanessa Castillo, Luisa Casella, Amanda Maloney, Stephanie Watkins

What is mold?

Life cycle of a mold from the Aspergillus genus

Several types of microorganisms can grow on photographic materials, the most common ones are bacterias and mold. Mold is a common term referring to filamentous fungal species from the Fungi Kingdom.

Life cycle of mold

Mold is present everywhere in the air in the form of mold fragments and reproductive cells, called spores. Fungi are heterotrophic organisms, which means that they cannot produce their own source of carbon to grow, it has to come from their environment. As a result, mold spores need to settle on an external source of carbon, which is called substrate, to grow. In favourable conditions, the spores then germinate and grow forming cylindrical branching filaments, called hyphaes. Once mature, the fungi can produce new spores that will contaminate other substrates.

For more information about mold biology, please refer to the section 1.2 of the BPG Mold page.

External factors influencing mold growth

Each fungal species has optimal environmental conditions in which they will grow best. However, they can still grow under less favorable conditions, the growth will be slower. Favorable conditions vary from species to species.

Several factors in the environment influence mould growth:

• Water: Relative humidity in the environment around the photograph has an influence on fungal growth because it influences the moisture content of the substrate on which it grows. The ideal relative humidity for fungal growth is between 65% and 70%, but growth can easily happen at lower (55%) or higher relative humidities.
• Nutrients: Photographic materials, such as cellulose (primary support) and proteins (image binder), serve as a source of carbon for mold growth.
• Temperature: Each species has a minimum, maximum and optimal temperature for growth. The most suitable temperature is between 23.8ºC and 28.7ºC; however, some species can grow at lower (2ºC) or higher (40ºC) temperatures.
• Light: The amount of light required for mold growth depends on species.
• Oxygen: Fungi are strict aerobic microorganisms; oxygen is indispensable to their growth.

For more information about external factors influencing mold growth, please refer to section 1.6 of the BPG Mold page.

Intrinsic factors influencing mold growth

Presence of silver

The presence of silver as an image forming material has an impact on fungal growth. It was reported that low-density image areas, with lower amounts of silver, are more prone to fungal growth; and that silver can completely inhibit fungal growth. It was also noted that fungal growth on silver images can modify the localization of the silver within the binder, which accumulates on the surface of the fungal cell wall.

Characteristics of gelatin emulsions

Tanning:

Tanning of emulsion during manufacturing or processing (fixing bath containing tanning agents) modifies its water content. Tanning, usually with chrome alum or formaldehyde, results in reticulation of the gelatin peptide chains, thus preventing water molecules from linking with the chains, and lowering its water sensitivity. Consequently, a tanned gelatin emulsion will have a water content between 3% and 9%, compared to a non-tanned gelatin emulsion whose water content is between 11% and 14%. Fungi prefer their substrate to have a water content between 8% and 10%, so they are more likely to attack a non-tanned gelatin emulsion.

Observations made by various authors sometimes contradict this information. Indeed, Abrusci et al. did not notice a difference in fungal development between tanned and non-tanned samples. On the other hand, Dalev et al. showed that the more an emulsion is tanned, the slower the enzymatic hydrolysis will be, with solutilisation time of gelatin solution by protease enzymes increasing with the degree of reticulation. Similarly, Lourenco and Sampaio noticed that early silver gelatin prints were more easily deteriorated by fungi, which could be explained by a lower degree of tanning.

Amorphous and crystal phases:

The structure of gelatin consists of amorphous phase (coil) and crystal phase (triple helix). A gelatin emulsion with more crystalline areas will be more resistant to fungal attack, as the gelatin chains are organized into triple helix and less accessible to react with external molecules.

Gel and solid phases:

Gelatin is a hygroscopic colloid, whose water content depends on the environmental temperature and relative humidity. The water content of gelatin influences its physical property, and two phases can be distinguished: a solid phase, under the glass-transition temperature, and a gel phase, above the glass-transition temperature. Water in gelatin is not easily accessible to fungi when the gelatin is in a solid phase, it is more vulnerable to fungal growth when it is in a gel phase. The gelatin gel phase can be reached during thermo-hygrometric variations: when the temperature and relative humidity increase, the water content of the emulsion augments as well, and can go over the glass-transition temperature. This gel phase can also be attained when there is condensation on the surface of the emulsion.

Effects of mold on photographic materials

Damage caused by mold

Mold damage may not always be visible to the eye, the fungal colony needs to be of a certain size and extent to start being visible.

Structural damages:

• Change in mechanical strength.
• Change in porosity.
• Increased wettability.
• Pitting of surfaces.
• Friability.
• Partial or total loss of binder and/or primary support.

Aesthetic damages:

• Stains.
• Change in gloss (from surface pitting).
• Change in color.

Degradation mechanisms

During fungal growth, hyphaes adhere to their substrate and can generate micro-fissures into the constituent materials. Fungi secrete metabolic products which can react chemically with the constituent materials and fragilize them through oxidation or enzymatic hydrolysis reactions.

Oxidation of proteins

Various fungal metabolic mechanisms, such as cellular respiration, can produce reactive oxygen species (ROS), which are highly reactive chemicals formed from dioxygen (O₂).

ROS are rejected in the substrate and oxidize proteins until full denaturation of the protein, which is the breakage of the alpha helix into a non-organised coil. The breakage of the amino-acid chains increases the availability of amines and acids for reaction with water, resulting in an increased sensitivity to water of the protein.

Some amino acids present in gelatin - methionine and tyrosine - are particularly sensitive to ROS and the secretion of ROS into the gelatin substrate results in its oxidation.

