Gelatin and air pollution

Pollutant gases are known to be very damaging to both the silver image and the gelatin binding of photographs. The mechanisms of photographic gelatin deterioration have received less attention than have those of silver imaging deterioration.

At the CRCDG, Thi-Phuong Nguyen, Bertrand Lavédrine, and Françoise Flieder investigated the degradation of photographic gelatin caused by nitrogen dioxide and sulfur dioxide. The behavior of photographic gelatin was studied by measuring the degree to which it swells and by conducting high-performance steric chromatography.

The investigation, performed on one unhardened gelatin and two hardened (and more water-resistant) gelatins, showed that all three are sensitive to pollutants. Two major effects were noted: the first was an increased swelling of the gelatin films in demineralized water; the second was a change in the steric exclusion chromatographic profile. Both phenomena indicate that air pollutants induce the hydrolysis of the gelatin macromolecules. Nevertheless, for the same exposure, the hardened gelatins showed less degradation than did the unhardened gelatin. Thus, even if hardening does not protect the photographic gelatin totally from attack by air pollution, it does slow the deterioration process (Nguyen et al 1997).

Monitoring of polyester film degradation

Early methods to characterize degradation of film yielded useful information, but they were time-consuming and destructive. To overcome these problems, color indicators were developed. Unfortunately, few tests exist for polyester film. Michele Edge from the

Manchester Metropolitan University, Department of Chemistry and Materials (UK), is studying new techniques for monitoring the breakdown of polyester film that is used as a support material for both moving images and sound. The investigations focus on the causes of polymer deterioration and on the features of the degradation process that can be quantified. New spectroscopic and optical methods to assess the condition of polyester film are being developed for these purposes (Edge, in press).

Risk assessment of nitrate- and safety-based film collections

Important for the long-term preservation of collections is risk assessment of the constituent materials. On the basis of the results of such an analysis, preservation priorities can be set.

Risk audits were developed for the nitrate- and triacetate (safety)-based motion picture film and for magnetic tape collections. Studies of these two types of materials were carried out by the Image Permanence Institute (IPI, Rochester, New York, USA) and by the now defunct National Media Laboratory (NML, St. Paul, Minnesota, USA), respectively. Their purpose was to assess the present condition of the LC’s film and magnetic tape holdings, evaluate the suitability of existing storage conditions, develop reliable data for planning future storage needs, and specify necessary requirements.

The IPI report reaffirmed choices about film storage that the LC had made in the 1970s; namely, that a cold, dry environment will significantly extend the life of the materials. Film and similar materials stored under ambient conditions would begin to show deterioration after about 40 years, whereas materials stored in the Library’s cold-storage vaults would show such effects after more than 900 years. Stored for most of its life under ambient conditions, black-and-white film in the Library’s holdings showed little image fading, while 40 percent of the color film exhibited moderate-to-severe color shifts. IPI concluded that placing all the film materials in cold storage would slow the deterioration enough to limit the acute threat, so that appropriate preservation and recovery efforts could subsequently be undertaken (Baker et al 1998; Reilly et al 1998).

Stability of nitrate and acetate film

Many photographic collections house numerous images on nitrate and acetate film, both of which are very unstable. Much research has been conducted into the causes of decay of these polymers.

Alain Louvet and Martine Gillet, from the CRCDG, studied film materials from the Harcourt Collection, which contains almost four million negatives, to find out more about these polymers and their stability. Nitrate films in different stages of decay were examined by means of GC, MS, and FTIR. Two plasticizers were identified: camphor and triphenylphosphate. A lack of camphor caused the polymer to become brittle. Triphenylphosphate reacted heavily with nitrogen dioxide, an air pollutant that is known to have an adverse effect on film. An alteration in the molecular structure of aged nitrate polymer was also demonstrated. Acetate films also contained triphenylphosphate as a plasticizer. The degradation of this softener seemed to occur at the same time as did that of the polymer itself. The release of acetic acid appeared to catalyze the decay of the triphenylphosphate (Louvet and Gillet 1999).

Surface tarnishing of daguerreotypes

In the past 15 years, conservators and scientists have studied the complex modes of deterioration of daguerreotypes. One important aspect is the surface tarnishing of the plate; however, because the layers of the tarnish are extremely thin, they are difficult to examine.

