Click for next page ( 50


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 49
Photographic Film Photography has been in existence for over 150 years, and the basic principle has remained essentially unchanged for black-and-white images. A photographic image is formed by the action of light on silver halide salts contained in a binder applied to a plastic substrate. This allows the exposed silver halide to be converted to metallic silver by the subsequent action of a photographic developer, yielding a reversal or negative image. The unexposed silver salts are removed in the photo- graphic processing. Other technologies are applicable for producing the photo- graphic image, but silver halide salts are by far the most used for recording black and white images. STRUCTURE The film's light-sensitive layer is called the photographic emulsion; it con- sists primarily of silver halide in a colloidal medium. The halide compound may be silver chloride or bromide or mixtures of the chloride, bromide, or iodide. The medium is usually gelatin, although the very early photographic materials employed albumen or collodion. The photographic emulsion also contains optical sensitizers, antifoggants, and other chemicals with specific purposes, such as hardening. Photographic emulsions vary in thickness but are generally from one to several ten-thousandths of an inch. Color emulsions are thicker than black-and- white emulsions. To provide the required mechanical integrity and to permit easy handling, photographic emulsions are coated onto a transparent base or support. Originally, glass was used as the major support when negatives were made on photographic plates. Photographic film came about because of the availability of flexible plastic supports. Film supports range in thickness from 0.0025 to 0.008 in. {65.5 to 215 ~m), depending on the chemical nature of the support and the end-use require- ments. The first plastic film support was cellulose nitrate, which was in commer- cialuse from about 1890 to 1950. It was one of the first plastics in existence, and its 49

OCR for page 49
50 PRESER VATION OF HIS TOPICAL RECORDS 1 " ~i ~... , i, . Microfilming~aboratoryat the NationaJArchives. Carefulinspection and quality contro~procediures assure integrity of records.

OCR for page 49
PHOTOGRAPHIC FILM 51 application marked the beginning of the widespread use of photography. It proved to be a very acceptable film base but suffered from high flammability and poor chemical stability. These drawbacks were recognized early, and there was consid- erable research after World War I to obtain a more stable material. In the 1 920s and 1930s, other derivatives of cellulose {i.e., cellulose acetate butyrate and cellulose acetate propionate) found commercial application for certain product types, pri- marily amateur and X-ray films. After World War II, cellulose triacetate was devel- oped, and the material replaced cellulose nitrate completely. The latter ceased to be manufactured in the United States for photographic film use early in the 1950s. While cellulose triacetate and other cellulose esters overcame the concerns about chemical stability, they were not the ideal film supports for all end uses. Many products require extremely good dimensional stability, primarily in the graphic arts and aerial photography fields. To meet this need, polyethylene tereph- thalate was introduced around 1956. It is commonly known as polyester film base, and it has since found very wide acceptance. Today most photographic films are coated onto either cellulose triacetate or polyester base. It is these two materials that must be considered for use by the National Archives. The cellulose ester supports are manufactured by the solvent-casting process. The cellulose ester, together with plasticizer, is dissolved in solvents. This viscous fluid is then spread evenly on a huge, highly finished casting drum or endless moving belt. The solvent is evaporated, and when the cast support is firm enough it is stripped from the drum or belt and is cured further in heated chambers. Polyester support is manufactured by a completely different process, since it is insoluble in all common solvents. It is melted and extruded through a precision- finished die to form a continuous sheet. To obtain the necessary physical proper- ties, it is subsequently stretched in both the machine and cross directions at elevated temperature. Finally, it is subjected to a temperature considerably above the stretching temperature while mechanically restrained to prevent shrinkage. This annealing causes the polymer chains to crystallize and imparts thermal stability to the support. Film supports are coated with various layers during manufacture to provide good adhesion with the photographic layer, to give static protection, or to elimi- nate light reflecting from the support surfaces back into the light-sensitive emul- sion. The photographic emulsion is coated onto the plastic support from aqueous solutions. The liquid emulsion flows onto the support from a coating hopper, and the water is then evaporated in curing sections. Depending on the product, the total emulsion may consist of one or many individual layers. Color emulsions have one or two layers for each color, along with filter layers. There can be as many as a dozen layers. Black-and-white archival emulsions have a total of one or two layers. APPLICATIONS Photography has been used to copy documents since the 1930s, when banks made photographic copies of checks. This approach has been widely expanded, and reduction printing is used for many types of documents, giving rise to the microfilm industry. There are multiple purposes for microfilming, as described by