Enzymatic hydrolysis

Fungal species can readily use simple sugars and amino acids to grow. In the presence of peptides or more complex sugars, such as cellulose, the fungi will need to produce enzymes specific to the materials - such as cellulase (for cellulose), protease (for proteins) or gelatinase (for gelatin) - in order to break it down and use it as a carbon source. Because simple sugars and amino acids are easier to assimilate, mould will be more likely to grow on an already broken down constituent material, from previous water or fungal damage.

Production of pigments

During growth, fungi can produce pigments, which can be located in the hyphaes, in the spores or secreted into the substrate. The production of pigments is species-dependent and is influenced by various factors such as the availability of nutrients, environmental factors or the presence of metal in the substrate. The formation of pigments can also result from the interaction between constituent materials and metabolic products.

Health and safety

Health risks associated with mold

Contact with mold can cause different types of reactions to human health:

• Allergic and hypersensitivity reactions: All mold species can induce allergic and hypersensitivity reactions as they are caused by the spores, mold fragments, biofilms and metabolic products secreted during mold growth. As the allergic and hypersensitivity reactions are caused by the presence of mold spores, fragments and biofilms, whether they are active or not, killing the mold does not eliminate the health hazard. The metabolic products are microbial volatile organic compounds (MVOCs), which are responsible for the moldy smell. Furthermore, MVOCs can be absorbed by porous substrates and be slowly released over time, the moldy smell can then indicate present or past mold growth.
• Irritant and toxic reactions: Irritant and toxic reactions (respiratory, immune or neurologic effects) are less common as they are caused by mycotoxins secreted by some mold species, usually as a response to a stressful growth environment.
• Infections: Mostly for people with severely compromised immune systems.

Sensitivity to mold varies from person to person, people with asthma or chronic conditions are more at risk. Moreover, the sensitivity increases with chronic exposure and permanent sensitization can occur over time. As a result there is no “safe” limit for mold levels, it is thus crucial to use protective equipment when working on contaminated photographs.

Protective equipment

Workspace

It is recommended to work in a space separate from non-contaminated materials and to isolate moldy materials to avoid cross-contamination.

The following workspaces are recommended for mold remediation treatments, depending on resources available, extent of contamination and size of photographs to be treated:

• Biosafety cabinet (with High Efficiency Particulate Absorbing (HEPA) filters).
• Fume hood.
• Indoor. In a space with no air exchange with the rest of the building. Air vents can be sealed with plastic sheets and tape.
• Outside. Under cover from the sun and far from building air intake.

Personal Protective equipment (PPE)

The mold exposure pathways into the human body are by inhalation, by ingestion, and by skin contact. It is necessary to protect your airways (nose and mouth), eyes and skin.

Airways protection:

• Disposable respirator, half-face respirator, full-face respirator or powered air purification respiratory systems (PAPRs).
• Particulate filters (N series), N100 (HEPA) is recommended.
• Organic vapour filters can be added to the particulate filter, when there is a mouldy smell (presence of MVOCs).
• /!\ Caution /!\ Filters have a limited lifetime, between 1 week and 1 month, depending on the model, frequency of use, air contamination and care.

Eyes protection:

• Goggles

Skin protection:

• Disposable gloves (PVC or nitrile)
• Protective clothing if there is a significant amount of mould. Disposable (high contamination) or reusable (medium or low contamination).
• Wash hands with soap and water after working on contaminated materials.

The type and extent of protection recommended varies depending on the size of the contamination. The Canadian Conservation Institute proposes a “Recommended Personal Protective Equipment” table to help determine which PPE to use, based on extent of contamination.

Should I use reusable or disposable PPE?

Reusable:

• Must not be worn outside of a contaminated area to avoid cross-contamination.
• Needs to be cleaned and disinfected after each use.
• Can be shared only when they are properly cleaned and disinfected.

Disposable:

• All disposable equipment is strictly personal and must not be shared.
• Gloves and protective clothing can be reused until they are damaged and do not fulfill their safety purpose anymore. If reused, they must be cleaned before and after every use (before removing them):
  • Gloves: with soap and water, as a regular hand washing process.
  • Protective clothing: carefully vacuumed.
• Discard in a sealed thick plastic bag in a regular garbage container. /!\ Caution /!\ Legislation may differ depending on location, consult local information sources.

Purpose of mold remediation for photographic materials

• Minimize the spread of mold within the collection

To prevent contamination of the photograph’s enclosure, surrounding photographs and storage space during use.

• Minimize detrimental health effects to users

To  lower health risks associated with the presence of mold during use.

• Minimize the effect on the collection

To prevent future growth of the mold currently contaminating the photograph, thus lowering the risk of damage.

Factors to consider

• How many objects need treatment?

Mass treatment vs. Itemized treatment. Some treatments are more appropriate for a mass/itemized treatment.

• What types of photographic materials need treatment? Are they in good, medium, poor condition?

Consider the effect of various mold remediation treatments on constituent materials. The effect of a treatment can vary depending on the condition of the photograph.

• Who uses the collection? How often?

Killing the fungi does not remove the risk of detrimental health effects for users. Photographs that are handled often may need additional preventive measures to protect users.

• How efficient is your climate control system?

Mould growth can be minimized by strict storage environment control. Photographs returning in a storage space with limited environmental control will be more at risk of future mold growth. Furthermore, mold damaged photographs are more susceptible to future mold growth because the constituent materials are already partially or completely broken down, as a result they are easier to assimilate by fungi.

• What are the short and long term effects of the treatment on health and the environment?

Consider the toxicity of the treatment to human health, the impact of the production and disposal of chemicals on the environment, and the potential retention of chemicals within porous constituent materials.