Three U.S. researchers, Lee Ann Daffner (Metropolitan Museum of Art, New York), Dan Kushel (Buffalo State College, Buffalo, Art Conservation Department), and John M. Messinger II (University at Buffalo, State University of New York, School of Dental Medicine) investigated the surface tarnish on daguerreotypes by means of short-wave UV illumination. The scope of the investigation included preliminary analysis, characterization of the tarnish, and a study of its rate of occurrence in 110 daguerreotype plates.

About half of plates showed some degree of fluorescence. The presence of this fluorescing tarnish may provide evidence of specific past treatments applied to the daguerreotype plates. The fluorescing tarnish can serve as a guide for monitoring the state of deterioration in daguerreotypes (Daffner et al 1996).

Vinegar syndrome

“Vinegar syndrome” is the process whereby acetic acid is released as a result of decay of acetate film. It is useful to monitor the progression of acid formation so that the condition of a film collection can be assessed more knowledgeably.

The acid-detection technologies of the IPI and of Dancan International Sales (Copenhagen, Denmark) were tested by Ed Zwaneveld and Jean Imbeau from the National Film Board of Canada (St. Laurent) and Ken Weissman from the Library of Congress. The results were presented at the 1999 conference of the Association of Moving Image Archivists (AMIA) in Quebec, Canada.

Zwaneveld and co-workers tested detection strips from Dancan International Sales (Danchek 2-hour acidity tester strip and Danchek monitor window button inserts) and from IPI (24-hour A-D strips). At an earlier stage, the National Film Board of Canada had already tested more than 10,000 film containers using IPI strips. The IPI strips gave more reliable results after 72 hours than they did after the recommended 24 hours. The Danchek strips and buttons also worked best after 72 hours, although the results showed a poor reproducibility as compared with the IPI strips. In general, IPI Film Condition Ratings and Recommended Actions were found to be most satisfactory (Zwaneveld and Imbeau 1999).

Ken Weissman did real-life comparisons of “litmus”-type acid-level detection strips from Dancan International Sales (Danchek 2-hour acidity tester strip), IPI (24 hour A-D strips), and J. T. Baker (Dual-Tint pH special indicator papers). IPI A-D and Danchek strips generally worked as advertised. Dual-Tint also worked well, but the results were subject to interpretation because there was no direct correlation to film deterioration levels (Weissman 1999).



In 1842, Sir John Herschel invented photographic contact printing, which he named cyanotype. The restyled ferroprussiate process, the leading method for photocopying until the mid-1950s, endowed the English language with a new word: the blueprint. The conservation of cyanotypes is a recent subject of preservation research.

Mike Ware, consultant to the National Museum of Photography, Film, and Television (UK) made a comprehensive study of cyanotypes, including their history, chemistry, and conservation. Using Herschel’s experimental notes, Ware elucidated the cyanotype process and established, through investigation of facsimile material, the damage brought about by three pathways of vulnerability:

  1. They were faded by visible light, but this reaction was substantially reversible if the cyanotype was moved to dark storage. Cyanotypes may be safely exhibited at up to 50 lux illumination.
  2. Prussian blue was rapidly and irreversibly hydrolyzed to ferrocyanide and hydrated ferric oxide (sensitivity to bleaching by alkali can be greatly diminished by treatment with nickel (II) salts).
  3. Significant amounts of image substance were irreversibly lost from cyanotypes in aqueous washing (Ware 1999a, 1999b).


Infestation by fungi can affect film in many ways. Fungi are difficult to identify, and currently available fungicides do not eliminate fungal contamination. At the CRCDG, Malalanirina Rakotonirainy, Fabien Fohrer, and Bertrand Lavédrine identified the main species responsible for degradation of the various films in the collection of the Archives du Film in Paris and developed an effective treatment.

To do so, they tested 15 products individually or in combination. The results of the initial in vitro experiments could be reproduced in situ. The findings demonstrated the effectiveness of formaldehyde and Aquasan, a commercial solution containing a quaternary ammonium chloride salt (Rakotonirainy et al, in press).