OCR for page 49
52 PRESERVATION OF HISTORICAL RECORDS Gille {1953), "to preserve documents from eventual and non-voluntary destruc- tion {fire, war, etc.J; to permit the communication {in the form of copies) of particularly valuable or fragile documents; to facilitate the consultation of docu- ments of special formats or volume; to enable the archives to satisfy requests for consultations of records outside the archives by sending copies." An additional purpose was described by Leisenger {1976) of NARS: ". . . the National Archives outlined a program for the rehabilitation of the damaged records. It was decided to preserve records by microfilming them whenever feasi- ble and to reserve the more costly laminating and rebinding processes for those records that are too fragile for microfilming, that have special values requiring their preservation in their original form." Crespo Nogueira { 1982) further identified the different purposes of filming an archive. She distinguished "reference microfilming, security microfilming, acquisition or complementary microfilming, space saving or substitution micro- filming, and publications microfilming. In most cases there will be no clear-cut separation between these various uses, which will frequently overlap. Whatever the primary reason for a microfilming operation may be, it will in most cases also serve some if not all of the other possible purposes. Undoubtedly, considering the various uses of microfilming, documentary preservation is the most frequent to appear as a subsidiary purpose." In the context of the current NARA concerns, substitution microfilming would be considered a preservation measure because of the poor physical condition of the original documents and the high possibility of their eventual loss. Crespo Nogueira also noted that "the usefulness of microfilm- ing as a method of preservation has been clear to archivists from the very beginning of archival microfilming, even if economic reasons have delayed its effective application. Sometimes, however, microfilm may be used as a substitute for more costly restoration which will then be restricted to the more fragile or most valu- able documents." Crespo Nogueira conducted an extensive survey of the microfilming practices of the national archives in various countries. Among the 48 replies received, only four indicate that a microfilming policy does not exist. In 39 countries, the pri- mary purpose of microfilming was archival preservation. PERMANENCE The permanence of photographic film is dependent on the stability of the film support, the image, the adhesive layer, and the adhesion between them. Each is here discussed in turn. The stability of the film support obviously varies with the support type. The chemical instability of cellulose nitrate has already been referred to, but this material has not been used as a film support in this country for over 30 years. The only concern today is that cellulose nitrate-base film is a fire hazard and should be stored accordingly. It should not be stored in the same storage areas as film having long-time value. Any degradation of the cellulose nitrate base can release nitrogen dioxide, which has a devastating effect on the silver image {Carroll and Calhoun, 1955~. The chemical stability of cellulose triacetate and polyester supports and films has been studied using high-temperature aging results to predict lower tempera