• What resources do you have access to?

Consider monetary, space and human resources needed vs. available.

Should I identify the mold species?

The effects of mold remediation techniques can differ from species to species. In most cases, the limiting factor is the treatment’s effects on photographic materials, rather than its efficiency on various mold species.

In the event of a mass mold contamination and for some types of treatments, it may be useful to identify the contaminating species. However, the identification is limited by the fact that any species identified on the surface of the photograph may have just deposited there, and may not be responsible for the contamination or visible damage. There is a risk of mis-identification of the contaminating mold species.

Mold remediation techniques

Terminology:

• Viable: mold that is able to grow (alive).
• Non-viable: mold that is unable to grow (dead or killed).

• Active mold / active growth: mold that has germinated and is growing (at any rate).
• Inactive mold / not actively growing / deactivated: mold that has stopped growing.

• Fungistatic treatment: treatment resulting in the deactivation of the mold, it is still viable.
• Fungicidal treatment: treatment resulting in the non-viability of the mold.

Thresholds of cleanliness

To approach mold remediation Eng Moore and Maitland proposed a framework dividing remediation in two thresholds of cleanliness. The first threshold goal is to minimize the spread of mold within the collection as well as the detrimental health effects to users; while the second threshold goal is to minimize the effects of mold growth on the collection.

The first threshold should be undertaken in all mold contamination situations and can be achieved with the following steps:

• Drying: to stop mold growth: most fungal filaments will become non-viable, while the spores will be deactivated but still viable and able to grow once conditions are favorable.
• Dry cleaning: to lower the overall amount of mold on the object by removing a maximum of loose spores, mold fragments and filaments from the surface of the photograph.
• Containing: to prevent mold spread and cross-contamination.
• Controlling the storage climate: to prevent future mold growth.

The first threshold is not fungicidal, but minimizes the risk of the same mold growing again and risks to human health by lowering the amount of mold present on the object. Indeed, as photographic materials are often porous (binders, paper supports), it may not be possible to remove all mold structures (viable and non-viable), embedded in the materials, without damaging the object.

In some cases, particularly when there is severe mold damage or when the environmental control is limited, the second threshold can be undertaken, and achieved with the following steps:

• Implementing threshold one.
• Killing the remaining mold: to prevent future growth of the same fungi, with a fungicidal treatment.

Implementation of the second threshold does not remove the risks to human health, because the embedded fungal structures are still present. Furthermore, fungicidal treatments are not a preventive measure. Treated objects are still susceptible to mold growth from newly deposited spores. They are also easier to contaminate than objects in good condition as the materials are already damaged.

Dry cleaning (Threshold 1)

Goal:  Minimize the spread of mold within the collection as well as the detrimental health effects to users.

Advantages:

• Not toxic for humans or the environment.
• Low budget.

Disadvantages:

• Not fungicidal.
• May not be possible to remove all the fungal fragments embedded in the materials.
• Does not remove mold-related risks to human health.

Factors to consider:

• Condition of the photograph:  for structurally weakened materials it is recommended to use a polyester screen as an interface over the object and/or as a filter on the vacuum cleaner’s nozzle.
• Suction of the vacuum: can be adjusted with miniature tips, a vacuum with adjustable suction, or by placing your hand to partially block the nozzle.

Tools:

• Personal protective equipment.
• Vacuum with HEPA filters or water trapping - to avoid releasing the mould spores into the air. Miniature tips can be added to adjust suction.
• Soft brushes - dedicated to mould remediation, do not use for other tasks to avoid cross-contamination.
• Polyester screen.
• Weights - should be easy to clean (non-porous materials), and can be separated from the object with a blotting paper if the surface is fragile.

Procedure:

• Wash your hands.
• Put your PPE on.
• Place the photograph on your working space.
• Maintain the photograph with weight(s).
• Place the vacuum tip above the photograph, approximately 10 cm away depending on the photograph’s condition and suction level.
• Delicately brush the mould toward the vacuum nozzle. Apply light pressure to avoid embedding the mould in the constituent material, particularly if it is porous.
• Methodically brush the whole surface of the photograph, even if there is no visible mould. Brush each side of the photograph twice, once vertically and the second time horizontally. Move the weight(s) to clean underneath it/them.
• Clean your tools and working space.
• Remove your PPE.
• Dispose of contaminated non-collection materials and disposable PPE.
• Wash your hands.

Fungicidal treatments (Threshold 2)

Goal: Minimize the effects of mold growth on the collection. In the context of heritage collections, killing all living spores with a sterilization treatment is not the goal as collections are not stored in a sterile environment. Fungicidal treatments aim at lowering the amount of viable fungal spores on a photograph to a level similar to the ones present in clean collection space.

Alcohols

Advantages:

• Low toxicity.
• Low budget.
• Several application methods, which can be adapted to the object.

Disadvantages:

• Detrimental effect on some photographic materials.
• Toxicity.
• Does not remove mold-related risks to human health.

