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close this book Prevention and treatment of mold in library collections with an emphasis on tropical climates: A RAMP study
close this folder 2. Mold
View the document 2.1 Structure of mold
Open this folder and view contents 2.2 Environmental and nutritional factors in growth and survival
View the document Literature cited

2. Mold

It may seem, in what follows, that an inordinate amount of attention is devoted to the structure and nature of mold. Because fumigation has for so long been the treatment of choice, there seems to be a feeling that information regarding the organism itself is irrelevant. Moreover, librarians are understandably frustrated by literature which urges them to consult a microbiologist or entomologist in order to identify the offending species. While it is to some extent true that one need not identify precisely the mold involved in order to treat it, an analysis of the problems associated with mold growth and the selection of an appropriate treatment must be based on some understanding of the organism. As Allsopp notes, it does not require a specialist to determine the hazards posed by most organisms. One can, after all, "observe a mouse or a bird and state accurately whether it is dead or alive. Organisms such as these can be seen and identified, and vital signs are easily recognized. Mice rarely lie stiff on their backs, motionless, with their feet in the air when they are alive. Microorganisms, however, pose problems..."1

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 structure of the mold organism is clearly understood, and the staff has some idea of the reasons for its occurrence and growth, recommendations in the literature can be more accurately evaluated, and 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. The mold organism must be clearly understood, since the nature of mold, the reasons for its occurance, and the stage of its development will determine the specific treatment and the time frame within which action should be taken.


2.1 Structure of mold

Mold is the commonly used term for cryptogamic fungi, i.e. fungi that propagate by means of spores. The prevention of mold growth through the exclusion of mold spores from the environment is not a viable option. Mold spores are ever present in virtually all environments and the distribution of species is relatively uniform world wide. The extreme micro-biological deterioration that occurs in tropical climates differs from that in temperate climates only in degree, not in kind. It is the result of optimum conditions rather than unique or particularly virulent strains. The isolation and identification of large numbers of fungi found in the tropics have failed to reveal any genera that can be singled out as either characteristically tropical, or limited to tropical areas.2

The majority of molds of concern to the librarian and the archivist are made up of two different structures, vegetative and reproductive. The vegetative portion is characterized by a branching of colorless threadlike filaments called hyphae. These hyphae, collectively referred to as mycelium, branch out across the paper or other substrate and are quite invisible to the unaided eye. They form the root system of the plant. Their presence preceeds the appearance of the visible mold growth. Once the mycelium are established, the mold reproduces by spores produced externally on the hyphae. In most of the mold which are of concern to librarians, the individual hyphae produces stalks known as conidiophores, which in turn produce phialides, which are the colored components of the mold. These are the reproductive structures.

Molds are admirably equipped by nature for survival. Of the spores produced, there are two general types. Some spores are produced rapidly and in large numbers, but have very little resistance to drying, sunlight and other adverse environmental factors. They make possible the rapid growth and development of colonies when conditions are favorable. Other spores are much more resistant to unfavorable conditions. These "hold-over" or resting spores enable the organism to survive over long periods of adverse conditions.3

In many molds, the flowering stage, which is evidenced by colored phialides is proceeded by a soft, grey, fuzzy growth visible to the unaided eye. If the mold is removed at this stage, before the flowering begins and the effect on the substrate is most severe, mold stains seldom occur. This is not to say that the substrate will not be damaged, but the damage may be greatly reduced.

The exact cause of the stains often seen after mold has been removed or in dead or dormant colonies is difficult to determine, as is the time frame within which the staining occurs. While staining usually seems to be the result of mature colonies that have been allowed prolonged growth and development, certain molds are known chromophores, and may produce extensive color changes in the substrate, even though their growth is limited.4 Belyakova has identified numerous genera which produce stains on paper due to the production of pigments by the fungi or to the mycelium, which penetrate the paper. The color of the stains is not an accurate guide to the specific mold which caused it. Penicillium frequentans for example produces yellow stains in some instances, pink stains in others.5 Much work remains to be done in order to determine whether staining is produced by the molds digesting the nutrients in the substrate and excreting the by products, as some sources suggest, as a result of acids produced during the hydrolysis of the cellulose, or simply by chromophores present in the cells of the mold itself.