OCR for page 49
PHOTOGRAPHIC FILM TABLE 5-1 Predicted Life of Photographic Film at 70F {21C), 50 percent Relative Humidity Base Life Reference Cellulose triacetate >300 years Adelstein and McCrea (1981 Polyester >2,000 years Adelstein and McCrea (1981 Polyester > 1,000 years Brown et al. ( 1984J 53 ture behavior {Adelstein and McCrea, 1981; Brown et al., 1984~. It was found that initial stability was followed by the change in viscosity and in mechanical proper- ties. These data were treated using the classical kinetic approach of Arrhenius for first-order chemical reactions, although it was well recognized that the change in mechanical properties involved many interactions that are much more complex. Nevertheless, the Arrhenius treatment allowed extrapolation of laboratory data to room-temperature storage conditions. Predictions by Adelstein and McCrea jl981) and Brown et al. ~1984) of the useful life of photographic film at recommended storage conditions are given in Table 5-1. These predictions are believed to be conservative because the property loss on which they were based did not represent a degree of degradation that rendered the film completely unusable. Arrhenius projections for emulsion decomposition were found under the same high-temperature incubations to be in excess of 2,000 years. The stability of the photographic image depends on the emulsion type, and these types may include black-and-white, color, diazo, vesicular, and thermally processed silver. Of primary concern to the archivist are black-and-white images formed by silver halide. However, it is very difficult to obtain good Arrhenius projections for this type of image because the photographic changes occurring are so small. For example, a black-and-white microfilm image was still very usable after 1,000 days of incubation at 70F t21C) and 50 percent RH {Adelstein and McCrea, 1977~. The resulting density changes were very small, and Arrhenius relationships cannot be obtained from such data. The only Arrhenius plots for a change in black-and-white image density were reported for X-ray films by Kopperl et al.; 1982~. These data indicated that a density change of 0.05 in the low-density or clear area for normally processed X-ray film would not be expected to occur within at least several centuries at recommended storage conditions. The stability of color images is quite different, and the utility of the Arrhenius plot to predict the fading of color images has been known for some time Adelstein et al., 1970; Bard et al., 1980; Seoka et al., 1982~. One obvious solution for preser- vation of color images is low-temperature {35F, or about 2C) storage, although this creates problems in the handling of the materials and is also expensive. It is not a practical or necessary consideration for the National Archives problem. Diazo films are used in the micrographic industry as a print material; these are dye images. Arrhenius projections have been useful in predicting the life of these products, and one study {Adelstein and McCrea, 1977) predicted useful life rang- ing from 10 to over 100 years under recommended storage conditions, depending on the product type. These products are considered medium-term or long-term films but are not suitable for archival storage. Vesicular films have images composed of small bubbles of nitrogen formed by

OCR for page 49
54 : ::< Eve .. PRESERVATION OF HISTORICAL RECORDS Mircofiln~ cabinets in National Archives microfilm reading room. Se~f-service microfilm storage cabinets allow researchers ready access to archival materials. Ok i~ ..... > ha.! . OCR for page 49
PHOTOGRAPHIC FILM 55 the decomposition of a diazonium salt. These films are used in the microfilm industry as a print material. Although good experience has been obtained with their image stability {Ram and Potter, 1970; Potter and Ram, 1975J, it is not possible to use Arrhenius relationships to predict the eventual life of these materi- als. Long-time behavior at room-temperature conditions cannot be predicted from short-term tests at elevated temperatures for this material. At the higher tempera- tures, the binder containing the nitrogen bubbles softens and the bubbles collapse, thereby destroying the image. Because of lack of knowledge of the eventual life of these films, they are not considered an archival medium. Thermally processed silver films are also used in the micrographics industry. A silver salt on a long-chain fatty acid is the image-forming material, and after exposure to light a silver image developed by heat is obtained from the decom- posed silver salt. Studies on the permanence of these images {Kurttila, 1977) predict a useful life varying up to 100 years. These films differ from conventional black-and-white silver images in that the latter have unwanted by-products removed during processing. Thermally developed silver films are not considered suitable as an archival medium. In addition to the stability of the film support and the photographic image layer, it is essential that there be good adhesion between the two and that this adhesion not deteriorate upon aging. Two adhesion tests are described in the American National Standards Institute specifications for archival film (ANSI/ ASC PH1.28 and PH1.41J. A tape stripping test measures the existing adhesion, and a humidity cycling test measures the stability of the adhesion after repeated cycles. This condition is particularly severe on adhesion properties. All photo- graphic films that are used for archival storage should pass these specifications. STORAGE STANDARDS The storage life of photographic film is dependent on the interactions among three separate factors: (aJ the film type, lb) the photographic processing, and IcJ the storage conditions. Both American national and international standards are now available that cover all three of these factors. Each is discussed here in turn. Inasmuch as this report deals primarily with the preservation of historical records and not with distribution of information on film, only black-and-white silver halide film will be considered. It is the only film type that is considered to be archival, i.e., suitable for the storage of information having permanent value. Other types of film {diazo, vesicular, etc. J are used only for producing inexpensive copies of the negative film. All films in use today must pass the requirements for safety film, to distinguish them from the older flammable and unstable nitrate- base film. The following approved American National Standards are applicable to archival films: ANSI PH1.25-1984, for Photography {FilmJ- Safety Photographic Film; ANSI/ASC PH1.28-1984, for Photography {FilmJ- Archival Records, Silver- GelatinType, on Cellulose Ester Base; andANSI/ASC PH1.41-1984, for Photogra- phy {Filmy Archival Records, Silver-Gelatin Type, on Polyester Base. Processing of exposed film can be successfully done in many different manu- facturers' brands of processors. There are procedures that must be followed to ensure that archival film is obtained. The specifications for maximum levels of residual silver halide salts and processing chemicals are given in ANSI/ASC speci