Factors to consider:

• Type of alcohol: The effectiveness of different types of alcohol varies with their carbon chain length: the shorter the chain, the more effective the alcohol is. As a result, methanol (CH₄O) is the most efficient alcohol for mould remediation; however, due to methanol high toxicity, ethanol (C₂H₆O) is usually preferred.
• Alcohol-water ratio: Research has shown that alcohols used undiluted have no antifungal action and that it is necessary to use it mixed with water to allow its penetration into the fungal cell. Depending on the type of alcohol and the alcohol-water ratio, the treatment can result in a fungistatic or a fungicidal effect. It was determined that 70:30 (v/v) ethanol-water mixture is most effective for a fungicidal effect, and is recommended by studies on written heritage collections.
• Application method: The efficiency of the treatment also depends on the fungal species, the contact time and the temperature, the higher the temperature is, the more efficient the treatment will be.   Several methods of implementation of the 70:30 (v/v) ethanol-water mixture have been studied for use on photographic materials:
  • By spraying: which results in a fungistatic effect with a 3 day delay of growth on paper.
  • By immersion: which results in a fungicidal effect on paper after 2.5 min, which was confirmed for chromogenic transparencies on cellulose acetate.
  • By vapours: which results in a fungicidal effect on paper and silver gelatin prints after two hours exposure.
• Effect on photographic materials: The effect of the treatment depends on the type of photographic material and the application method:
  • By immersion: The immersion of paper in ethanol-water 70:30 (v/v) solution does not modify its colour or mechanical properties. The immersion of chromogenic prints on resin coated paper in a ethanol-water 50:50 (v/v) solution causes partial dissolution of the dyes, the extent of the damage depended on the contact time. Similar effect was observed with the immersion of Ektachrome® films in an ethanol-water 70:30 (v/v) solution; as well as strong and irreversible planar distortions to the cellulose acetate support. However, the treatment plan developed by Pietsch and Fernandes with immersion of Kodachrome and Ektachrome films in successive ethanol-water baths (30:70 and 70:30 v/v) does not result in changes to colour or mechanical properties of the photographs.
  • By vapours: Various types of alcohols and water-alcohol ratios have been tested for vapour treatment.
    • Exposure to 50:50 and 95:5 (v/v) ethanol-water vapours for 21 days at 45°C results in modification of the silver image density, reddish coloration and yellowing of silver gelatin prints on baryta paper.
    • Exposure to 96:4 (v/v) butanol-water vapours for 48 hours at 26 °C does not induce colour changes to silver gelatin prints on baryta paper; however, treated samples were more faded and yellowed after aging than untreated samples. This treatment also caused some decrease of the gelatin degree of polymerization, resulting in a lower viscosity.
    • Exposure to 70:30 (v/v) ethanol-water vapours for 2 hours at 22°C did not modify colour or mechanical properties of paper. However, the treatment induced strong and irreversible planar distortions to cellulose acetate support and colour changes to chromogenic prints, the change being more important and recurrent in recent prints.
• Condition of the photograph: Treatment by vapour may be more suitable for photographs with fragile surface or poor physical integrity. A damaged photograph may be more vulnerable to treatment as the alcohol may react differently with damaged materials than with non-damages ones.
• Inscriptions: Some inks can be sensitive to ethanol and/or water. Treatment should be modified accordingly.

Gamma radiations

Advantages:

• Treatment of a large quantity of objects at once.
• No residues after treatment.

Disadvantages:

• Specialized installation required.
• High budget.
• Detrimental effect on some photographic materials (cumulative).
• Does not remove mold-related risks to human health.

Factors to consider:

• Radiation dose: Gamma radiation is obtained by nuclear fission of Cobalt 60, the exposure dose is expressed in Gray (Gy). The lethal dose for mold varies based on species, development stage (spores versus filaments) and the treatment conditions (temperature and oxygen levels). It has been found to be between 3 and 25 kGy, depending on the implementation conditions.
• Effects on photographic materials:
  • Physical effects: The treatment causes depolymerisation and oxidation of cellulose, proportional to the irradiation dose, similar to ones observed during aging, with lower folding endurance and tear resistance, increased yellowing, and embrittlement.  However, it does not modify the mechanical properties of cellulose nitrate and acetate, after exposure to 25 kGy. It also lowers gelatin emulsion viscosity after exposure to 2.2 kGy, but it does not modify the surface aspect of the emulsion after exposure to a dose up to 200 kGy.
  • Visual changes: Results obtained vary depending on the treatment implementation conditions, and where authors place the threshold for acceptable or visible color change. The treatment has been found to induce some color changes on albumen prints, x-ray films, monochrome cellulose acetate films, and chromogenic prints; as well as an increase in optical absorption of silver gelatin cellulose acetate films. Some authors observed more fading and yellowing of irradiated silver gelatin prints on baryta paper after light aging compared to control samples; whereas others did not find a difference. No change in color has been found after temperature aging of irradiated silver gelatin and chromogenic films on cellulose nitrate and acetate; nor after dark aging of chromogenic prints.
  • Susceptibility to fungal growth after treatment: Exposure to gamma radiation up to 100 kGy does not increase silver gelatin prints’ susceptibility to mold growth.

Commercial biocides

Buendia and Hoyos Velasco studied two commercially available biocides, Citricidal®️ and Éviter®️. The first one is composed of a citrus extract diluted in water or ethanol and the second one is composed of titanium dioxide nanoparticles diluted in ethanol.

Advantages:

• Commercially available
• No known toxicity for humans.

Disadvantages:

• Adverse short-term effect on photographic materials.
• Constant changes to products' composition.
• Does not remove mold-related risks to human health.

Factors to consider:

• Application method: The efficiency of both products depends on the fungal species and application method.
  • Citricidal®️: Application by spraying the biocides on both recto and verso of silver gelatin prints on baryta paper, three times, resulted in a fungistatic effect on four fungal species. Uneven application of the product, by spray, could explain the fungistatic results obtained. The product needs to be rinsed after application to avoid residues.
  • Eviter®️:  Application by spraying the biocides on both recto and verso of silver gelatin prints on baryta paper, three times, resulted in a fungistatic effect on one fungal species and a fungicidal effect on three fungal species. Uneven application of the product, by spray, could explain the fungistatic results obtained.
• Effects on photographic materials:
  • Citricidal®️: Leaves aluminum residues on the surface of silver gelatin prints on baryta papers, visible as amber coloured stains ; and a greasy texture on paper .  
  • Eviter®️: Leaves aluminum and chloride residues on the surface of silver gelatin prints on baryta papers and results in optical changes in brightness and gloss of the prints.
• Condition of the photograph: Citricidal®️ needs to be rinsed after application, which is not applicable to photographs with important mold growth and poor cohesion of constituent materials.