In addition to the cryptogamic fungi, which are the primary focus of this study, two other types of mold may cause damage to library materials. Foxing, the common designation for the small brown spots that appear in old papers, is a mystery yet to be resolved. Its exact nature and cause remain uncertain. Dard Hunter noted that books papers before 1501 seldom showed signs of foxing and attributed its occurance after that date to the increased demand for paper which caused paper makers to reduce the amount of water used and did not allow enough time for "the proper cleansing of the fibers."6 In the 1920's Beckwith found that foxing was usually associated with the presence of iron in the paper,7 leading others to believe that it is the result of metals left in the paper during manufacture, and that it's incidence coincided with the invention of the Hollander beater in the late 17th century. While trace elements of iron may be a necessary component, the presence of foxing, called hoshi (stars), in very old Japanese papers produced using traditional beating and sheet formation techniques would seem to indicate that iron left in the paper as a result of Western manufacturing processes is not the sole cause. Though foxing has yet to be produced on demand in the laboratory, many now believe foxing to be a form of micro-biological growth. In 1984, a Japanese researcher, using a scanning electron microscope isolated and identified the fungi Aspergillus glaucus and Aspergillus restrictus which he believes to be the cause of foxing.8 Whatever the cause, it seems certain that its incidence is increased by high temperatures, high humidity, and by proximity to poor quality materials. That it does indeed damage paper is evidenced by differential wetting characteristics of foxed papers during conservation treatment.

Slime molds, which are relatively rare on finished materials, most commonly occur during paper manufacture. These organisms are usually destroyed by various chemicals and by the heat of the drying process. Their presence however may serve to weaken paper and make it more vulnerable to deterioration when combined with adverse environmental conditions later.


2.2 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. George9 notes that it is a common misconception that light is required for mold growth. Unlike most plants, virtually all molds lack chlorophyl 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.10


2.2.1 Temperature

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 59 to 95 F (15 to 35 C), although there are forms which will grow at almost freezing and others which thrive at over 150 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.11

Sykes, speaking of bacteria, says:

Refrigeration at low 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.12

Given the existence of the "hold-over" spores, this undoubtedly applies to molds as well.


2.2.2 Moisture

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 petrie 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 hygoscopic 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.


2.2.3 Nutrients

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 occuring 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 glycerine.13


Literature cited

1. Dennis Allsopp. "Biology and Growth Requirements of Mould and Other Deteriogenic Fungi." Journal of Society of Archivists, Vol. 7:8 October, 1985. p.530

2. R.A. St. George, et al. "Biological Agents of Deterioration." Deterioration of Materials, Greathouse & Wessel, p. 179.

3. St. George, p. 183.

4. T.D. Beckwith, et al. "Deterioration of Paper: The Cause and Effect of Foxing." UCLA Publications in the Biological Sciences. Vol.1:13, 1940. p.331.

5. L.A. Belyakova. "The Mold Species and Their Injurious Effects on Various Book Materials." Collection of Materials on the Preservation of Library Resources, Nos. 2 & 3. Translated from Russian, National Science Foundation and Council on Library Resources, 1964. pp. 183-184.

6. Dard Hunter. Papermaking, the History and Technique of an Ancient Craft. New York, Dover, 1978. p. 154.

7. Beckwith, pp. 299-300.

8. Hideo Asai. "Microbiological Studies on Conservation of Paper and Related Cultural Property: Part I." Studies in Conservation, No. 23, March, 1984. pp. 33-39. In Japanese. Abstracted in English in Art and Archaelogy Technical Abstracts.

9. St. George, p. 186.

10. Belyakova, p. 73.

11. St. George, p.186.

12. G. Sykes. Disinfection and Sterilization. London, Spon. p.183.

13. St. George, p. 186-187.