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

A black-and-white photograph treated with the mordanҫage process shows characteristic 'veils' deposited on dark areas of the print.

Enlarge / A black-and-white
photograph treated with the mordanҫage process shows
characteristic ‘veils’ deposited on dark areas of the print.
(credit: Fudala and
Jones/Analytical Chemistry
)

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 protege, Elizabeth Opalenik,
is perhaps the best-known photographer who uses the process.

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Source: FS – All – Science – News
Scientists unlock the chemical secrets of a 19th-century photography technique