It is currently not recommended to use Citricidal®️ and Éviter®️ to treat photographic materials.

Ethylene oxide

Fungicidal properties of ethylene oxide are known since 1929, it was widely used in conservation in the 1960s; however, because of its toxicity, its use was restricted in the USA, Canada and some European countries. It is still in use in some countries (e.g. France).

Advantages:

• Treatment of a large quantity of objects at once.
• Fungicidal, insecticidal and bactericidal.
• Low cost

Disadvantages:

• Detrimental effect on some photographic materials.
• Long retention time in some photographic materials.
• High toxicity.
• Does not remove mold-related risks to human health.

Factors to consider:

• Gas mixture: At room temperature, ethylene oxide is a colorless gas, flammable and explosive in contact with air (at concentrations superior to 3%). To mitigate the risks of ignition or explosion, ethylene oxide is mixed with another gas: carbon dioxide, nitrogen, or hydrofluorocarbon derivatives, among others. The carbon dioxide and ethylene oxide mixture is the most economical, and several volume ratios (v/v) are used: 30:70, 15:85 and 10:90.
• Temperature and relative humidity during treatment: The efficiency of the treatment increases with temperature and with the humidify content of mold spores.
• Effect on photographic materials: Treatment with ethylene oxide breaks gelatin chains and  lowers the degree of polymerization, thus influencing the viscosity of the emulsion. Treatment of cellulose nitrate films results in slight shrinking of the film; and cellulosic materials are more hydrophilic after treatment, thus more susceptible to future fungal attack.
• Retention time: Ethylene oxide is easily absorbed by constituent materials, and can form chemical bonds particularly with plastic materials. Given its toxicity,  ethylene oxide retention time in materials must be taken into account for users safety. Seven days of off-gassing are enough for silver gelatin prints to reach a level of 2 ppm (legal threshold for human safety in France), plastic films (cellulose nitrate, cellulose acetate and polyester) require 42 days to reach the same threshold.

Notes on freezing

Freezing is not a fungicidal technique. Freezing will kill the hydrated fungal filaments and spores by physical damage of their structure during the formation of ice crystals and chemical damage from changes in pH and ions concentration; however, dry spores will remain viable. Furthermore, to completely stop fungal growth, photographs need to be frozen at -20°C to completely stop fungal growth. Above this temperature, liquid water is still available in the constituent materials, and fungal growth will occur at a very slow rate. Freezing can be used to buy time in an emergency situation.

Historical treatments

Chemical treatments

Formaldehyde

Formaldehyde (CH₂O) is a colorless, odorless gas, which is carcinogenic to humans. Its fungicidal properties were studied in conservation in the 1960s. Various treatment times were recommended and it was widely used in libraries and archives. Further research in the 1990s evidenced the limit of this treatment because of the required off-gassing time, the risk of polymerization of formaldehyde on the surface of documents - creating a superficial film, and the adverse effects on cellulose - such as depolymerization, pH change, loss of flexibility and protein reticulation.

Phenol derivatives

Thymol (2-isopropyl-5-methylphenol) presents a genotoxic risk to humans. It can be used either as a gas or diluted in ethanol and results in fungistatic effects, as it does not kill fungal spores. It was widely used in conservation in the 1980s and the implementation varied from institution to institution, from 1 to 90 g/m³ concentration during 24 hours to 3 weeks. The implementation conditions are crucial for the efficiency of the treatment as thymol can promote mold growth if used at inefficient concentrations. Treatment with thymol can result in various detrimental effects on photographic materials: lower folding endurance, lower mechanical resistance, yellowing of paper, and oxidation of silver images.

O-Phenylphenol (C₁₂H₁₀O), is available under different trade names, such as Preventol, O Topane and Dowicide. In the 1980s it was recommended as a substitute for thymol for its lower toxicity to humans; however, it is toxic for aquatic environments. Early studies showed a fungicidal effect at low concentrations (10-50 ppm) in solution applied directly to the photograph or as a fumigant; however, as a gas it has low volatility and requires long off-gassing periods. Later studies at the end of 1980s and in the 1990s showed that O-Phenylphenol only has a fungistatic effect, and evidenced damages to photographic materials via paper discoloration, depolymerization of proteins, and effects equivalent to accelerated aging.

Quaternary ammonium salts

Quaternary ammonium salts (Quats) were discovered in 1851 and have been used for disinfection of photographic materials, using zinc fluorosilicate or Hyamine 1622 (Röhm & Haas) in an aqueous solution. Some Quats are highly toxic to humans with detrimental effects on the immune system. The disinfection treatment can be dangerous for water-damaged photographs as quats are applied by immersion in an aqueous solution. Furthermore, Hyamine 1622 adversely reacts with cyan dyes present in color photographs.

Kodak Film Cleaner

Kodak Film Cleaner (1,1,2-trichloro- 1,1,2-trifluoroethane) has some detrimental effects on humans. It was widely used to disinfect photographs and showed no adverse effect on the gelatin binders and plastic supports. This product was phased out after the 1994 Protocol of Montreal as an ozone-depleting substance.

