Understanding Mold (edited)                                                 Home Up

     by Mary Wood Lee -- June 1988

See Also:
Mold & Mildew Summary
Prevention of Mold
Treatment of Mold
Storage Materials

Because the nature of molds is so poorly understood, their appearance is often cause for disproportionate alarms and excursions, with cries for institution wide fumigation, the formation of committees, and often, a lamentable level of inaction. Much of the older and some of the current literature recommends that items be isolated in plastic bags, to await fumigation or other treatment or that the mold be brushed from the surface of the item. Once the staff has some idea of the reasons for its occurrence and growth, informed decisions can be made as to the appropriate treatment. For example, in the above instance, placing the item in a plastic bag at the first visible sign of the mold will simply create a micro-climate that may actually accelerate the growth of the colonies, possibly doing serious damage while treatment is awaited or debated. Simply brushing the mold away will only remove the visible portion of the mold, scattering the spores, and pressing the invisible sub-structure down onto the surface of the item. Treatment techniques will be dealt with in detail in a later section, but are mentioned now to stress the importance of this section and the section that follows. Together they will provide the basis for informed decision making.

Environmental and nutritional factors in growth and survival

Most of the information available on the growth and development of mold is derived from laboratory cultures rather than on site studies. This information is therefore not always relevant to the growth and development of the same organism in the library environment. It is however, accurate to say that three factors are essential for the growth and survival of molds: the correct temperature, adequate moisture, and proper nutrients. St. George notes that it is a common misconception that light is required for mold growth. Unlike most plants, virtually all molds lack chlorophyll and therefore, light plays no role in their development. Colonies thrive in the dark, since for some varieties, exposure to ultra-violet light is injurious or lethal.


There are three critical temperatures for mold, the temperature below which no growth occurs, the temperature above which no growth occurs, and the temperature at which most rapid growth takes place. Most microbial forms grow in temperatures ranging from 59º to 95º F (15º to 35° C), although there are forms which will grow at almost freezing and others which thrive at over 150º F. The average optimum for mold growth is usually stated to be in the vicinity of 86° F. The optimum temperature for the growth of specific molds is difficult to determine, in part because of variables in other environmental conditions, and in part because the culturing of organisms in the laboratory is a very different matter than the growth of the same organism in more natural surroundings.

It should be noted that the temperature below which no growth occurs is not synonymous with the temperature at which the potential for growth is destroyed. Many molds can survive periods of several months at sub-zero temperatures, but are less tolerant of alternating below-freezing and above-freezing temperatures.

Sykes, speaking of bacteria, says: “Refrigeration at low temperatures...is popularly considered to be fatal to all forms of life. Whilst this may be true for the larger forms of organized life, it is certainly not true for the smaller plant life, including micro-organisms....sometimes the death rate is as high as 99% but once frozen at a sufficiently low temperature the surviving cells can be preserved for long periods.”


The amount of moisture required for mold development is seldom addressed in the microbiological literature. In the laboratory molds are cultured in media with a high moisture content, but the precise level is seldom mentioned in their reports. The covered Petri dish creates a microclimate where the mold can flourish undisturbed. With regard to the growth of mold outside the laboratory, sources do indicate that the hygroscopic nature of materials affects the growth of mold. Materials which absorb and hold moisture from the air require lower levels of ambient relative humidity than do less hygroscopic materials. Thus, in a non-laboratory environment, the mold has at its disposal two sources of moisture, the air surrounding the item and the moisture held by the item itself.


The elements required for the growth of fungi include carbon, hydrogen, oxygen, nitrogen, sulfur, potassium, and magnesium. Trace elements such as iron, zinc, copper, manganese, and in some cases, calcium may also be required. Certain of the vitamins are also needed. Most naturally occurring compounds can be utilized by fungi as sources of carbon and energy. Cellulose provides many of these elements, as do animal and vegetable fats and their component acids and glycerin.