OCR for page 49
56 PRESER VATION OF HISTORICAL RECORDS fications PH1.28 and PH1.41. The most critical processing chemical is the thiosulfate ion, which can cause image staining if at too high a level. It can be determined by the methylene blue method as described in ANSI/ASC PH4.8- 1985, Residual Thiosulfate and Other Chemicals in Film, Plates, and Papers Determination and Measurement. Archival storage conditions are absolutely essential for the films. The full potential of storage life will only be realized if the film is stored under optimum conditions. These conditions are under the control of the film user, and the user's role in film preservation must be understood by all who are concerned with the film life. The only real disadvantage to photographic film is the necessity for a controlled environment for archival storage. The average temperature should not be over 68F {20C), and the relative humidity should ideally be between 30 and 40 percent. Silver halide film should be stored by itself in a secure controlled environ- ment. If it is stored with other material, particular care should be taken to remove all materials that give off contaminants. The air should be filtered for particle contaminants, and harmful gases should be removed or neutralized. Examples of harmful gases are nitrogen dioxide, peroxides, ammonia, sulfur dioxide, and some paint fumes that may cause deterioration of the base and a chemical degradation of the photographic image. Adverse experience has been reported when photographic images are exposed to nitrogen dioxide {Carroll and Calhoun, 1955~. This is of practical importance in that nitrate-base films should never be stored in the same area as films having archival value. Another adverse reaction to air pollutants is the appearance of microspots or microblemishes when film is exposed to ozone, peroxides, or oxidizing gases {McCamy, 1964; Henn et al., 1965~. Although changes in photographic processing have resulted in film becoming much more resistant to this type of defect, care must be exercised so that photographic images are not stored in an oxidizing atmosphere tWeyde, 1972~. These concerns about the environment also apply to other storage media and are discussed in greater detail in Chapter 3. Even though proper storage is a neces- sity for archival permanence of film, it is achievable with presently known tech- nology and readily available machinery. In addition to the storage environment, there are specifications for enclosure materials {cartons, cans, jackets, etc.) that are stored with the film. The following standards must be followed: ANSI PH 1.43- 1985, for Photography {Film) Storage of Processed Safety Film; and ANSI PH1.53-1984, for Photography {Processing)-Processed Films, Plates, and Papers Filing Enclosures and Containers for Storage. In addition to these standards on film material, photographic processing, and storage, there are also the following standards on the formats used to make reduced photographic copies on microfilm and the required inspection and quality control: ANSI/NMA MS23-1983, Practice for Operational Procedures Inspec- tion and Quality Control of First-Generation, Silver-Gelatin Microfilm of Docu- ments; and ANSI/NMA MS14-1978, Specifications for 16 and 35-mm Microfilms in Roll Form. Great care has been taken to ensure that manufacturers, users, industry asso- ciations, and standards bureaus have all had appropriate input into these stan- dards. Consequently, they represent the best information available and should be used as the foundation for any National Archives filming program.

