Scientists unlock the chemical secrets of a 19th-century photography technique

Mordançage is a photographic process that yields striking black-and-white photographs characterized by ghostly veiling effects. Scientists from George Mason University have recently figured out precisely what is happening chemically during the process, according to a recent paper in Analytical Chemistry.

Mordançage has its roots in a late 19th-century method of changing a film negative to a positive, first documented by a man named Paul Liesegang in 1897. In the 1960s, French photographer Jean-Pierre Sudre further refined the technique to produce silver gelatin prints and dubbed it “Mordançage.” It’s also known as “etch-bleaching,” because it uses an acid-copper bleaching solution to dissolve the darker parts of the silver gelatin layer so that it partially lifts away from the print. Those areas can either be rubbed away, creating an image reversal, or retained to produce a veiling effect.

The solution is then rinsed off in a water bath (an additional stop-bath step is optional), then the print is redeveloped to restore the black color before being dried and pressed flat. The result: those dark areas that had lifted from the paper during the earlier stage of the process are preserved to produce the ghostly final veils. Sudre’s American protegé, Elizabeth Opalenik, is perhaps the best-known photographer who uses the process.

Mordançage works best for images with lots of black or detailed dark patterns since darker areas experience more dissolution. The most common solution consists of copper chloride, citric acid (or glacial acetic acid), and hydrogen peroxide mixed with distilled water.

There have been several patents relating to a similar chemical process, according to GMU chemists Caroline Fudala and Rebecca Jones, the authors of this latest paper. This suggests that the copper chloride plays a significant role in the initial bleaching. Hydrogen peroxide is known to soften gelatin during bleaching and hence likely also plays a role. Opalenik has said that paper type is also a factor in controlling the process.

But there have been no rigorous experiments to back up these assertions. “Clearly, the various components of the process are interacting with the silver gelatin photographic print,” the authors wrote. “However, the chemical details and processes, especially regarding the formation of veils, has remained obscured.”

So Fudala and Jones performed a series of experiments to put those assertions to the test. First, they created black-and-white photographs using 35mm film and standard darkroom procedures, using different kinds of photographic papers (multi-grade resin-coated, fiber based, and cotton rag). They used a filter to increase contrast in the images and an enlarger to project images onto the photographic paper.

Once the prints were dry, they subjected them to the Mordançage process. During these experiments, Fudala and Jones measured how much hydrogen peroxide was absorbed by the paper, and they noted correlations between how long a print was submerged in the solution and the type of photographic paper used. They also performed spectral and pH measurements to determine the effects of the copper chloride.

They found that the hydrogen peroxide and acetic acid served to soften the photographic paper. That meant the copper chloride was better able to permeate it and oxidize the metallic silver, turning it to silver chloride as the surface layers lift off as veils. The veils darken again during redevelopment because the silver chloride returns to metallic silver. The emulsion lift continues in the water bath phase, while the optional stop-bath step reacts with residual hydroxide to neutralize redevelopment.

“The final drying step gives the artist one opportunity for control,” the authors wrote. “When making the final transfer from the water-bath solution, the veils succumb to gravity and, depending upon how the print is finally moved from the solution, reattach to the substrate in strategic locations.”

The GMU experiments also confirmed Opalenik’s assertion that the type of photographic paper used matters. Cotton-rag paper is the most permeable and had the most dramatic reaction to the Mordançage solution, i.e., the greatest emulsion lift, compared to the resin-coated and fiber-based photograph papers. So Fudala and Jones used cotton-rag photographic paper to perform control experiments to explore further the effects of each of the solutions’ chemical components (copper chloride, hydrogen peroxide, and glacial acetic acid).

The results: prints processed in a solution with just copper chloride and water bleached the print but did not produce any emulsion lift. Prints processed in a solution of hydrogen peroxide and water, or acetic acid and water, showed no bleaching or emulsion lift at all. It takes the combination of all three components in the solution to get the full veiling effect. Finally, as expected, the longer a print was in the solution, the more emulsion lift occurred, and the more pronounced the resulting veiling effect in the final photograph.

DOI: Analytical Chemistry, 2019. 10.1021/acs.analchem.9b03205″  (About DOIs).

https://arstechnica.com/?p=1597167