Humidification
Humidification is not necessary or desirable unless the prevailing relative humidity is under 15%—under 30% for polyester-base films—for long periods of time and unless the film is used frequently and physical troubles are encountered. Neither water trays nor saturated chemical solutions should be used for humidification of storage cabinets because of the more serious danger of overhumidification. Even humidification controlled by instrumentation is risky unless “fail-safe” devices are installed.
Fungus Growth
Fungus spores are found in the air everywhere. They germinate and grow under favorable conditions. When silver-gelatin films are stored for any length of time in an atmosphere having a relative humidity of above 60%, fungus (often called mold or mildew) has a tendency to grow on the emulsion surface, the back of the film, or on the film reel. The higher the relative humidity, the greater the chance of fungus attack and the more abundant its growth. The only real protection against fungus growth is to make certain that conditions are unfavorable for its growth.
If fungus growth progresses far enough, it can cause serious and permanent damage to film. This takes the form of distortion of the emulsion and eventually causes chemical breakdown so that the gelatin becomes sticky and readily soluble in water. Water or water solutions should not be used for the removal of fungus growth because either may lead to disintegration of the image. Clean affected film by wiping it with a soft plush or cotton pad moistened with an approved film-cleaning liquid, such as isopropyl alcohol. This procedure is described in detail in a separate Kodak Pamphlet.10
Fungus can grow on most surfaces, including diazo and vesicular films; however, these films are more resistant to fungus growth than silver-gelatin films. If necessary, clean these film types by wiping with a soft, plush or cotton pad, moistened with water. Do not use solvents on diazo, vesicular, or thermally processed silver films.
The Kodak Disaster Recovery Laboratory offers no charge evaluation of suspect microfilm and can be reached at 1-800-EKC-TEST (1-800-352-8378) or 1-585-253-3907.
Microscopic Blemishes
Some processed silver-gelatin microfilms in storage for two to twenty years have developed microscopically small colored spots or blemishes. The fogged leader at the outside of the roll is most frequently affected by the blemishes, which are generally red or yellow in color and smaller in size than the image characters (e.g., a typewritten letter reduced 20X) on the microfilm. On occasion, these spots progress further into the roll and appear in image areas. A more detailed description of the blemishes and of the techniques used in inspecting microfilm is given in National Bureau of Standards Handbook 96.11
The spots are caused by local oxidation of image silver, resulting in the formation of minute deposits of yellow- or red-colored colloidal silver.12,13 Possible oxidizing agents entering from outside the roll of microfilm are aerial oxygen, whose action on the film is strongly accelerated by moisture, and atmospheric contaminants such as peroxides, ozone, sulfur dioxide, hydrogen sulfide, and nitrogen oxides, which all occur in industrial atmospheres.
Assuming that your microfilm was processed to established quality control standards, microscopic blemishes (redox) may occur due to the oxidation of the metallic silver image by the presence of oxidants in the storage area: peroxides, ozone, nitrogen oxides, oil-based paint fumes, organic solvents, floor-cleaning agents, cardboard cartons, excessive temperature and humidity,
or a variety of other materials that generate peroxides which attack silver microfilms.
The use of low concentrations of potassium iodide in the fixing bath (0.2 g/L) has been recommended and has been found to provide a good degree of protection against these blemishes.13 Kodak Microfilm and Prostar Fix Solutions contain this stability-enhancing iodide.
Once the deterioration of the microfilm occurs, it cannot be reversed. Silver film duplication, Kodak Brown Toner, and the use of molecular sieves can be used to stabilize the reaction.