OCR for page 49
PHOTOGMPHIC FILM MICROFILM USES 57 The versatility of microfilms makes them easily adaptable to various archival needs. The choice of format may be dictated by many factors: 1. Size and volume of original records 2. Physical condition, binding, and storage method of original records 3. Frequency of use of records 4. Needs or demand for distribution of information Economics and money available Equipment and manpower available The most economical format is roll film, particularly 16-mm when using a high-reduction filming. The roll is less expensive to purchase, and filming takes only half the time of a step-and-repeat camera, which uses a fiche {i.e., sheet film) format. Large files of records with infrequent research use are best suited for the roll format {Powell, 1985~. The microfiche format is usually best when the use level is high and large numbers of copies are to be distributed. The other advantage of fiche is the ability to have a complete publication Project or file) on one fiche. Fiche readers are less expensive than roll-film readers. With the versatility and flexibility of microfilm, uses are limited only by one's ingenuity and/or needs. Filmed images can be used as originally filmed or changed. For example, if roll film is used, it can later be put in fiche format by specialized equipment that cuts and jackets the film to form microfiche. If the information on film warrants the expense of optical disk, then the film can be scanned and the image also put on optical disk. Film can be indexed for quick retrieval. The index can be completed manually and then filmed for preservation, security, and distribution, or it can be done on computer and distributed on computer-output microfilm. It is recommended that documents to be filmed have an image or frame number and a blip that can be read {counted) electronically to facilitate eventual indexing. Microfilm has the advantage of data compaction. Depending on the format used, the storage space saved over that needed for the original documents ranges from 92 to 95 percent. This is illustrated in Table 5-2, which shows the average number of pages in a microfilm roll and microfiche. TRENDS The present trend in the micrographics industry is toward 16-mm film and away from 35-mm film. There is also a definite trend toward computer-aided retrieval. An index is made with the help of computer technology, and the desired file or frame location is reported to the researcher. This can be an "on-line" system or a fiche produced from computer-output microfilm. A new technique for enhancement of images that are not easily readable is reportedly being prepared for commercial availability. There continues to be a despecialization in the use of film. At one time only specialists filmed and processed microfilm. Now micrographics is being used in an

OCR for page 49
58 PRESERVATION OF HISTORICAL RECORDS TABLE 5-2 Format and Compaction of Microfilm Film Size Roll film C 16 mm x 125 it 24 X 24 X 42 X 42 X 15 X 19 X Number of Pages Reduction per Rolla Formatb 35 mm x 125 it 6000& 3000~ 60003 3000& 2000e 2200 Fiche 4 x 6 in. 24 X 98 48 X 420 Cine mode, double page Comie mode, single page Cine mode, double page Comie mode, single page Comic mode aPages filmed simplex {one document at a time} comic formal, 2 (8~/z x 11 in.) pages per exposure. bCine: Documents filmed with pages or frames arranged as in a motion picture film with the writing perpendicular to the edges of the film; Comie: Documents filmed with images running with top of the document at top of film and writing from left to right. Thickness of polyester-base film is 0.004 in. The horizontal space on 1 6-mm film is the same for both 24 X and 42 X reduction, as the cameras have a fixed film advance. The advantage of higher reduction filming is that single large documents or two smaller documents can be filmed in a single exposure. eAdditional frames can be realized on 35-mm film by filming in cine mode. Office environment along with word processors, filing cabinets, etc., by regular office personnel. ADVANTAGES AND DISADVANTAGES Advantages Film has a number of advantages as an archival material: 1. Except for physical details such as color notations, watermarks, etc., rnicro- film preserves the intellectual content {the written word} in the original form because it is an exact picture of the original. 2. The life of microfilms is predictable, provided that present standards are met. 3. There is significant compaction of data. 4. Copies of microfilm are easily and economically made on silver, diazo, or vesicular film. 5. Retrieval of film is easily accomplished. If the system is correctly designed, retrieval of any roll or fiche should be less than 5 minutes. 6. The micrographics industry is a mature industry, with accepted standards already in existence. 7. Micrographics is not a high-technology industry requiring rapidly outdated hardware that is software- or computer-dependent; quite simple optical devices can be used to read the film.

