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.