Oxyden reduced atmospheres

Oxygen reduced atmospheres are widely used for the disinsectization of heritage collections. The method consists in reaching an oxygen level lower than 0.1% in a sealed environment, either by replacing oxygen with nitrogen or carbon dioxide (dynamic anoxia), or by introducing oxygen absorbers in the chamber (static anoxia). Since fungi are strict aerobic organisms, the removal of oxygen prevents their development, they enter into dormancy. The Centre de Recherche pour la Conservation des Collections (CRCC) tested anoxia treatments on 37 fungal strains: ten were completely destroyed but the others remained viable. Buendia tested static anoxia treatment, which was not successful to slow down or inhibit mold growth. 

Radiations

Radiations are electromagnetic waves characterized by a frequency and a wavelength. Their interaction with constituent materials and fungi depends on their energy levels.

Ultraviolet radiations

Ultraviolet (UV) radiation does not penetrate in materials, their wavelengths are between 10 and 450 nm. The fungicidal effect of UV radiation on paper documents is described in literature at various wavelengths. Flieder and Capderou mention UV-A (350-450 nm) whereas Nittérus talks about UV-C (230-275). Their use on photographic materials is not adapted as UV radiation has cumulative damaging effects, such as cellulose depolymerisation.

Micro-waves

Microwaves correspond to a wavelength of one meter to a few millimeters in the air, they are more commonly characterized by their frequency, in gigahertz. The production of microwaves generates heat through two mechanisms:

1. Electrical conduction: microwaves generate an electrical current in the conductive media, which creates heat by Joule effect;
2. Dielectric effect: polar molecules orient themselves according to the direction of the electric field, creating a mechanical oscillation. At the mechanical resonance frequency of the molecule (geometry and mass dependent) the amplitude is maximum, increasing the temperature. This effect is predominant in paper.

The first studies on the use of microwaves for the disinfection of heritage date back to 1947; however, this technique became applicable only in 1986 with the study by A. Brandt. The exposure to microwaves has a fungicidal effect provided that the temperature is above 55°C, the treated objects are thin (less than 2 cm) and humid. This fungicidal treatment does not damage paper; however, Gillet and Garnier report damages after treatment of gelatin emulsions weakened by mold.  

Further reading

General information on mold

• Guild S., MacDonald M., Strang T., Mould prevention and collection recovery: Guidelines for heritage collections, Technical Bulletin 26, Ottawa, ON: Canadian Conservation Institute, 2020 <https://publications.gc.ca/collections/collection_2020/pch/CH57-3-1-26-2020-eng.pdf> (accessed November 2024).
• Florian M-L., Fungal facts: solving fungal problems in heritage collections, London: Archetype Publications Ltd., 2002.
• Merritt J., Brewer T., “Mold: Prevention Of Growth In Museum Collections”, National Park Services Conserve-O-Gram, 3, 4, 2007 <https://www.nps.gov/museum/publications/conserveogram/03-04.pdf> (accessed November 2024).  

Effect of mold on photographic materials

• Abrusci C., Marquina D., Del Amo A., Catalina F., “Biodegradation of cinematographic gelatin emulsion by bacteria and filamentous fungi using indirect impedance technique”, International Biodeterioration & Biodegradation, 60, 2003, pp. 137-143.
• Abrusci C., Marquina D., Santos A., Del Amo A., Catalina F., “A chemiluminescence study on the degradation of gelatin. Biodegradation by bacteria and fungi isolated from cinematographic films”, Journal of Photochemistry and Photobiology. A: Chemistry, 185, 2007, pp. 188-197.
• Bard C.C., “Biodeterioration of photographs”, In: Barry S., Houghton D.R., Llewellyn G.C., O’Rear C.E. (Eds.). Biodeterioration 6: Papers Presented at the Sixth International Biodeterioration Symposium, Washington, D.C., August 1984 (Wallingford: CAB International, 1986), pp. 379–382.
• Borrego S., Guiamet P., Gómez de Saravia S., Batistini P., Garcia M., Lavin P., Perdomo I., "The quality of air at archives and the biodeterioration of photographs”, International Biodeterioration & Biodegradation, 64, 2010, pp. 139-145.
• Dalev P., Vassileva E., Mark J E., Fakirov S., “Enzymatic degradation of formaldehyde crosslinked gelatin”, Biotechnology techniques, 12, 12, 1998, pp. 889-892.
• Florian M.-L., “Water, heritage photographic material and fungi”, Topics in Photographic Preservation, 10, 2003, pp. 60-73, <https://resources.culturalheritage.org/pmgtopics/2003-volume-ten/10_07_Florian.pdf> (accessed November 2024).
• Furic G., Les hospices civils de Lyon à l’exposition internationale urbaine de 1914. Étude et restauration de trois photographies sur papier au gélatino-bromure d’argent montées sur toile tendue sur châssis. Étude de la dégradation des papiers au gélatino-bromure d’argent à l’humidité, Mémoire de fin d’étude, Saint-Denis La Plaine: Institut national du patrimoine, 2002, <https://mediatheque-numerique.inp.fr/documentation-oeuvres/memoires-diplome-restaurateurs-patrimoine/hospices-civils-lyon-lexposition-internationale-urbaine-lyon-1914-etude-restauration-trois-photographies-sur-papier-au-gelatino-bromure> (accessed November 2024).
• Lourenço M. J. L., Sampaio J. P., “Microbial deterioration of gelatin emulsion photographs: a case study”, Topics in Photographic Preservation, 12, 2007, pp. 19-34,<https://resources.culturalheritage.org/pmgtopics/2007-volume-twelve/12_06_Lourenco.pdf> (accessed November 2024).
• Matè D., Schlocchi M. C., “Danni estetici prodotti da microfunhi su carte fotografiche b/n ottenute con procedimenti argentici”, Kermes, 55, 2004, pp. 57-65.
• Schlocchi M. C., Daminao E., Matè D., Colaizzi P., Pinzari F., “Fungal biosorption of silver particles on 20th century photographic documents”, International Biodeterioration & Biodegradation, 84, 2013, pp. 367-371.