OCR for page 49
PHOTOGRAPHIC FILM Disadvantages The disadvantages of film are as follows: 59 1. Microfilm can be damaged or destroyed by storage at high humidity. 2. Microfilm can be damaged by gaseous pollutants in the storage environ ment. 3. If the original documents are brittle or damaged, it may be difficult or impos sible to film them. 4. Copying onto microfilm requires verification as to photographic quality and content. 5. Microfilm can be damaged by careless handling. REFERENCES Adelstein, P. Z., and J. L. McCrea. 1977. Dark image stability of diazo film. J. Appl. Photogr. Eng., 3~3, Summer}:173-177. Adelstein, P. Z., and J. L. McCrea. 1981. Stability of processed polyester base photographic films. J. Appl. Photogr. Eng., 7(6, December): 160-166. Adelstein, P. Z., C. L. Graham, and L. E. West. 1970. Preservation of motion-picture color films having permanent value. J. Soc. Motion Pict. Telev. Eng., 79(November1:1011-1018. Bard, C. C., G. W. Larson, H. Hammond, and C. Packard. 1980. Predicting long-term dark storage dye stability characteristics of color photographic products from short-term tests. J. Appl. Photogr. Eng., 6(April):42-45. Brown, D. W., R. E. Lowry, and L. E. Smith.1984. Prediction of the Long Term Stability of Polyester- Based Recording Media. NBSIR 84-2988 December). Gaithersburg, Maryland: U.S. Depart- ment of Commerce, National Bureau of Standards. Carroll, J. F., and J. M. Calhoun.1955. Effect of nitrogen oxide gases on processed acetate film. J. Soc. Motion Pict. Telev. Eng., 64tSeptember):501-507. Crespo Nogueira, C.1982. The use of microfilm as a means of archival preservation. Pp.3-8 in Proc. 21stInt. Conf. Round Table on Archives, KualaLumpur (November-December 1982). Gille, B. 1953. Esquisse diun plan de normalization pour le microfilmage des archives. Archivim m:87-103. Paris: International Council on Archives. Henn, R. W., D. G. Wiest, and B. D. Mack. 1965. Microscopic spots in processed microfilm: The effect of iodide. Photogr. Sci. Eng., 9~3, May-June):167-173. Kopperl, D. F., G. W. Larson, B. A. Hutchins, and C. C. Bard. 1982. A method to predict the effect of residual thiosulfate content on the long-term image-stability characteristics of radiographic films. J. Appl. Photogr. Eng., 8~2, April}:83-89. Kurttila, K. R. 1977. DrY silver film stabilitY. J. Micro.gr., 10(3, JanuaryJ:113-117. Leisenger, A. H. 1976. Report of the microfilming committee of the International Council on Archives. Archivim XVI:140. Paris: International Council on Archives. McCamy, C. S. 1964. Inspection of Processed Photographic Record Films for Aging Blemishes. National Bureau of Standards Handbook 96 (January 24). Potter, E. W., and A. T. Ram. 1975. Stability of vesicular microfilm images III. Paper presented at Annual Society of Photographic Science and Engineers Conference, Denver, Colorado, May 1975. Powell, T. F. 1985. The miracle of microfilm: The foundation of the largest genealogical record collection in the world. Microform Rev., 14j3, Summer):148-156. Ram, A. T., and E. W. Potter.1970. Stability of vesicular microfilm images II. Photogr. Sci. Eng., 14(4, July):283-288. Seoka, Y., S. Kubodera, T. Aono, and M. Hirano. 1982. Some problems in the evaluation of color image stability. J. Appl. Photogr. Eng., 812, April):79-82. Weyde, E.1972. A simple test to identify gases which destroy silver images. Photogr. Sci. Eng., 16(4, July-August} :283-286.

OCR for page 49
- E: ~ ' - - ~ :~ F~ :~: - - ~: Magnetic tape storage area. Vast amounts of data can be preserved compactly by using magnetic recording media.