Laser cleaning of daguerreotypes

Removal of tarnish from daguerreotypes is a long-standing problem. In the past, many daguerreotypes were severely damaged, often to the complete destruction of the silver image, as a consequence of inappropriate cleaning. At present, electrocleaning is the only known method of cleaning gilded daguerreotypes safely; however, it is not suitable for the treatment of ungilded and colored types.

Three Israeli researchers, Igor Turovets (Intel Electronics Ltd.), Michael Maggen (Israel Museum, Jerusalem), and Aaron Lewis (Hebrew University, Jerusalem), studied the old methods of cleaning daguerreotypes and developed a new technique that uses an excimer laser. Their goal was to develop a cleaning method that does not alter the optical properties of these early photographs-e.g., that does not etch or pit the polished silver layer substrate and does not alter or damage the image particles. Their laser-cleaning method permits the cleaning of gilded and ungilded plates, local tarnish removal, and removal of tarnish without immersion in solvents and chemicals (Turovets et al 1998).


Climate-controlled macro- and microenvironments

The preservation of moving images on photographic film is a prime concern today. The serious problems of decay of cellulose triacetate films are well known to film archivists. The development of an adequate conservation approach remains a challenge.

Jean-Louis Bigourdan and James M. Reilly from the IPI worked on a preservation strategy for acetate film based on an environmental assessment and condition survey. They investigated the effectiveness of both the climate-controlled macroenvironment and the microenvironment. At room temperature, they found that tight enclosures had a detrimental effect on film stability. However, open enclosures did not significantly reduce the acid content of the acetate base film. Microenvironments created by absorbents or low preconditioned RH were found to extend the longevity by a factor of three to four. Macroenvironments using low temperatures showed greater potential to improve film stability than did microclimates. The studies demonstrated the utility of the TWPI (Time-Weighted Preservation Index) in evaluating the efficiency of a storage area (Bigourdan and Reilly 1999).

Climate standards

Recommendations for proper storage temperatures and relative humidities are the subject of continued attention in preservation research. The development of climate standards for photographic materials is of special interest, given that safe conditions for these materials generally differ from those for other artifacts.

For the Smithsonian Institution, Mark McCormick-Goodhart studied the allowable temperature and RH ranges for the safe use and storage of photographic materials. The findings indicated that to maintain a constant level of moisture, the RH of a storage environment must be reduced by 3 percent to 4 percent for every 10C drop in temperature. The author concluded that it is extremely important to avoid environmental conditions that cause photographic gelatin to cross its glass transition temperature and revert to a gel state. The acceptable range for storage temperatures was -25C to 25C (McCormick-Goodhart 1996).

Cold storage

In recent years, we have gained a new understanding of the effects of temperature and humidity on photographic materials. Cold storage is now considered the highest standard of care, but it requires the moisture content to be maintained within a safe range.

For the Smithsonian Institution, Mark McCormick-Goodhart from Old Town Editions Inc. (Alexandria, Virginia, USA), evaluated three methods of cold storage that are economical for historical societies and individuals:

  1. Custom dehumidified cold-storage vaults. McCormick-Goodhart is working with Wilhelm Imaging Research, Inc., to investigate housing methods for photographs and motion picture films stored at -20°C in moderately priced, commercially available non-humidity-controlled freezer units.
  2. Specific packaging techniques. The Smithsonian Institution developed a critical moisture indicator (CMI) package that, unlike the high-moisture-barrier package, is a reusable design. As such, it is preferred over the Film Institute Conditioning Apparatus (FICA) system developed at the Swedish Film Institute.
  3. Sealed-gasket cabinets. Models have been developed for the use of sealed-gasket cabinets for passive climate control (McCormick-Goodhart 1999).

Environment and enclosures

The effects of temperature and relative humidity on photographic film are well established. To protect these materials against negative influences of the environment, they are often put in some kind of enclosure.

James M. Reilly, Douglas W. Nishimura, Peter Z. Adelstein, Jean-Louis Bigourdan, and Catherine Erbland, at the IPI, participated in a project entitled “Environment and Enclosures in Film Preservation.” This project completed a cycle of research into the ways in which the storage environment, together with storage enclosures (sleeves, boxes, cans, and cabinets), can be used to extend the useful life of cinema, still, and micrographic films.