Fungal species identified on photographic materials

• Abrusci C., Martín-González M., Del Amo A., Collado J., Platas G., “Isolation and identification of bacteria and fungi from cinematogrpahic films”, International Biodeterioration & Biodegradation, 56, 2005, pp.58-68.
• Bogomolova E. V., Ivanova A.M., Kirtsideli I.Y., Mel’Nik V.A., Sokolenko D.V., “Micromycetes complexes on photographs from old collections (1839-1912)”, Topics in Photographic Preservation, 12, 2007, pp. 55-63, <https://resources.culturalheritage.org/pmgtopics/2007-volume-twelve/12_11_Bogomolova.pdf> (accessed November 2024).
• Lourenço M. J. L., Sampaio J.P., “A deterioração microbiologica de especies fotograficas con emulsão de gelatina: isolamento de microorganismos contaminantes de três colecções", Conservar patrimonio, 2, 2005, pp. 13-19.
• Zyska B., Cieplik Z. T., Wójcik A. R., Lozlowska R., “Microbial deterioration of historical glass plate negatives”, in Biodeterioration 7, Selected papers presented at the Seventh International Biodeterioration Symposium, London;New York: Elsevier Applied Science, 1988, pp. 428-135.
• Zyska B., “Fungi isolated from library materials: a review of the literature”, International Biodeterioration & Biodegradation, 40, 1, 1997, pp. 43-51.

Health & safety

• Ammann H., “Is Indoor Mold Contamination a Threat to Health?”, Journal of Environmental Health, 64, 6, 2002, pp. 43–44. <https://www.mold-survivor.com/harrietammann.html> (accessed November 2024).
• Center for Disease Control, “Mold Health Problems”, National Institute for Occupational Safety and Health, March 2024 <https://www.cdc.gov/niosh/mold/health-problems/index.html> (accessed November 2024).
• Salkinoja-Salonen M. S., Peltola J., Andersson M. A., Saiz-Jimenez C., “Microbial toxins in moisture damaged indoor environment and cultural assets”, In Molecular Biology and Cultural Heritage, ed. C. Saiz-Jimenez, The Netherlands: Swets & Zeitlinger, 2003, pp. 93-105.
• United States Environmental Protection Agency, “A brief guide to mold, moisture and your home”, March 2024 <https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home> (accessed November 2024).

Mold Remediation techniques for photographic materials

• Eng Moore T., Maitland C., How do we assess mould levels?, 2021 <https://canadianconservationconsortium.ca/en/assess-mould-levels/> (accessed November 2024).

Dry cleaning

• Guild S., MacDonald M., Strang T., “Cleaning by vacuuming”, in Mould prevention and collection recovery: Guidelines for heritage collections, Technical Bulletin 26, Ottawa, ON: Canadian Conservation Institute, 2020 <https://publications.gc.ca/collections/collection_2020/pch/CH57-3-1-26-2020-eng.pdf> , pp. 52-58.

Fungicidal treatments

• Nittérus M., “Fungi in archives and libraries, A literary survey”, Restaurator, Vol. 21, 2000, pp. 25-40.
• Sequeira S. O., Fungal biodeterioration of paper: Development of safer and accessible conservation treatments, PhD in Heritage Conservation and Restoration, Specialty in Conservation Sciences, Universidade Nova de Lisboa, 2016.