Implications for library materials

Virtually all organic materials are susceptible to some species of mold and therefore to mold growth. The organic materials in library collections include, but are not limited to: cellulosic fiber; sizes and fillers of starch, casein and gelatine; natural adhesives, including starch paste made from vegetable matter and glues from animal skins; some synthetic adhesives; leather; and the gelatine on negatives and photographic prints. In addition, dust and dirt can provide additional nutrients required by the mold. All of these materials are hygroscopic, that is, they attract and hold moisture.

Despite this overall vulnerability, a variety of factors will affect the actual growth of mold within the library collection. Certain papers, leathers, bookcloths and adhesives are more susceptible to mold growth than others. In most cases the librarian has little control over the composition of the materials in the collection. However, a knowledge of the nature of those materials is necessary in order to make informed decisions as to why the infestation has occurred, how to treat those items obviously affected, and whether it is likely that the problem will spread throughout the collection.

For example:

- The appearance of mold on only the leather bound books indicates that the active spores are specialized in their nutrient requirements. Since molds are selective, if no cloth covered or paperbound books in the immediate vicinity are involved, emergency treatment can be concentrated on the leather volumes.

- If the growth appears only around the head cap, or on the edges of the text next to the turn ins on the boards, it is likely that the nutrient source is the adhesive used in the binding.

- If only a few ranges or a few stack sections in the area are affected, the problem is most likely one of a microclimate. The affected items can be removed and efforts to modify the environment can be localized to that area.

Innumerable examples could be given, however the point is that a knowledge of the materials, an analysis of the nature of the problem, and an understanding of the interaction between the two can greatly reduce the potential damage.

Vulnerability of materials

In order to prevent mold growth, or to treat it effectively once it has developed, it is not necessary to identify which of the thousands of genera of mold may be involved. It is however necessary to understand the basic structure of the mold organism and the manner in which it takes advantage of favorable conditions. This means that librarians must assume responsibility for a wide range of knowledge concerning the materials in their collections as well as the nature of the threat in order to make informed decisions regarding appropriate treatments.

Paper - cellulose, sizes, coatings

In 1940, Beckwith and his co-workers isolated 55 different mold cultures from old book papers, including eleven genera, of which Penicillium and Aspergillus were the most commonly found. In the study, spores were removed from the papers, transferred to a culture medium and grown under laboratory conditions. This is not to say that all of them would have been able to use the paper as a medium for growth, but certainly some of the strains of Aspergillus and Penicillium would be likely to attack cellulose or one of the numerous paper additives, sizes, fillers or coatings. At least 180 genera or species of mold are known cellulose destroyers, i.e., they use the cellulose fiber as a nutrient.

Other molds that do not actually consume cellulose may damage paper by weakening the fiber bonding as they feed on other materials in the paper. The fillers, sizes and coatings added to the paper during manufacture to improve printability, texture, color or brightness are a potential source of nutrients, and may include starch, gelatine and casein. Rosin size was found by Beckwith to inhibit fungal growth; however, rosin is acidic and has been found to accelerate the chemical deterioration of paper and its presence is not cause for rejoicing. Very little is known about the various synthetic sizes, as much of the research in this area took place before they were in common use.

Paper in bound volumes is less vulnerable to high ambient relative humidity than unbound paper. Cryptogamic fungi seldom occur in closed volumes under such conditions, but rather on the bindings and on unbound sheets of paper exposed during prolonged periods of dampness. Foxing, on the other hand, is commonly found in text blocks.

In cases of flood or other severe wetting, book paper may be considered to be more vulnerable, since the bulk of the volume and the compression of the paper at the spine slow the drying process considerably.


Pastes (made from vegetable starches), glues (made from animal products) and gums (made from vegetable resins) are all subject to mold growth to varying degrees. The use of excessive amounts of adhesives may be one factor in promoting the growth of mold. With regard to the application of adhesives, more in not necessarily better.

Synthetic adhesives, including polyvinyl acetate emulsions (the so called "white glues" which vary enormously in composition and properties), pressure sensitive adhesives on tapes and labels, heat set adhesives such as those used in dry mount papers, and aerosol spray adhesives are more resistant to mold, but not entirely immune. They are solvent based, and therefore dry quickly. However, their poor aging properties and the fact that solvents are required for their removal make them undesirable for the repair of torn or damaged paper.

