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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 3. Implications for library materials
Open this folder and view contents 3.1 Vulnerability of materials
Open this folder and view contents 3.2 Environmental factors
View the document Literature cited

3. 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, ho,; 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.


3.1 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.


3.1.1 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.1 In the study, spores were removed from the papers, transfered 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.2

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;3 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 know 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.


3.1.2 Bookcloth

Many bookcloths, including those of cotton and linen, are cellulosic and are vulnerable to the same range of mold species that affect paper. Like paper, the fillers and coatings added during manufacture provide an additional source of nutrients. The unsized cloth frequently used in bindings from India and Southeast Asia is particularly vulnerable. Because it is often quite thin, the adhesive used in attaching the cloth to the boards often penetrates the weave of the cloth, allowing mold to grow on the surface. Starch filled buckram, commonly used in more temperate climates is also an excellent source of nutrients. Manmade fibers, or natural fibers coated with synthetic resins, i.e., peroxylin cloth and acrylic coated buckram are more resistant to mold, but not entirely immune. No literature was found regarding the affect of dyes on mold growth, although dye. have been found to have considerable effect on the resistance of textiles to photochemical action (some accelerating deterioration and others providing protection).4


3.1.3 Leather

Tanned leather is more resistant to mold growth than untanned leather. Chrome tanned leathers are relatively impervious, vegetable tanned leathers considerably less so. Book leathers are, unfortunately, vegetable tanned, chrome leathers being used primarily in shoes, luggage and other such items.

Studies indicate that mold growth does not affect leather in the same way that it does cellulose. The mold apparently does not attack the hide-tannin complex itself.

Barghoorn has demonstrated that invasion and destruction of the collagen aggregates of the hide substance does not occur; and Hyde, Musgrave and Mitton have shown that vegetable-tanned leathers suffer surprisingly little damage through even fairly heavy and prolonged mold growth. Experimental evidence indicates that the major cause of tropical deterioration of leather is hydrolytic breakdown due to the high atmospheric humidity and temperature and to their effect on interfiber lubrication, the extent of the hydrolysis being dependent upon the pH of the leather.5

Thus, it seems that the components of leather which support mold growth are the lubricants, the conditioning materials and the finish. It would seem from the literature cited above that high ambient relative humidity rather than mold damage is the primary cause of deterioration of leather in tropical climates.

Oiling of leathers, which many libraries have viewed primarily as a cosmetic treatment, may in fact be the most viable way of protecting leather in a tropical environment. Some libraries in tropical climates have avoided leather dressings, fearing that the use of oils and lubricants would promote mold growth. However, since any resultant mold growth is superficial and causes no structural damage to the leather, and since the application of a leather dressing prevents the hydrolytic damage that is the chief cause of deterioration, the use of leather dressings of appropriate composition should be considered beneficial.

With regard to the choice of a particular leather dressing, experience in the tropics indicates that a very light coat of neatsfoot oil and lanolin, allowed to dry for 24 hours and then buffed with a soft cloth works well. Leather dressings containing wax, including one developed by the British Museum, do not harden satisfactorily in warm humid climates, and the surfaces of treated items tended to stick together when returned to the stacks.


3.1.4 Adhesives

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 undesireable 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 reversability. Proper application and thorough drying of the adhesive film provided the best protection. Repairs to bindings are perhaps best done with good quality PVA.


3.1.5 Film and related materials

All photographic materials have in common a substrate of gelatin which carries the emulsion of silver halide particles that produce the image. The film base may be nitrate, acetate, polyester, glass or paper and the format may be a negative, a photograph, or a reel of microfilm, but all have a gelatin layer. As with the gelatin sizes used in paper, photographic gelatin provides a nutrient for mold growth, which can penetrate the emulsion layer, damaging the image. The polymers that provide the base for contemporary film stock are generally very resistant to fungal attack,6 however paper and glass supports are both vulnerable. Glass plate negatives can actually be etched by fungi, and combined with the damage to the silver halide layer, can render the negative completely useless.

Gelatin is relatively stable as long as it is kept dry. In high humidities gelatin begins to swell and if exposure is prolonged, becomes sticky. This can occur at relative humidities as low as 60%.7


3.2 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 occurance of mold growth.


3.2.1 Circulation

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.


3.2.2 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 measurments 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 70 F, the RH is 50%, it requires only a ten degree drop in temperature to raise the RH to 70%. Plenderleith and Werner8 include a chart which shows the curves relating a reduction in temperature to the corresponding rise in relative humidity. 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 airconditioning 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.9 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.


3.2.3 Temperature

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 emphasised 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.


Literature cited

1. St. George, p. 179.

2. Belyakova, p. 184.

3. Beckwith, p. 307.

4. Carl J. Wessel. "Textiles and Cordage." Deterioration of Materials, Greathouse & Wessel, p. 474-479.

5. Robert M. Lollar. "Leather." Deterioration of Materials, Greathouse & Wessel, p. 152-153.

6. Charleston C. Baird and David F. Kopperl. "Treating Insect and Microorganism Infestation of Photographic Collections." Second International Symposium: The Stability and Preservation of Photographic Image, August 15-28, 1985. Springfield, VA., Society of Photographic Scientists and Engineers, p. 53.

7. Fleming, p.363.

8. H.J. Plenderleith and A.E.A. Werner. The Conservation of Antiquities and Works of Art, 2nd ed. London, Oxford University Press, 1971. p.6.

9. Beckwith, p. 331.