Alcohols

• Bacílková B., “Study on the effect of butanol vapours and other alcohols on fungi”, Restaurator, 27, 3, 2006, pp. 186-199.
• Jacek B., “Erkennen und Behandlung von Mikroorganismen auf Fotografien (Teil 2)”, Rundbrief Fotografie, 11, 4, 2004, pp. 5-11.
• Lucas C., Deniel F., Dantigny P., “Ethanol as an antifungal treatment for silver gelatin prints: implementation methods evaluation”, Restaurator, 38, 3, 2017, pp. 235-248, <http://photographconservation.ca/wp-content/uploads/2017/09/LUCAS_DENIEL_DANTIGNY_2017_Ethanol-as-an-Antifungal-Treatment-for-Silver-Gelatin-Prints-Implementation-Methods-Evaluation.pdf> (accessed November 2024).
• Lucas C., Rottier V., Déniel F., Dantigny P., “Traitement aux vapeurs d’éthanol de photographies gélatino-argentiques et de papiers altérés par des micro-organismes”, Support Tracé, 18, 2018, pp. 167-173, <http://photographconservation.ca/wp-content/uploads/2020/02/LUCAS_ROTTIER_DENIEL_DANTIGNY_Traitement-aux-vapeurs-d%E2%80%99%C3%A9thanol-des-photographies-g%C3%A9latino-argentiques-et-de-papiers-alt%C3%A9r%C3%A9s-par-des-micro-organismes_Support-Trac%C3%A9_2018.pdf> (accessed Novemebr 2024).
• Lucas C., Hill G., Binnie N. E., “Disinfection of photographic materials with ethanol vapours: Preliminary evaluation of the effects on chromogenic prints”, Journal of the Canadian Association for Conservation, 45, 2020, pp. 29-50, <http://photographconservation.ca/wp-content/uploads/2024/10/LUCAS-et-al_2020_Disinfection-of-photographic-materials-with-ethanol-vapours-Preliminary-evaluation-of-the-effect-on-chromogenic-prints.pdf> (accessed November 2024) .
• Meier C., “Schimmelpilze auf papier. Fungizide wirkung von isopropanol und ethanol”, Papier Restaurierung, 7, 1, 2006, pp. 24-31.
• Nittérus M., “Ethanol as fungal sanitizer in paper conservation”, Restaurator, 21, 2000, pp. 101-105.
• Pietsch K., “Die Restaurierung der durch Schimmel angegriffenen Farbdias Ed van der Elskens am Nederlands Fotomuseun - Teil 2”, Beiträge zur Erhaltung von Kunst- un Kulturgut, 2, 2017.
• Pietsch K., Fernandes L. O., “Moldy Matters: Conserving Ed van der Elsken’s 42,000 colour slides”, Topics in Photographic Preservation, 18, 2019, pp. 167-180, <https://faic.wpenginepowered.com/pmg-topics/wp-content/uploads/sites/9/2022/11/T18-167-180-Pietsch-Formatted.pdf> (accessed November 2024).
• Sequeira S. O., Phillips A. J. L., Cabrita E. J., Macedo M. F., “Ethanol as an antifungal treatment for paper: Short-term and long-term effects”, Studies in Conservation, 62, 1, 2016, pp. 1-9.
• Tomšová K., Ďurovič M., Dránková K., “The effect of disinfection methods on the stability of photographic gelatin”, Polymer Degradation Stability, 129, 2016, pp. 1-6.
• Tomšová K., Ďurovič M., “Influence of disinfection methods on the stability of black and white silver gelatin prints”, Journal of Cultural Heritage, 24, 2017, pp. 78-85.
• Weiß D., “Ethanol und Chlorometakresol als Fungizide”, Papier Restaurierung, 7, 2006, pp. 32-39.

Gamma radiation

• Adamo M., Magaudda G., Trionfetti Nisini P., Tronelli G., "Susceptibility of cellulose to attack by cellulolytic microfungi after gamma irradiation and ageing", Restaurator, 24, 3, 2003, pp. 145-151.
• Adamo M., Cesareo U., De Francesco M., Matè D., “Gamma radiation treatment for the recovery of photographic materials: results achieved and prospects”, Kermes, Vol. 86, 2012, pp. 45-53.
• Bousta F., Bouvet S., Cortella L., Courselaud M., Pellizzi E., Leplat J., François A., “Le traitement des archives aux rayonnements gamma : vers une méthodologie”, Support Tracé, n°18, 2018, pp. 147-158.
• Havermans J., Post Ageing effect van bestraalde fotografische materialen. Gamma Desinfectie deelrapport 3, Unpublished report, TNO Building Materials, Section on Conservation Science, 2011.
• Mitran A., Ponta C. C., Danis A., “Traitement antimicrobien des films cinématographiques au moyen du rayonnement gamma”, in La conservation à l’ère du numérique, Actes des quatrièmes journées internationales d’études de l’ARSAG, Paris: Groupe Lienhart Press, 2002, pp. 235-248.
• Nagai M. L. E., de Souza Santos P., Otubo L., Oliveira M. J. A., Vasquez P., “Gamma and electron beam irradiation effects for conservation treatment of cellulose triacetate photographic and cinematographic films”, Radiation Physics and Chemistry, Vol. 182, 2021.
• Tomšová K., Ďurovič M., Dránková K., “The effect of disinfection methods on the stability of photographic gelatin”, Polymer Degradation Stability, 129, 2016, pp. 1-6.
• Tomšová K., Ďurovič M., “Influence of disinfection methods on the stability of black and white silver gelatin prints”, Journal of Cultural Heritage, 24, 2017, pp. 78-85.
• Vellati, D., Adamo M., Matè D., Pinzari F., Ruggiero D., Residori L., Schlocchi M. C., “Assays on the resistance of photographic paper treated with gamma rays to fungal biodeterioration”, in 15th International Biodeterioration and Biodegradation Symposium, Vienne, 2011, pp. 19-24.

Commercial biocides

• Buendía, S. A. R, Evaluación de cuatro alternativas para el control de hongos en papel de pulpa de trapo, Tesis, México: Escuela Nacional de Conservación, Restauración y Museografía, 2017.
• Hoyos Velasco S. A., Evaluación de dos productos biocidas en impresiones fotográficas de plata gelatina en soporte de papel de fibra, Tesis, México: Escuela Nacional de Conservación, Restauración y Museografía, 2018.
• Hoyos Velasco S. A., “Safely handling moldy photographs might be worth a few stains”, Topics in Photographic Preservation, Volume 18, 2018, pp.238-246, <https://faic.wpenginepowered.com/pmg-topics/wp-content/uploads/sites/9/2022/11/T18-238-246-Hoyos-Velasco-Formatted.pdf> (accessed November 2024).

Case studies

• Caldararo N, Griggs C., “Preliminary report on the conservation of slides with special reference to the removal of mold”, Topics in Photographic Preservation, 9, 2001, pp. 97-102, <https://resources.culturalheritage.org/pmgtopics/2001-volume-nine/09_07_Caldararo.pdf> (accessed November 2024).
• Pietsch K., Fernandes L. O. 2019. “Moldy Matters: Conserving Ed van der Elsken’s 42,000 colour slides”, Topics in Photographic Preservation, 18, pp. 167-180, <https://faic.wpenginepowered.com/pmg-topics/wp-content/uploads/sites/9/2022/11/T18-167-180-Pietsch-Formatted.pdf> (accessed November 2024).

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