[Image above] An example of the composite positive transparencies from the Animal Locomotion series (Series 1478, Plate 598). This series of glass photographic plates are widely regarded as the immediate predecessor to the development of motion pictures. Credit: National Museum of American History
Glass is a remarkably stable material that can last for millions of years in the right conditions. That does not mean it is immune to environmental stimuli, however.
Take, for example, silicate glasses. In aqueous environments, silicate glasses will react with water and begin to dissolve. This dissolution is governed by several operative mechanisms, namely ion exchange, matrix dissolution, accelerated matrix dissolution, and surface layer formation.
Geologists have intensively investigated these reactions to better understand the geochemical/biogeochemical weathering that governs element cycles in the ocean and Earth’s crust. But in the past few decades, these reactions also gained prominence in the energy community because borosilicate glasses are commonly used to immobilize nuclear waste. Understanding silicate dissolution is key to predicting radionuclide releases from these waste forms.
While these two communities conduct the bulk of research on silicate dissolution, there are other areas that benefit from this understanding of the dissolution process. For example, museum conservators working to preserve glass photographic plates. When the daguerreotype, the first photographic process, debuted to the public in the 1830s, the material used to capture the images was a highly polished, silver-plated copper sheet. This material served as the basis of photography for the next 20 years, until the invention of a collodion wet plate process in the 1850s.
The collodion wet plate process involved pouring a 2% solution of collodion over a glass plate and then placing the plate in a solution of silver nitrate. When removed from the silver, the collodion film contained a translucent yellow compound of light-sensitive silver iodide. The plate was exposed while still wet and then developed by inspection under red light.
Unlike the daguerreotype process, which only produced a single image at a time, the collodion wet plate process created a negative image that could be copied numerous times onto paper for wide distribution. However, disadvantages of this technique include the need for a darkroom and a time limit on developing the plate before the collodion emulsion dried.
In the 1870s, a dry plate process was developed. Unlike the collodion wet plate process, which required the plate to be prepared just before exposure and developed immediately after, the dry plate process involved coating a glass plate with a gelatin emulsion of silver bromide. The prepared plate could then be stored until exposure, and after exposure, it could be brought back to a darkroom for development at leisure.
The dry plate process allowed for the mass production of both negatives and positive transparencies, popularly known as lantern slides. Some of the most well-known positive transparencies produced using this technique are from the 1887 Animal Locomotion series by Eadweard Muybridge, which depict humans and various animals performing simple motions and are widely regarded as the immediate predecessor to the development of motion pictures.
The Smithsonian Institution’s National Museum of American History houses 528 composite positive transparencies from the Animal Locomotion series. (The composites consist of individual transparencies arranged in sequence on a larger glass support panel.) As these photographs are now approximately 150 years old, they are beginning to exhibit a range of degradation phenomena and products, including alteration of the glass plates.
“Alteration” refers to the dissolution mechanism mentioned above, formation of surface (alteration) layers. Silica from the glass, along with other glass components or exogenous elements supplied by the solution, form an amorphous, porous, and hydrated interfacial surface layer by two processes.
- Reorganization via hydrolysis/condensation reactions, in which SiO2 tetrahedra are not completely detached from the glassy network.
- Dissolution and reprecipitation of species in the aqueous solution.
Even without alteration, glass photographic plates commonly experience delamination of the image-containing gelatin layer from the glass support. This delamination occurs because gelatin coatings, when subjected to significant changes in relative humidity, tend to curl, creating tensile stress at the interface with the photographic support. Understanding the role an alteration layer may play in this existing delamination process can aid conservators in improving preservation protocols.
In a recent paper, researchers from the Smithsonian Museum Conservation Institute worked with a collections specialist in the Photographic History Collection at the National Museum of American History to explore the role of the alteration layer in delamination of the Animal Locomotion glass plates.
For their study, they examined previously broken fragments of both the image-bearing glass plates and larger support glass panels. Compositions of the glasses were obtained through quantitative scanning electron microscope–energy-dispersive spectrometry, while electron imaging was used to perform cross-sectional analysis.
Imaging identified a visible sodium-depleted alteration layer (~5 μm thick) on the image-bearing glass plates. The layer was apparent on both the gelatin-coated and uncoated faces of the glass. In scanning electron microscope images of the glass, the researchers clearly saw that as the gelatin lifted off the glass, it carried with it the alteration layer (seen as the bright white line under the delaminated gelatin layer in the backscattered electron image below).
This observation “suggests that while the delamination of the image layer observed in these objects is driven by the tensile stress caused by the curling of the polymeric gelatin, the interfacial point of failure is not between the gelatin and the glass, but between the glass alteration layer and the underlying unaltered glass,” the researchers write.
As such, “It is plausible that had the glass not been altered, the curling of the gelatin would not have been sufficient to cause delamination from an unaltered glass surface,” they add.
The researchers conclude that further modeling is needed to fully explain the relationship between the alteration layer and delamination of the gelatin. Additionally, further modeling is needed on the mechanical properties and dissolution mechanisms of glasses with more complex chemistries, such as these photographic plates.
The paper, published in Journal of the American Ceramic Society, is “Glass alteration in 19th century glass photographic plates: Potential role in gelatin delamination” (DOI: 10.1111/jace.18880).