Despite the possibility of mold, pastes and gums are recommended for mending of paper due to their reversibility. Proper application and thorough drying of the adhesive film provided the best protection. Repairs to bindings are perhaps best done with good quality PVA.

Environmental factors

The five critical environmental factors for the growth and development of mold in library collections are:

- The presence of mold spores
- A source of nutrients
- Adequate moisture
- Suitable temperature for a particular variety of mold.
- Limited air circulation

It is obvious that the first two factors are completely beyond the control of librarians. The presence of spores and the source of nutrients are a given in library collections. Only the last three factors can be manipulated or controlled in order to prevent the occurrence of mold growth.


Of these three, circulation is one of the most critical, and the most often neglected. The literature often mentions in passing the importance of good air circulation. Unfortunately, the significance of this factor, particularly in areas where the environment is not temperature and humidity controlled, has been largely overlooked. Air movement causes the evaporation of moisture, lowering the surface temperature. This is evident to anyone who has ever experienced the cooling effect of a sudden breeze on a hot still day. Good air circulation in the library results in the evaporation of moisture, lowers the surface temperature, and alters two of the environmental factors on which mold growth depends.

It is, in general, much less expensive to move existing air around, thereby modifying the temperature and humidity than it is to introduce an artificially created supply of air with characteristics radically different than that of the surrounding air. Good air circulation can do much to reduce the problems associated with lack of control of conditions three and four.

Relative humidity

Paper, cloth and leather are all hygroscopic, that is, they absorb moisture from the air and retain it. Thus, in humid climates, most materials in the library contain a relatively high percentage of water. In these conditions, even a slight increase in ambient relative humidity is enough for the item to sustain mold growth, if the other requirements are present.

There are several different ways to measure moisture. Absolute Humidity is the weight of water in a given volume of air (g/m3 ). Moisture content is the weight of water in any given material (kg/kg). Both of these measurements are variable, i.e., warm air can hold more moisture than cold air, and the moisture content of materials varies with the absolute humidity of the surrounding air. Neither absolute humidity nor moisture content can be effectively determined in a library environment. Therefore, the only useful measure from the point of view of collections maintenance is that of Relative Humidity (RH). Relative humidity is the amount of water in a given volume of air relative to the maximum amount of water air can hold at that temperature, and is expressed as a percentage.

When warm air is cooled it can hold less moisture. This moisture condenses on the surface of items or is absorbed by them if they are hygroscopic. If, for example, at 70º F, the RH is 50%, it requires only a ten-degree drop in temperature to raise the RH to 70%. In humid tropical climates lowering the temperature without reducing the relative humidity can result in rampant mold growth, as many institutions have discovered to their dismay after installing a series of window air conditioners in an attempt to improve their environment. While air conditioning does remove some moisture from the air, and is generally adequate in a more temperate environment with a naturally lower ambient RH, in tropical climates with year round RH of 80 to 90%, a window air conditioning unit cannot remove enough of the moisture to prevent the cooled air from reaching the dew point.

The literature contains a variety of recommendations for RH levels that will prevent the growth of mold. They range from a high of 60% to a low of 45%, and seem to have declined steadily over the years. In 1940, Beckwith found that of the molds in his experiment, none would grow at a relative humidity below 75%, even when additional nutrients were added to the culture. While not definitive, this would help to explain why tropical libraries and museums (whose RH is seldom as low as 60%, let alone 45%) are not constantly blanketed in mold. Certainly lower relative humidities are safer, but it is apparent that the incidence of growth can be minimized at significantly higher levels of humidity.

Because relative humidity is so dependent on temperature, all figures are relative, and subject to a number of variables. As seen above, a change in one results in a change in the other and achieving the correct balance is the critical factor.


There is a strong inclination to attempt to modify the environment through changes in temperature alone, in part because temperature is the factor to which human beings are most sensitive. High temperatures do have a detrimental effect on library materials, and these have been so emphasized in the literature that they have tended to obscure the effects of lowering the temperature without regard to the relative humidity. As with most other environmental issues, easy answers and quick fixes tend to create problems that, in the long run, are often more damaging than the original problem.

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