PHOTOGRAPHIC MATERIALS SCIENCE AT THE CONSERVATION ANALYTICAL LABORATORY
MARKH. MCCORMICK-GOODHART, CONSERVATION ANALYTICAL LABORATORY,
SMITHSONIAN INSTITUTION
The Conservation Analytical Laboratory (CAL) established a photographic materials science program in 1988. Fundamental research is conducted on the deterioration of photographic materials, and assistance is given to conservators and curators at the Smithsonian Institution with material analyses, process identification, and preservation issues. Results from two areas of recent research are summarized:
THE DETERIORATION OF WET-PLATE NEGATIVES
The National Portrait Gallery has 5445 wet-plate negatives made by the Mathew Brady Studios during the 1860s. These plates now comprise the Frederick Hill Meserve collection and share a common heritage with wet-plate negatives at the Library of Congress and at the National Archives. Bulk glass composition was determined by electron microprobe analysis and confirmed that numerous batches of predominantly soda lime glass were used by the Washington and New York studios over the span of several years. However, the chemically deteriorated images resided on only two batches of glass, and these plates account for 11% of the collection. The two batches were less durable glass, characterized by high alkali oxide content and lower alkaline earth oxide content. Sodium leaching from the glass into the collodion and varnish layers promoted cracking and flaking, and in the most severe cases, total saponification of the varnish. The saponification reaction was identified by FTIR analysis of the degraded varnishes. No plates were found where present image quality could be attributed solely to cellulose nitrate degradation or to poor processing by the Brady studios. Because the varnish and collodion layers are very thin (typically less than 10 microns total), comparatively small amounts of glass corrosion at the collodion-glass interface initiate chemical degradation of the image coatings. Nevertheless, the wet-plate process was executed on reasonably stable glass in most cases, and these images have endured remarkably well.
COLD STORAGE ENVIRONMENTS FOR PHOTOGRAPHIC MATERIALS
The benefits of cold storage for slowing the deterioration of photographic materials, especially chromogenic color systems, are widely recognized. Unfortunately, implementation is challenging and much difficulty stems from the methods of achieving and maintaining the recommended humidity levels within the cold storage facility. Concerns about stress on the objects as they are cycled between storage and user environment have also been raised.
Under the direction of Dr. Marion Mecklenburg, CAL does extensive materials testing of polymers. The response of multi-layered polymeric structures to changes in temperature and relative humidity have been successfully predicted at CAL by computer modeling based on the method of finite element analysis. The first application of the computer modeling technique was in the study of paintings. During 1992, the same methodology has been extended to photographic materials with outstanding results. The dimensional response and internal stress in a Cibachrome color print was accurately calculated as it responded to changes in relative humidity and temperature. A 30 percent change in relative humidity was of particular interest, because it can easily occur in the real world when a film is conditioned to low relative humidity prior to or upon entering cold storage. A stress level over 4000 PSI was developed in the Cibachrome gelatin layers due to this drop in relative humidity. Lowering temperature from 24°C to -18°C caused a 1200 PSI stress increase. The stress levels are additive when changes occur in both temperature and humidity. Also, the general behavior modeled for Cibachrome is expected to hold true for all photographic materials containing gelatin layers.
New photographic materials can withstand stresses in excess of 5000 PSI without crack initiation or delamination, but these levels cannot be ignored, especially where older samples with weakened adhe-sion are present in a collection. For example, the "channeling" phenomenon in deteriorating acetate base film collections can be triggered. The important point for cold storage designs and for storage and use environments in general is that humidity induced stresses are best managed by avoiding moisture des-orption paths. Often utilized conditioning procedures establish moisture equilibration but do not eliminate the majority of the active stress.
Finally, the practical chemical benefits of low humidity versus moderate humidity cold vaults were re-examined using existing dye stability data. The objective was to determine how much chemical stabi 1 i ty would really be sacrificed if the humidity controls were raised, for example, from 30%RH to 50% RH. An important aspect of this evaluation was that time out of storage was included in the overall rating. A table accompanies this report. It lists effective dye fading rates for various temperature, relative humidity, and time out of storage combinations. Small amounts of time out of storage were determined to play a critical role in the overall effectiveness of cold storage when temperatures approach commercial freezer levels of —18°C. One or two extra days per year out of storage can cancel out the chemical stability improvement that would be expected by setting low relative humidity within the vault or sealed package. As the storage temperature increases, time out of storage becomes less significant. Low relative humidity then appears to give a chemical benefit. The paradox is that this same benefit can be met at moderate humidity simply by selecting a slightly lower temperature value.
In conclusion, the fading rate data demonstrate that the temperature parameter can be independently set to achieve any sustainable level of chemical stability. Coincidentally, the stress analysis shows that lowering temperature is also far less stressful than lowering humidity. Therefore, the humidity parameter seems better suited to the purpose of managing mechanical stress rather than fine tuning chemical stability goals. None of the facts presented in this report justify the acceptance of high humidity conditions. High humidity conditions are well documented in terms of their danger to photographic materials, but an analytical approach that considers both chemical and structural stability does provide a rational basis for understanding the total role of the humidity and temperature parameters. Hopefully, this research
will contribute to a greater understanding of useful storage and exhibition environments for photographic collections.
RELEVANT PAPERS
M.H. McCormick-Goodhart, "An Analysis of Image Deterioration in Wet-Plate Negatives from the Mathew Brady Studios." Journal of Imaging Science and Technology. Vol. 36, No 3., 297-305, 1992.
Submitted or In Press:
M.H. McCormick-Goodhart and M.F. Mecklenburg, "Cold Storage Environments for Photographic Materials. "Submitted to the Journal of Imaging Science and Technology. June, 1992.
M.F. Mecklenburg, C.S. Tumosa, and M.H. McCormick-Goodhart, "A General Method for Determining the Mechanical Properties Needed for the Computer Analysis ofPolymeric Structures Subjected to Changes in Temperature and Relative Humidity." Materials Issues in Art & Archaeology III. Materials Research Society Proceedings, Vol. 283, P.B.Vandiver, J. Druzik, G.S. Wheeler, and I.C. Freestone, Eds., Pittsburgh, Pa., (in Press, 1992).
M.H. McCormick-Goodhart, "Glass Corrosion and its Relation to Image Deterioration in Collodion Wet-plate Negatives," Conference '92: The Imperfect Image, Photographs their Past, Present and Future. The Centre for Photographic Conservation: London, England (in Press, 1992).
Mark H. McCormick-Goodhart joined the Smithsonian Institution in 1988 as a research photographic scientist at the Conservation Analytical Laboratory. He holds a B.S. degree in Photographic Science from Rochester Institute of Technology, and was formerly employed by Energy Conversion Devices, Inc., from 1976 to 1988, where he was granted eight U.S. patents related to non-silver film and electronic imaging technology. His present research concerns the effects of environment on structural properties of photographs, and cold storage of photographic materials.
† Average dark fading rates for chromogenic color dyes relative to an environmental condition of 24°C, 40% RH. Table values are reciprocals to the effective fading rate. †† The standard condition (24°C, 40% RH) was assumed during the time that an object is out of cold storage. Underlined table values correspond to rates and conditions listed in "Conservation of Photographs". Publication No. F-40, Eastman Kodak Co.,1985. All other values in the 0 days/year column are interpolated from the primary data.