Small but Mighty: Nano-Cellulose Applications in Photograph Conservation | by Cleveland Museum of Art | CMA Thinker | Sep, 2022

By Marissa Maynard

The Cleveland Museum of Art’s (CMA) superb photography collection encompasses the history of the medium and offers a tremendous range, from cased photographs to salted paper prints to colored digital prints. For most photographic processes, the image layer is composed of silver particles in an emulsion. Some of the trickiest conservation treatments for photographs are in-painting and creating fills for missing emulsion. It is difficult to match the color and gloss that vary from photo to photo, and even when highly nuanced, these variations are perceptible to the human eye. As part of my Andrew W. Mellon Fellowship in the CMA’s conservation department, I am conducting research on the application of cast nanocellulose films in photograph conservation (fig. 1), which has the potential to expand treatment options for a wide variety of photographic materials in the CMA’s collection.

These following three black-and-white photographs nicely illustrate the varied tonal range we encounter with photographs. Although not captured here, each photograph also varies significantly in gloss: the platinum print (fig. 2) is essentially without gloss (matte); the albumen print (fig. 3) carries a soft sheen (semigloss); the silver gelatin print (fig. 4) has a shiny finish (high gloss) associated with a surface burnished by drying against a highly polished metal surface in a process called ferrotyping. Two of the works below (2009.13 and 1976.156) are currently on view in gallery 242B in the permanent collection rotation Text and Image in Southern Asia​.

Before discussing the nitty gritty of my project, let’s take a closer look at nanocellulose. A natural polymer that comes from plants, cellulose is the main component of paper. Nanocellulose is a highly purified and processed cellulose composed of minute fiber fragments that are measured on a molecular scale. Developed originally for the pharmaceutical industry as an antimicrobial coating, it has moved into other industries, including food as a replacement for carbohydrate fillers, and the tech industry as a substrate for electronic devices.

Nanocellulose, which is still fairly new to the conservation field, can be used for both structural and cosmetic treatments. It has a defined structure and can be cast to form thin transparent sheets (fig. 5) with properties useful to both photograph and paper conservation treatments. There are different types of nanocellulose, but conservators tend to work with cellulose nanofibrils (CNFs), which are composed of entangled fibrils (tiny fibers) and have a flexibility similar to paper. The high tensile strength of nanocellulose makes it a suitable material for repairing tears in paper. Conservators also value nanocellulose for its transparency, variable gloss levels, and ability to take up color, all features that expand its use to cosmetic treatments such as filling and disguising losses to the emulsion layer and image in a photograph.

Intrigued by the material’s potential in conservation treatments, my research was designed to explore several properties of nanocellulose. In the context of tear repair and image-loss compensation on a photograph, thickness and tensile strength are important working properties. Gloss, transparency, and color are important optical properties for aesthetic fills.

Nanocellulose is fairly easy to prepare for casting, requiring just water, something to stir with, and a plastic petri dish or a silicone mold. For my project, I used a 2.4% suspension of microfibrillated (MFC) nanocellulose in water.

First, I mixed a small amount of the stock nanocellulose suspension with water and allowed it stir for 10 minutes. In a separate beaker I mixed a coloring material with a small amount of water. Altogether, I tested four different coloring materials: watercolor, QoR colors or watercolors soluble in both ethanol and water, Carriage House pigments (colorant used for dying paper pulp), and acrylics. When adequately mixed, the nanocellulose and color solutions were combined and stirred for another 10 minutes. Once a homogenous dispersion was achieved, I poured equal amounts into three-inch silicone molds and let them air dry for about a week (fig. 6).

Once the material was dry, I carefully removed the sheets from the molds and took pictures using different light sources (fig. 7) to document the results. Transmitted light (below right) uses light passing through the sheet and is ideal for evaluating color distribution. I also described my observations regarding color distribution, degree of gloss (how shiny or matte) on both sides of the cast sheets, how easily the sheets came out of the mold, and whether any color transferred when handled. We don’t want color to transfer to our photographs! I used all these observations to inform and modify my procedure to improve the sheets in each subsequent trial. The goal was an intact uniformly thick sheet with even color dispersion.

The next hurdle was how to apply or adhere the nanocellulose sheets to a photograph. I tried several adhesives including gelatin, methylcellulose, Klucel G, and wheat starch paste, all of which are commonly used in paper and photograph conservation. I applied them to a practice piece of Japanese paper (fig. 8) and made note of which worked best.

Once the trials and prep work were complete, the next step was to practice using a variety of the most successful samples. I ordered a set of 100 vintage photographs from eBay that included albumen and silver gelatin prints and exhibited a range of condition problems. I made sure to select a variety of tone and gloss for my practice treatments.

For a photograph with missing emulsion (fig. 9), I chose a cast nanocellulose with the closest tonal match. I applied methylcellulose to the loss and then quickly placed the piece of cast nanocellulose over the loss using Mylar as a barrier (figs. 10, 11). Due to its thinness and sensitivity to moisture, the nanocellulose can easily be impressed with texture — including fingerprints — so the Mylar barrier is necessary. No one wants extra fingerprints on their photographs!

This research continues; for example, another important property to match with nanocellulose fills is surface texture. The preliminary test cases show that toned nanocellulose is a possible treatment solution to compensate for image loss in complex photographic materials.