The project has yielded several findings:

  • With the exception of completely permeable enclosures such as cardboard boxes, most enclosures had the desirable effect of slowing the rate at which the moisture content of the film inside approaches the equilibrium RH outside the enclosure. However, the thermal equilibration rate was relatively unaffected by enclosures.
  • Enclosures could moderate daily cycling of relative humidity and even seasonal drift, depending on their moisture-buffering capacity.
  • Data on moisture conditioning and thermal equilibration rates of photographic film provided background information on the use of microenvironments.
  • The capacity of buffered paper to neutralize exposure to acetic acid was not directly correlated to its initial alkaline reserve. A significant amount of absorbed acetic acid can coexist with residual alkaline reserve in the paper structure (Bigourdan et al 1996).
  • Buffered and nonbuffered paper enclosures placed in contact with degrading acetate films acted as acid-receptors. As a result of the presence of alkali reserve, buffered paper reduced the acidity; however, the practical impact on the film was limited.
  • The extrapolation to room temperature of results obtained at 50C in a sealed bag remained problematic.
  • Investigations at room temperature showed that the use of a paper envelope was a secondary contributor to the vinegar syndrome. This risk was not eliminated by the use of buffered paper.
  • Although segregation of degrading objects, rehousing, and efficient ventilation are important for collection management, control of temperature and relative humidity remains the most effective preservation strategy for acetate film collections.
  • Studies of moisture-equilibrium rates at low temperatures provided more comprehensive data than had been previously available. These investigations indicated that low-temperature equilibration is slow, but appreciable.
  • On the basis of these findings, the use of moisture-proof packing is recommended for storage in frost-free freezers or in cold storage vaults where the RH exceeds 40 percent.
  • Analysis of gelatin-degradation studies at conditions both above and below the glass transition temperature showed that linear Arrhenius relationships may be obtained, although contradicting results have been reported.

Light conditions

Many photographs and graphic documents quickly deteriorate as a result of exposure to light. Reliable measurement of the light conditions is therefore necessary for effective preservation management. The Blue Wool Standard, a light dosimeter, is often used; however, this scale is not sensitive enough for exhibition of very fragile artifacts such as early or color photographs, which should not be exposed to more than 12,000 lux hours per year.

Bertrand Lavédrine from the CRCDG studied a new light dosimeter for the exhibition of photographs and sensitive artifacts: the Blue Pink Scale (BPS). The BPS enables a precise and sensitive quantification of light exposure. It can monitor low light energy, (i.e., between 5000 and 100,000 lux hours) and allows a visual judgment of the amount of radiation. When exposed to light, the BPS, a card support with a bluish polymer, turns from bluish-purple, to pink, and finally to gray (Lavédrine 1999).


The zeolite molecular traps of MicroChamber’s archival-quality paperboard (see pp. 22-23) make it possible to deal with both airborne pollutant gases and the byproducts of media deterioration. MicroChamber has been widely used in the passive conservation of photographic and film materials.

Brenda Keneghan and Elizabeth Martin from the Photographic Studio of the Victoria and Albert Museum (London, UK) have investigated the use of molecular sieves for extending the life of cellulose acetate photographic negatives. The efficiency of Type 4A molecular sieve as a scavenger for acetic acid was measured using GC/MS. Initial results show a significant reduction of the concentration of acetic acid when the sieve was used over a three-month period. The active lifetime of the scavenger, along with the possible changes in the physical properties of the negatives, is under investigation (Keneghan and Martin 1998; Rempel 1996).

Polystyrene products

There has been debate over whether polystyrene products are safe for storing museum objects. Graphic boards made from polystyrene are often used in storage cabinets to mount slides and other photographic materials. The use of “food-grade” polystyrene, which is safe for food products, has been suggested, but there are conflicting recommendations about whether to use it in conservation.

Scott Williams from the CCI is investigating the suitability of polystyrene products for archival storage. Naturally aged samples will be analyzed for additives and decomposition products; accelerated-aging tests will also be conducted. Williams has already analyzed several products containing polystyrene foams and reported them as safe for conservation applications.